A REAL-TIME EXAMINATION OF LEXICAL AMBIGUITY RESOLUTION FOLLOWING LESIONS OF THE DOMINANT NONTHALAMIC SUBCORTEX

The role of the basal ganglia in human language function remains unknown, despite a corpus of literature documenting the association between vascular lesions of the dominant nonthalamic subcortical (NS) region and disordered language. Theories of subcortical language function have been postulated (e.g., Crosson, 1985; Wallesch & Papagno, 1988), however, research in this field has remained largely data-driven, providing limited descriptions of individuals with vascular NS lesions in terms of performance on standard off-line language measures. This approach has failed to reveal the underlying nature of these language deficits Âlocally in terms of various dynamic and temporally constrained linguistic and nonlinguistic component processes. The current series of studies are based largely on the premise that such empirical data has the potential to speak more directly to the cogency of current theories proposing a subcortical role in language or related cognitive functions. The present thesis investigated the performance of individuals with dominant chronic vascular NS lesions, compared to matched control subjects, individuals with ParkinsonÂs disease (PD) (also assumed to have NS dysfunction) and subjects with cortical lesions (CL), on a series of experiments which allowed for the real-time examination of language processing, manipulating the degree to which automatic and attentional/strategic processing is invoked. The theoretical underpinning of these experiments hinges primarily on the proposed role of frontal-subcortical systems in mediating aspects of language via attentional/strategic mechanisms. Accordingly, it was hypothesised that the locus of impairment for individuals with NS lesions would be centred selectively on those facets of language processing which require increased recourse to these proposed frontal-subcortical cognitive capacities. The language abilities of 15 subjects with chronic dominant NS lesions, 15 matched control subjects, 14 matched subjects with CL, and 12 matched individuals with PD were examined initially on the Western Aphasia Battery (WAB) and the Boston Naming Test (BNT). Most NS subjects were classified as non-aphasic according to the WAB, however, circumscribed deficits were evidenced, typically in generative and confrontation naming. In contrast, the CL group showed significant deficits on most aspects of the WAB compared to matched normal control subjects, and presented with a more severe impairment than NS subjects overall on the WAB and in confrontation naming and repetition. The PD group performance was not significantly different from the matched control group, while PD subjects performed better than the NS group overall on the WAB. The same cohort of NS, CL, PD, and control subjects undertook a battery of complex language measures designed to place a range of higher-order cognitive demands on the language processing system. This battery included subtests from the Test of Language Competence-Expanded Edition (TLC-E), the Test of Word Knowledge (TOWK), and The Word Test-Revised (TWT-R). The NS, CL, and PD subjects presented with marked disturbances in those tasks involving cognitive-linguistic flexibility, sentence formulation, indeterminacy of meaning, and metalinguistic manipulation of the lexical-semantic system. Collectively, the off-line results suggest that those aspects of language processing which are more heavily reliant on higher-order cognitive capacities are selectively compromised in subjects with NS lesions and PD. This assumption was further examined and substantiated in a series of on-line lexical ambiguity priming experiments performed by a subset of the original NS subjects (n = 10), matched control subjects (n = 10), matched CL subjects (n=10), and matched PD subjects (n = 10). When lexical ambiguities were presented in a single word context as word triplets, NS subjects showed rapid nonselective lexical activation, suggesting that intact lexical-semantic information could be accessed via automatic routines, similar to control subjects. Unlike control subjects, however, NS subjects were unable to sustain any form of significant activation, implying a selective impairment in the ability to manipulate lexical-semantic information through attentional/controlled processing. This breakdown was qualitatively different to the controlled processing disturbance evidenced by CL subjects, who maintained nonselective meaning facilitation over time, while PD subjects showed a pattern of selective priming consistent with a reduction in attentional processing. The emerging picture of a dissociation between intact automatic processing and compromised attentional/strategic lexical processing in the NS subjects was further elucidated in an experiment examining the processing of unequibiased lexical ambiguities in isolation. In this study, NS and PD subjects showed rapid nonselective meaning facilitation, again implying intact automatic lexical processing. While control and CL subjects evidenced multiple meaning activation followed by selective facilitation of the dominant meaning, NS and PD subjects were unable to achieve selective meaning facilitation, instead showing a protracted period of nonselective lexical activation. This finding suggested that when ambiguities were encountered in isolation, there was not an absolute breakdown in attentional processing per se, but rather a circumscribed deficit in the selective attentional engagement of the semantic network on the basis of meaning frequency, possibly implicating a disturbance of inhibitory mechanisms within the semantic network. A cross-modal priming experiment was used to investigate how lexical ambiguities were processed and resolved in a biased sentential context. Initially, lexical activation for the neurological patient groups appeared influenced by contextual information to a greater extent than in normal controls, which may indicate delayed lexical decision making or disturbed automatic lexical access. Only the PD and NS individuals failed to then maintain selective facilitation of the contextually appropriate meaning, suggesting a breakdown in the attention-based control and maintenance of semantic activation on the basis of integrated sentential constraints. This finding was extended in another cross-modal priming experiment, where NS and PD subjects appeared unable to use discourse-level information to select meanings and develop topical inferences via attentional/strategic mechanisms, while CL subjects showed a selective disturbance of inference development. The results of this thesis have served to delineate certain dynamic aspects of language processing in individuals with NS lesions in terms of automatic lexical processing components and processes involving the attentional/strategic selection of meaning on the basis of meaning frequency and various types of contextual information. In general, the NS group showed a demarcation between intact automatic processing and a breakdown in attentional/strategic processing which was manifest differently depending on the conditions under which processing was invoked. Furthermore, the performance of NS subjects on attentional operations was able to be dissociated under certain conditions from CL group performance and was similar to the PD groupÂs performance in certain instances. These preliminary findings are consistent with recent theories proposing a role for frontal-subcortical systems in the Âtop-down modulation of semantic processing via executive attentional and strategic mechanisms. Although a disturbance in these systems provides a parsimonious explanation of the NS and PD group performance, such conclusions are drawn tentatively with the caveat that the precise neuropathological basis of cognitive-linguistic deficits in these individuals remains unclear at present