Cerebral hemispheric mechanisms in the retrieval of ambiguous word meanings

BRAIN

AND

LANGUAGE

33, 86-103 (1988)

Cerebral Hemispheric Mechanisms in the Retrieval of Ambiguous Word Meanings CURT BURGESS University

of Rochester AND

GREG University

B.

SIMPSON

of Nebraska

at Omaha

Targets related to ambiguous primes were projected to the left and right visual fields in a lexical priming experiment with stimulus onset asynchronies (SOA) of 35 and 750 msec. Right visual field results were similar to our earlier results with central projection (G. B. Simpson & C. Burgess, 1985,Journal of Experimental Psychology: Human Perception and Performance, 11, 28-39). Facilitation was found for the more frequent meaning at both SOAs and a decrease in facilitation for the less frequent meaning at the longer SOA. In contrast, left visual field results indicated a decay of facilitation for the more frequent meaning at the longer SOA, while activation for the subordinate meaning increased. Results suggest that, while automatic processing occurs in both hemispheres, only the left hemisphere engages in controlled processing of ambiguous word meanings. In addition, the present results support the idea that the right hemisphere has a special role in ambiguity resolution and that the right hemisphere lexicon possesses a richer endowment than earlier thought. Q 1988 Academic press, Inc. We thank Christine Chiarello, Mike Tanenhaus, Susan Garnsey, Thomas G. Bever, Robert R. Peterson, Padraig O’Seaghdha, and two anonymous reviewers for their helpful comments and criticisms. Technical support from Frank Clark, Ed Furman, and Kristine Salomon is greatly appreciated. This research was presented at the 1986BABBLE Conference and at the 1986 Psychonomic Society Meeting. Curt Burgess is now at the University of Rochester. This research was part of the M. A. thesis of the first author at the University of Nebraska at Omaha under the supervision of the second author. Preparation of this manuscript was supported in part by a University of Rochester Rush Rhees Fellowship awarded to the first author and by a U.S. Department of Education Grant GO08630072to the Kansas Bureau of Child Research and the Learning Disabilities Institute of the University of Kansas, while the second author was a fellow in the Child Language Program of the University of Kansas. I extend my appreciation to the other thesis committee members: 86 0093-934X/88 $3.OO Copyright All rights

Q 1988 by Academic Press. Inc. of reproduction in any form reserved.

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A central component of reading comprehension concerns the retrieval of word meanings and only following such retrieval can the integrative processes of comprehension occur. The manner in which word meanings are stored in and retrieved from the lexicon are questions that have received considerable attention both in cognitive psychology (Becker, 1980; Meyer & Schvaneveldt, 1971; Neely, 1977; Stanovich & West, 1979) and in the neurosciences as well (Chiarello, 1985; Krashen, 1976; Searleman, 1977; Zaidel, 1978). It has been suggested that research into the neuropsychological underpinnings of word recognition could potentially tell us much about comprehension processes in general (see Posner, 1981, 1984; Posner, Pea, & Volpe, 1982). The present research was designed to explore the differential representation in the cerebral hemispheres of a restricted domain of words: those with more than one distinct meaning. It is generally accepted that the association cortex of the left hemisphere is primarily responsible for language processes, at least among the righthanded (Kolb & Whishaw, 1980). At the same time, it is now conceded that the right hemisphere also makes some contribution to language comprehension and reading processes (Krashen, 1976; Searleman, 1977; Zaidel, 1983b, 1985; cf. Gazzaniga, 1983). A special case of lexical retrieval involves the processing of ambiguous words, that is, words that have more than one meaning. Most words possess some indeterminacy in their meanings, so ambiguity may be a general characteristic that pervades natural language processing (Simpson, 1984; Simpson, Burgess & Peterson, 1987; Swinney, 1982). Any complete language comprehension model, then, would need to account for lexical ambiguity. The present paper will first review models of lexical ambiguity and the processes involved in the retrieval of the meanings of these words. The neuropsychological literature will then be examined with regard to differences and similarities between the cerebral hemispheres in lexical memory. Finally, an experiment will be reported that represents a first attempt to explore the differential representation of the meanings of ambiguous words in the two hemispheres. Lexical Ambiguity

Simpson (1984; also see Taft, 1984) recently reviewed the literature on the effect of lexical ambiguity on word recognition, and concluded that three models of ambiguity processing have emerged. The contextdependent model states that the meanings of ambiguous words are activated by the context of the sentences in which they occur, so that only the contextually appropriate meaning of the ambiguous word is activated. Frank Clark, Wayne Harrison, and Ken Deffenbacher. Reprint requests and correspondence should be sent to Curt Burgess, Psychology Department, University of Rochester, River Campus, Rochester, NY 14627.

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This model (Glucksberg, 1984; Glucksberg, Kreuz, & Rho, 1986; Schvaneveldt, Meyer, & Becker, 1976;Simpson, 1981)may be the most intuitively appealing, but is the least supported by the research (Simpson, 1984). In fact, there is research that shows activation for meanings that would not be appropriate for the sentence context (Onifer & Swinney, 1981; Seidenberg, Tanenhaus, Leiman, & Bienkowski, 1982;Tanenhaus, Leiman, & Seidenberg, 1979). The single access model proposes that the meanings of an ambiguous word are retrieved serially, according to their frequency (Forster & Bednall, 1976; Hogaboam & Perfetti, 1975). The most frequent meaning is retrieved first, and the search stops if that meaning is appropriate in context. If it is not, the next most frequent meaning is selected. This serial selfterminating search continues until a fit is made with the context. In the absence of context, retrieval favors the dominant (most frequent) meaning (Simpson, 1981). Finally, the multiple access model states that all word meanings are retrieved upon the presentation of an ambiguous word, after which context influences the selection of the appropriate meaning. Context affects the selection process, but lexical activation occurs automatically for all meanings (Holley-Wilcox & Blank, 1980; Lucas, 1984; Onifer & Swinney, 1981; Seidenberg et al., 1982; Swinney, 1979; Tanenhaus et al., 1979). The multiple access model differs from the single access view in that the multiple access model claims that all meanings are activated in parallel. In contrast, the single access model claims that only the most frequent meaning is retrieved and if a fit with context is not made then retrieval for the next most frequent meaning occurs. Onifer and Swinney (1981) interpret their results in favor of a multiple access model where all word meanings are accessed regardless of the frequency of association between the ambiguous word and its meaning. Seidenberg et al. (1982) have suggested that the speed of activation of ambiguous word meanings is a function of their frequency. Thus, there are two possible accounts of multiple access models which are distinguished by their frequency contraints: frequency independent multiple access and frequency coded multiple access. Recent research by Simpson and Burgess (1985) suggeststhat a frequency coded multiple access model can account for the activation patterns in the processing of ambiguous words in isolation (i.e., not embedded in biasing context). Subjects made timed lexical (word/nonword) decisions to target letter strings that were preceded (primed) by ambiguous words. A word target was related either to the dominant (more frequent) or the subordinate (less frequent) meaning of an ambiguous word prime. For example, an ambiguous word such as BANK would be followed by either MONEY or RIVER as a target, corresponding to the dominant and subordinate meanings, respectively. In unrelated conditions, these targets

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were preceded instead by another ambiguous word to which they were not related. The interval between the onset of the prime and the target (stimulus onset asynchrony, or SOA) was varied from 16 to 750 msec. The results showed that the time-course of semantic activation varied as a function of meaning frequency. Dominant meanings showed facilitation at all SOAs. The subordinate meanings showed no facilitation at the briefest SOA, but activation built as SOA increased, so that by 300 msec, the subordinate meaning showed the same level of facilitation as the dominant. It appears from these results that there is multiple access of meanings of ambiguous words as both meanings were available by 300 msec. However, the rate at which activation occurs is sensitive to the frequency of the meanings. After 300 msec, Simpson and Burgess (1985) found that facilitation for the subordinate meaning declined, but that the facilitation for the dominant meaning was maintained. It is usually assumed that context effects that occur at very short SOAs are traceable to an automatic process of spreading activation among lexical representations in memory (Collins & Loftus, 1975; Neely, 1976, 1977; Stanovich & West, 1979, 1981, 1983). When the ambiguous prime is recognized, its memory representation is activated, and this activation spreads to the representations of related words, decreasing the amount of stimulus information needed for their subsequent recognition. The results obtained by Simpson and Burgess at the early SOAs suggests that the dominant meaning is more accessible from the representation of an ambiguous word than is the subordinate. Changes at long SOAs, on the other hand, are thought to reflect the conscious allocation of processing resources to prime-related information (Posner & Snyder, 1975; Tanenhaus et al., 1979). This hypothesis is usually confirmed by including a neutral prime condition against which facilitation and inhibition can be computed by subtraction of the neutral from the related and unrelated response times. If the subject has allocated attention to information consistent with the prime, then presentation of unrelated information necessitates a time-consuming redirection of that attention, leading to slower responses to unrelated targets than to those primed by a neutral stimulus (Neely, 1976, 1977; Posner & Snyder, 1975; Stanovich & West, 1979, 1981). In a subsequent experiment, Simpson and Burgess (1985) confirmed that the decline of activation of the subordinate meaning at long SOAs was indeed due to inhibition of that meaning. These results are suggestive of a two-stage model of ambiguous word recognition, whereby word meanings are first activated automatically, in order of their frequency, followed by a stage in which attention is allocated to the dominant meaning. Once attention is directed to the dominant meaning, additional time is required to reallocate attention to the subordinate. This difficulty in reallocating resources results in inhibition for responses to words related to the subordinate meaning. These results implicate active,

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capacity-limited processes as well as automatic spreading activation as components of ambiguous word processing. Hemisphere Asymmetries in Lexical Representation The issues central to the lexical ambiguity literature are also important in understanding neuropsychological models of word recognition. Chiarello (1985; Chiarello & Church, 1986) has shown that lexical information is available for processing in each hemisphere, but that the hemispheres ditfer with respect to lexical organization and retrieval processes. Chiarello (1985) investigated three kinds of lexical priming: orthographically related stimuli (BEAK-BEAR), phonologically related stimuli (JUICE-MOOSE), and semantically related stimuli (INCH-YARD). These stimuli were used in two divided visual field experiments to ascertain the relative contributions of automatic and controlled processes to lexical access in the two hemispheres by manipulating the probability of related word pairs. In one experiment, related trials occurred only 25% of the time, while in the other, they occurred 75% of the time. This manipulation was intended to enable subjects to use controlled processes when the proportion of related trials was high, but not when it was low. The two experiments, therefore, were intended to tap automatic (low proportion related trials) and controlled (high proportion related trials) word recognition processes. The results from her automatic priming experiment showed that semantic priming occurred in both hemispheres, but that greater priming occurred in the right hemisphere. However, in the controlled priming experiment the retrieval process appeared to be quite different. Controlled semantic priming still occurred in both hemispheres, but was now larger in the left hemisphere. In light of the specialization of the left hemisphere for language function, we might expect a general superiority of context effects in that hemisphere for both automatic and controlled priming. However, Zaidel(1983a) obtained results consistent with those of Chiarello, namely, greater facilitation with a priming task in the right hemisphere. Chiarello explained her results by first assuming, based on results by Zaidel (1978; also Ellis & Shepherd, 1974; Mannhaupt, 1983), that the right hemisphere lexicon consists of words that are of high frequency, concreteness, and imageability (cf. Lambert, 1982a, 1982b; Moscovitch, 1981), and that these words are a subset of the words contained in the left hemisphere. Second, she assumes (see Anderson, 1976) that an inverse relationship exists between the amount of activation spreading to a single network node and the extent of the semantic network. A richer semantic network in the left hemisphere, then, would lead to a smaller spreading activation effect for any given node. The larger facilitation in the left hemisphere for controlled processing, on the other hand, may be due to a superiority of that hemisphere at attaining a preparatory state (Cohen, 1975). That left-hemisphere involvement is necessary for controlled semantic processing

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receives additional support from the work of Milberg and Blumstein (1981; Blumstein, Milberg, & Shrier, 1982).In a lexical decision experiment, patients with left-hemisphere lesions showed no deficit in the automatic retrieval of word meanings (see also Sidtis, 1985), yet they were unable to retrieve word meanings for use in a controlled semantic decision. The present experiment aimed to explore the availability of dominant and subordinate meanings of ambiguous words to the left and right cerebral hemispheres, and the speed with which they may be retrieved. Subjects were foveally presented ambiguous words, and then made lexical decisions to letter strings presented in the left or right visual field. As in Simpson and Burgess (1985), word targets were related to the ambiguous prime through the dominant or subordinate meaning. Two SOAs were used (35 and 750-msec) in order to separate automatic and controlled processing. Because of the rapid onset of spreading activation, it is assumed that the short SOA should tap automatic processes only (Hasher & Zacks, 1979;Posner & Snyder, 1975;Simpson & Lorsbach, 1983).Such processing should be present in both hemispheres (Chiarello, 1985), but restricted to the dominant meaning (Simpson & Burgess, 1985). The 750-msec SOA, on the other hand, is assumed to be long enough to allow controlled processing to occur, and such processing should be present only in the left hemisphere. Therefore, at the longer interval we would expect inhibition of the subordinate meaning in the left hemisphere, but not in the right. The fate of the subordinate meaning in the right hemisphere is more difficult to predict. If the subordinate meaning is activated in the right hemisphere, it may then simply be left to decay, rather than undergoing the active suppression expected in the left hemisphere. On the other hand, if the right-hemisphere lexicon is truly restricted to a subset of that in the left, then possibly only dominant meanings are represented in the right. In this case, we would not expect facilitation for subordinate meanings in the right hemisphere. The patterns of facilitation observed for the different meanings in the two hemispheres should allow us to separate these alternatives. METHOD

Subjects Subjects were 48 undergraduate psychology students who agreed to participate for extra credit. English was their native language. All subjects had normal or corrected-to-normal vision and were rightzhanded, with a noninverted writing posture (see Levy, 1982; Levy & Reid, 1976). Stimuli One hundred twenty homographs were selected from the Nelson, McEvoy, Walling, and Wheeler (1980) norms. Two associates were selected for each homograph. One associate was related to the homograph through

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BURGESS AND SIMPSON TABLE EXAMPLES

Target condition Subordinate Dominant Nonword

I

OF STIMULUS

ITEMS

Relatedness

Prime

Target

Unrelated Neutral Related Unrelated Neutral Related Word Neutral

Riddle Bank Riddle Bank Bear -

River River River Money Money Money Glorb Glorb

its dominant meaning, and one through a subordinate meaning (see Table 1 for sample stimuli). Unrelated trials were formed by pairing homographs with unrelated associates. This was done separately for dominant and subordinate conditions. Dominant and subordinate associates did not differ in length, t(238) = 0.75, p > .05, or in printed frequency (Kucera & Francis, 1967), t(238) = -1.41, p > .05. A second set of ambiguous primes was selected for nonword trials. Nonword targets were formed by replacing letters of words, while maintaining pronounceability. Apparatus A modified Commodore Model 2001 Pet microcomputer was used to present the stimuli. Subjects viewed the stimuli through a tachistoscopelike apparatus, consisting of a telescoping lightproof masonite box mounted on the CRT. A rubber facepiece was used so that the subject’s head could be comfortably placed against the apparatus. The onset and offset of the prime and target were controlled with a circuit that allowed the screen to be written while blank and then “flashed” on (or off) within a single raster scan. Screen intensity was diminished to 50% of the minimum factory capability to decrease phosphor persistence. Design and Procedure The experimental design was a 2 x 2 x 2 x 3 mixed factorial, with the between-subjects factor corresponding to SOA. The within-subjects factors corresponded to visual hemifield, meaning dominance, and targetprime relatedness. Six word lists were formed so that, across lists, dominant and subordinate targets followed related, neutral, and unrelated primes an equal number of times and were presented to each visual hemifield an equal number of times. Unrelated trials were constructed from a homograph and an

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unrelated dominant or subordinate associate. Four subjects saw each of the six lists; of these, two saw the stimuli in one order, and two in the reverse order. Subjects participated in the experiment individually. They were seated in front of the microcomputer and viewed the stimuli through the viewing hood. All stimuli were presented vertically to avoid directional scanning bias (see Bradshaw, Nettleton, & Taylor, 1981). The maximum vertical visual angle subtended by a word was 5.2” with 2.0” fovea1 eccentricity to the left or right. Responses were made by pressing one of two response buttons on a response box placed to the right of the computer. Responses were made with the index finger of the right hand. Subjects were instructed to rest the index finger lightly between the two buttons and respond with the smallest possible excursion in order to keep the motor component of the response time at a minimum. A trial consisted of a fixation point followed by an ambiguous word or neutral prime followed by the target. First, a fixation point (a period) appeared in the center of the screen. The prime was presented 2 set later in the same location for 35 msec, and was then masked for the balance of the duration of the SOA (0 or 715 msec) with a graphics character (a filled circle) in each letter location.’ Subjects were instructed not to respond to the prime, but to attend to it, as it would help them make the lexical decision to the target. The target followed, and was randomly presented 2” either to the left or right of fixation for 185 msec, and was then masked for 50 msec. The screen was blanked immediately following the presentation of the mask. Presentation of the target initiated a software millisecond timer, which stopped when the subject responded “WORD” (right button) or “NONWORD” (left button). If an error occurred the word “ERROR” was immediately presented vertically on the far left side of the screen. The response began a 5-set intertrial interval. Stimulus presentation and all timing events were controlled by the computer. Before the experiment began, subjects were given three blocks of practice trials (20 trials per block). Subjects received response latency and error rate feedback after each block.* ’ It was important to know that our (flash) mask did not compromise the ability of the first word to prime a related target. We feel confident that this was not the case. First, after practice trials subjects were usually able to report the prime. More importantly, priming did occur at the 35-msec SOA in the RVF (40 msec for the dominant associate, 22 msec for the subordinate). Likewise, priming occurred in the LVF. ’ Our pilot studies showed that error rates decreased approximately 30% and RTs were quicker when we increased practice trials to three blocks and incorporated RT and accuracy feedback after each block. Subjects were coached to keep errors to a minimum as well as RT as fast as possible. Error feedback on each trial on which an error ocurred was also helpful in this respect. We considered this important since RT can be a more sensitive measure than error rates (see Young, 1982).

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BURGESS AND SIMPSON TABLE 2 MEAN LEXICAL DECISION LATENCIES (IN MSEC) AND ERROR PROPORTIONSFOR EACH TARGET CONDITION

Target type Target

UNR

SD

NEU

(35 msec-Right visual jield) Sub 458(.21) 72 414(.16) Dom 416(.13) 62 392(. 13) (35 msec-Left visual jield) Sub 437(.23) 49 469(. 18) 440(. 14) Dom 459(.16) 64 (750 msec-Right visual field) Sub 440(.20) 82 428(.18) Dom 442(.15) 80 511(.13) (750 msec-Leji visual Jield) Sub 476(.19) 84 518(.15) Dom 448(.18) 70 443(. 17)

SD

REL

SD

PRIM

FAC

INH

54 58

436(.17) 376(.13)

66 62

22 40

-22 12

44 28

60 67

436(.20) 410(.15)

57 59

1 49

34 34

-33 19

71 97

486(. 14) 397(.07)

82 84

-46 45

-58 114

12 -69

105 72

442(. 14) 419(.10)

78 71

34 29

76 24

-42 5

Note. Numbers in parentheses are error proportions. Priming (PRIM) is the difference between related and unrelated conditions; Facilitation (FAC) is the difference between related and neutral conditions; Inhibition (INH) is the difference between unrelated and neutral conditions.

RESULTS

Mean lexical decision latencies for correct word responses, along with corresponding error proportions, are shown in Table 2 To facilitate understanding of the results, it should be noted that faster responses on related than on neutral trials is an indication of facilitation due to the activation of the related meaning. Contextual inhibition, on the other hand, is indicated by slower responses to unrelated targets than to those primed by the neutral signal. If these relatedness effects are larger for dominant-meaning trials, a single access view is supported. If multiple access obtains, however, no dominance x relatedness interaction is expected. The latencies underwent a 2 (SOA) x 2 (hemifield) x 2 (dominant vs. subordinate) x 3 (related vs. unrelated vs. neutral) mixed analysis of variance, with the sole between-subjects factor being SOA. A parallel analysis was carried out on the error proportions, but the discussion will focus on the latency results. Examination of Table 2 suggests that the neutral trials do not provide a reliable baseline against which to measure facilitation and inhibition. Indeed, the range of response latencies to the neutral trials was much greater than that of either the related or unrelated trials, particularly at the 750-msec SOA, where it is argued that attentional processing may be occurring. Chiarello (1985) noted a similar problem with neutral trials in a hemispheric asymmetry study. Jonides and Mack (1984) have argued

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that specific attentional responses may develop to neutral trials, and in the present case this problem may be exacerbated by the use of vertical presentation. A test for homogeneity of variance of related, neutral, and urelated trials was significant, Fmax(2, 47) = 3.27, p = .047, confirming that the three relatedness conditions were not consistent in their degree of variability. However, when the neutral trials were not included in the analysis, homogeneity of variance was restored, Fmax(1, 47) = 1.66, p = .204. Therefore, the data were analyzed without the neutral trials, in a 2 (SOA) x 2 (hemifield) x 2 (dominant vs. subordinate) x 2 (related vs. unrelated) analysis of variance. With the deletion of neutral trials, we are no longer able to make strong statements concerning facilitation and inhibition effects, but will instead discuss the overall context effects (which we will simply call priming), based on the differences between the related and unrelated conditions. Responses were faster to targets presented to the right hemifield,3 F( 1, 46) = 4.28, p = .044, to related targets, F( 1, 46) = 12.29, p < .OOl, and to dominant targets, F(1, 46) = 27.15, p < .OOl. Visual hemifield and dominance interacted, F(1, 46) = 8.27, p = .004, as did relatedness and dominance F(1, 46) = 17.66, p < .OOl. The interaction of most interest is the four-way SOA x hemifield x relatedness x dominance interaction, F(1, 46) = 5.05, p = .029. This interaction was examined by testing the simple interaction effects for the SOA x dominance x relatedness interaction separately for each hemifield. This and all subsequent simple effects were calculated with the weighted average of the between-subjects error term and the error term for the four-way interaction (Kirk, 1982). In the left visual field, the three-way interaction of SOA x relatedness x dominance was significant, F(1, 46), = 4.45, p = -040. Therefore, the dominance x relatedness simple effects were calculated for each SOA. Relatedness and dominance marginally interacted at the 35-msec SOA, F(1, 46) = 4.01, p = .051. Simple main effects tests showed that priming occurred (49 msec) at the 35-msec SOA for the dominant target, F(1,46) = 143.58, p < .OOl, but not for the subordinate target (only 1 msec difference), F(1, 46) = .34. There was no interaction between relatedness and dominance at the 750-msecSOA, F( 1,46) = 1.54, indicating an equivalent amount of priming for both the dominant and subordinate meaning (29 and 34 msec, respectively). These results indicate that when the targets are presented to the left visual field, only the dominant meaning is activated at the short SOA, while both meanings are primed by 750 msec. 3 Consistent with similar research (Chiarello, 1985),there were no visual field differences for the nonword trials suggesting that the RVF advantage for word trials was not a result of using a right-hand response task.

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. q ‘\

50 -

3 P ‘E E

o-

. MomJ ‘\\ ‘\\

-50 -

RVF

‘\ b

35

Sub

-

750

I

I

50 -

es0 _ LVF

I

I 35

750 SOA

FIG. 1. Mean priming of dominant (Dam) and subordinate (Sub) associates at 35 and 750-msec SOAs for the left and right visual field.

In the right visual field, the three-way interaction of SOA, relatedness, and dominance was again significant, E’(1, 46) = 5.23, p = .027, so separate dominance x relatedness simple effects were again tested at each SOA. At the 35msec SOA, there was no interaction between relatedness and dominance, F(1, 46) = .94. Related targets led to faster responses than unrelated, F(1, 46) = 5.84, p = .024, for both dominant and subordinate targets, indicating that both meanings are primed at 35 msec when targets are presented to the right visual field. At the 750msec SOA, the interaction effect between relatedness and dominance was significant, F(1, 46) = 5.11, p = .028. Simple main effects tests showed that whereas there was priming (45 msec) for the dominant target at the long SOA, F(1, 46) = 523.04, p < .OOl, subordinate targets led to slower responses than did unrelated words, F(1, 46) = 184.14, p < .OOl. By 750 msec, then, only the dominant meaning is still active when targets are presented to the right visual field. These priming results are shown in Fig. 1. The analysis of error rates showed that fewer errors were made on related targets than on unrelated targets, F(1, 46) = 17.26, p < .OOl. Also, fewer errors were made on dominant than on subordinate targets, F(1, 46) = 32.00, p < .OOl. Relatedness and dominance interacted, F(1, 46) = 5.81, p = .020, because fewer errors occurred with related targets

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in both the dornmant condition, t(47) = 3.44, p = .OOl, and the subordinate condition, t(47) = 2.77, p = .008. The important point about these error proportions is that they do not suggest any speed-accuracy trade-off that would qualify the response latency findings. DISCUSSION

The intent of this experiment was to compare the retrieval of ambiguous word meanings when presented to the left and right visual fields. The right visual field results are very similar to those described by Simpson and Burgess (1985), which did not consider hemispheric functioning. Both of these experiments show greater facilitation for the dominant meaning than for the subordinate meaning at the brief SOA (35 msec in the present experiment, 16 msec in Simpson & Burgess). The facilitation for the dominant meaning is maintained at the longer SOA (750 msec) just as Simpson and Burgess found. However, both experiments show a sharp decline in activation of the subordinate meaning at the longer SOA. Simpson and Burgess suggest that inhibition of the subordinate meaning occurs and accounts for the decline in activation. While the neutral trials necessary for a straightforward discussion of inhibition effects were not used in the analyses of the present data, the results for related and unrelated stimuli are consistent with those of Simpson and Burgess and therefore suggestive of the conclusion that the allocation of attention to the dominant meaning results in inhibition of the subordinate meaning. The left visual field results are in marked contrast to the right visual field findings (or those of Simpson and Burgess, 1985). Priming effects for the dominant meaning at the 35-msec SOA are similar in the left and right visual fields (40 msec vs. 49 msec, respectively). However, by 750 msec, activation for the dominant meaning declines in the left visual field, and the subordinate meaning now shows priming (34 msec). These results support Chiarello’s (1985) finding that greater controlled semantic priming occurs in the left hemisphere than in the right. The apparent controlled processing is represented by the fact that subordinaterelated targets lead to slower responses than do completely unrelated words at the 750-msec SOA when presented to the right visual field. Such an effect is absent in the left visual field. The right visual field results are similar to those that Simpson and Burgess (1985) found in their Experiment 2. Simpson and Burgess confirmed in their Experiment 3 that this decline in performance for the subordinate meanings was due to inhibition of those meanings. Greater priming was not found in the left visual field, contrary to Chiarello’s (1985) results. In her automatic processing experiment, there was greater semantic facilitation in the left visual field. The rationale for finding greater facilitation in the left visual field is that these words are a subset of the words contained in the left hemisphere lexicon (Zaidel,

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1978), and that an inverse relationship exists between the amount of activation spreading to a single network node and the extent of the semantic network (Anderson, 1976). The richer semantic network in the left hemisphere should therefore lead to smaller spreading activation effects for any given node. One possible reason for the discrepancy between the present findings and those of Chiarello and of Zaidel(1983a) may involve the nature of the stimuli. Chiarello used words that were imageable and concrete (e.g., HORSE, TIGER, RING, WATCH), as did Zaidel (e.g., DOG, APPLE). While some words in the present study were imageable and concrete (e.g., ROPE, HOUSE, CLUB), many others were not (e.g., NAME, UNION, NAG, WEIGH). A comparison of the present findings to those of Chiarello (1985) might suggest that the lexicon in the right hemisphere may be organized differently for imageable, concrete words than for more abstract or less imageable words. However, recent work by Chiarello, Sehehi, and Nuding (1987) found no visual field difference between abstract and concrete words in a low probability (automatic processing) manipulation (also see Marcel & Patterson, 1978). Replication of the greater left visual field priming effect is in order before further speculation can be justified. The fact that activation for the subordinate meanings (which are less concrete and imageable) did increase by 750 msec suggests that the right-hemisphere lexicon may be more richly endowed than earlier thought (Zaidel, 1978). The most important contribution of this study is the demonstration that right visual field results were consistent with the results with central projection reported in the cognition literature. At present, it is not completely clear what implications should be drawn from the left visual field results, which showed a decline of facilitation for the more frequent meaning at the longer SOA, while activation for the subordinate meaning increased. One possibility is that the right hemisphere retrieves the subordinate meaning from the left hemisphere. This argument would make more sense for the subordinate meanings than for the dominant meanings. Another explanation is a model of hemispheric functioning in which the left hemisphere calls upon the right hemisphere via some gating mechanism to access memory information when it is needed. For example, when the subordinate meaning is needed, but sufficient time has elapsed so that it is inhibited in the left hemisphere, access could occur from the right hemisphere. The present results suggest that the activation of the subordinate word meaning is building in the right hemisphere and therefore available for retrieval (see Burgess 62 Simpson, in press, for a complete discussion of such a model). This model would predict that right-hemisphere-damaged patients would have difficulty in making associations with subordinate associates, but that left-hemisphere-damaged patients would experience the same difficulty with dominant associates in a task requiring controlled processing. This result can be found with several of

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the stimuli in an experiment reported in Brownell, Potter, Michelow, and Gardner (1984). While their purpose was to evaluate the sensitivity of right- and left-hemisphere-damaged patients to connotatively and denotatively associated words, it can be seen that several of their items can be considered ambiguous. For example, WARM can be related to either COLD or LOVING. WARM is more frequently related to COLD (see Shapiro & Palermo, 1968), and so our results predict that left-hemispheredamaged patients would have difficulty making the WARM-COLD association, but would be spared the WARM-LOVING association. Likewise, right-hemisphere-damaged patients would be capable of making the WARM-LOVING association, but not WARM-COLD. Brownell et al. found this to be an accurate description of their patients with several ambiguous words (WARM, COLD, DEEP, SHALLOW). These results argue against a model where the right hemisphere needs to retrieve the subordinate meaning from the left hemisphere. It may be that right-hemisphere processing plays a special role in the resolution of indeterminacy of meaning. Such indeterminacies exist in the form of indirect speech acts, metaphors, idioms, connotative meaning, as well as word ambiguity. Such a notion is supported by the finding that right-hemisphere lesions are associated with literal interpretation of indirect speech acts (Hirst, LeDoux, & Stein, 1984), idioms (Myers & Linebaugh, 1981), metaphors (Stachowiak, Huber, Poeck, & KerschenSteiner, 1977), humor (Wapner, Hamby, & Gardner, 1981) and in difficulty understanding connotative meaning (Brownell et al., 1984). This brief review of the neuropsychological literature relevant to the notion of indeterminacy of meaning suggests that the processes involved in indeterminacy resolution are dissociable from processes responsible for literal computation or use of the more frequent expression. Although the mechanism is not explicit, the right hemisphere may well play a major role in the comprehension of indeterminacy (see Simpson et al., 1987, for a review of the general issues concerning indeterminacy). Implicit in our interpretation is the idea that retrieval and processing actually occur in the hemisphere receiving the stimulus. We think that this is the case for several reasons. If hemispheric transmission were required for the entire array of sensory information, rather than just the lexical decision, one might expect more errors when stimuli were projected to the inappropriate hemisphere (see Cohen, 1982). Our error data do not suggest this. Other semantic priming research similar to ours has not found response-hand differences, and has found that semantic priming occurs in both hemispheres. In addition, the clinical literature suggests that the right hemisphere is somehow involved in ambiguity resolution, which is consistent with the present findings with normal subjects. Investigating a wider range of SOAs (as Simpson and Burgess, 198.5, did) would further our understanding of the activation timecourse of the

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subordinate meaning in the cerebral hemispheres. Finally, it will be important to put ambiguous words in sentential contexts to determine how meaning dominance interacts with appropriateness, and whether this interaction differs with words projected to the two hemispheres. In the meantime, these results indicate basic differences between the two hemispheres with respect to the processing of words with more than one meaning. They suggest that both meanings are available to each hemisphere, but that the processes involved in the meanings’ retrieval are quite different. It has been suggested elsewhere that ambiguity may be useful as a means of exploring other aspects of word recognition and language processing in general (Simpson, 1984; Swinney, 1982). For example, ambiguity research may help to distinguish among general models of lexical retrieval (Simpson, 1984), and to trace developmental changes in word recognition skills (Simpson & Foster, 1986). The present results indicate that it may also be a valuable tool for separating language processes associated with the two cerebral hemispheres. REFERENCES Anderson, J. R. 1976. Lnnguage, memory, and thought. Hillsdale, NJ: Erlbaum. Becker, C. A. 1980. Semantic context effects in visual word recognition: An analysis of semantic strategies. Memory & Cognition, 8, 493-512. Blumstein, S. E., Milberg, W., & Shrier, R. 1982. Semantic processing in aphasia: Evidence from an auditory lexical decision task. Brain and Language, 17, 301-315. Bradshaw, J. L., Nettleton, N. C., & Taylor, M. J. 1981. The use of laterally presented words in research into cerebral asymmetry: Is directional scanning likely to be a source of artifact? Bruin and Language, 14, l-14. Brownell, H. H., Potter,, H. H., Michelow, D., & Gardner, H. 1984. Sensitivity to lexical denotation and connotation in brain-damaged patients: A double dissociation? Bruin and Language,

22, 253-265.

Burgess, C., & Simpson, G. B. (in press). Neuropsychology of lexical ambiguity resolution: The contribution of divided visual field studies. To appear in S. L. Small, G. W. Cottrell, & M. K. Tanenhaus (Eds.), Lexical ambiguity resolution in the comprehension of human language. Los Altos, CA: Morgan Kaufmann Publishers. Chiarello, C. 1985. Hemisphere dynamics in lexical access: Automatic and controlled priming. Bruin nnd Language, 26, 146-172. Chiarello, C., & Church, K. L. 1986. Lexical judgments after right- or left-hemisphere injury. Neuropsychologia, 24, 623-630. Chiarello, C., Senehi, J., & Nuding, S. (1987). Semantic priming with abstract and concrete words: Differential asymmetry may be post-lexical. Bruin and Language. 31, 43-60. Cohen, Cl. 1975. Hemispheric differences in the utilization of advance information. In P. Rabbitt & S. Dornic (Eds.), Attention andperformance New York/London: Academic Press. Vol 5. Cohen, G. 1982. Theoretical interpretations of lateral asymmetries. In J. G. Beaumont (Ed.), Divided visualfield studies of cerebral organisation. New York/London: Academic Press. Collins, A. M., & Loftus, E. F. 1975. A spreading-activation theory of semantic processing. Psychological

Review,

82, 407-428.

Ellis, H. D., & Shepherd, J. W. 1974, Recognition of abstract and concrete words presented in left and right visual fields. Journal of Experimental Psychology, 103, 1035-1036.

HEMISPHERIC

MECHANISMS

101

Forster, K. I., & Bednall, E. S. 1976. Terminating and exhaustive search in lexical access. Memory & Cognition, 4, 53-61. Gazzaniga, M. S. 1983. Right hemisphere language following brain bisection: A 20-year perspective. American Psychologist, 38, 525-537. Glucksberg, S. 1984. How people use context to resolve ambiguity: Implications for an interactive model of language understanding. Invited address at the meeting of the Eastern Psychological Association, Baltimore, MD. Glucksberg, S., Kreuz, R. J., & Rho, S. 1986. Context can constrain lexical access: Implications for models of language comprehension. Journal of Experimental Psychology: Learning, Memory, and Cognition, 12, 323-335. Hasher, L., & Zacks, R. T. 1979. Automatic and effortful processes in memory. Journal of Experimental Psychology: General, 108, 356-388. Hirst, W., LeDoux, J., & Stein, S. 1984. Constraints on the processing of indirect speech acts: Evidence from aphasiology. Brain and Language, 23, 26-33. Hogaboam, T. W., & Perfetti, C. A. 1975. Lexical ambiguity and sentence comprehension. Journal of Verbal Learning and Verbal Behavior, 14, 265-274. Holley-Wilcox, P., & Blank, M. A. 1980. Evidence for multiple access in the processing of isolated words. Journal of Experimental Psychology: Human Perception and Performance, 6, 75-84. Jonides, J., & Mack, R. 1984. On the cost and benefit of cost and benefit. Psychological Bulletin,

96, 29-44.

Kirk, R. E. 1982. Experimenlal design. Monterey, CA.: Brooks/Cole. Kolb, B., & Whishaw, I. Q. 1980.Fundamentals of human neuropsychology. San Francisco: Freeman. Krashen, S. D. 1976. Cerebral asymmetry. In H. Whitaker & H. Whitaker (Eds.), Studies in neurolinguistics. New York: Academic Press. Vol. 2, pp. 157-191. Kucera, H., & Francis, W. 1967. Computational analysis ofpresent-day American English. Providence, RI: Brown Univ. Press. Lambert, A. J. 1982a. Right hemisphere language ability: 1. Clinical evidence. Current Psychological

Reviews,

2, 77-94.

Lambert, A. J. 1982b. Right hemisphere language ability: 2. Evidence from normal subjects. Current Psychological Reviews, 2, 139-152. Levy, J. 1982. Handwriting posture and cerebral organization: How are they related? Psychological Bulletin, 91, 589-608. Levy, J., & Reid, M. 1976. Variations in writing posture and cerebral organization. Science, 194, 33. Lucas, M. 1984. Frequency and context effects in lexical ambiguity resolution (Tech. Rep. URCS-14). Rochester, NY: University of Rochester, Cognitive Science Technical Report. Mannhaupt, H. R. 1983. Processing of abstract and concrete nouns in a lateralized memory search task. Psychological Research, 45, 91-105. Marcel, A. J., & Patterson, K. E. 1978. Word recognition and production: Reciprocity in clinical and normal studies. In J. Requin (Ed.), Attenrion and performance, Hillsdale, NJ: Erlbaum. Vol. 7. Meyer, D. E., & Schvaneveldt, R. W. 1971. Facilitation in recognizing pairs of words: Evidence of a dependence between retrieval operations. Journal of Experimentnl Psychology,

90, 227-234.

Milberg, W., & Blumstein, S. E. 1981. Lexical decision and aphasia: Evidence for semantic processing. Brain and Language, 14, 371-385. Moscovitch, M. 1981. Right-hemisphere language. Topics in Language Disorders, 1, 4161. Myers, P. S., & Linebaugh, C. W. 1981. Comprehension of idiomatic expressions by righthemisphere-damaged adults. In R. H. Brookshire (Ed.), Clinical aphasiology: Conference proceedings, 1981. Minneapolis, MN: BRK.

102

BURGESS AND SIMPSON

Neely, J. H. 1976. Semantic priming and retrieval from lexical memory: Evidence for facilitatory and inhibitory processes. Memory & Cognition, 4, 648-654. Neely, J. H. 1977.Semantic priming and retrieval from lexical memory: Roles of inhibitionless spreading activation and limited-capacity attention. Journal of Experimental Psychology: General, 106, 226-254. Nelson, D. L., McEvoy, C. L., Walling, J. R., & Wheeler, J. W., Jr. 1980. The University of South Florida homograph norms. Behavior Research Methods and Instrumentation, 12, 16-37. Onifer, W., & Swinney, D. A. 1981. Accessing lexical ambiguities during sentence comprehension: Effects of frequency of meaning and contextual bias. Memory & Cognition, 9, 225-236.

Posner, M. I. 1981. Cognition and neural systems. Cognition, 10, 261-266. Posner, M. I. 1984. A framework for relating cognitive to neural systems (Tech. Rep. No. 4-2). Portland: University of Oregon, Cognitive Neuropsychology Laboratory. Posner, M. I., Pea, R., 62 Volpe, B. 1982. Cognitive neuroscience: Developments toward of science of synthesis. In J. Mehler & E. C. T. Walker (Eds.), Perspectives on mental representation Hillsdale, NJ: Erlbaum. Pp. 251-276. Posner, M. I., & Snyder, C. R. R. 1975. Attention and cognitive control. In R. L. Solso (Ed.), Information processing and cognition: The Loyola symposium. Hillsdale, NJ: Erlbaum. Pp. 55-85. Schvaneveldt, R. W., Meyer, D. E., & Becker, C. A. 1976. Lexical ambiguity, semantic context, and visual word recognition. Journal of Experimental Psychology: Human Perception

& Performance,

2, 243-256.

Searleman, A. 1977. A review of right hemisphere linguistic capabilities. Psychological Bulletin,

84, 503-528.

Seidenberg, M. S., Tanenhaus, M. K., Leiman, J. M., & Bienkowski, M. 1982. Automatic access of the meanings of ambiguous words in context: Some limitations of knowledgebased processing. Cognitive Psychology, 14, 489-537. Shapiro, S. I., & Palermo, D. S. 1968.An atlas of normative free association data. Psychonomic Monograph Supplements, 2, (Whole No. 28), 219-250. Sidtis, J. J. 1985. Bilateral language and commissurotomy: Interactions between the hemispheres with and without the corpus callosum. In A. Reeves (Ed.), Epilepsy and the corpus callosum. New York: Plenum. Pp. 369-380. Simpson, G. B. 1981. Meaning dominance and semantic context in the processing of lexical ambiguity. Journal of Verbal Learning and Verbal Behavior, 20, 120-136. Simpson, G. B. 1984. Lexical ambiguity and its role in models of word recognition. Psychological Bulletin, 96, 316-340. Simpson, G. B., & Burgess, C. 1985. Activation and selection processes in the recognition of ambiguous words. Journal of Experimental Psychology: Human Perception and 11, 28-39. Performance, Simpson, G. B., Burgess, C., & Peterson, R. R. 1987. Comprehension processes and the indeterminacy of meaning (Tech. Rep. No. URCS-39). Rochester, NY: University of Rochester, Cognitive Science Technical Report. Simpson, G. B., 62 Foster, M. 1986. Lexical ambiguity and children’s word recognition. Developmental Psychology, 22, 147-154. Simpson, G. B., & Lorsbach, T. G. 1983. The development of automatic and conscious components of contextual facilitation. Child Development, 54, 760-772. Stachowiak, F. J., Huber, W., Poeck, K., & Kerschensteiner, M. 1977.Text comprehension in aphasia. Brain and Language, 4, 177-195. Stanovich, K. E., & West, R. F. 1979. Mechanisms of sentence context effects in reading: Automatic activation and conscious attention. Memory & Cognition, 7, 77-85. Stanovich, K. E., & West, R. F. 1981. The effect of sentence context on ongoing word

HEMISPHERIC

MECHANISMS

103

recognition: Tests of a two-process theory. Journal of Experimental Psychology: Human Perception

and Performance,

I, 658472.

Stanovich, K. E., & West, R. F. 1983. On priming by a sentence context. Journal Experimental

Psychology:

General,

of

112, l-36.

Swinney, D. A. 1979. Lexical access during sentence comprehension: (Re)Consideration of context effects. Journal of Verbal Learning and Verbal Behavior, 18, 645-659. Swinney, D. A. 1982. The structure and time-course of information interaction during speech comprehension: Lexical segmentation, access, and interpretation. In J. Mehler & E. C. T. Walker (Eds.), Perspectives on mental representation. Hillsdale, NJ: Erlbaum. Pp. 151-167. Taft, M. 1984. Exploring the mental lexicon. Australiun Journal of Psychology, 36, 3546. Tanenhaus, M. K., Leiman, J. M., & Seidenberg, M. S. 1979. Evidence for multiple stages in the processing of ambiguous words in syntactic contexts. Journal of Verbal Learning and Verbal Behavior,

18, 427-440.

Wapner, W., Hamby, S., & Gardner, H. 1981. The role of the right hemisphere in the apprehension of complex linguistic materials. Brain and Language, 14, 15-33. Young, A. W. 1982. Methodological and theoretical bases of visual hemifield studies. In J. G. Beaumont (Ed.), Divided visual Jield studies of cerebral organisafion. New York/London: Academic Press. Pp. 1l-27. Zaidel, E. 1978. Lexical organization in the right hemisphere. In Buser & Rougeul-Buser (Eds.), Cerebral correlates of conscious experience. Amsterdam: Elsevier/North Holland Biomedical. Pp. 177-197. Zaidel, E. 1983a. Disconnection syndrome as a model for laterality effects in the normal brain. In J. B. Hellige (Ed.), Cerebral hemisphere asymmetry: Method, theory. and application. New York: Praeger. Pp. 95-151. Zaidel, E. 1983b. On multiple representations of the lexicon in the brain-The case of two hemispheres. In M. Studdert-Kennedy (Ed.), Psychobiology of language. Cambridge, MA: MIT Press. Pp. 105-125. Zaidel, E. 1985. Language in the right hemisphere. In D. F. Benson & E. Zaidel (Eds.), The dual brain: Hemispheric specialization in humans. New York: Guilford Pp. 205231.