Meaning Selection and the Subcortex: Evidence of

Journal of Psycholinguistic Research, Vol. 35, No. 1, January 2006 (© 2005) DOI: 10.1007/s10936-005-9003-6

Meaning Selection and the Subcortex: Evidence of Reduced Lexical Ambiguity Repetition Effects Following Subcortical Lesions David A. Copland1 Recent research indicates that individuals with nonthalamic subcortical (NS) lesions can experience difficulties processing lexical ambiguities in a variety of contexts. This study examined how prior processing of a lexical ambiguity influences subsequent meaning activation in 10 individuals with NS lesions and 10 matched healthy controls. Subjects made speeded lexical decisions on related or unrelated targets following homophone primes. Homophones were repeated with different targets biasing the same or different meanings on the second presentation. The effects of prime-target relatedness, interstimulus interval (200 or 1250 ms), and same vs. different meaning repetition were examined. Both the patient and control groups showed priming when the same homophone meaning was biased on repetition. When a different meaning was biased on the second presentation, no priming was evident in the controls, while facilitation remained present for the NS group, consistent with aberrant meaning selection and deactivation processes. These findings are discussed in terms of age and task-related repetition effects and current conceptions of frontal–subcortical involvement in cognition. KEY WORDS: lexical ambiguity; semantic priming; subcortical aphasia; meaning suppres- sion; inhibition.

INTRODUCTION Despite numerous clinical–anatomical studies of individuals with subcortical lesions, the role of the basal ganglia in language processing remains elusive. Earlier theories of subcortical language function (Crosson, 1985; Wallesch & Papagno, 1988) have difficulty accounting for the lack of a coherent profile of subcortical aphasia based on aphasia battery performance and the possibility that language deficits following David Copland is supported by the Australian Research Council. 1 Centre for Research in Language Processing and Linguistics, Division of Speech Pathology, The University of Queensland, Brisbane, Queensland, Australia. email: [email protected] 51 0090-6905/06/0100–0051/0 © 2005 Springer Science+Business Media, Inc.

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nonthalamic subcortical (NS) lesions arise from distant cortical dysfunction (see Nadeau & Crosson, 1997). Yet evidence has emerged that basal ganglia lesions can lead to subtle deficits in complex language functions which may be masked by the effects of cortical hypoperfusion and undetected by standard aphasia batteries. Specifically, Mega and Alexander (1994) and Copland et al. (2000a) found that NS vascular lesions are associated with deficits in more executive or complex aspects of language including sentence generation, and interpreting ambiguous or figurative passages. Importantly, a subsequent series of studies observed a similar profile of impaired ambiguity resolution in individuals with vascular NS lesions and individuals with Parkinson’s disease, which features altered striatal output through degeneration of the nigrostriatal dopaminergic system (Gerfen, 1992; Mink, 1996), and is not generally subject to the same possibility of cortical hypoperfusion as vascular lesions. More specifically, these studies revealed a selective impairment in the attention-based resolution of lexical ambiguities presented in isolation (Copland, 2003) or in a lexical, sentential or discourse context (Copland et al., 2000b, 2000c, 2001). In particular, subcortical dysfunction appeared to disrupt the suppression of inappropriate meanings on the basis of meaning frequency or integrated contextual information via controlled/attentional procedures. The present study is concerned with further exploring the nature of this postulated faulty suppression mechanism. Lexical ambiguity processing has been primarily concerned with the ‘local’ context preceding an ambiguous word, and its effect on lexical access and meaning selection. Experiments typically provide a sentence containing an ambiguity, present the target word, record response times, and then repeat the process with a new context containing a new ambiguity. Both Simpson and Kellas (1989) and Simpson and Kang (1994) observed that this discrete trial presentation is not reflective of normal discourse. In discourse, a topic or concept is typically raised and this interpretation will be maintained over several sentences (Simpson & Kang, 1994). For instance, the sentence “I opened a cheque account at the bank” might easily be followed by the sentence “I’m using the bank John uses”. In this way, the context of a word in discourse extends beyond the limits of the sentence in which it is contained. Importantly, it seems likely that this extended context would include the previous processing of the ambiguity, such that the second sentence would be interpreted differently if preceded by a different sentence. Based on this pretext, Simpson and colleagues set out to examine the effects of a prior occurrence of an ambiguity on the later processing of that same ambiguity. Simpson and Kellas (1989) found that the first presentation of a target related to one meaning of an ambiguity resulted in a response to

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a target related to the other meaning which was slower than responses to unrelated targets. This response pattern was demonstrated regardless of the order of the dominant and subordinate presentations. Further, the effect of prior processing was still apparent when the subsequent presentation was separated by 12 intervening word pairs. From this observation, Simpson and Kellas (1989) assumed that if one meaning of an ambiguous word is involved in processing which requires an overt response, the other meaning will be actively suppressed to such an extent that it is less available for access than unrelated information for a considerable amount of time. Simpson and Kang (1994) replicated these findings and went on to show that this suppression was quite specific, in that inhibition only occurred for that information which was in direct competition with the previously processed meaning. Through using the prior processing of ambiguous words as the context, the studies of Simpson and Kang (1994) and Simpson and Kellas (1989) have produced the strongest evidence yet of active suppression affecting unselected meanings. Such a mechanism makes intuitive sense as language processing efficiency would be enhanced by a process which obviates the need to reprocess a meaning which is still inappropriate or irrelevant. Further, these studies provide an experimental paradigm which reflects an important aspect of normal ambiguity processing in discourse. The data from these ambiguity repetition experiments speak directly to the vexing question of how meanings of lexical ambiguities are dealt with when they are not selected due to incongruency with context or a weaker meaning frequency. Candidate mechanisms include decay, attentional withdrawal, and active suppression of the inappropriate meaning, however, evidence for these various mechanisms often varies as a function of the task involved. The active suppression mechanism proposed by several authors (Gernsbacher, 1990; Simpson & Burgess, 1985; Simpson & Kang, 1994) differs from the concept of inhibition within a controlled processing framework as it is selective and does not occur as a by-product of attention being committed elsewhere. Nor does it occur as the rejected meaning is no longer attended to. Instead, the meaning no longer required is actively dampened. The ‘structure building’ framework espoused by Gernsbacher (1990) incorporates such an active suppression mechanism. According to this framework, the goal of comprehension is to develop a coherent mental structure which represents the meaningful units being comprehended, such as a sentence. Suppression occurs when the memory traces representing contextual information transmit processing signals which serve to dampen the activation of inappropriate meanings, as this information is no longer needed for the structure being constructed. With regards to the findings of Simpson and colleagues discussed above, it

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appears that an overt response towards one meaning of an ambiguity, in the form of a lexical decision on a biased target, is sufficient to activate this suppression mechanism in the course of structure building. The present study sought to further examine the cognitive mechanisms underlying faulty ambiguity resolution following subcortical lesions. Previous findings show that inappropriate or weaker meanings remain active following subcortical dysfunction, however, the postulated deficit in meaning suppression has not been clearly dissociated from faulty attentional withdrawal or decay (Copland, 2003; Copland et al., 2000b, 2001). All of these candidate mechanisms remain possible, as control subjects in these previous studies generally showed no significant difference between targets related to non-selected meanings and unrelated targets, whereas meaning suppression would be clearly indicated when non-selected meaning targets are significantly slower than unrelated targets. This precise finding was observed in the ambiguity repetition studies of Simpson and colleagues discussed above. Accordingly, this paradigm provides an ideal avenue for further examining whether meaning suppression is disrupted following subcortical lesions. In the present study, a word pair repetition priming paradigm was employed, based on Simpson and Kang (1994), where on the first presentation an unequibiased lexical ambiguity prime was paired with a target biasing its dominant or subordinate meaning (e.g. bank–money, bank–river) and on the second presentation the same ambiguity is paired with another target which is congruent or incongruent with the meaning biased on the first presentation (e.g. bank–coin, bank–stream). It was hypothesised that healthy individuals would show meaning suppression on the second presentation for all targets biasing a different meaning to that biased on the first presentation, regardless of meaning dominance. In contrast, compromised meaning suppression was predicted in individuals with NS lesions, as demonstrated by continued facilitation of targets incongruent with the first presentation meaning.

METHODS Subjects The NS group consisted of 10 participants (6 females, 4 males) who met the following criteria: (1) CT or MRI-confirmed lesions visible only in subcortical regions, excluding the thalamus, following a single cerebrovascular accident (CVA); (2) no previous history of head trauma, dementia, brain tumour, cerebral abscess or alcoholism; (3) right-handedness, monolingual

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in English and no reported visual and/or hearing abnormality; (4) at least 6 months post-onset at the time of testing, and (5) able to perform the lexical decision task. All lesion sites were confirmed by a radiologist. Neuroradiological, demographic and language performance data for the NS subjects is presented in Table I. All NS subjects obtained Western Aphasia Battery (WAB) (Kertesz, 1982) Aphasia Quotients above the 93.8 aphasia cut-off. The control group comprised 10 nonneurologically impaired individuals matched to the NS participants for age, sex, and educational level. Control subjects were excluded if: (a) they had a history of neurological disease or head trauma; (b) they had a history of alcohol abuse; (c) they had defective vision and/or hearing which would affect the validity of the task performance; and (d) English was not their first language. There were no significant differences (p > .05) between the control group and the NS subjects in years of education, gender, and age (Control Education M = 11.5 years, SD = 2.76; NS Education M = 11.2, SD = 2.78; Control age M = 54.00, SD = 11.65; NS age M = 56.00, SD = 12.37). Materials The present experiment used auditorily presented word pairs. The first word presented was a lexical ambiguity, representing the prime, followed by a target which was either a nonword or a real word which was related or unrelated to the prime word. Twenty lexical ambiguities and their associates were selected on the basis of a pretest which was carried out to obtain regional and age-appropriate stimuli in terms of meaning dominance for lexical ambiguities and strength of association for targets. The pretest involved a group of 50 neurologically intact elderly adults (age range 60–89) who were presented with 79 lexical ambiguities (selected from the norms of Nelson et al. (1980) and Twilley et al., (1994)), and were asked to provide a word related to the first meaning of the ambiguity which came to mind, followed by any secondary meaning associates. Twenty lexical ambiguities were chosen which had (a) two distinct meanings, (b) common related associates which were signaled by the respondents, (c) a dominant meaning which was provided by the subjects as the first related meaning at least 70% of the time, and (d) a subordinate meaning which was provided second at least 70% of the time. Two dominant and subordinate associates were selected for each of the 20 lexical ambiguities, based on the most common responses given in the pretest. There was no significant difference (p365

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8 1

3 >365

Time of scan post-stroke* CN, IC Adjacent to CN BG, CR IC, HCN, LN IC, BG, PVWM PVWM, CS, LN, IC, EC P, IC LN, CS IC DWM

Lesion site

49 26 9 24

13

51

18 32

71 78

Months post-stroke

9.9 10.0 10.0 10.0

9.9

10.0

9.8 10.0

9.7 10.0

Comp

95.7 95.0 96.4 97.8

98.8

99.6

96.5 96.8

95.4 98.0

AQ

nonaphasic nonaphasic nonaphasic anomic

nonaphasic

nonaphasic

nonaphasic anomic

nonaphasic nonaphasic

Class.

Note:∗ reported in days; I = infarct; H = hemorrhage; IC = internal capsule; GP = globus pallidus; PVWM = periventricular white matter; EC = external capsule; BG = basal ganglia; CS = centrum semiovale; LN = lentiform nucleus; HCN = head of caudate nucleus; P = putamen; CN = caudate nucleus; DWM = deep white matter; CR = corona radiata; AQ = WAB aphasia quotient; Comp = WAB comprehension summary.

Age

Case

Education (years)

Table I. Summary of Nonthalamic Subcortical Subject Characteristics

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lexical ambiguity (from Nelson et al. (1980) or Twilley et al. (1994)), so that comparisons of related and unrelated pair latencies were based on responses to the same target words presented in different sessions. Lexical ambiguities were repeated with 8, 10, or 12 intervening word-pair trials where the targets in presentation 2 biased the same meaning of the ambiguous prime (congruent e.g. bank–stream, bank–river) or a different meaning of the prime (incongruent e.g. bank–stream, bank–money) biased in presentation 1. Meaning dominance was also manipulated such that the first and second presentation targets biased the dominant or subordinate meaning of the ambiguity. Table II provides examples of the conditions employed. Twenty nonword pairs were also constructed, with 10 pairs consisting of a homophone (from Nelson et al. (1980); Twilley et al. (1994)) followed by a pronounceable nonword, and 10 pairs including a nonambiguous word followed by a pronounceable nonword. The probability of seeing a word or nonword target was .50 in any session. Eight session lists were constructed, with each homophone appearing twice in each session in one of the four conditions stated above at 200 ms ISI and 1250 ms ISI (different ISIs per session). The same set of nonword pairs was used twice in each session. The order of critical and filler word pairs was pseudorandomized in each session, with the condition that no more than three real word or nonword targets were presented in succession. All stimuli were spoken by a female speaker with neutral intonation in a sound-proof booth and digitized with a sampling rate of 22 kHz directly into an IBM-compatible computer. Word pairs were then constructed, where identical words were represented by the same physical token. An ISI of 200 ms and 1250 ms was placed in between each word

Table II. Sample Stimuli Presentation 1 Condition Congruent Dom–Dom Sub–Sub Incongruent Sub–Dom Dom–Sub Unrelated Dom Sub

Presentation 2

Prime

Target

Prime

Target

bank bank

save stream

bank bank

money river

bank bank

stream save

bank bank

money river

calf bat

save stream

calf bat

money river

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pair, with four sessions consisting of pairs with an ISI of 200 ms, and four sessions using word pairs with an ISI of 1250 ms. Apparatus and Procedure The experiment was conducted using an IBM-compatible lap-top computer with Pentium processor, sound-card, millisecond timer, and freefield speakers. The computer presented the word pairs in free-field and recorded the time elapsed from the offset of the target to the response made by the mouse button press in milliseconds. The time-out was set to 5000 ms, after which a no-response was recorded and the next trial began. All reaction times were saved directly onto computer. Subjects were tested in 8 single sessions including 10 practice trials. Participants were instructed that they would hear a real word followed by either another real word or a nonword and were to make speeded lexical decisions using a GO–NO GO response procedure on the second word. There was an inter-trial interval of 4 s.

RESULTS Results from presentation 1 have been described previously (Copland, 2003). Statistical analyses were carried out on correct Yes lexical decisions for presentation 2 to investigate the effects of ambiguity repetition. Errors were made on less than 5% of trials for both groups. Outliers (responses > 2SDs from the subject means) were identified and replaced with a Tukey’s biweight mean estimator (less than 3% for both groups). A repeated measures ANOVA was carried out with RTs as the dependent variable with the between-subjects factor of group, and the within subjects factors of ISI (200, 1000), Congruency (Same, Different), Dominance (Dominant, Subordinate), and Relatedness (related, unrelated). The mean group RTs per condition are shown in Table III. There were significant main effects for Relatedness (F (1, 18) = 69.119, p< .001), indicating overall priming, Dominance (F (1, 18) = 41.907, p < .001), reflecting slower latencies for dominant related and unrelated targets, and Group (F (1, 18) = 19.123, p < .001), reflecting faster overall latencies for the control group (Controls M = 452 ms; NS Group M = 630 ms) . A significant interaction was obtained for Relatedness × Group (F (1, 18) = 6.612, p = .019), indicating greater priming effects for the NS group. Other significant interactions were ISI × Relatedness (F (1, 18) = 6.153, p = .023), Congruency × Relatedness (F (1, 18) = 4.984, p = .039), and importantly, Congruency × Relatedness × Group (F (1, 18) = 4.804, p = .042).

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Table III. Mean RTs as a Function of Group and Condition Control group (n = 10) Condition ISI 200 ms Same Dom–Dom Sub–Sub Different Sub–Dom Dom–Sub ISI 1250 ms Same Dom–Dom Sub–Sub Different Sub–Dom Dom–Sub

Related

Unrelated

NS group (n = 10) Related

Unrelated

419 (52) 357 (61)

545 (135) 462 (84)

562 (89) 517 (109)

703 (138) 673 (128)

424 (84) 433 (74)

485 (97) 417 (67)

498 (137) 570 (128)

755 (189) 691 (108)

430 (63) 372 (61)

532 (157) 470 (99)

580 (82) 553 (100)

647 (127) 655 (137)

497 (105) 440 (64)

499 (77) 451 (53)

658 (207) 543 (110)

717 (209) 670 (160)

Note: NS = Nonthalamic Subcortical. ISI = Interstimulus Interval. Standard Deviations are in parentheses. Dom = Dominant. Sub = Subordinate.

This significant three-way interaction was further investigated by planned pairwise comparisons of related vs. unrelated targets with dominant and subordinate conditions combined (see Fig. 1). For the control group, significant priming was observed for concordant targets at short (F (1, 9) = 20.016, p = .002) and long ISIs (F (1, 9) = 23.931, p = .001), but not for discordant targets (p > .05). For the NS group, significant priming was observed at the short ISI for the concordant (F (1, 9) = 28.133, p < .001) and discordant condition (F (1, 9) = 23.129, p = .001) and at the long ISI for both concordant (F (1, 9) = 14.896, p = .004) and discordant conditions (F (1, 9) = 7.945, p = .020). In summary, on the second presentation, the control group showed significant priming for same meaning targets, but not for different meaning targets, whereas the NS group showed significant priming for related targets, regardless of whether the same or different meaning was biased on the second presentation. Comparisons between related and unrelated conditions did not identify any significant suppression effects (i.e. significantly slower latencies for related compared to unrelated conditions).

DISCUSSION The present study investigated lexical ambiguity repetition effects in individuals with vascular NS lesions and matched controls using a

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160 140

200 1250

120 mean priming (msec)

100 80 60 40 20 0

Same Different CONTROL GROUP

Same Different NS GROUP

Fig. 1. Mean priming effects for the two groups as a function of meaning congruency of targets and interstimulus interval.

semantic priming paradigm. As predicted, the NS group showed priming for related trials regardless of meaning congruency with the previous presentation, meaning dominance, or ISI. The control group evidenced priming for targets related to the same meaning of the ambiguity previously biased, however, when different meanings were biased on the second presentation, there was no evidence of significant facilitation or meaning suppression. The control data will be discussed first with regards to previous findings and the influence of task effects and aging on ambiguity repetition. The NS group data will then be considered in terms of the possible mechanisms underlying faulty ambiguity resolution following NS lesions. The control group showed lexical ambiguity repetition effects in terms of both facilitation and diminished priming. First, repetition of an ambiguity with a target related to the same meaning resulted in facilitation of that target relative to an unrelated target. This facilitation effect was robust in the sense that it was observed for both subordinate and dominant related targets and occurred at both short and long ISIs. As previously reported in this cohort and elsewhere (Copland 2003; Simpson & Burgess, 1985), facilitation of the subordinate meaning is usually absent at longer ISIs when an ambiguity is presented once. In this context, the present findings suggest that the first response to the subordinate meaning associate resulted in sustained activation which was witnessed on the second presentation. Interestingly, previous studies of lexical ambiguity word-pair repetition have reported that repetition effects were predominantly manifest in suppression for different meanings, whereas facilitation of same meaning targets was reduced or absent (Simpson & Kang, 1994; Simpson & Kellas, 1989). Both of these studies were on young

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controls and employed speeded naming rather than lexical decision. It has been proposed that speeded naming emphasises orthography-to-phonology pathways whereas lexical decision relies on familiarity and meaning information (Balota et al., 2001), and in this instance the focus on semantic information may have resulted in facilitation more than previously found in naming studies (the impact of task differences will be discussed further below). When different meanings were biased on the second presentation, the control group showed no difference between related and unrelated target latencies. The lack of facilitation for related targets suggests that priming was diminished as a consequence of a competing meaning being biased on the first presentation. As previously reported (Copland, 2003), this cohort evidenced priming on the first presentation for both dominant and subordinate targets at the short ISI and for dominant targets at the long ISI. The fact that priming was no longer present for dominant or subordinate associates at both short and long ISIs on the incongruent second presentation suggests a powerful meaning selection mechanism which enables a temporary restructuring of connections within semantic networks (Simpson & Kellas, 1989), such that automatic spreading activation or controlled priming is no longer witnessed under these conditions. Contrary to predictions and previous investigations of lexical ambiguity repetition (Simpson & Kang, 1994; Simpson & Kellas, 1989), the control group did not show clear evidence of suppression (i.e. related targets slower than unrelated targets), but instead, showed a lack of facilitation which allows for the possibility of suppression, attentional withdrawal or decay for the unselected meanings. It is suggested that a mechanism of meaning decay is difficult to reconcile with the observation that ambiguities presented on a second occasion with a short ISI were not facilitated, suggesting a powerful temporary alteration within the lexical–semantic network. The lack of clear evidence for suppression in the current study may relate to the task used, given the view that naming tasks are more likely to invoke inhibition of competing meanings compared to lexical decisions, based on the specificity required in naming according to (Neill, 1989). It should be noted that Balota and others (Balota et al., 2001) have argued that lexical decision tasks actually involve more semantic processing than naming tasks which emphasise orthographic–phonological analysis, however, such an interpretation does not account for the present findings. It is suggested that the use of a paradigm which requires more semantic processing and integration such as relatedness judgment or sentence integration may provide clearer evidence of meaning suppression. In addition to task differences, the lack of strong evidence for suppression in the control group may relate to the older age of this cohort

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compared to previous ambiguity repetition studies. There is evidence that older adults have difficulties inhibiting irrelevant information (Hasher & Zacks, 1988), and more specifically, in resolving lexical ambiguities in sentences through rejection of inappropriate meanings (Faust et al., 1997). Yet Brady et al. (1996) found no significant effect of age in an ambiguity repetition task which required relatedness judgment, suggesting the need to further examine the effect of age on the present lexical decision paradigm using a young comparison group. It was hypothesised that the NS group would show impaired meaning suppression based on previous deficits in lexical ambiguity resolution and postulated subcortical functions in information suppression and selection. Indeed, the NS group did not show any effect of the previous ambiguity presentation on subsequent priming, compared to the absent priming in the control group for different meaning targets. Consistent with previously reported findings of single ambiguity presentation in this cohort (Copland, 2003), the NS subjects showed facilitation for related targets regardless of meaning dominance or ISI. This finding again confirms that lexical representations for ambiguities are intact and are able to be accessed following subcortical damage, consistent with the view that such representations are stored cortically (Crosson et al., 1997). The capacity of the NS group to suppress meanings cannot be properly judged in the current study, given that the control group did not show clear evidence of suppression under these conditions. Regardless, it is evident that the NS subjects had difficulties reducing the level of activation for competing meanings such that one meaning was selectively facilitated over time. This finding is consistent with previously reported findings for the first presentation of the ambiguity (Copland, 2003), where sustained subordinate meaning facilitation suggested a failure in inhibitory mechanisms which are thought to occur based on meaning frequency in the absence of context. Other related findings suggest that NS individuals have difficulties deactivating either subordinate or dominant meanings based on integrated contextual information through attentional/controlled mechanisms (Copland et al., 2000b, 2000c, 2001). In the present case, requiring the NS subjects to make an overt response towards a meaning on the first presentation does not subsequently invoke the suppression or deactivation of competing meanings. It remains to be seen whether a more demanding task such as relatedness judgment where semantic information is more consciously retrieved would invoke suppression or deactivation of competing meanings in this group. The present findings of impaired meaning selection and deactivation are consistent with the notion that basal ganglia damage may interrupt mechanisms underlying the selection and inhibition of competing

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representations of various types. For instance, Mink (1996) proposed that the basal ganglia act to select appropriate motor programs and inhibit competing mechanisms. With regards to cognition, evidence from populations with basal ganglia dysfunction has led to view that the basal ganglia may be involved in selecting salient information and ignoring or inhibiting irrelevant or competing information (e.g., Levin et al., 1989; Stout et al., 2001; see Zgaljardic et al. (2003)). In terms of possible frontal–subcortical circuits, a series of neuroimaging studies have implicated the anterior cingulate in situations of increased response competition (e.g. Carter et al., 2000). At the level of semantics, it has been proposed that striatocortical circuits involving the anterior cingulate and the basal ganglia are involved in selection from competing representations including meanings associated with lexical ambiguities, however, this suggestion has not been directly tested (Early et al., 1989; Posner & DiGirolamo, 1998). Crosson and colleagues recently reported neuroimaging evidence consistent with a related pre-SMA-dorsal caudate-ventral anterior thalamic loop supporting lexical retrieval during word generation, and postulated that this circuit may both monitor conflict between competing potential words and bias responses towards one of the potential items (Crosson et al., 2003). Based on the present findings, the proposed role of this frontal–subcortical circuit may extend beyond word generation to the mechanisms of meaning deactivation which support the maintenance of a selective semantic bias over time. The fact that previous presentations of an ambiguity and associated target had no influence on subsequent processing of the same ambiguity may reflect a wider disruption in context processing. From this perspective, the first presentation of an ambiguity represents a form of context which is not utilized to influence the subsequent selection of meanings from the lexicon. Similarly, previous studies in this cohort have found a prolonged period of encapsulated lexical access where context in the form of a lexical item or a sentence or paragraph is not integrated in order to select from competing meanings (Copland et al., 2000b, 2001). This generalized deficit in context processing is consistent with the view that the frontal–subcortical circuits are involved in the maintenance and integration of contextual information of various types in order to select appropriate behaviours and responses. Houk (2001) recently argued that in an analogous manner to voluntary movement control, the basal ganglia may act through prefrontal connections to instantiate the working memory of particular thoughts selected from a large array of alternatives, echoing a previous proposal regarding lexical selection and the basal ganglia (Wallesch & Papagno, 1988). Houk’s (2001) proposal remains to be further specified and tested in terms of identifiable cognitive processes,

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and the present findings do not relate directly to working memory as it is usually operationalized. However, the present data provide some tentative support in the sense that subcortical damage may disrupt the selective engagement of a representation which is used to govern meaning selection on subsequent occasions. The question arises then, as to whether these findings reflect the inability to maintain this meaning bias only over a longer period of time (i.e. at least eight intervening trials). Indeed, it has been recently proposed that the basal ganglia maintain a bias towards a particular representation or lexical item across time (Crosson et al., 2003). Further testing the effect of lexical ambiguity repetition over shorter durations may provide information concerning the temporal dimensions of this identified disturbance. In summary, the present study identified a disturbance in meaning deactivation for repeated lexical ambiguities in individuals with NS lesions. This finding is consistent with previous findings of impaired meaning selection and ambiguity resolution in this population and more general notions that the basal ganglia are involved in cognitive processes governing the selection of salient representations and the inhibition of competing or irrelevant information. A caveat applies to interpretations regarding the neural bases of these disturbances and the implications for normal basal ganglia function; namely, that there is a distinct possibility that NS vascular lesions can lead to cortical hypoperfusion which could also account for the present findings (Nadeau & Crosson, 1997). Data on matched individuals with Parkinson’s disease and cortical vascular lesions are currently being collected to address this issue. In addition, the use of similar tasks combined with functional neuroimaging in healthy individuals should further clarify the neural substrates supporting meaning selection and ambiguity resolution.

REFERENCES Balota, D. A., Cortese, M. J., & Wenke, D. (2001). Ambiguity resolution as a function of reading skill, age, dementia, and schizophrenia. In D. S. Gorfein (Ed.) On the Consequences of Meaning Selection: Perspectives on Resolving Lexical Ambiguity (pp. 87–102). Washington: APA Books. Brady, C. B., Balota, D., & Faust, M. (1996). Inhibitory Processes in a Homograph Disambiguation task: Evidence for a Dissociation between Healthy Aging and Alzheimer’s Disease. Poster presented at the Cognitive Aging Conference, Atlanta. Carter, C. S., Macdonald, A. M., Botvinick, M., Ross, L. L., Stenger, A., Noll, D., & Cohen, J. D. (2000). Parsing executive processes: Strategic vs. evaluative functions of the anterior cingulate cortex. Proceedings of the National Academy of Sciences, USA., 97, 1944–1948.

Ambiguity Repetition and Subcortical Lesions

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Copland, D. (2003). The basal ganglia and semantic engagement: Potential insights from semantic priming in individuals with subcortical vascular lesions, Parkinson’s disease, and cortical lesions. Journal of the International Neuropsychological Society, 9, 1041–1052. Copland, D. A., Chenery, H. J., & Murdoch, B. E. (2000a). Persistent deficits in complex language function following dominant nonthalamic subcortical lesions. Journal of Medical Speech-Language Pathology, 8, 1–15. Copland, D. A., Chenery, H. J., & Murdoch, B. E. (2000b). Understanding ambiguous words in biased sentences: Evidence of transient contextual effects in individuals with nonthalamic subcortical lesions and Parkinson’s disease. Cortex, 36, 601–622. Copland, D. A., Chenery, H. J., & Murdoch, B. E. (2000c). Processing lexical ambiguities in word triplets: Evidence of lexical–semantic deficits following dominant nonthalamic subcortical lesions. Neuropsychology, 14, 370–390. Copland, D. A., Chenery, H. J., & Murdoch, B. E. (2001). Discourse priming of homophones in individuals with dominant subcortical lesions, cortical lesions, and Parkinson’s disease. Journal of Clinical and Experimental Neuropsychology, 23, 338–356. Crosson, B. (1985). Subcortical functions in language: A working model. Brain and Language, 25, 257–292. Crosson, B., Benefield, H., Cato, M., Sadek, J., Moore, A., et al., (2003). Left and right basal ganglia and frontal activity during language generation: Contributions to lexical, semantic, and phonological processes. Journal of the International Neuropsychological Society, 9, 1061–1077. Crosson, B., Moberg, P., Boone, J., Gonzalez Rothi, L., & Raymer, A. (1997). Categoryspecific naming deficit for medical terms after fominant thalamic/capsular hemorrhage. Brain and Language, 60, 407–442. Early, T. S., Posner, M. I., Reiman, E. M., & Raichle, M. E. (1989). Left striato-pallidal hyperactivity in schizophrenia part II: Phenomenology and thought disorder. Psychiatric Developments, 2, 109–121. Faust, M. E., Balota, D., Duchek, J., Gernsbacher, M. A., & Smith, S. (1997). Inhibitory control during sentence comprehension in individuals with dementia of the Alzheimer type. Brain and Language, 57, 225–253. Gerfen, C. R. (1992). The neostriatal mosaic: Multiple levels of compartmental organization in the basal ganglia. Annual Review of Neuroscience, 15, 285–320. Gernsbacher, M. A. (1990). Language Comprehension as Structure Building. New Jersey: Lawrence Erlbaum. Hasher, L., & Zacks, R. T. (1988). Working Memory, comprehension, and aging: A review and a new view. In G. H. Bower (Ed.), The Psychology of Learning and Motivation, Vol. 22 (pp. 193–225). New York: Academic Press. Houk, J. C. (2001). Neurophysiology of frontal-subcortical loops. In D. G. Lichter & J. L. Cummings (Eds.), Frontal–Subcortical Circuits in Psychiatry and Neurology (pp. 92–113). New York: Guilford. Kertesz, A. (1982). The Western Aphasia Battery. New York: Grune and Stratton. Kucera, H., & Francis, W. N. (1967). Computational Analysis of Present-day American English. Providence, RI: Brown University Press. Levin, B., Llabre, M., & Weiner, W. (1989). Cognitive impairments associated with early Parkinson’s disease. Neurology, 39, 557–561. Mega, M. S., & Alexander, M. P. (1994). Subcortical aphasia: The core profile of capsulostriatal infarction. Neurology, 44, 1824–1829. Mink, J. W. (1996). The basal ganglia: Focused selection and inhibition of competing motor programs. Progress in Neurobiology, 50, 381–425.

66

Copland

Nadeau, S. E., & Crosson, B. (1997). Subcortical Aphasia. Brain and Language, 58, 355–402. Neill, T. W. (1989). Lexical ambiguity and context: An activation-suppression model. In D. S. Gorfein (Ed.), Resolving semantic ambiguity. New York: Springer-Verlag. Nelson, D. L., McEvoy, C. L., Walling, J. R., & Wheeler, J. W. (1980). The University of South Florida homograph norms. Behavior Research Methods & Instrumentation, 12(1), 16–37. Posner, M. I., & DiGirolamo, G. J. (1998). Executive attention: Conflict, target detection, and cognitive control. In R. Parasuraman (Ed.), The Attentive Brain (pp. 401–423). Cambridge, MA: MIT Press. Simpson, G. B., & Burgess, C. (1985). Activation and selection processes in the recognition of ambiguous words. Journal of Experimental Psychology: Human Perception and Performance, 11(1), 28–39. Simpson, G. B., & Kang, H. (1994). Inhibitory processes in the recognition of homograph meanings, Inhibitory Processes in Attention, Memory, and Language (pp. 359–381). New York: Academic Press. Simpson, G. B., & Kellas, G. (1989). Dynamic contextual processes and lexical access. In D. S. Gorfein (Ed.), Resolving Semantic Ambiguity. New York: Springer-Verlag. Stout, J. C., Wylie, S. A., Simone, P. M., & Siemers, E. R. (2001). Influence of competing distractors on response selection in Huntington’s disease and Parkinson’s disease. Cognitive Neuropsychology, 18, 643–653. Twilley, L. C., Dixon, P., Taylor, D., & Clark, K. (1994). University of Alberta norms of relative meaning frequency for 566 homographs. Memory and Cognition, 22(1), 111–126. Wallesch, C. W., & Papagno, C. (1988). Subcortical Aphasia. In F. C. Rose, R. Whurr, & M. A. Wyke (Eds.), Aphasia. London: Whurr Publishers. Zgaljardic, D. J., Borod, J., Foldi, N., & Mattis, P. (2003). A review of the cognitive and behavioral sequelae of Parkinson’s Disease: Relationship to frontostriatal circuitry. Cognitive and Behavioral Neurology, 16, 193–210.