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Word recognition and orthographic context effects in a letter-by-letter reader
BRAIN
AND
LANGUAGE
36,
357-376 (1989)
Word Recognition and Orthographic Context Effects in a Letter-by-Letter Reader DANIEL N. BUB Montreal
Neurological
Institute
and Centre Hospitalier
C&e-des-Neiges
SANDRA BLACK Sunnybrook
Medical
Centre
AND JANICE HOWELL St. Joseph’s Hospital
The performance of letter-by-letter readers when attempting to decipher written material gives the impression that words fail to directly evoke any higher-level representation. As a consequence, spelling patterns appear to be treated perceptually as if they were a collection of random letters. We tested the hypothesis that words are no longer mapped onto orthographic descriptions by examining the ability of a letter-by-letter reader to identify letters in familiar words, pseudowords, and random strings. A clear effect of orthographic context was obtained on the accuracy of letter recognition, indicating that spelling patterns do gain access to more central components of the reading mechanism. The implications of this result for our understanding of the syndrome are discussed. 8 1989 Academic PI~SS, Inc.
Certain acquired dyslexics are unable to identify a word without laboriously naming or sounding out each letter-hence the use of the term letter-by-letter reading to describe the syndrome (Patterson & Kay, 1982), which has also been called spelling dyslexia (Kinsbourne & Warrington, 1962) or word-form dyslexia (Warrington & Shallice, 1980). Early interpreters of this disorder considered it to be the outcome of a more general The preparation and execution of this manuscript was supported by grants from the Medical Research Council of Canada and the Fonds de la Recherche en Sante du Quebec. Address correspondence and reprint requests to Dr. Daniel N. Bub, Montreal Neurological Institute, 3801 University, Montreal, Quebec, Canada H3A 2B4. 357
0093-934x/89 $3.00 Copyright Q 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.
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BLACK,
AND
HOWELL
disturbance in the ability to encode a number of separate visual forms, because of the observed association between letter-by-letter reading and simultanagnosia (Wolpert, 1924, Kinsbourne & Warrington, 1%2). Patients with damage to the occipitotemporal region of the left hemisphere can usually recognize a single percept at brief exposure durations but in many cases are unable to recover the identity of additional elements from a display of two or more items, even when the material is presented for a relatively protracted interval (Kinsboume & Warrington, 1962, 1%3). The claim was made that the breakdown in the simultaneous perception of multiple forms extended to the synthesis of a word from its component letters. Thus, according to Kinsboume and Warrington (1%2), “Only one letter can be read at a time and the interval before the visual system is ready for perception of the next is so long that reading must be a laborious hardship” (p. 481; also see Levine & Calvanio, 1978). On theoretical grounds, there is reason to doubt this interpretation of letter-by-letter reading. The simultanagnosic patient has difficulty retrieving many perceptual units presented at the same time, be they letters, numbers, or line drawings. Written words, of course, also have the status of perceptual wholes once they have been categorized-we may therefore expect simultanagnosia to interfere with the reading of several items presented together, but the deficit should not impair the recognition of an isolated word. Evidence provides support for a functional distinction between the two syndromes. Patients described in a group study by Warrington and Rabin (1971) were found to be very impaired on visual span tasks, yet none of them appeared to fit the clinical description of a letter-by-letter reader. More recently, Warrington and Shallice (1980) have documented a case of letter-by-letter reading who showed only a moderate reduction in visual span, incommensurate with his profound reading disorder. These authors conclude that letter-by-letter reading cannot be attributed to visual or perceptual deficits. They offer the following interpretation of the syndrome: A specialized mechanism, termed the visual word-form system, exists in the left posterior cortex, which parses letter strings into familiar units (ranging in size from graphemes to syllables and whole words), and categorizes them perceptually. This automatic synthesis of letters into higher order reading units allows for rapid and accurate identification of written words. According to Warrington and Shallice, damage to the word-form system produces letter-by-letter reading-the patient can no longer extract familiar perceptual units from a spelling pattern and is forced to read by explicitly identifying each separate letter in the word. While this explanation is certainly attractive (see Patterson & Kay, 1982, for an evaluation), we should point out that no direct evidence has yet been obtained for the claim that the functional impairment responsible
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for letter-by-letter reading is confined to the visual word-form system. Warrington and Shallice have demonstrated only that the syndrome cannot be the outcome of damageto peripheral mechanisms involving the allocation of attention to a string of letters. The word-form system becomes a reasonable candidate as the functional locus of the disorder, but it does not follow from the available results that the assumption is necessarily correct. Patterson and Kay (1982) have provided a different account of letter-by-letter reading, similar to the classical interpretation of pure alexia given by Dejerine (1892). In their version, letter-by-letter reading is due to a disconnection between graphemic analyzers and the wordform system, such that the normal parallel mapping of abstract letter identities onto word-level representation has given way to an effortful sequential process. Our aim in this paper is to obtain relevant evidence for or against the notion that letter-by-letter reading is caused by damage to orthographic word forms. Such evidence would require a two-step procedure: first, the general location of the impairment must be found within a functional architecture of the normal reading mechanism by measuring the influence of particular variables on the performance of the dyslexic patient. Warrington and Shallice (1980), for example, note that the familiarity of words and their semantic properties (i.e., concreteness) had no effect on the accuracy with which their patients could decipher written words. There was also no indication that any recognition or comprehension took place without the letter-by-letter procedure used for explicit report.’ The authors infer that “. , . the letter-by-letter strategy is an attempt to compensate for damage to a stage in the reading process prior to phonological or semantic analysis” (p. 108). Patterson and Kay (1982) have confirmed this interpretation via similar methods in a later investigation of the disorder. A description of the approximate locus of impairment in letter-byletter reading does not in itself justify an explanation of the syndrome based on the concept of a word-form system-we know only that the damage occurs before the word has gained access to higher-level routines that extract sound and meaning. In order to obtain reasonable proof that directly implicates this component, we need to invoke some theoretically motivated claim about the word-form system that would predict a certain result if the damage occurred at the hypothesized location (see Bub & ’ The case documented by Shallice and Saffran (1986) evinced partial comprehension of words he was unable to explicitly identify. Similar capabilities have been reported in a few other letter-by-letter readers (e.g., Coslett & S&ran, in press). Tacit recognition does not appear to be a general feature of the syndrome; however, we have recently examined five letter-by-letter readers on a forced-choice comprehension test of written words presented at 500 msec and have not observed above-chance performance in any of the patients.
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Bub, 1988, for discussion of methodological issues in cognitive neuropsychology). Our approach to the problem relies on the fact that stored knowledge about the orthographic description of words normally enhances recovery of their constituent letters. Numerous experiments, beginning with observations made by Cattell in 1886, have demonstrated that subjects are able to recognize briefly displayed words more accurately than random letter strings. The processing stage responsible for the word superiority effect proved difficulty to specify initially-for example, the effect might simply be due to the subject’s better ability at guessing the identity of a word than a random letter array under limited viewing conditions, or to poorer retention of unfamiliar letter strings in memory. A procedure devised by Wheeler (1970) and Reicher (1969) however, has shown clearly that the advantage for words must be perceptual in origin. The general method behind these experiments is as follows: A fourletter word or random letter sequence is briefly presented to the subject on each trial, followed by a pattern mask. Immediately after this event, two choices are displayed; one of these is the target item, the other is a foil which dilfers from the target by one letter only. Reliable performance cannot be based on guesswork in this task, because the two alternatives are equally plausible. Furthermore, the use of a forced-choice procedure with a brief delay between the stimulus and the response minimizes the load on short-term memory (McClelland & Johnston, 1977).Any advantage in the recognition of words (e.g., McClelland, 1976; Johnston, 1978) can reasonably be attributed to the fact that the letters constituting them are more accurately encoded than a group of random letters-or even a single letter (c.f. McClelland & Johnston, 1977)-that fails to activate a familiar orthographic structure. The knowledge mediating this effect is stored in the visual word-form system and may no longer be available to the letter-by-letter reader. We take it that the hypothesis originally proposed by Warrington and Shallice (1980) dictates that the impairment to the visual word-form system in letter-by-letter reading is so extensive that spelling patterns have the perceptual status of random letter strings to the patient. This basic assumption follows from the concept of the word-form system as a hierarchically organized component with levels of orthographic representation (graphemes, subsyllabic units, syllables, and morphemes) that are increasingly vulnerable to brain damage (c.f. Shallice & McCarthy, 1985). The most extreme condition takes place when the units extracted from print are reduced to single letters. Shallice (1981) writes that the characteristics of letter-by-letter reading, or “word- form dyslexia,” indicate “an artificial mode of reading, where letters are explicitly identified serially . . . and then the word probably recognized from its auditory spelling.
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No higher-level information seems to be available to the patient” ;d. ‘189). In this paper we examine the ability of a letter-by-letter reader to perceptually extract orthographic information from written words. If the current understanding of the syndrome is correct, orthographic structure would have little or no effect on the patient’s ability to recognize a briefly presented string of letters, and the word superiority effect observed in normal readers should be abolished. CASE HISTORY
J.V., a 51-year-old teacher of physics and biology at a high school in Ontario, underwent neurosurgery in 1982 for removal of a benign left temporo-occipital meningioma. Postoperatively, he demonstrated no language deficits other than a very mild anomia, but complained of severe reading difficulties. The impairment eventually forced him to resign his position as head of the Science Department 6 months after surgery. He was seen for a preliminary neuropsychological assessment in September 1983, at which time he continued to manifest a profound dyslexia. No word, even a very short familiar one (e.g., BOY), could be recognized without laboriously decoding the constituent letters. On the average, he required between 3 and 4 set to read a three-letter item. Sentence reading was carried out with such effort that comprehension was impossible unless the material was analyzed repeatedly. Preliminary testing on the Western Aphasia Battery disclosed no significant language deficits in the spoken modality. Writing and spelling were normal; J.V. could accurately write orthographically regular and irregular words taken from a wide range of frequency values. A CT scan obtained after surgery indicated a large hypodense region affecting most of the left occipital lobe (areas 17, 18, and 19) with some sparing laterally and superiorly. The lesion engulfed splenial fibers of the corpus callosum radiating through the forceps major to the left occipital lobe. It extended anteriorly into the periventricular area of the left occipital horn, interrupting optic radiations and other white matter tracts deep to the posterior angular gyrus. The damage effectively prevents access to the left temporoparietal area from both visual cortices and is characteristic of patients with pure alexia and a dense right homonymous hemianopsia. EXPERIMENTAL
TESTS
(1) The Effect of Length on Word Recognition Patients who read letter by letter exhibit dramatic effects of word length on performance, a result that is considered a hallmark of the syndrome. In order to document the extent of J.V.‘s reliance on a letterby-letter strategy, we presented him with two reading tasks in which this variable was systematically manipulated.
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BUB, BLACK, AND HOWELL 10.0
8.0 SECONDS 6.0
4.0
I// I
I
I
I
I
3
4
5
6
WORD LENGTH
FIG. 1. Naming latency as a function of written word length.
(a) Naming latency. Eight lists of words containing three-, four-, five-, or six-letter items were constructed and typed in bold uppercase print on individual sheets of paper. For each word length, a list of 15 stimuli was chosen from higher-frequency items (greater than 55 per million; Kucera & Francis, 1967) and a second list was compiled from lower frequency values (less than 10 per million). All words were monosyllabic. J.V. was asked to read each list from beginning to end and his overall speed for the complete set of 15 words was measured with a stopwatch. Test lists were administered in random order, and J.V. was given one practice sequence of three-letter items before commencing. Results of the average response time per word length are displayed in Fig. 1. Reading latencies are extremely slow even for short words and increase directly with the number of letters. Performance was unaffected by word frequency. J.V. made few errors and self-corrected on all but three of them. (b) Lexical decision speed. A word-nonword decision task was used to obtain a further assessment of J.V.‘s letter-by-letter reading without asking him to explicitly produce the spoken form of a written word. Sixty trials were prepared, (30 words and 30 pronounceable nonsense words), consisting of an equal number of four-, six-, and eight-letter items. Frequency was balanced across word length. Nonwords were created by changing two letters in a set of words that were matched for length to the target items. Stimuli were presented tachistoscopically in a pseudorandom order and were positioned so that the last letter occurred slightly to the left of fixation, to compensate for J.V.‘s dense right visual field deficit. He was asked to indicate his response by pressing one of two keys with his left (“No”) or right (“Yes”) index finger. The keypress stopped a millisecond timer that was triggered by the onset of a
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WORD RECOGNITION
I
ERRORS
NEGATIVE
POSITIVE
DECISION LATENCY
3.0 2.5 2.0 4
6
0
WORD FIG.
2. Lexical decision speed as a function of word length.
stimulus. Each target item was displayed until a response was made, at which time it was replaced by a blank white field containing a black dot in the centre. J.V. was instructed to look directly at the dot at the start of a trial and to respond as soon as he had classified the word or nonword. We emphasized both speed and accuracy of responding. Reaction time and error rates on positive and negative trials are indicated in Fig. 2. Lexical decisions are carried out very slowly and are heavily influenced by length. The slope of the function for nonwords is slightly less than words, most probably due to the fact that J.V. could reach a decision about many negative items on the basis of the first few letters. In general, the results of the two reading tests confirm that J.V. is totally reliant on a letter-by-letter strategy to extract higher-level information from print. (2) Recognition of Letters in Context To what extent do the surface characteristics of J.V.‘s disorder bear on the nature of the underlying functional impairment to his reading mechanism? Certainly, his approach to word recognition may lead one to assume that no direct contact is possible between letter descriptions and visual word forms. The labored decoding of individual letters, however, could be the result of an attempt to compensate for a breakdown in processing that occurs after the structure of a word has been activated. If such activation from print is weak, J.V. might use letter names as a
364
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BLACK,
AND
HOWELL
way of generating an additional source of input to the word-form system, In a recent paper, Shallice and Saffran (1986) have documented a letterby-letter reader who was capable of extracting partial semantic information from written words presented too briefly for him to explicitly identify. While this preserved ability has not been found in other patients (c.f. Warrington & Shallice, 1980; Patterson & Kay, 1982; note l), the evidence does suggest the artificial strategy used for complete recognition may be misleading and that print may gain automatic entry to more central components of the reading mechanism. The question of interest is whether J.V. can perceptually synthesize any higher-level orthographic code from written words, even though his reading gives the general impression that the units of processing are confined to single letters. The search for an answer requires a test of J.V.‘s ability to recover letters from a familiar spelling pattern compared to the letters in a random string. Normal readers can use context to enhance their perception of letters under conditions of pattern-masking, because the output of letter analyzers is reinforced by the visual descriptions of whole-word and subword units (c.f. McClelland & Rumelhart, 1981). The lack of a contextual influence on J.V.‘s performance would be consistent with the view that letter-by-letter reading is the outcome of a major impairment to the word-form system. Three groups of 48, four-letter strings were obtained, group 1 consisting of real words, group 2 of orthographically legal pseudowords, and group 3 of random letter sequences. Words were chosen to be high in frequency (greater than 55 per million) and pseudowords were constructed so that they closely matched the bigram frequencies of the word stimuli. The random letter sequences were of zero approximation to English orthography. Stimuli were presented in black uppercase OS-in. font on a white background and subtended a horizontal angle of approximately 1.5 degrees from the viewing point. Each target item was exposed tachistoscopically in J.V.‘s intact visual field for a limited duration (the final letter occurred slightly to the left of a central fixation point) and was followed by a random pattern mask displayed for 500 msec (see Fig. 3). Immediately after the.mask, J.V. was given two choices printed in block letters on a card, one of which was the target item, the other differing from the target by a single letter. On word trials, the negative choice was always another legitimate word that matched the frequency of the target. An equal number of substitutions involved the first, second, third, and fourth positions in all three groups of items (N = 12 substitutions per letter position). Thus, J.V.‘s recognition of letters was probed across the entire four-letter array. Target items were displayed according to a blocked design, with letter strings occurring first, followed by pseudowords and familiar words. The
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WORD RECOGNITION *
FIXATION
WORD TARGET
~ FORCED CHOICE
#W## CARE
[500
tlSEC.1
CART
FIG. 3. Design of experiment on the word superiority effect.
position of the correct choice and the foil (left or right) was randomized over trials, as was the location of the critical letter (1, 2, 3, or 4) that differed between the members of each pair. During a practice session, the exposure duration of the stimulus was carefully adjusted so that J.V. reached an overall accuracy of roughly 75%. The value producing this level of performance turned out to be 650 msec and was checked for reliability before we conducted the experiment. To compare the nature of J.V.‘s recognition with an appropriate control, we selected an age-matched patient (P.J.) who also had a dense right visual field deficit caused by a stroke to the posterior regions of the left hemisphere, but whose reading was unaffected by the damage. In other respects, P.J. is very similar to J.V.; his production of written and spoken language was normal and he experienced no comprehension difficulties when tested. The exposure duration at which 75% of P.J.‘s responses were correct on the probe test was found to be 120 msec, substantially lower than the value obtained for J.V. Figure 4 depicts the relative ability of the patients to identify letters in each of the four probe locations as a function of stimulus type. P.J. demonstrates a U-shaped curve on random letters typical of normal encoding; the string is processed from the outer to the inner elements, so that medial positions are less accurately identified than the letters in external positions (Rumelhart & McClelland, 1981). The curve for word stimuli remains flat (c.f. Rumelhart & McClelland, 1981) and reveals a clear influence of orthographic context on letter recognition (x2 = 4.8, p < .05). The serial position effects obtained for J.V. on random letter strings indicate that he processes them from left to right; moreover, he appears to be very limited in the number of letters he can recover from an unstructured array; his recognition of the initial two letters is accurate, but deteriorates significantly over the next two positions. In fact, J.V.‘s performance remains at chance when he is required to discriminate either of these letters from an alternative choice. This pattern is consistent
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366
CONTROL
1
2
3
LETTER POSITION
J.V.
4
1
2
3
4
LETTER POSITION
FIG. 4. Percentage correct of letter recognition in words, pseudowords, and random letter strings.
with an impairment in the simultaneous perception of visual forms (simultanagnosia), a disorder often found in association with letter-by-letter reading (c.f. Kinsboume & Warrington, 1962; Levine & Calvanio, 1978). The nature of J.V.‘s responding on words and pseudowords, however, implies that his perceptual deficit must affect the retrieval of multiple letters after they have been identified. In contrast to the very marked impact of serial position on the recognition of random letter strings, J.V. demonstrates no loss of accuracy over the length of the array when he can make use of orthographic context to facilitate the recovery of letter codes. Both words (x2 = 4.4, p < .05) and pseudowords (x2 = 7.5, < 0.025) yield a clear advantage relative to strings that cannot be mapped onto a higher-level description. We note that words and pseudowords produce equivalent superiority effects, consistent with results obtained for normal readers; any discrepancy between these items (i.e., more accurate perception of letters in familiar words than pseudowords) is typically quite small on a forced-choice procedure and varies substantially between individuals (Manelis, 1974). The evidence does not provide support for the claim that letter-byletter readers have no visual access to orthographic representation. We cannot know from the present results whether whole-word knowledge is completely intact, since J.V. derives the same magnitude of perceptual advantage from words and pseudowords. To examine the full integrity of the word-form system would demand a more sensitive test yielding a reliable measure of word-specific activation. We can, however, reject the strongest version of the explanation originally proposed by Warrington and Shallice (1980), namely that the syndrome is due to such a major
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WORD RECOGNITION TABLE 1 J.V.‘s RESPONSESON EXPLICIT NAMING OF LETTERS IN FAMILIAR WORDS
Stimulus Went Kind Name Food Send Life Lose Boat Lack Then Cold Roof Fond Word Heat
Response W,E . . . Kin? N,A . . . . Foo . . Sin, Sun, Seen? Life L, 0, . .Loss? Boo L,A . . The? Cool Roof F,A... Work Heat Shop
reduction in the magnitude of the encoding units derived from print that only information about single letters can be processed. J.V. clearly has access to some orthographic knowledge; though the level of description he retains may or may not extend to whole words. (3) Explicit Word Report versus Probe-Recognition While carrying out the forced-choice task, J.V. reported that he felt very uncertain about the identity of the last two letters even when the targets were familiar words. In fact, he maintained that most of his responses were “guesses,” and expressed astonishment on being told that his recognition of letters in words had been very accurate. We examined J.V.‘s ability to explicitly recover the letters making up a word by presenting him with a subset of the items used in the proberecognition experiment. Sixteen words were displayed to J.V. under the same conditions as before (a target for 650 msec, followed by a randompattern mask), without a choice of reponse alternatives. J.V. was told that four-letter strings making up real words would be briefly presented to him and was simply asked to report as many of the letters as he could recognize. Table 1 provides a list of his answers. He clearly lacks details about the final two letters, and therefore fails to identify a majority of the words. Analysis of the total percentage of letters recognized in the four positions, without taking correct order into account, further demonstrates the effect of serial position on the accuracy
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60%
-
LETTER POSITIOR
FIG. 5. Total percentage correct across letter position on explicit report of four-letter words.
of J.V.‘s explicit report (see Fig. 5)-only 52% of the penultimate letters are retrieved, and performance drops to 25% correct on the final position. We conclude that J.V. does not have access to a complete description of all the letters in briefly presented words. Under conditions of full report, the nature of the constraints on his reading appear similar to the constraints on probe recognition of unpatterned letter sequences. These results might have led us to infer that J.V.‘s encoding of words is restricted to an incomplete analysis of letters carried out from left to right. But the use of a forced-choice procedure reveals a substantial influence of orthographic context on all letter positions in words and pseudowords, suggesting that more information has been activated than can be retrieved for an explicit naming response. Analysis of J.V.‘s performance on forcedchoice responding to words, adjusted for the fact that he successfully identified 3/16 items on full report, continued to yield higher accuracy than random letter sequences (x2 = 7.8, p < .05).* Knowledge of the target may be difficult to retrieve for J.V. because ’ The adjustment was achieved by adding 3 to the number of correct responses in each letter position for random sequences. The use of a forced-choice procedure to assess word recognition in normal subjects has yielded only a small advantage relative to free reports when the critical letter was not reported correctly (Johnston, 1978). This result implies that very little information available to the normal reader is not uncovered by the method of free report. The fact that some discrepancy occurs, however, is consistent with the difference between full report and probe recognition observed in J.V. The question of whether our finding represents a more extreme, pathological form of recognition awaits a detailed comparison of the letter-by-letter reader’s performance under the two modes of responding.
WORD RECOGNITION
369
the activation from print is pathologically weak. If the correct entry in the word-form system has been activated, however, the meaning of the target may also be weakly available, yielding enough subthreshold information to produce tacit comprehension of written material. The recent demonstration by Shallice and Saffran (1986) that a letter-by-letter reader appeared capable of partial understanding when he could not overtly identify a word may therefore illustrate the same general phenomenon underlying the implicit contextual effects observed in J.V. We argue that the syndrome co-occurs with the preservation of whole-word codes, and that the damagehas not eliminated direct accessto higher-level orthographic representation. Finally, it should be noted that the evidence obtained from J.V. rules out a potential artifact that might have complicated our interpretation of the word-superiority effect. A discrepancy between the recognition of random letters and words or pseudowords could simply be due to the fact that J.V. has enough time to convert the items displayed at 650 msec into auditory code. Familiar words and pronounceable nonwords are easier to remember than an unrelated sequence of letters, yielding a spurious influence of orthographic context on probe recognition. J.V.‘s failure to explicitly recover all the letters in words presented under the same conditions that permit highly accurate discrimination leads us to reject this alternative explanation and to infer that the results have a strong perceptual basis. (4) A Further Analysis
of Letter Processing
Several authors have noted that letter-by-letter readers may often experience difficulty recognizing letters. Patterson and Kay (1982) reported that patients tended to mis-identify letters in words they were attempting to decode; analysis of the errors confirmed that they bore a strong visual relationship to the target. Friedman and Alexander (1984) have argued that a disturbance in the automatic perception of visual forms is the primary cause of letter-by-letter reading; the patient they described had elevated perceptual thresholds (compared to normal controls) for identifying letter strings, words, and pictures displayed tachistoscopically. While this explanation cannot suffice, given that the reading disorder has been observed in at least one patient who demonstrated no major impairment in the ability to encode a string of letters (Wanington & shallice, 1980), the labored process by which letters are named to activate word-level units does raise questions about the general status of letter recognition and its possible contribution to the dramatic effect of word length on reading performance that characterizes the syndrome. If the only demand for the letter-by-letter reader was to explicitly identify the letters making up a word via a normal procedure before it could be read, we would expect sequential but relatively fast processing of written
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A A
20
A B
20
FIG. 6. Design of cost-benefit procedure.
material. The fact that performance is generally very slow and effortful indicates that some aspect of the letter recognition process may be damaged. Our aim is to examine further the hypothesis that J.V.‘s occasional misnaming of letters in words is the result of a breakdown in the automatic encoding of letter-identities from print. The experiment designed to investigate this question was based on a procedure devised by Posner and Snyder (1975a, 1975b), who attempted to isolate the effect of automatic priming on reaction time to pairs of letters in a same-different comparison task. Trials consisted of a neutral warning signal (e.g., a plus sign) or a letter that matched the comparison items on a certain number of positive trials (see Fig. 6). The authors maintained that the probability of the prime as a valid cue to the identity of the target letters would determine the amount of active processing committed to it-if the correspondence occurred very infrequently (e.g., on 20% of positive trials), the prime may not receive much attention and would need little encoding capacity. According to Posner and Snyder, the passive build-up of information from a stimulus that demands no conscious allocation of resources produces a facilitating effect on the subsequent availability of the perceptual code (benefit), without any inhibitory effects (cost) ensuing when the prime does not match the target and attention must be directed to the output of a new set of analyzers. By contrast, the development of conscious expectancies, taking place when the prime is correlated with the target on the majority of positive trials, yields both cost and benefit; the subject fixes attention on the prime and must actively redirect the mechanism to cope with the onset of a different target. If J.V. is unable to process letters efficiently, we can expect a negative prime to yield inhibition even under conditions of low predictability between the prime and the target letters. Attention must be switched from the one to the other, and J.V.‘s perceptual encoding of letters may no longer be skilled enough to make this process automatic (c.f. LaBerge & Samuels, 1974). The experimental session involved 200 trials, each consisting of a
371
WORD RECOGNITION TABLE 2 REACTION TIME ON POSITIVE TRIALS TO NEUTRAL, FACILITATING, AND INHIBITORY Reaction
Neutral (+ AA) Facilitating (A AA) Inhibiting (B AA)
CUES
time
Control
J.V.
420 (37) 376 (58) 402 (59)
658 (102) 638 (119) 700 (126)
warning cue displayed tachistoscopically for 500 msec, followed by a 200-msec pause and then a pair of target uppercase letters that were either the same (e.g., AA) or different (e.g., AB). The cue was a neutral plus sign (+) on 20 of the positive trials, a letter that was physically identical to the target letters on 16 trials (P PP), and a different letter (J PP) on the remaining 80 positive trials. Thus, the majority of letter cues (more than 80%) were negatively related to the ensuing targets, a contingency that normally does not exert an inhibitory influence on the activation of letter codes. J.V. was instructed to look directly at the cue and to get ready for the occurrence of the letter pair requiring a same-different judgment. He was asked to respond as quickly and as accurately as possible, depressing a key with the index finger of his right hand to indicate a positive decision, and to use his left index finger for a negative decision. The key-press stopped a millisecond clock triggered by the onset of the target display, which remained ilhuninated until the execution of a response. A series of 30 practice trials was administered before the actual experiment. All stimuli occurred slightly to the left of a central fixation point, and the target letters were positioned along a vertical axis. The influence of positive and negative primes on “same” judgments can be observed in Table 2. A normal control subject, matched to J.V. for age, sex, and general level of education demonstrates the anticipated facilitation effect on reaction times (t(33) = 2.94,~ < .Ol), without concomitant inhibition. By contrast, J.V. yields less-pronounced facilitation but very marked inhibition,3 which just falls short of significance (t(97) = 1.9, p = .06). Errors were too few (less than 5% of responses) for a statistical analysis. The results imply that J.V. does experience some limitation in the automatic components of letter recognition. The comparison task requires 3 J.V.‘s failure to demonstrate significant facilitation from a matching prime (e.g., A AA) is reminiscent of the observation that conditions producing larger cost in normal subjects also tend to yield smaller benefits (Stanovich & West, 1981; Eisenberg & Becker, 1982).
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only that he determine whether two letters are physically identical, yet the presence of another letter before the target pair exerts a substantial distracting effect. The nature of the inhibitory priming is not entirely clear, given that the explanation offered by Posner and Snyder (1975a) has met with some dispute (c.f. Henderson, 1982). The evidence, however, is consistent with the claim that letter perception is a sufficiently demanding process for J.V. that attention must be actively switched from one element to another. DISCUSSION
The attempts of a letter-by-letter reader to identify written words certainly gives one the impression that each letter is treated as an isolated unit to arrive at the meaning and pronunciation. We have tested the hypothesis that the syndrome reflects the complete absence of higher-level contextual influences on the encoding process, by examining J.V.‘s ability to recognize letters in familiar words, orthographically legal pseudowords, and random letter sequences. Normal readers show a clear effect of orthographic structure in this task, which aids their recovery of the letter descriptions in words and pseudowords. J.V., though requiring much longer exposure durations to obtain the same level of accuracy as a matched control patient, nevertheless demonstrates a comparable influence of context extending across all four letter positions of the target string. Evidence suggests that at least some of the activated information mediating this orthographic superiority effect may not be sufficiently detailed for an overt naming response-J.V. is unable to explicitly report all the letters in familiar words presented at the same exposure duration yielding nearperfect recognition under conditions of forced choice, and his performance deteriorates systematically over the array. Given that a measure based on a two-choice procedure is less sensitive than full report, we need to be cautious about the interpretation of the observed discrepancy. The complete absence of any serial position effect on probe recognition scores, however, and their high level of accuracy, does indicate that some information is available to J.V. that is not captured by free report. What are the implications of the present results for a theory of letterby-letter reading? Normal sensitivity to orthographic context involves the contribution of both word-specific (lexical) activation and the activation of subword units. Attempts to isolate the properties of pseudowords responsible for their perceptual enhancement have uncovered attributes like positional constraints (e.g. McClelland & Johnston, 1977) and pronounceability (e.g., Massaro, Venezky, & Taylor, 1979), though the status of the latter variable remains controversial. More importantly, neither dimension has provided a clear theoretical reason for assuming a distinction between the mechanism that extracts spelling units from a letter array and the one that extracts whole words. Recent explanations of the pseu-
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doword advantage utilize the concept of spreading activation between the members of an orthographic neighborhood within the word-form system (c.f. McClelland & Rumelhart, 1981). On this account, words and pseudowords “differ merely in the fact that the region of orthographic space excited by their display contains, in the case of a real word, a representation of the word itself” (Henderson, 1982; p. 272). The presence of a word and pseudoword advantage in a letter-by-letter reader, then, may be taken as relevant evidence against the claim that the patient is denied access to the visual word-form system, though we cannot reach any definite conclusions about the overall integrity of the mechanism (e.g., only a subset of whole-word descriptions may be available), nor do we know whether word-level activation and inhibition are operating normally. Rival analyses of letter-by-letter reading suggest two possible ways of interpreting J.V.‘s preserved sensitivity to orthographic context: The word-form system may be intact, but the output of peripheral letter analyzers to this component may require a slow, sequential operation (Patterson & Kay, 1982). The word-superiority effect can still occur under conditions that force letter-by-letter reading in normal subjects, provided the rate of letter processing falls within certain limits. Forster (1980) used a serial incremental display of letters and found a reliable influence of context on letter identification at fast rates of presentation (40 msec/letter) but not at a slower rate (400 msec/letter). More recently, Chastain (1984) obtained a word-superiority effect for targets with letters in reversed order (e.g., TAH), a manipulation that produced a serial, right-to-left mode of encoding. If this letter-by-letter approach mimics the reading procedure utilized by J.V., we can view his recognition of words as a sequential operation that continuously receives feedback from higher levels of representation during the analysis of each letter position. Certain aspects of J.V.‘s performance are not inconsistent with the foregoing hypothesis-for example, the magnitude of the word-superiority effect increases systematically across the location of the probe in the fourletter array-but these data can also be handled by a theory incorporating the assumption of parallel letter entry to the word-form system (e.g., Rumelhart, 1970). Distinguishing between the two alternatives is crucial to a better understanding of the syndrome and would involve an assessement of the relationship between changes in the accuracy of letter processing and the effective duration of the target (c.f. Adams, 1979). If letter-by-letter readers are not impaired in the transfer of letters to the word-form system, the functional damage causing the syndrome must occur after word-level representations are contacted. We assume from J.V.‘s response to orthographic context that at least some whole-word information remains intact, and that partial loss of word forms cannot be responsible for the striking nature of his disorder. Patients with surface
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dyslexia also appear to have lost a significant proportion of word descriptions at the orthographic level, yet their performance may show none of the characteristics found in letter-by-letter reading. M.P., for example, described by Bub, Cancelliere, and Kertesz (1985) had clearly sustained major damage to the whole-word reading procedure,4 but was nevertheless capable of normal speed and accuracy when she translated print (orthographically regular words or nonsense words) into sound. The extremely slow responses of the letter-by-letter reader, coupled with the massive effect of word length on performance, cannot be simply attributed to a more extreme version of the word-form deficit that evidently does exist as an important component of surface dyslexia (Patterson, Marshall, & Coltheart, 1985). To account for letter-by-letter reading in terms of an impaired wordform system without assuming the total destruction of its contents, Shallice and Saffran (1986) propose that damage has occurred to the mechanism by which a given word is activated and competing possibilities inhibited. Tacit recognition (and presumably the word-superiority effect as well) may then be the due to weak output generated by the system. Evaluating this interpretation would require more detailed information on the nature of whole-word activation in the syndrome. Finally, a test of J.V.‘s ability to encode letters in isolation has revealed the possibility of a subtle disturbance that may account for the extreme slowness of his letter-by-letter reading strategy. We have found that the mere presence of a letter before the onset of another letter pair inhibits their analysis, even though the situation has been designed to minimize attention to the prime. While it is unlikely that the dyslexia is caused by this perceptual disturbance, which may well arise beyond the passive activation of letter identities, the impairment will add to the difficulties experienced by the patient and could affect the nature of performance on many word recognition tasks. REFERENCES Adams, M. .I. 1979. Models of word recognition. Cognitive Psychology, 11, 133-176. Bub, J., & Bub, D. 1988. On the methodology of single-case studies in cognitive neuropsychology. Cognitive Neuropsychology, 5, 565-582. Bub, D., Cancelliere, A., & Kertsez, A. 1985. Whole-word and analytic translation of spelling to sound in a non-semantic reader. In K. E. Patterson, J. C. Marshall, & M. Coltheart (Eds.), Surface dyslexia. Hillsdale, NJ: Erlbaum. Cattell, J. M. 1886. The time taken up by cerebral operations. Mind, 11, 220-242. 4 Regularization errors (e.g., BEAR read as “beer”) are caused by failure to locate the whole-word address for converting print to sound, and the consequent assembly of pronunciation is based on a more regular correpondence (e.g., the most common rendering of the pattern EAR-as in HEAR, NEAR, GEAR, etc.-differs from the pronunciation of exception words like BEAR and PEAR).
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Chastain, G. 1984. Directional letter-by-letter analysis and the word-superiority effect. Memory and Cognition, 12, 195-201. Coslett, B., & Satfran, E. Preserved reading in pure alexia. Brain, in press. Dejerine, J. 1892. Contribution a I’etude anatomochimique des differentes varittts de &cite verbale. Compte Rendu Hebdomadaire des Sct!ances et Me’moires de la SocitW de Biologie, 4, 61-90. Eisenberg, P., & Becker, C. A. 1982. Semantic context effects in visual word recognition, sentence processing, and reading: Evidence for semantic strategies. Journal of Experimental
Psychology:
Human Perception
and Performance,
8, 739-155.
Forster, K. I. 1980. Absence of lexical and orthographic effects in a same-different task. Memory and Cognition, 8, 210-215. Friedman, R. B., & Alexander, M. P. 1984. Pictures, images and pure alexia: A case study. Cognitive Neuropsychology, 1, 9-23. Henderson, L. 1982. Orthography and word recognition in reading. New York/London: Academic Press. Johnston, J. C. 1978. A test of the sophisticated guessing theory of word perception. Cognitive Psychology, 10, 123-153. Kinsbourne, M., & Warrington, E. K. 1962. A disorder of simultaneous form perception. Brain, 85, 461-486. Kinsbourne, M., & Warrington, E. K. 1963. The localizing significance of simultaneous form perception. Brain, 86, 397-415. KuEera, H., & Francis, W. N. 1967. Computational analysis of present-day American English. Providence, RI: Brown Univ. Press. Laberge, D., & Samuels, S. J. 1974. Towards a theory of automatic information processing in reading. Cognitive Neuropsychology, 6, 293-323. Levine, D. N., & Calvanio, R. 1978. A study of the visual defect in verbal alexia-simultangnosia. Brain, 101, 65-81. Manelis, L. 1974.The effect of meaningfulness in tachistoscopic word perception. Perception and Psychophysics, 16, 182-192. Massaro, D. W., Venezky, R. L., & Taylor, G. A. 1979.Orthographic regularity, positional frequency and visual processing of letter strings. Journal of Experimental Psychology, 108, 107-124. McClelland, J. L. 1976. Preliminary letter identification in the perception of words and nonwords. Journal of Experimental Psychology: Human Perception and Performance, 2, 80-91. McClelland, J. L., & Johnston, J. C. 1977. Preliminary letter identification in the perception of words and non-words. Journal of Experimental Psychology: Human Perception and Performance, 2, 80-91. McClelland, J. L., & Rumelhart, D. E. 1981. An interactive activation model of context effects in letter perception: Part 1. An account of basic findings. Psychological Review, 88, 375-407. Patterson, K., & Kay, J. 1982. Letter-by-letter reading: Psychological descriptions of a neurological syndrome. Quarterly Journal of Experimental Psychology, 34A, 411-441. Patterson, K. E., Marshall, J. C., & Coltheart, M. 1985. Surface dyslexia. Hillsdale, NJ: Erlbaum. Posner, M. I., & Snyder, C. R. R. 1975a. Attention and cognitive control. In R. L. Solso (Ed.), Information processing and cognition: The Loyola symposium. Hillsdale, NJ: Erlbaum. Posner, M. I., & Snyder, C. R. R. 1975b. Facilitation and inhibition in the processing of signals. In P.M.A. Rabbitt & S. Dornic (Eds.), Attention nnd Performance. New York: Academic Press. Vol. 5.
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Reicher, Cl. M. 1%9. Perceptual recognition as a function of meaningfulness of stimulus material. Journal of Experimental Psychology, 81, 274-280. Rumelhart, D. E. 1970. A multicomponent theory of the perception of briefly exposed visual displays. Journal of Mathematical Psychology, 7, 191-218. Rumelhart, D. E., & McClelland, J. L. 1981. An interactive activation model of context effects in letter perception: Part 2: The contextual enhancement effect and some tests and extensions of the model. Psychological Review, 89, 60-94. Shallice, T. 1981. Neurological impairments of cognitive processes. Brirish Medical Bulletin, 37, 187-192. Shallice, T., & McCarthy, R. 1985. Phonological reading: From patterns of impairment to possible procedures. In K. E. Patterson, J. C. Marshall, &KM. Coltheart (Eds.), Surface dyslexia. London: Erlbaum. Shallice, T., & Saffran, E. 1986. Lexical processing in the absence of explicit word identification: Evidence from a letter-by-letter reader. Cognitive Neuropsychology, 3,429458. Shallice, T., Warrington, E. K., & McCarthy, R. 1983.Reading without semantics. Quarterly Journal of Experimental Psychology, 35A, 111-138. Stanovich, K. E., & West, R. F. 1981. The effect of sentence context on ongoing word recognition: Tests of a two-process theory. Journal of Experimental Psychology: Human Perception and Performance, 7, 658-672. Warrington, E. K., & Rabin, P. 1971. Visual span of apprehension in patients with unilateral cerebral lesions. Quarterly Journal of Experimental Psychology, 23, 423-431. Warrington, E. K., & Shallice, T. 1980. Word-form dyslexia. Brain, 103, 99-l 12. Wheeler, D. D. 1970. Processes in word recognition. Cognitive Psychology, 1, 59-85. Wolpert, I. 1924. Die Simultanagnosie-Storng der Gesamtauffassung. Zeitschrift fur die gesamte Neurologie und Psychiatric, 93, 397-415.
AND
LANGUAGE
36,
357-376 (1989)
Word Recognition and Orthographic Context Effects in a Letter-by-Letter Reader DANIEL N. BUB Montreal
Neurological
Institute
and Centre Hospitalier
C&e-des-Neiges
SANDRA BLACK Sunnybrook
Medical
Centre
AND JANICE HOWELL St. Joseph’s Hospital
The performance of letter-by-letter readers when attempting to decipher written material gives the impression that words fail to directly evoke any higher-level representation. As a consequence, spelling patterns appear to be treated perceptually as if they were a collection of random letters. We tested the hypothesis that words are no longer mapped onto orthographic descriptions by examining the ability of a letter-by-letter reader to identify letters in familiar words, pseudowords, and random strings. A clear effect of orthographic context was obtained on the accuracy of letter recognition, indicating that spelling patterns do gain access to more central components of the reading mechanism. The implications of this result for our understanding of the syndrome are discussed. 8 1989 Academic PI~SS, Inc.
Certain acquired dyslexics are unable to identify a word without laboriously naming or sounding out each letter-hence the use of the term letter-by-letter reading to describe the syndrome (Patterson & Kay, 1982), which has also been called spelling dyslexia (Kinsbourne & Warrington, 1962) or word-form dyslexia (Warrington & Shallice, 1980). Early interpreters of this disorder considered it to be the outcome of a more general The preparation and execution of this manuscript was supported by grants from the Medical Research Council of Canada and the Fonds de la Recherche en Sante du Quebec. Address correspondence and reprint requests to Dr. Daniel N. Bub, Montreal Neurological Institute, 3801 University, Montreal, Quebec, Canada H3A 2B4. 357
0093-934x/89 $3.00 Copyright Q 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.
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disturbance in the ability to encode a number of separate visual forms, because of the observed association between letter-by-letter reading and simultanagnosia (Wolpert, 1924, Kinsbourne & Warrington, 1%2). Patients with damage to the occipitotemporal region of the left hemisphere can usually recognize a single percept at brief exposure durations but in many cases are unable to recover the identity of additional elements from a display of two or more items, even when the material is presented for a relatively protracted interval (Kinsboume & Warrington, 1962, 1%3). The claim was made that the breakdown in the simultaneous perception of multiple forms extended to the synthesis of a word from its component letters. Thus, according to Kinsboume and Warrington (1%2), “Only one letter can be read at a time and the interval before the visual system is ready for perception of the next is so long that reading must be a laborious hardship” (p. 481; also see Levine & Calvanio, 1978). On theoretical grounds, there is reason to doubt this interpretation of letter-by-letter reading. The simultanagnosic patient has difficulty retrieving many perceptual units presented at the same time, be they letters, numbers, or line drawings. Written words, of course, also have the status of perceptual wholes once they have been categorized-we may therefore expect simultanagnosia to interfere with the reading of several items presented together, but the deficit should not impair the recognition of an isolated word. Evidence provides support for a functional distinction between the two syndromes. Patients described in a group study by Warrington and Rabin (1971) were found to be very impaired on visual span tasks, yet none of them appeared to fit the clinical description of a letter-by-letter reader. More recently, Warrington and Shallice (1980) have documented a case of letter-by-letter reading who showed only a moderate reduction in visual span, incommensurate with his profound reading disorder. These authors conclude that letter-by-letter reading cannot be attributed to visual or perceptual deficits. They offer the following interpretation of the syndrome: A specialized mechanism, termed the visual word-form system, exists in the left posterior cortex, which parses letter strings into familiar units (ranging in size from graphemes to syllables and whole words), and categorizes them perceptually. This automatic synthesis of letters into higher order reading units allows for rapid and accurate identification of written words. According to Warrington and Shallice, damage to the word-form system produces letter-by-letter reading-the patient can no longer extract familiar perceptual units from a spelling pattern and is forced to read by explicitly identifying each separate letter in the word. While this explanation is certainly attractive (see Patterson & Kay, 1982, for an evaluation), we should point out that no direct evidence has yet been obtained for the claim that the functional impairment responsible
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for letter-by-letter reading is confined to the visual word-form system. Warrington and Shallice have demonstrated only that the syndrome cannot be the outcome of damageto peripheral mechanisms involving the allocation of attention to a string of letters. The word-form system becomes a reasonable candidate as the functional locus of the disorder, but it does not follow from the available results that the assumption is necessarily correct. Patterson and Kay (1982) have provided a different account of letter-by-letter reading, similar to the classical interpretation of pure alexia given by Dejerine (1892). In their version, letter-by-letter reading is due to a disconnection between graphemic analyzers and the wordform system, such that the normal parallel mapping of abstract letter identities onto word-level representation has given way to an effortful sequential process. Our aim in this paper is to obtain relevant evidence for or against the notion that letter-by-letter reading is caused by damage to orthographic word forms. Such evidence would require a two-step procedure: first, the general location of the impairment must be found within a functional architecture of the normal reading mechanism by measuring the influence of particular variables on the performance of the dyslexic patient. Warrington and Shallice (1980), for example, note that the familiarity of words and their semantic properties (i.e., concreteness) had no effect on the accuracy with which their patients could decipher written words. There was also no indication that any recognition or comprehension took place without the letter-by-letter procedure used for explicit report.’ The authors infer that “. , . the letter-by-letter strategy is an attempt to compensate for damage to a stage in the reading process prior to phonological or semantic analysis” (p. 108). Patterson and Kay (1982) have confirmed this interpretation via similar methods in a later investigation of the disorder. A description of the approximate locus of impairment in letter-byletter reading does not in itself justify an explanation of the syndrome based on the concept of a word-form system-we know only that the damage occurs before the word has gained access to higher-level routines that extract sound and meaning. In order to obtain reasonable proof that directly implicates this component, we need to invoke some theoretically motivated claim about the word-form system that would predict a certain result if the damage occurred at the hypothesized location (see Bub & ’ The case documented by Shallice and Saffran (1986) evinced partial comprehension of words he was unable to explicitly identify. Similar capabilities have been reported in a few other letter-by-letter readers (e.g., Coslett & S&ran, in press). Tacit recognition does not appear to be a general feature of the syndrome; however, we have recently examined five letter-by-letter readers on a forced-choice comprehension test of written words presented at 500 msec and have not observed above-chance performance in any of the patients.
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Bub, 1988, for discussion of methodological issues in cognitive neuropsychology). Our approach to the problem relies on the fact that stored knowledge about the orthographic description of words normally enhances recovery of their constituent letters. Numerous experiments, beginning with observations made by Cattell in 1886, have demonstrated that subjects are able to recognize briefly displayed words more accurately than random letter strings. The processing stage responsible for the word superiority effect proved difficulty to specify initially-for example, the effect might simply be due to the subject’s better ability at guessing the identity of a word than a random letter array under limited viewing conditions, or to poorer retention of unfamiliar letter strings in memory. A procedure devised by Wheeler (1970) and Reicher (1969) however, has shown clearly that the advantage for words must be perceptual in origin. The general method behind these experiments is as follows: A fourletter word or random letter sequence is briefly presented to the subject on each trial, followed by a pattern mask. Immediately after this event, two choices are displayed; one of these is the target item, the other is a foil which dilfers from the target by one letter only. Reliable performance cannot be based on guesswork in this task, because the two alternatives are equally plausible. Furthermore, the use of a forced-choice procedure with a brief delay between the stimulus and the response minimizes the load on short-term memory (McClelland & Johnston, 1977).Any advantage in the recognition of words (e.g., McClelland, 1976; Johnston, 1978) can reasonably be attributed to the fact that the letters constituting them are more accurately encoded than a group of random letters-or even a single letter (c.f. McClelland & Johnston, 1977)-that fails to activate a familiar orthographic structure. The knowledge mediating this effect is stored in the visual word-form system and may no longer be available to the letter-by-letter reader. We take it that the hypothesis originally proposed by Warrington and Shallice (1980) dictates that the impairment to the visual word-form system in letter-by-letter reading is so extensive that spelling patterns have the perceptual status of random letter strings to the patient. This basic assumption follows from the concept of the word-form system as a hierarchically organized component with levels of orthographic representation (graphemes, subsyllabic units, syllables, and morphemes) that are increasingly vulnerable to brain damage (c.f. Shallice & McCarthy, 1985). The most extreme condition takes place when the units extracted from print are reduced to single letters. Shallice (1981) writes that the characteristics of letter-by-letter reading, or “word- form dyslexia,” indicate “an artificial mode of reading, where letters are explicitly identified serially . . . and then the word probably recognized from its auditory spelling.
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No higher-level information seems to be available to the patient” ;d. ‘189). In this paper we examine the ability of a letter-by-letter reader to perceptually extract orthographic information from written words. If the current understanding of the syndrome is correct, orthographic structure would have little or no effect on the patient’s ability to recognize a briefly presented string of letters, and the word superiority effect observed in normal readers should be abolished. CASE HISTORY
J.V., a 51-year-old teacher of physics and biology at a high school in Ontario, underwent neurosurgery in 1982 for removal of a benign left temporo-occipital meningioma. Postoperatively, he demonstrated no language deficits other than a very mild anomia, but complained of severe reading difficulties. The impairment eventually forced him to resign his position as head of the Science Department 6 months after surgery. He was seen for a preliminary neuropsychological assessment in September 1983, at which time he continued to manifest a profound dyslexia. No word, even a very short familiar one (e.g., BOY), could be recognized without laboriously decoding the constituent letters. On the average, he required between 3 and 4 set to read a three-letter item. Sentence reading was carried out with such effort that comprehension was impossible unless the material was analyzed repeatedly. Preliminary testing on the Western Aphasia Battery disclosed no significant language deficits in the spoken modality. Writing and spelling were normal; J.V. could accurately write orthographically regular and irregular words taken from a wide range of frequency values. A CT scan obtained after surgery indicated a large hypodense region affecting most of the left occipital lobe (areas 17, 18, and 19) with some sparing laterally and superiorly. The lesion engulfed splenial fibers of the corpus callosum radiating through the forceps major to the left occipital lobe. It extended anteriorly into the periventricular area of the left occipital horn, interrupting optic radiations and other white matter tracts deep to the posterior angular gyrus. The damage effectively prevents access to the left temporoparietal area from both visual cortices and is characteristic of patients with pure alexia and a dense right homonymous hemianopsia. EXPERIMENTAL
TESTS
(1) The Effect of Length on Word Recognition Patients who read letter by letter exhibit dramatic effects of word length on performance, a result that is considered a hallmark of the syndrome. In order to document the extent of J.V.‘s reliance on a letterby-letter strategy, we presented him with two reading tasks in which this variable was systematically manipulated.
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BUB, BLACK, AND HOWELL 10.0
8.0 SECONDS 6.0
4.0
I// I
I
I
I
I
3
4
5
6
WORD LENGTH
FIG. 1. Naming latency as a function of written word length.
(a) Naming latency. Eight lists of words containing three-, four-, five-, or six-letter items were constructed and typed in bold uppercase print on individual sheets of paper. For each word length, a list of 15 stimuli was chosen from higher-frequency items (greater than 55 per million; Kucera & Francis, 1967) and a second list was compiled from lower frequency values (less than 10 per million). All words were monosyllabic. J.V. was asked to read each list from beginning to end and his overall speed for the complete set of 15 words was measured with a stopwatch. Test lists were administered in random order, and J.V. was given one practice sequence of three-letter items before commencing. Results of the average response time per word length are displayed in Fig. 1. Reading latencies are extremely slow even for short words and increase directly with the number of letters. Performance was unaffected by word frequency. J.V. made few errors and self-corrected on all but three of them. (b) Lexical decision speed. A word-nonword decision task was used to obtain a further assessment of J.V.‘s letter-by-letter reading without asking him to explicitly produce the spoken form of a written word. Sixty trials were prepared, (30 words and 30 pronounceable nonsense words), consisting of an equal number of four-, six-, and eight-letter items. Frequency was balanced across word length. Nonwords were created by changing two letters in a set of words that were matched for length to the target items. Stimuli were presented tachistoscopically in a pseudorandom order and were positioned so that the last letter occurred slightly to the left of fixation, to compensate for J.V.‘s dense right visual field deficit. He was asked to indicate his response by pressing one of two keys with his left (“No”) or right (“Yes”) index finger. The keypress stopped a millisecond timer that was triggered by the onset of a
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I
ERRORS
NEGATIVE
POSITIVE
DECISION LATENCY
3.0 2.5 2.0 4
6
0
WORD FIG.
2. Lexical decision speed as a function of word length.
stimulus. Each target item was displayed until a response was made, at which time it was replaced by a blank white field containing a black dot in the centre. J.V. was instructed to look directly at the dot at the start of a trial and to respond as soon as he had classified the word or nonword. We emphasized both speed and accuracy of responding. Reaction time and error rates on positive and negative trials are indicated in Fig. 2. Lexical decisions are carried out very slowly and are heavily influenced by length. The slope of the function for nonwords is slightly less than words, most probably due to the fact that J.V. could reach a decision about many negative items on the basis of the first few letters. In general, the results of the two reading tests confirm that J.V. is totally reliant on a letter-by-letter strategy to extract higher-level information from print. (2) Recognition of Letters in Context To what extent do the surface characteristics of J.V.‘s disorder bear on the nature of the underlying functional impairment to his reading mechanism? Certainly, his approach to word recognition may lead one to assume that no direct contact is possible between letter descriptions and visual word forms. The labored decoding of individual letters, however, could be the result of an attempt to compensate for a breakdown in processing that occurs after the structure of a word has been activated. If such activation from print is weak, J.V. might use letter names as a
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way of generating an additional source of input to the word-form system, In a recent paper, Shallice and Saffran (1986) have documented a letterby-letter reader who was capable of extracting partial semantic information from written words presented too briefly for him to explicitly identify. While this preserved ability has not been found in other patients (c.f. Warrington & Shallice, 1980; Patterson & Kay, 1982; note l), the evidence does suggest the artificial strategy used for complete recognition may be misleading and that print may gain automatic entry to more central components of the reading mechanism. The question of interest is whether J.V. can perceptually synthesize any higher-level orthographic code from written words, even though his reading gives the general impression that the units of processing are confined to single letters. The search for an answer requires a test of J.V.‘s ability to recover letters from a familiar spelling pattern compared to the letters in a random string. Normal readers can use context to enhance their perception of letters under conditions of pattern-masking, because the output of letter analyzers is reinforced by the visual descriptions of whole-word and subword units (c.f. McClelland & Rumelhart, 1981). The lack of a contextual influence on J.V.‘s performance would be consistent with the view that letter-by-letter reading is the outcome of a major impairment to the word-form system. Three groups of 48, four-letter strings were obtained, group 1 consisting of real words, group 2 of orthographically legal pseudowords, and group 3 of random letter sequences. Words were chosen to be high in frequency (greater than 55 per million) and pseudowords were constructed so that they closely matched the bigram frequencies of the word stimuli. The random letter sequences were of zero approximation to English orthography. Stimuli were presented in black uppercase OS-in. font on a white background and subtended a horizontal angle of approximately 1.5 degrees from the viewing point. Each target item was exposed tachistoscopically in J.V.‘s intact visual field for a limited duration (the final letter occurred slightly to the left of a central fixation point) and was followed by a random pattern mask displayed for 500 msec (see Fig. 3). Immediately after the.mask, J.V. was given two choices printed in block letters on a card, one of which was the target item, the other differing from the target by a single letter. On word trials, the negative choice was always another legitimate word that matched the frequency of the target. An equal number of substitutions involved the first, second, third, and fourth positions in all three groups of items (N = 12 substitutions per letter position). Thus, J.V.‘s recognition of letters was probed across the entire four-letter array. Target items were displayed according to a blocked design, with letter strings occurring first, followed by pseudowords and familiar words. The
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FIXATION
WORD TARGET
~ FORCED CHOICE
#W## CARE
[500
tlSEC.1
CART
FIG. 3. Design of experiment on the word superiority effect.
position of the correct choice and the foil (left or right) was randomized over trials, as was the location of the critical letter (1, 2, 3, or 4) that differed between the members of each pair. During a practice session, the exposure duration of the stimulus was carefully adjusted so that J.V. reached an overall accuracy of roughly 75%. The value producing this level of performance turned out to be 650 msec and was checked for reliability before we conducted the experiment. To compare the nature of J.V.‘s recognition with an appropriate control, we selected an age-matched patient (P.J.) who also had a dense right visual field deficit caused by a stroke to the posterior regions of the left hemisphere, but whose reading was unaffected by the damage. In other respects, P.J. is very similar to J.V.; his production of written and spoken language was normal and he experienced no comprehension difficulties when tested. The exposure duration at which 75% of P.J.‘s responses were correct on the probe test was found to be 120 msec, substantially lower than the value obtained for J.V. Figure 4 depicts the relative ability of the patients to identify letters in each of the four probe locations as a function of stimulus type. P.J. demonstrates a U-shaped curve on random letters typical of normal encoding; the string is processed from the outer to the inner elements, so that medial positions are less accurately identified than the letters in external positions (Rumelhart & McClelland, 1981). The curve for word stimuli remains flat (c.f. Rumelhart & McClelland, 1981) and reveals a clear influence of orthographic context on letter recognition (x2 = 4.8, p < .05). The serial position effects obtained for J.V. on random letter strings indicate that he processes them from left to right; moreover, he appears to be very limited in the number of letters he can recover from an unstructured array; his recognition of the initial two letters is accurate, but deteriorates significantly over the next two positions. In fact, J.V.‘s performance remains at chance when he is required to discriminate either of these letters from an alternative choice. This pattern is consistent
BUB, BLACK, AND HOWELL
366
CONTROL
1
2
3
LETTER POSITION
J.V.
4
1
2
3
4
LETTER POSITION
FIG. 4. Percentage correct of letter recognition in words, pseudowords, and random letter strings.
with an impairment in the simultaneous perception of visual forms (simultanagnosia), a disorder often found in association with letter-by-letter reading (c.f. Kinsboume & Warrington, 1962; Levine & Calvanio, 1978). The nature of J.V.‘s responding on words and pseudowords, however, implies that his perceptual deficit must affect the retrieval of multiple letters after they have been identified. In contrast to the very marked impact of serial position on the recognition of random letter strings, J.V. demonstrates no loss of accuracy over the length of the array when he can make use of orthographic context to facilitate the recovery of letter codes. Both words (x2 = 4.4, p < .05) and pseudowords (x2 = 7.5, < 0.025) yield a clear advantage relative to strings that cannot be mapped onto a higher-level description. We note that words and pseudowords produce equivalent superiority effects, consistent with results obtained for normal readers; any discrepancy between these items (i.e., more accurate perception of letters in familiar words than pseudowords) is typically quite small on a forced-choice procedure and varies substantially between individuals (Manelis, 1974). The evidence does not provide support for the claim that letter-byletter readers have no visual access to orthographic representation. We cannot know from the present results whether whole-word knowledge is completely intact, since J.V. derives the same magnitude of perceptual advantage from words and pseudowords. To examine the full integrity of the word-form system would demand a more sensitive test yielding a reliable measure of word-specific activation. We can, however, reject the strongest version of the explanation originally proposed by Warrington and Shallice (1980), namely that the syndrome is due to such a major
367
WORD RECOGNITION TABLE 1 J.V.‘s RESPONSESON EXPLICIT NAMING OF LETTERS IN FAMILIAR WORDS
Stimulus Went Kind Name Food Send Life Lose Boat Lack Then Cold Roof Fond Word Heat
Response W,E . . . Kin? N,A . . . . Foo . . Sin, Sun, Seen? Life L, 0, . .Loss? Boo L,A . . The? Cool Roof F,A... Work Heat Shop
reduction in the magnitude of the encoding units derived from print that only information about single letters can be processed. J.V. clearly has access to some orthographic knowledge; though the level of description he retains may or may not extend to whole words. (3) Explicit Word Report versus Probe-Recognition While carrying out the forced-choice task, J.V. reported that he felt very uncertain about the identity of the last two letters even when the targets were familiar words. In fact, he maintained that most of his responses were “guesses,” and expressed astonishment on being told that his recognition of letters in words had been very accurate. We examined J.V.‘s ability to explicitly recover the letters making up a word by presenting him with a subset of the items used in the proberecognition experiment. Sixteen words were displayed to J.V. under the same conditions as before (a target for 650 msec, followed by a randompattern mask), without a choice of reponse alternatives. J.V. was told that four-letter strings making up real words would be briefly presented to him and was simply asked to report as many of the letters as he could recognize. Table 1 provides a list of his answers. He clearly lacks details about the final two letters, and therefore fails to identify a majority of the words. Analysis of the total percentage of letters recognized in the four positions, without taking correct order into account, further demonstrates the effect of serial position on the accuracy
368
BUB, BLACK, AND HOWELL
60%
-
LETTER POSITIOR
FIG. 5. Total percentage correct across letter position on explicit report of four-letter words.
of J.V.‘s explicit report (see Fig. 5)-only 52% of the penultimate letters are retrieved, and performance drops to 25% correct on the final position. We conclude that J.V. does not have access to a complete description of all the letters in briefly presented words. Under conditions of full report, the nature of the constraints on his reading appear similar to the constraints on probe recognition of unpatterned letter sequences. These results might have led us to infer that J.V.‘s encoding of words is restricted to an incomplete analysis of letters carried out from left to right. But the use of a forced-choice procedure reveals a substantial influence of orthographic context on all letter positions in words and pseudowords, suggesting that more information has been activated than can be retrieved for an explicit naming response. Analysis of J.V.‘s performance on forcedchoice responding to words, adjusted for the fact that he successfully identified 3/16 items on full report, continued to yield higher accuracy than random letter sequences (x2 = 7.8, p < .05).* Knowledge of the target may be difficult to retrieve for J.V. because ’ The adjustment was achieved by adding 3 to the number of correct responses in each letter position for random sequences. The use of a forced-choice procedure to assess word recognition in normal subjects has yielded only a small advantage relative to free reports when the critical letter was not reported correctly (Johnston, 1978). This result implies that very little information available to the normal reader is not uncovered by the method of free report. The fact that some discrepancy occurs, however, is consistent with the difference between full report and probe recognition observed in J.V. The question of whether our finding represents a more extreme, pathological form of recognition awaits a detailed comparison of the letter-by-letter reader’s performance under the two modes of responding.
WORD RECOGNITION
369
the activation from print is pathologically weak. If the correct entry in the word-form system has been activated, however, the meaning of the target may also be weakly available, yielding enough subthreshold information to produce tacit comprehension of written material. The recent demonstration by Shallice and Saffran (1986) that a letter-by-letter reader appeared capable of partial understanding when he could not overtly identify a word may therefore illustrate the same general phenomenon underlying the implicit contextual effects observed in J.V. We argue that the syndrome co-occurs with the preservation of whole-word codes, and that the damagehas not eliminated direct accessto higher-level orthographic representation. Finally, it should be noted that the evidence obtained from J.V. rules out a potential artifact that might have complicated our interpretation of the word-superiority effect. A discrepancy between the recognition of random letters and words or pseudowords could simply be due to the fact that J.V. has enough time to convert the items displayed at 650 msec into auditory code. Familiar words and pronounceable nonwords are easier to remember than an unrelated sequence of letters, yielding a spurious influence of orthographic context on probe recognition. J.V.‘s failure to explicitly recover all the letters in words presented under the same conditions that permit highly accurate discrimination leads us to reject this alternative explanation and to infer that the results have a strong perceptual basis. (4) A Further Analysis
of Letter Processing
Several authors have noted that letter-by-letter readers may often experience difficulty recognizing letters. Patterson and Kay (1982) reported that patients tended to mis-identify letters in words they were attempting to decode; analysis of the errors confirmed that they bore a strong visual relationship to the target. Friedman and Alexander (1984) have argued that a disturbance in the automatic perception of visual forms is the primary cause of letter-by-letter reading; the patient they described had elevated perceptual thresholds (compared to normal controls) for identifying letter strings, words, and pictures displayed tachistoscopically. While this explanation cannot suffice, given that the reading disorder has been observed in at least one patient who demonstrated no major impairment in the ability to encode a string of letters (Wanington & shallice, 1980), the labored process by which letters are named to activate word-level units does raise questions about the general status of letter recognition and its possible contribution to the dramatic effect of word length on reading performance that characterizes the syndrome. If the only demand for the letter-by-letter reader was to explicitly identify the letters making up a word via a normal procedure before it could be read, we would expect sequential but relatively fast processing of written
370
BUB, BLACK, AND HOWELL
A A
20
A B
20
FIG. 6. Design of cost-benefit procedure.
material. The fact that performance is generally very slow and effortful indicates that some aspect of the letter recognition process may be damaged. Our aim is to examine further the hypothesis that J.V.‘s occasional misnaming of letters in words is the result of a breakdown in the automatic encoding of letter-identities from print. The experiment designed to investigate this question was based on a procedure devised by Posner and Snyder (1975a, 1975b), who attempted to isolate the effect of automatic priming on reaction time to pairs of letters in a same-different comparison task. Trials consisted of a neutral warning signal (e.g., a plus sign) or a letter that matched the comparison items on a certain number of positive trials (see Fig. 6). The authors maintained that the probability of the prime as a valid cue to the identity of the target letters would determine the amount of active processing committed to it-if the correspondence occurred very infrequently (e.g., on 20% of positive trials), the prime may not receive much attention and would need little encoding capacity. According to Posner and Snyder, the passive build-up of information from a stimulus that demands no conscious allocation of resources produces a facilitating effect on the subsequent availability of the perceptual code (benefit), without any inhibitory effects (cost) ensuing when the prime does not match the target and attention must be directed to the output of a new set of analyzers. By contrast, the development of conscious expectancies, taking place when the prime is correlated with the target on the majority of positive trials, yields both cost and benefit; the subject fixes attention on the prime and must actively redirect the mechanism to cope with the onset of a different target. If J.V. is unable to process letters efficiently, we can expect a negative prime to yield inhibition even under conditions of low predictability between the prime and the target letters. Attention must be switched from the one to the other, and J.V.‘s perceptual encoding of letters may no longer be skilled enough to make this process automatic (c.f. LaBerge & Samuels, 1974). The experimental session involved 200 trials, each consisting of a
371
WORD RECOGNITION TABLE 2 REACTION TIME ON POSITIVE TRIALS TO NEUTRAL, FACILITATING, AND INHIBITORY Reaction
Neutral (+ AA) Facilitating (A AA) Inhibiting (B AA)
CUES
time
Control
J.V.
420 (37) 376 (58) 402 (59)
658 (102) 638 (119) 700 (126)
warning cue displayed tachistoscopically for 500 msec, followed by a 200-msec pause and then a pair of target uppercase letters that were either the same (e.g., AA) or different (e.g., AB). The cue was a neutral plus sign (+) on 20 of the positive trials, a letter that was physically identical to the target letters on 16 trials (P PP), and a different letter (J PP) on the remaining 80 positive trials. Thus, the majority of letter cues (more than 80%) were negatively related to the ensuing targets, a contingency that normally does not exert an inhibitory influence on the activation of letter codes. J.V. was instructed to look directly at the cue and to get ready for the occurrence of the letter pair requiring a same-different judgment. He was asked to respond as quickly and as accurately as possible, depressing a key with the index finger of his right hand to indicate a positive decision, and to use his left index finger for a negative decision. The key-press stopped a millisecond clock triggered by the onset of the target display, which remained ilhuninated until the execution of a response. A series of 30 practice trials was administered before the actual experiment. All stimuli occurred slightly to the left of a central fixation point, and the target letters were positioned along a vertical axis. The influence of positive and negative primes on “same” judgments can be observed in Table 2. A normal control subject, matched to J.V. for age, sex, and general level of education demonstrates the anticipated facilitation effect on reaction times (t(33) = 2.94,~ < .Ol), without concomitant inhibition. By contrast, J.V. yields less-pronounced facilitation but very marked inhibition,3 which just falls short of significance (t(97) = 1.9, p = .06). Errors were too few (less than 5% of responses) for a statistical analysis. The results imply that J.V. does experience some limitation in the automatic components of letter recognition. The comparison task requires 3 J.V.‘s failure to demonstrate significant facilitation from a matching prime (e.g., A AA) is reminiscent of the observation that conditions producing larger cost in normal subjects also tend to yield smaller benefits (Stanovich & West, 1981; Eisenberg & Becker, 1982).
372
BUB, BLACK, AND HOWELL
only that he determine whether two letters are physically identical, yet the presence of another letter before the target pair exerts a substantial distracting effect. The nature of the inhibitory priming is not entirely clear, given that the explanation offered by Posner and Snyder (1975a) has met with some dispute (c.f. Henderson, 1982). The evidence, however, is consistent with the claim that letter perception is a sufficiently demanding process for J.V. that attention must be actively switched from one element to another. DISCUSSION
The attempts of a letter-by-letter reader to identify written words certainly gives one the impression that each letter is treated as an isolated unit to arrive at the meaning and pronunciation. We have tested the hypothesis that the syndrome reflects the complete absence of higher-level contextual influences on the encoding process, by examining J.V.‘s ability to recognize letters in familiar words, orthographically legal pseudowords, and random letter sequences. Normal readers show a clear effect of orthographic structure in this task, which aids their recovery of the letter descriptions in words and pseudowords. J.V., though requiring much longer exposure durations to obtain the same level of accuracy as a matched control patient, nevertheless demonstrates a comparable influence of context extending across all four letter positions of the target string. Evidence suggests that at least some of the activated information mediating this orthographic superiority effect may not be sufficiently detailed for an overt naming response-J.V. is unable to explicitly report all the letters in familiar words presented at the same exposure duration yielding nearperfect recognition under conditions of forced choice, and his performance deteriorates systematically over the array. Given that a measure based on a two-choice procedure is less sensitive than full report, we need to be cautious about the interpretation of the observed discrepancy. The complete absence of any serial position effect on probe recognition scores, however, and their high level of accuracy, does indicate that some information is available to J.V. that is not captured by free report. What are the implications of the present results for a theory of letterby-letter reading? Normal sensitivity to orthographic context involves the contribution of both word-specific (lexical) activation and the activation of subword units. Attempts to isolate the properties of pseudowords responsible for their perceptual enhancement have uncovered attributes like positional constraints (e.g. McClelland & Johnston, 1977) and pronounceability (e.g., Massaro, Venezky, & Taylor, 1979), though the status of the latter variable remains controversial. More importantly, neither dimension has provided a clear theoretical reason for assuming a distinction between the mechanism that extracts spelling units from a letter array and the one that extracts whole words. Recent explanations of the pseu-
WORD RECOGNITION
373
doword advantage utilize the concept of spreading activation between the members of an orthographic neighborhood within the word-form system (c.f. McClelland & Rumelhart, 1981). On this account, words and pseudowords “differ merely in the fact that the region of orthographic space excited by their display contains, in the case of a real word, a representation of the word itself” (Henderson, 1982; p. 272). The presence of a word and pseudoword advantage in a letter-by-letter reader, then, may be taken as relevant evidence against the claim that the patient is denied access to the visual word-form system, though we cannot reach any definite conclusions about the overall integrity of the mechanism (e.g., only a subset of whole-word descriptions may be available), nor do we know whether word-level activation and inhibition are operating normally. Rival analyses of letter-by-letter reading suggest two possible ways of interpreting J.V.‘s preserved sensitivity to orthographic context: The word-form system may be intact, but the output of peripheral letter analyzers to this component may require a slow, sequential operation (Patterson & Kay, 1982). The word-superiority effect can still occur under conditions that force letter-by-letter reading in normal subjects, provided the rate of letter processing falls within certain limits. Forster (1980) used a serial incremental display of letters and found a reliable influence of context on letter identification at fast rates of presentation (40 msec/letter) but not at a slower rate (400 msec/letter). More recently, Chastain (1984) obtained a word-superiority effect for targets with letters in reversed order (e.g., TAH), a manipulation that produced a serial, right-to-left mode of encoding. If this letter-by-letter approach mimics the reading procedure utilized by J.V., we can view his recognition of words as a sequential operation that continuously receives feedback from higher levels of representation during the analysis of each letter position. Certain aspects of J.V.‘s performance are not inconsistent with the foregoing hypothesis-for example, the magnitude of the word-superiority effect increases systematically across the location of the probe in the fourletter array-but these data can also be handled by a theory incorporating the assumption of parallel letter entry to the word-form system (e.g., Rumelhart, 1970). Distinguishing between the two alternatives is crucial to a better understanding of the syndrome and would involve an assessement of the relationship between changes in the accuracy of letter processing and the effective duration of the target (c.f. Adams, 1979). If letter-by-letter readers are not impaired in the transfer of letters to the word-form system, the functional damage causing the syndrome must occur after word-level representations are contacted. We assume from J.V.‘s response to orthographic context that at least some whole-word information remains intact, and that partial loss of word forms cannot be responsible for the striking nature of his disorder. Patients with surface
374
BUB, BLACK, AND HOWELL
dyslexia also appear to have lost a significant proportion of word descriptions at the orthographic level, yet their performance may show none of the characteristics found in letter-by-letter reading. M.P., for example, described by Bub, Cancelliere, and Kertesz (1985) had clearly sustained major damage to the whole-word reading procedure,4 but was nevertheless capable of normal speed and accuracy when she translated print (orthographically regular words or nonsense words) into sound. The extremely slow responses of the letter-by-letter reader, coupled with the massive effect of word length on performance, cannot be simply attributed to a more extreme version of the word-form deficit that evidently does exist as an important component of surface dyslexia (Patterson, Marshall, & Coltheart, 1985). To account for letter-by-letter reading in terms of an impaired wordform system without assuming the total destruction of its contents, Shallice and Saffran (1986) propose that damage has occurred to the mechanism by which a given word is activated and competing possibilities inhibited. Tacit recognition (and presumably the word-superiority effect as well) may then be the due to weak output generated by the system. Evaluating this interpretation would require more detailed information on the nature of whole-word activation in the syndrome. Finally, a test of J.V.‘s ability to encode letters in isolation has revealed the possibility of a subtle disturbance that may account for the extreme slowness of his letter-by-letter reading strategy. We have found that the mere presence of a letter before the onset of another letter pair inhibits their analysis, even though the situation has been designed to minimize attention to the prime. While it is unlikely that the dyslexia is caused by this perceptual disturbance, which may well arise beyond the passive activation of letter identities, the impairment will add to the difficulties experienced by the patient and could affect the nature of performance on many word recognition tasks. REFERENCES Adams, M. .I. 1979. Models of word recognition. Cognitive Psychology, 11, 133-176. Bub, J., & Bub, D. 1988. On the methodology of single-case studies in cognitive neuropsychology. Cognitive Neuropsychology, 5, 565-582. Bub, D., Cancelliere, A., & Kertsez, A. 1985. Whole-word and analytic translation of spelling to sound in a non-semantic reader. In K. E. Patterson, J. C. Marshall, & M. Coltheart (Eds.), Surface dyslexia. Hillsdale, NJ: Erlbaum. Cattell, J. M. 1886. The time taken up by cerebral operations. Mind, 11, 220-242. 4 Regularization errors (e.g., BEAR read as “beer”) are caused by failure to locate the whole-word address for converting print to sound, and the consequent assembly of pronunciation is based on a more regular correpondence (e.g., the most common rendering of the pattern EAR-as in HEAR, NEAR, GEAR, etc.-differs from the pronunciation of exception words like BEAR and PEAR).
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Chastain, G. 1984. Directional letter-by-letter analysis and the word-superiority effect. Memory and Cognition, 12, 195-201. Coslett, B., & Satfran, E. Preserved reading in pure alexia. Brain, in press. Dejerine, J. 1892. Contribution a I’etude anatomochimique des differentes varittts de &cite verbale. Compte Rendu Hebdomadaire des Sct!ances et Me’moires de la SocitW de Biologie, 4, 61-90. Eisenberg, P., & Becker, C. A. 1982. Semantic context effects in visual word recognition, sentence processing, and reading: Evidence for semantic strategies. Journal of Experimental
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Forster, K. I. 1980. Absence of lexical and orthographic effects in a same-different task. Memory and Cognition, 8, 210-215. Friedman, R. B., & Alexander, M. P. 1984. Pictures, images and pure alexia: A case study. Cognitive Neuropsychology, 1, 9-23. Henderson, L. 1982. Orthography and word recognition in reading. New York/London: Academic Press. Johnston, J. C. 1978. A test of the sophisticated guessing theory of word perception. Cognitive Psychology, 10, 123-153. Kinsbourne, M., & Warrington, E. K. 1962. A disorder of simultaneous form perception. Brain, 85, 461-486. Kinsbourne, M., & Warrington, E. K. 1963. The localizing significance of simultaneous form perception. Brain, 86, 397-415. KuEera, H., & Francis, W. N. 1967. Computational analysis of present-day American English. Providence, RI: Brown Univ. Press. Laberge, D., & Samuels, S. J. 1974. Towards a theory of automatic information processing in reading. Cognitive Neuropsychology, 6, 293-323. Levine, D. N., & Calvanio, R. 1978. A study of the visual defect in verbal alexia-simultangnosia. Brain, 101, 65-81. Manelis, L. 1974.The effect of meaningfulness in tachistoscopic word perception. Perception and Psychophysics, 16, 182-192. Massaro, D. W., Venezky, R. L., & Taylor, G. A. 1979.Orthographic regularity, positional frequency and visual processing of letter strings. Journal of Experimental Psychology, 108, 107-124. McClelland, J. L. 1976. Preliminary letter identification in the perception of words and nonwords. Journal of Experimental Psychology: Human Perception and Performance, 2, 80-91. McClelland, J. L., & Johnston, J. C. 1977. Preliminary letter identification in the perception of words and non-words. Journal of Experimental Psychology: Human Perception and Performance, 2, 80-91. McClelland, J. L., & Rumelhart, D. E. 1981. An interactive activation model of context effects in letter perception: Part 1. An account of basic findings. Psychological Review, 88, 375-407. Patterson, K., & Kay, J. 1982. Letter-by-letter reading: Psychological descriptions of a neurological syndrome. Quarterly Journal of Experimental Psychology, 34A, 411-441. Patterson, K. E., Marshall, J. C., & Coltheart, M. 1985. Surface dyslexia. Hillsdale, NJ: Erlbaum. Posner, M. I., & Snyder, C. R. R. 1975a. Attention and cognitive control. In R. L. Solso (Ed.), Information processing and cognition: The Loyola symposium. Hillsdale, NJ: Erlbaum. Posner, M. I., & Snyder, C. R. R. 1975b. Facilitation and inhibition in the processing of signals. In P.M.A. Rabbitt & S. Dornic (Eds.), Attention nnd Performance. New York: Academic Press. Vol. 5.
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Reicher, Cl. M. 1%9. Perceptual recognition as a function of meaningfulness of stimulus material. Journal of Experimental Psychology, 81, 274-280. Rumelhart, D. E. 1970. A multicomponent theory of the perception of briefly exposed visual displays. Journal of Mathematical Psychology, 7, 191-218. Rumelhart, D. E., & McClelland, J. L. 1981. An interactive activation model of context effects in letter perception: Part 2: The contextual enhancement effect and some tests and extensions of the model. Psychological Review, 89, 60-94. Shallice, T. 1981. Neurological impairments of cognitive processes. Brirish Medical Bulletin, 37, 187-192. Shallice, T., & McCarthy, R. 1985. Phonological reading: From patterns of impairment to possible procedures. In K. E. Patterson, J. C. Marshall, &KM. Coltheart (Eds.), Surface dyslexia. London: Erlbaum. Shallice, T., & Saffran, E. 1986. Lexical processing in the absence of explicit word identification: Evidence from a letter-by-letter reader. Cognitive Neuropsychology, 3,429458. Shallice, T., Warrington, E. K., & McCarthy, R. 1983.Reading without semantics. Quarterly Journal of Experimental Psychology, 35A, 111-138. Stanovich, K. E., & West, R. F. 1981. The effect of sentence context on ongoing word recognition: Tests of a two-process theory. Journal of Experimental Psychology: Human Perception and Performance, 7, 658-672. Warrington, E. K., & Rabin, P. 1971. Visual span of apprehension in patients with unilateral cerebral lesions. Quarterly Journal of Experimental Psychology, 23, 423-431. Warrington, E. K., & Shallice, T. 1980. Word-form dyslexia. Brain, 103, 99-l 12. Wheeler, D. D. 1970. Processes in word recognition. Cognitive Psychology, 1, 59-85. Wolpert, I. 1924. Die Simultanagnosie-Storng der Gesamtauffassung. Zeitschrift fur die gesamte Neurologie und Psychiatric, 93, 397-415.