Deep dyslexia, imageability, and ease of predication

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24, 1-19

(1985)

Deep Dyslexia, Imageability,

and Ease of Predication

GREGORY V. JONES University

of Bristol

A development of existing theories of deep dyslexia is outlined. It proposes that the effects of imageability upon the ease of reading of words by deep dyslexics occur as a result of variation in the ease with which individual words summon semantic predicates, on the basis of which reading responses can be made. Ease-of-predication scores are obtained for a corpus of nouns and found to be, as hypothesized, closely related to imageability scores. It is shown that the other major characteristics of deep dyslexia can also be accounted for by this proposed mechanism. These include the well-established effects of syntactic category (in particular, of the distinction between content and function words) upon reading. Further evidence is provided that this effect may be attributed to variation in ease of predication. %: 1985 Academic Press. Inc.

The ease with which a word gives rise to an image has been shown recently to be a powerful determinant of reading performance, in particular in the syndrome of deep dyslexia. However, the explanation for this phenomenon is at present uncertain. As noted by Baddeley, Ellis, Miles, and Lewis (1982, p. 196), “the process whereby imageability influences readability is obviously a puzzle for any theory of reading at present. Hence, despite the fact that imageability is a potent variable, it has virtually no explanatory power.” It is proposed here that the apparent role of imagery in reading and its disorders may be accounted for in terms of a variable termed ease of predication. The latter is first described, and then shown to provide a simple explanation of imageability effects in reading, and in particular a coherent account of major aspects of deep dyslexia. A word’s imageability has been proposed by Paivio (1971, 1983) to be an indication of the readiness with which it is encoded by the imagery The results of Experiment 1 were reported at the Cambridge, 1982, meeting of the Experimental Psychology Society. I am grateful to two anonymous reviewers for comments and to Brian Alner for help in carrying out Experiment 2. Send requests for reprints to Gregory V. Jones, Department of Psychology, University of Bristol, 8-10 Berkeley Square, Bristol, BS8 IHH, England.

0093-934X185

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Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.

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component of an imagery-verbal dual-coding memory system. Jorm (1979a, 1979b) has suggested that imageability affects performance in developmental dyslexia via an effect upon the direct visual route to reading which is revealed when the usual phonological route is unavailable. Contrary to this, however, recent evidence suggests that the use of the phonological route declines rather than increases with age (Doctor & Coltheart, 1980; see also Baddeley et al., 1982). In addition, it is not clear why the efficiency of direct lexical access itself should be related to variation in the subsequent modality of storage of a lexical item. An alternative approach is to assume that there is some underlying variable that is capable of being used to explain systematic differences both in the ease with which a word gives rise to the experience of mental imagery and in the ease with which that word can be read in particular circumstances. The point at which these two factors intersect presumably lies within the semantic domain, and it is therefore here that the search for an underlying factor may be made. Modern psychological theories of meaning have taken a diversity of forms but have generally made the common assumption that the element representing a word in semantic memory is associated with a number of features or, more fully, predicates (e.g., Anderson, 1976; Anderson & Bower, 1973; Collins & Loftus, 1975; Jones, 1983b; Smith, Shoben, & Rips, 1974). This assumption has been widely made not only in work primarily concerned with sentence verification, but also in work in a number of related areas such as those concerned with category prototypes (e.g., Jones, 1982; Rosch & Mervis, 1975; Smith & Medin, 1981), basic categories (e.g., Jones, 1983a; Murphy & Smith, 1982; Rosch, Mervis, Gray, Johnson, & Boyes-Braem, 1976), similarity (e.g., Tversky, 1977; Tversky & Gati, 1982), metaphor (e.g., Ortony, 1979), and episodic memory (e.g., Jones, 1976, 1978; Tulving, 1983). From this point of view, the most parsimonious explanation which could in principle account for both phenomena being related is that variability in ease of imaging and in ease of reading both reflect variability in the associated distributions of predicates for individual words. At present, relatively little is known about the predicate distributions of particular words. Subjects have been asked to generate lists of explicit predicates for particular types of words by Rosch et al. (1976) and by Ortony (1979). Both studies were carried out for relatively specific purposes, however. Rosch et al. were studying a possible criterion for identifying basic categories: the highest level of a hierarchy at which the members of a category still possess many predicates in common. For the hierarchy Levis, pants, clothing, for example, the basic level turned out to be pants. Many predicates such as “are made of cloth” and “have two legs” were generated for punts, while the only additional predicate generated for Levis was “are blue,” and only a relatively small number of predicates such as “keeps you warm” were generated for clothing. Ortony

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examined similes such as “billboards are like warts.” Subjects generated predicates for both billboards and warts, and these were examined for commonality. It was found, as hypothesized, that these predicates [or as they are also termed by Ortony; see Rumelhart and “subschemata,” Ortony (1977)] in general overlapped at a predicate which is frequently generated for the comparator term (warts) but only infrequently generated for the main term (billboards)-in this case, “are ugly.” The present study differs from the two previously described in that it attempts to investigate quantitative rather than qualitative aspects of the predicates associated with individual words. For the purposes of both the previous studies it was necessary to elicit predicates individually. This meant that Ortony (1979), for example, was able to investigate only 10 words per subject. The approach to be adopted here, on the other hand, is that useful prediction of the role of individual words in deep dyslexia is possible on the basis of knowledge of only the overall ease of generating predicates, regardless of their identities. This means that it was appropriate to obtain an overall measure of ease of predication for each member of a relatively large set of words. The central hypothesis to be examined concerns the apparent dependency of deep dyslexics’ reading of nouns upon the imageability of each word (e.g., Coltheart, 1980a). It is widely accepted (as cited earlier) that the explanation for this finding is at present obscure, although there is probably some consensus surrounding the view of Shallice and Warrington (1980, p. 139) “that in our view imagery is not a relevant process.” The explanation which is investigated here is a development of that proposed by Morton and Patterson (1980a). They postulate that in deep dyslexia reading output is possible only when mediated via the cognitive system. According to the present view, word-derived input accesses the cognitive system by attempting to summon one or more matching predicates. Subsequently, verbal output occurs only on the basis of this predicational information. Occasionally, deep dyslexic output is indeed directly predicative in form, as when the prototypical deep dyslexic patient G.R. (Marshall & Newcombe, 1966, 1973) read “enemy” as “I know it . . . something . . . different countries fighting together . . . spy” (Newcombe & Marshall, 1980). Usually, however, task constraints are met by the production of a single word as response. Thus it follows that it is the ease with which predicates of the stimulus word can be summoned which is the determinant of its likelihood of being read correctly. Given that a word’s imageability is known already to be strongly related to this likelihood (Coltheart, 1980a; Shallice & Warrington, 1980), it follows that if the ease of predication hypothesis is correct there should be a close relation between ease of predication and imageability. Experiment 1 was carried out in order to test this prediction empirically, and further exposition of the ease of predication account of deep dyslexia is postponed until after its description.

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EXPERIMENT

1

Pilot observations indicated that many subjects were unfamiliar with the term “predication.” Hence in gathering the data to be reported, the phrase “ease of predication” was operationalized as “ease of putting words into simple factual statements.” To clarify the task further, examples were given showing that a wide variety of knowledge was eligible for inclusion. Method The 30 subjects were students at Bristol University. Each subject rated the same set of 125 words that are shown in Table 1. These were selected as those nouns common to the studies of Paivio, Yuille, and Madigan (1968) and of Brown and Ure (1%9). To conform to British usage the three words “candy,” “humor,” and “kerosene” were replaced in the rating booklets by the equivalent “sweets,” “humour,” and “paraffin,” respectively. Each subject viewed the words in a different randomized order, with the orders for onehalf of the subjects being the exact reverse of those for the other half. The words were arranged on four successive pages of a booklet. Alongside each word the numerals 1 to 7 were printed, with 1 and 7 representing lowest and highest ease of predication, respectively. The first page of the booklet contained the instructions, which were also read aloud by the experimenter. Subjects. Materials.

TABLE EASE OF PREDICATION

1 OF NOUNS

Word

Mean

SD

Word

Agility Anger Amy Beggar Blossom Bowl Breast Butterfly Capacity Child City Comedy Contents Corpse Custom Dirt Door Dress Engine Excuse Flower Frog Girl Green

2.67 2.70 5.63 5.50 5.17 5.57 5.27 5.97 3.00 6.27 5.83 3.27 3.33 5.37 3.37 4.53 5.90 6.03 5.47 3.20 6.17 6.50 6.20 4.57

.84 1.02 1.19 1.14 .95 1.28 1.31 1.03 1.49 .83 1.29 1.44 1.52 1.50 1.30 1.43 1.30 1.00 1.22 1.42 .87 .68 1.16 2.21

Ambulance Anxiety Baby Bird Book BOY Butter Candy Chair Church Clothing Comparison Context Cottage Death Doctor Dream Earth Errand Fire Friend Fur Glacier Grief

Mean 5.87 2.30 6.40 6.63 6.20 6.43 5.87 5.97 6.47 5.80 5.97 2.67 2.37 6.03 3.93 5.63 3.87 5.33 3.67 5.53 5.03 5.13 5.13 2.77

SD

1.14 1.18 .77 .67 1.10 .90 1.20 1.03 .78 1.32 1.10 1.15 1.43 1.10 1.64 1.59 1.76 1.18 1.27 1.31 1.61 1.55 1.41 1.30

DEEP DYSLEXIA TABLE Word

Mean

SD

Hammer Health History Horse Hostage Industry Jelly Justice Kindness Kiss Love Memory Moment Month Mountain Ocean Orchestra Party Pencil Plant Power Pride Rattle River Salute Shadow Sickness Star Table Thief Tobacco Trouble Truth Vehicle Violation Vision Water Wine World

5.87

3.57 3.53 6.73 4.50 4.53 5.10 2.60 2.70 4.47 2.30 2.97 2.60 5.03 5.87 5.77 5.90 5.43 6.20 6.07 3.17 2.40 4.43 6.20 4.00 3.97 3.83 5.60 6.27 5.40 5.53 2.90 2.13 5.90 2.47 3.03 5.87 6.10 5.37

1.01 1.17 1.59 .58 1.36 1.46 1.45 1.25 1.12 1.38 1.60 1.33 1.45 1.65 1.20 1.07 1.09 1.41 .96 .78 1.34 1.07 1.19 .66 1.70 1.S6 1.34 1.13 1.17 1.19 1.22 1.30 1.14 1.35 1.55 1.38 1.36 .84 1.52

Instructions.

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l-Continued

Word Hatred Hide Home Hospital Humor Ink Joy Kerosene King Lake Malice Menace Money Mother Nectar Opinion Paper Patent Person Poetry Prairie Priest Revolt Salad Seat Ship Square Street Theory Time Tree Trumpet Vanity Village Virtue Warmth Window Woman

-

Mean

SD

2.53 3.30 5.03 5.93 2.57 5.53 2.50 5.03 5.77 6.03 2.27 2.90 5.63 5.93 4.20 3.07 5.63 2.60 4.77 3.57 4.93 5.60 3.57 5.60 6.00 6.47 5.10 5.67 3.10 3.50 6.47 5.97 2.53 5.90 2.23 3.13 6.17 6.20

1.20 1.64 1.56 1.05 1.25 1.38 I.11 1.38 1.22 1.00 1.08 1.27 1.43 1.28 1.56 1.46 1.33 I .45 1.83 1.76 1.39 1.07 1.22 1.16 1.05 .68 1.65 1.12 1.73 1.74 .78 .96 1.22 .92 1.04 1.20 1.09 1.16

The instructions were similar in form to those of Paivio et al. (1968). They

were as follows. Words differ in the ease with which what they refer to can be described by simple factual statements. Some words can be put into statements quite easily and quickly, while for others this can be done only with difficulty or not at all. The purpose of this experiment is to rate a list of 125 words as to the ease or difficulty with which they can be put into simple factual statements.

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As an example, the word “dog” would probably be judged as very easy to make simple factual statements about, because it can readily be put into statements such as the following: A dog is a type of animal. A dog often lives in a kennel. A dog barks when angry. A dog can be pedigree or mongrel. A dog has four legs. A dog is called a puppy when young. A dog wags its tail when pleased. A dog can be as small as a Chihuahua. A dog can be as large as a St. Bernard. A dog sometimes chases a cat. As a contrasting example, the word “idea” would probably be judged as very difficult to make simple factual statements about. Because words also differ in many other ways (such as in how easy they are to mentally image or to categorize), it is important that in making your ratings you attend only to the ease with which each word can be put into simple factual statements. Your ratings will be made on a seven-point scale, where 1 is the low end of the ease-of-putting-into-statements scale, and 7 is the high end of the ease-ofputting-into-statements scale. Make your rating by putting a circle around the number from 1 to 7 that best indicates how easy it is to put the word into simple factual statements. The words that are most difficult to put into statements should be given a rating of 1; words that are easiest to put into statements should be given a rating of 7. Words that are intermediate in ease-of-putting-into-statements should of course be rated appropriately between these two extremes, with a rating of 4 representing the average level of easiness. Feel free to use the entire range of ratings from 1 to 7, but don’t be concerned about how often you use a particular rating as long as it represents your true judgement. Work at a reasonable pace, but try to give for each word your best judgement as to the ease with which it can be put into simple factual statements. If necessary, you can refer back to these instructions when rating the words on the following pages. Do you have any questions?

Results Table 1 shows the mean and standard deviation of the ease-of-predication score for each word. The mean scores yielded an overall mean and standard deviation of 4.70 and 1.40, respectively. The reliability of the mean scores was assessed by randomly dividing the set of participants into two halves. It was found that the correlation between the two subgroups of mean scores took the value 4123) = .95. The present scores were compared with the imageability values reported by Paivio et al. (1%8). Imageability scores were higher than corresponding ease of predication scores in all but six cases, a highly significant difference on a sign test. The six words for which ease of predication scores were higher than imageability scores were context, excuse, history, moment, month, and theory. In the opposite direction, there were six words for which imageability scores were more than two scale units higher than

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ease of predication scores: anger, green, joy, kiss, love, and warmth. In spite of these differences, the two sets of scores were very highly correlated overall, r(123) = X3. Discussion

The results of this experiment demonstrate that there is, as predicted, a very high degree of commonality in assessments of the ease with which predicates of a word are summoned and of the ease with which images of that word can be formed. The ease of predication measure therefore provides for the first time evidence in favor of the frequently voiced hypothesis (Anderson & Bower, 1973; Kieras, 1978; Shallice & Warrington, 1980) that apparent effects of imageability may be mediated via a previously unspecified semantically defined variable with which it is very closely correlated. The relation observed here is indeed so close that it is not practicable to separate out the effects of the two variables by orthogonal manipulation or by partial correlation techniques. Instead, qualitative rather than quantitative analysis appears appropriate, with the variables compared in terms of the adequacy of their theoretical accounts of empirical phenomena. As noted previously, the concept of ease of predication provides a principled account of variation in the probability with which individual nouns are read correctly by deep dyslexics. This account is parsimonious because it does not require the assumption of separate imageable and abstract semantics, with only the latter damaged in deep dyslexia (e.g., Morton & Patterson, 1980a). The account is shown next to embrace also other outstanding features of the symptomatology of deep dyslexia. A second characteristic of deep dyslexia is the production in reading of semantic paralexias-responses that are incorrect but whose meaning is related to that of the stimulus word. The example of patient G.R. reading “enemy” ultimately as “spy” (Newcombe & Marshall, 1980) has already been cited. It is evident that semantic paralexias receive a natural interpretation within the present framework as output derived, like correct responses, from the predicational information retrieved by the stimulus word. Predicates that are insufficiently detailed to specify the stimulus word uniquely will nevertheless delimit the correct semantic domain. It is thus predicted that the average ease-of-predication values of words producing semantic paraiexias should be nearly as high as those of words read correctly, and considerably higher than those of words yielding no response. The preceding prediction may be examined by means of the responses given by two deep dyslexic patients, P.D. (Kapur & Perl, 1978) and K.F. (Shallice & Warrington, 1975) to the 125 words of Table 1, which have been reported [along with the responses to the remainder of the corpus of Brown and Ure (1969)] in Coltheart, Patterson, and Marshall (1980, Appendix 2). The mean ease-of-predication values for correct responses,

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semantic paralexias, and omissions are 5.33,5.87, and 3.82, respectively. The numbers of semantic paralexias for both these patients were quite small (four for P.D. and six for K.F.), and hence analysis of variance showed no significant difference between semantic paralexias and correct responses, though ease-of-predication values for omissions were indeed significantly lower. A recent study by Barry (1984), however, reported the effects of ease of predication upon the reading of G.R., the deep dyslexic patient described originally by Marshall and Newcombe (1966, 1973), who makes a rather higher proportion of semantic errors in reading. Barry was interested in the consistency with which words were read on two separate trials by G.R. He reported that, consistent with the present account, the mean ease-of-predication scores of the 22 words read as semantic paralexias on both trials (4.36) was both significantly less than that of words read correctly on both trials (5.76) and significantly greater than that of words that were not read on either trial (2.93). Three further characteristics of deep dyslexia are also readily accounted for. The first consists of the production of visual paralexias in readingresponses whose spellings are similar to those of the stimuli. As an example, Patterson’s patient P.W. read “innate” as “inn” and “campaign” as “camping” (Coltheart et al., 1980, Appendix 2). Following Morton and Patterson (1980a), the present approach attributes the formation of visual paralexias to the excitation of an adjacent input logogen when the correct input logogen has failed to summon a predicate (in Morton and Patterson’s terms, has failed to access a semantic code). The second characteristic consists of the production of derivational paralexias in reading (see Patterson, 1980). As an example, Patterson’s patient P.W. read “hurting” as “hurt” and “territory” as “territorial” (Coltheart et al., 1980, Appendix 2). It is clear that derivational paralexias can generally be classified also as semantic paralexias or visual paralexias (or both), and thus may be formed in the ways already discussed. Semantically, for example, the high degree of overlap of the predicational information associated with different forms of the same lexeme (e.g., “decide,” “decision, ” “decisive,” etc.) may result in the output form differing from the input one. The third characteristic consists of the inability to read nonwords. The present account again follows Morton and Patterson (1980a) in attributing this aspect of the syndrome to the reliance of deep dyslexics’ reading upon the lexically accessed cognitive system (here postulated to be predicatively organized). Nonwords simply cannot gain admittance to the system. Finally, we come to what is generally recognized to be the remaining major characteristic of deep dyslexia (e.g., Coltheart, 1980a; Geschwind, 1981; Patterson, 1981; Shallice & Warrington, 1980). This is the existence of particular difficulties in reading certain syntactic categories of words. Nouns are read better than adjectives, which are read better than verbs,

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and all of these are read better than function words such as prepositions and auxiliary verbs. Morton and Patterson (1980a) suggest that both this phenomenon and the occurrence of derivational paralexias indicate damage to a linguistic processor within the cognitive system. But the present explanation proposes that the syntactic category effect, like the derivational paralexia one, is not a consequence of linguistic malfunction. Instead, it is simply a manifestation of reliance upon the cognitive route to reading. According to this view, the ordering observed among different syntactic categories is a reflection of the ease with which they summon matching predicates. Experiment 2 was carried out in order to put this explanation to the test. EXPERIMENT 2

This experiment was conducted to examine the hypothesis that the ordering of ease-of-predication scores for different syntactic categories would be the same as that found in the reading of deep dyslexics. Its method was similar to that of Experiment 1. Method Subjects. The 14 subjects were students at Bristol University. Materials. Each subject rated the same set of 100 words that are shown in Table 2. These were selected as members of five subsets of 20 words each: high-imagery nouns, low-imagery nouns, adjectives, verbs, and function words. All words had frequencies of occurrence of 100 or more per million in the Thorndike-Lorge (1944) word count, and were chosen to be as unambiguous as possible in syntactic category. High-imagery and low-imagery nouns had imagery values that were greater than 6.00 or less than 4.50, respectively, in the study of Paivio et al. (1968). Function words are clearly more heterogeneous than the other categories, and thus for this category five members of each of four groups were selected. These were personal pronouns, relative pronouns and interrogatives, prepositions and conjunctions, and auxiliaries (see Quirk, Greenbaum, Leech, & Svartvik, 1972). Each subject viewed the set of 100 words in a different random order. Instructions. The first and the last three paragraphs of the instructions were identical to those of Experiment 1 (except that the first paragraph referred to 100 instead of 125 words). The other instructions were simplified slightly, as follows. As an example, consider the word “dog.” This could be put into simple factual statements such as the following: A dog is a type of animal. A dog often lives in a kennel. Because words also differ in many other ways, it is important that in making your ratings you attend only to the ease with which each word can be put into simple factual statements.

Results Table 2 shows the mean and standard deviation of the ease-of-predication score for each word. Page’s test showed that there was a significant trend in the predicted direction, L = 754, p < .Ol, with the means for high-imagery nouns, low-imagery nouns, adjectives, verbs, and function

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EASE OF PREDICATION

TABLE 2 OF FIVE CATEGORIESOF WORD

SD

Mean

SD

nouns Car City Gentleman Lake Mountain Ocean River Valley Window Woods

6.71 6.79 6.21 6.50 6.79 6.64 6.86 6.14 6.71 6.86

.61 .42 1.31 .77 .42 .63 .36 1.03 .61 .36

nouns Duty Fact Health History Knowledge Length Method Month Soul Truth

5.21 5.36 5.00 5.93 4.79 5.14 5.71 6.43 4.29 4.79

1.81 1.65 1.92 1.14 1.81 1.68 1.20 .76 2.49 2.01

3.86 3.71 2.00 4.21 4.00 4.43 3.64 5.00 3.86 3.00

Adjectives 2.11 Beautiful 1.82 Different .79 Famous 2.19 Former 1.80 Golden 1.74 Heavy 1.98 Pleasant 1.92 Simple 1.99 Sudden 1.88 Wonderful

4.93 3.71 4.64 3.86 4.00 4.36 3.57 4.07 3.36 4.07

1.94 2.20 1.69 1.95 1.92 2.02 1.91 2.13 1.78 1.48

2.93 3.29 3.86 3.71 3.71 3.29 3.64 4.71 4.14 3.86

.92 1.73 1.79 3.12 1.59 1.38 1.98 2.09 1.95 1.35

3.86 3.50 4.14 3.64 3.64 3.00 3.50 4.00 4.43 4.36

1.88 2.21 1.79 2.10 2.02 1.71 1.79 1.80 1.79 1.63

Word

Mean

Word

Body Child Door Hall Meat Newspaper Officer Sea Village Woman

6.50 6.64 6.57 6.14 6.86 6.64 6.43 6.64 6.86 6.71

High-imageability .94 .64 .94 1.56 .36 .50 1.09 .84 .36 .73

Direction Effort Hatred Heaven Hour Law Life Moment Opinion Thought

4.71 4.14 5.64 4.79 6.21 5.36 5.07 4.64 5.00 5.36

Low-imageability 1.54 1.83 1.34 2.36 1.19 1.65 1.73 1.95 1.52 1.39

Afraid Bright Every Foreign Fresh Happy Important Rich Soft Various Accept Believe Continue Eat Grow Include Lose Prepare Sing Understand

Verbs Ask Consider Destroy Enjoy Hear Know Obtain Receive Sit Write

DEEP DYSLEXIA TABLE Word Him” It” You” At’ Inb That’ What’ Who’ Has“ Shalf

Mean 2.43 2.43 3.14 1.57 2.07 2.29 1.71 2.29 1.64 1.79

SD

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Word

Function words 1.87 I” 1.99 She” 2.11 Andb 1.60 But6 1.39 OUP 1.90 This’ 1.00 Which 2.14 Didd 1.15 ISd .97 Wouldd

Mean

SD

3.50 2.86 1.57 1.29 2.86 2.29 1.79 1.79 1.43 1.64

2.41 1.99 .85 .61 1.88 1.73 1.19 1.12 .65 1.60

a Personal pronouns. b Prepositions and conjunctions. ’ Relative pronouns and interrogatives. ’ Auxiliaries.

words being 6.62, 5.17, 3.89, 3.80, and 2.12, respectively. Analysis of variance demonstrated a significant effect of word group, F(4, 52) = 54.54, p < .OOl. This effect was generalizable beyond the words sampled, min F’(14, 52) = 2.27, p < .05 (Clark, 1973). Newman-Keuls’ test showed that all pair-wise comparisons between categories, except between adjective and verb, were significant at the .Ol level. An additional analysis was carried out upon the data for the four types of function word. The mean ease-of-predication scores for personal pronouns, relative pronouns and interrogatives, prepositions and conjunctions, and auxiliaries were 2.87, 2.07, 1.87, and 1.66, respectively. Analysis of variance demonstrated a significant effect of function word type, F(3, 39) = 6.67, p < .OOl, which was generalizable beyond the words sampled, min F’(11, 39) = 2.99, p < .Ol. Newman-Keuls’ test showed that, at the .05 level, personal pronouns yielded higher scores than each of the three other types of function word, but that the latter did not differ significantly from each other. Discussion The results of this experiment supported the hypothesis that the easeof-predication scores for different syntactic classes of word would vary in the same way as the ease of reading these words does for deep dyslexic patients. A subsidiary analysis examined variation within the general class of function words, and it was found here that personal pronouns yielded higher ease-of-predication scores than did the other types of function word studied. As far as I know, a difference of this kind has not been postulated in the deep dyslexia literature. However, it is suggestive that

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Morton and Patterson (1980b) found in their study of the patient P.W. that the proportion of function words which he read either correctly or as another semantically related function word was highest for personal pronouns; one difference, though, is that Morton and Patterson included possessive pronouns within their personal pronouns category. But further evidence consistent with the present account (which suggests that deep dyslexics’ reading is using a purely semantic route) is provided by the ingenious triad method used by Morton and Patterson to demonstrate that, even for function words, P.W. is sensitive to major semantic dimensions such as gender and number, though not to purely syntactic variables such as case or part of speech. More generally, this finding is also highly consistent, as Morton and Patterson (1980b) themselves note, with the basic dissociation between semantic and syntactic neuropsychological systems proposed by Marin, Saffran, and Schwartz (1976). GENERAL

DISCUSSION

The results of the experiments reported here provide support for the usefulness of the ease-of-predication concept. Much of the reading performance of deep dyslexic patients (see Coltheart, 198Oa) may be accounted for in terms of the sparing of a semantic reading route in which output is possible only on the basis of predicational information excited by input logogens. It is possible that this reading route is utilized also in other forms of dyslexia, for example, developmental dyslexia (Baddeley et al., 1982). Baddeley et al. in fact observed a sensitivity to words’ imageability values, attributed here to the role of predication, in the reading not only of developmental dyslexics but also that of normal controls. However, given the inaccuracy of the purely predicational route to reading, it would appear unlikely that it is normally utilized by adult readers. Consistent with this, Richardson (1976) found no effect of imagery value upon word pronunciation latency for normal readers. It might be asked why a predicational route to reading should exist, if it is not normally utilized by adult readers. The answer, of course, is that the route represents a facility which is usually subordinated to more precise mechanisms. An analogy may be made, for example, with a “lipreading route” to the understanding of speech. In normal listeners, such a route is generally subordinated to auditory perceptual mechanisms. But if the latter are damaged, even a profoundly deaf person may obtain an at least partial understanding of speech via lip-reading. Neural damage does not create the predicational route to reading any more than it creates the lip-reading route to speech comprehension. Instead, it may create the circumstances in which the output of such a route is not neglected, as it usually is, in favor of other sources of information. In a phrase of Geschwind (1981), the behavior after brain damage reveals “previously present but suppressed processes” (p. 833). It might further be enquired what are the normally utilized mechanisms

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for reading, if not predicational. In view of the very large volume of existing research devoted to this question [see Henderson (1982) for a recent review], it is not possible to attempt to answer this question adequately within the space of the present article. Implicit in the locating of the present proposal within the framework described by Morton and Patterson (1980a) there is a subscription to the modified logogen model of reading mechanisms (Morton, 1979) adopted by Morton and Patterson. However, since the predicational reading route is assumed to become of central importance in deep dyslexia only when normal reading processes are rendered unavailable, it would seem that the account of the former (i.e., the predicational reading route) that has been given here is unlikely to impose severe constraints upon theories of the latter (i.e., the normal reading process). One further question which might be posed in the present context asks how, if the predicational route is unreliable, we can access an individual word’s precise meaning with an orthographic input. However, this question is itself based on a premise which is almost certainly incorrect-the assumption that precise lexical meanings in fact exist. A variety of linguistic evidence inconsistent with this assumption has been reviewed by Lyons (1981), while influential psychological work (e.g., Anderson & Ortony, 1975) has also demonstrated experimentally that the meaning of a word is not a precise entity but instead something that encompasses a wide range of possibilities. Conversely, however, this indeterminacy at the lexical level is fully compatible with the predicational approach adopted here. The present approach has been shown to be one which provides a particularly parsimonious account of deep dyslexia. In its general form it derives from that of Morton and Patterson (198Oa), but has the advantage that it is necessary to postulate the existence of damage only to normal reading mechanisms, and not of additional damage to abstract-word semantics or linguistic processors (Morton & Patterson, 1980a). However, the model should only be regarded as the outline of a comprehensive theory. This is because the latter would need to specify explanations not only of deep dyslexia’s major characteristics but of minor ones also. It is to be hoped, nevertheless, that explanations of more detailed findings can be deduced from the model’s general principles. Three examples may be given. First, deep dyslexic patients are sometimes aware that their semantic paralexias are incorrect responses, and sometimes not. According to the present view, it might be expected that patients could usually detect poor agreement between the predicates that have been summoned and the output that they have given rise to unless either of two conditions held. Difficulty should arise either if the semantic paralexia were almost synonymous with the stimulus word (in which case their predicates would be very similar), or if the two words were both to have low levels of

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ease of predication (in which case the basis for detection of an error is impoverished). The available evidence suggests that this is indeed the pattern of awareness of their semantic paralexias which is experienced in practice by deep dyslexics (Patterson, 1978). A second detailed finding is that the errors made by deep dyslexic patients in reading function words often consist of other function words. The best-documented evidence for this phenomenon comes from the study of the patient P.W. reported by Morton and Patterson (1980b). In that particular case, a very simple explanation for the phenomenon is available. The data of the study were obtained by presenting P.W. repeatedly with lists of function words to read. The composition of the error corpus might be expected therefore to reflect a response set in favor of producing only function words as answers. Nevertheless, the phenomenon of function-word substitution is also apparent in other corpora. For example, Shallice and Warrington report the one-word error responses made by the deep dyslexic patient K.F. when attempting to read all words with frequencies of at least 100 per million in the ThorndikeLorge (1944) count (Coltheart et al., 1980, Appendix 2). Of these, 24 are clearly function words (a few others, such as “except,” are ambiguous), with 11 of these producing other function words as responses. The production of this sizable proportion of function-word paralexias is expected on the ease-of-predication account because they may arise in the ways previously discussed, as either semantic or visual errors (or both). For example, “am” was read as “be” (semantic), “yet” was read as “yes” (visual), and “upon” was read as “up” (semantic or visual); overall, the average proportion of the stimulus letters that were repeated in the patient’s response, for these function-word substitutions, was more than half, indicating a sizable visual contribution. A third detailed finding is that the visual errors made by deep dyslexic patients tend to have higher imageability values than the stimulus words themselves (Shallice & Coughlan, 1980; Shallice & Warrington, 1975). This result is to be expected on the present account which, as noted earlier, attributes the formation of visual errors to the excitation of adjacent input logogens when correct input logogens have failed to summon their predicates. Visual paralexic responses will derive from these secondarily excited logogens only if they in turn succeed in summoning their predicates. But the likelihood of their doing so is governed by the relevant ease-ofpredication values: The higher are the latter, the greater is the chance that a response corresponding to the secondarily excited logogen can be made via the predicational route. Overall, therefore, errors are expected to have relatively high ease-of-predication values, which means that, because of the finding reported here of a close relation between easeof-predication and imageabiliy values, they will tend also to have relatively high imageability values. Although the present article has focused upon the syndrome of deep

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dyslexia, it is appropriate to consider also relevant aspects of several other disorders. First, there is evidence that, in anemic dementia, semantic processing may be disrupted while reading ability is preserved (Schwartz, Marin, & Saffran, 1979; Schwartz, Saffran, & Marin, 1980>, exhibiting therefore the complement of the pattern of disorder displayed in deep dyslexia. Schwartz and her colleagues studied a patient, W.L.P., who suffered from presenile dementia, and found that she made many errors in a task such as sorting words into categories (e.g., “animal name”) but very few errors in reading even irregularly pronounced words (e.g., “leopard”). In this patient the normal reading processes responsible for the transformation of print into speech appear therefore to be relatively spared, while the semantic processes that are generally instigated by lexical access (and which are here postulated to be predicational in nature) are severely damaged, providing additional support for the distinction between these two types of process which has been assumed by the present article. Second, there is evidence that certain symptoms of deep dyslexia but not others are exhibited in the syndrome of phonological dyslexia (often alternatively termed phonological alexia) in which there is again great difficulty in correctly reading nonwords (Beauvois & DCrouesnC, 1979a; Patterson, 1982; Shallice & Warrington, 1980). In particular, Beauvois and her colleagues (Beauvois & DCrouesnC, 1979b; Beauvois, DCrouesnC, & Saillant, 1980) reported that when their prototypical phonological dyslexia patient, R.G., was reading words, many of the relatively small number of errors that he made were derivational (22%) or visual (35%) in nature, but none was purely semantic. The framework adopted in the present article suggests that the absence of semantic paralexias is due to the predicational route not being utilized by this patient (because a reading route utilizing lexically accessed phonology is still substantially intact), and similarly suggests that the derivational errors were in this case only visual in origin (rather than either visual or semantic as in deep dyslexia), In one recently reported phonological dyslexia case, A.M. (Patterson, 1982), the number of nonderivational visual errors was relatively low. However, in the reported corpus of all 26 paralexic errors made by A.M. [when reading the 100 words listed by Schonell(1942), and the 200 words listed by Klee and Legge (1976)], the average length of word that was read incorrectly was 7.7 letters, and the average number of these letters appearing in the corresponding paralexia was 6.6 letters. Given this extremely high degree of visual similarity between stimuli and responses overall, it is not surprising that they tended to be derivatives of each other. Third, an investigation reported by Warrington (1981) is of considerable interest. What Warrington found was that a particular brain-damaged patient, C.A.V., exhibited for a short period a greater impairment in

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reading high-imageability words than in reading low-imageability words (C.A.V.‘s cognitive functions recovered over the period of approximately 2 weeks during which he was tested, though he unfortunately died shortly afterward). As examples, the patient was unsuccessful on each of four separate occasions in attempting to read either “bowl” or “daughter,” but was successful on all four occasions in reading both “luck” and “evidence.” Warrington hypothesized that this pattern of reading was the consequence of greater damage suffered by the representations of high-imageability words. Equivalently, it would be possible to attribute the pattern of reading to greater damage suffered by the representations of high ease-of-predication words. An alternative approach is also possible, however, and asserts that similar damage occurred for all words. Thus this explanation is, in one respect at least, more parsimonious. The posited nature of C.A.V.‘s deficit is that the output of the predicational route (which, as outlined in the present article, is often not identical with the original stimulus word) was not suppressed as it normally is, but instead interfered with normal reading processes. Thus the reading of high-imageability words suffered relatively highly from activity along the unreliable predicational route, which interfered with reliable reading processes (utilizing lexically accessed phonology, since C.A.V.‘s reading of nonwords was poor) that had remained intact. The resulting conflict between potential predicational and normal outputs would be expected to lead to the omission of any response at all, which Warrington (1981) indeed found to be the principal type of error. An alternative consequence is that an input logogen adjacent to the original one would be excited instead, leading (as observed) to frequent visual errors. According to the present account, those errors actually produced have themselves escaped interference from the predicational route, and thus should have relatively low imageability values. This result also was observed by Warrington (1981). Finally, it should be noted that the present account of deep dyslexia has been couched in functional rather than neurological terms, and as such is not tied to a particular neurological pattern of brain damage. However, previous workers have concluded (Coltheart, 1980b; Saffran, Bogyo, Schwartz, & Marin, 1980) that deep dyslexia may be demonstrating the operation of the reading process when this is subserved by the right hemisphere alone. For further development of the right-hemisphere hypothesis, and a critique of it, see also Landis, Regard, Graves, and Goodglass (1983) and Marshall and Patterson (1983), respectively. In principle, the right-hemisphere hypothesis appears compatible with the predicational account advanced here, if it is assumed that the predicational route (unlike the damaged normal reading mechanisms) is subserved either by the right hemisphere alone or else by both right and left hemispheres.

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