Bilateral capacity for speech sound processing in auditory comprehension: evidence from Wada procedures

Data from lesion studies suggest that the ability to perceive speech sounds, as measured by auditory comprehension tasks, is supported by temporal lobe systems in both the left and right hemisphere. For example, patients with left temporal lobe damage and auditory comprehension deficits (i.e., Wernicke's aphasics), nonetheless comprehend isolated words better than one would expect if their speech perception system had been largely destroyed (70-80% accuracy). Further, when comprehension fails in such patients their errors are more often semantically-based, than-phonemically based. The question addressed by the present study is whether this ability of the right hemisphere to process speech sounds is a result of plastic reorganization following chronic left hemisphere damage, or whether the ability exists in undamaged language systems. We sought to test these possibilities by studying auditory comprehension in acute left versus right hemisphere deactivation during Wada procedures. A series of 20 patients undergoing clinically indicated Wada procedures were asked to listen to an auditorily presented stimulus word, and then point to its matching picture on a card that contained the target picture, a semantic foil, a phonemic foil, and an unrelated foil. This task was performed under three conditions, baseline, during left carotid injection of sodium amytal, and during right carotid injection of sodium amytal. Overall, left hemisphere injection led to a significantly higher error rate than right hemisphere injection. However, consistent with lesion work, the majority (75%) of these errors were semantic in nature. These findings suggest that auditory comprehension deficits are predominantly semantic in nature, even following acute left hemisphere disruption. This, in turn, supports the hypothesis that the right hemisphere is capable of speech sound processing in the intact brain.     

Verbal retention deficits in aphasic and amnesic patients.

Five different groups of patients (aphasics, alcoholic Korsakoffs, nondominant hemisphere patients, alcoholics, and control patients) were asked to detect either repeated letters, repeated words, rhyming words, or words from the same category during the reading of a list. It was discovered that the number of intervening words had a greater effect on the aphasics for all conditions than it did for the other groups. However, when the rate of presentation was slowed, the aphasics showed considerable improvement while the other groups maintained their same level of performance. The Korsakoff patients were impaired only on the semantic task (same category inclusion) and did not improve at the slower presentation rate. An interpretation based on speed and level of information processing abilities is given.     

Cognitive processing of tokens and their description in aphasia

In a dual-reaction time task aphasics (N = 21) and right-hemisphere (RH) controls (N = 24) had to decide whether a list of features given verbally or pictorially correctly described the picture of a token. Although the error rates were extremely low, aphasics made significantly more errors than RH controls. There were no significant differences between the groups in latencies when pictures of tokens were presented; the groups differed drastically, however, when confronted with lists of features. The findings are interpreted as indicating a general deficit in the short-term storage of highly specific information.     

The appreciation of antonymic contrasts in aphasia

To secure information on the ability of aphasic patients to comprehend antonymic relations, the kinds of confusions typically made, and the extent to which antonymic sensitivity depends upon mode of presentation and task demands, three linguistic and nonlinguistic tests were administered to aphasic patients, right hemisphere-damaged patients, and non-neurological controls. Although difficulty with antonymous relations was found among all the organic patients, the kinds of problems evidenced and the relative profile of difficulties differed across populations. Among the principal findings were the generally preserved sensitivity to antonymy found among anterior (particularly Broca's) aphasics; a surprising insensitivity to antonymy and a preference for synonyms, found among right hemisphere patients, particularly on the nonlinguistic tasks; a relative preservation of sensitivity to antonymy on nonlinguistic tasks, coupled with a loss of such sensitivity on linguistic tasks, found among posterior (particularly Wernicke's) aphasics; a proclivity toward stereotypical correct responses among the organic patients; and an absolutely worse performance by right hemisphere patients on tasks involving antonymic relations in pictures and abstract designs.     

Lexical knowledge in anterior and posterior aphasics

A semantic categorization task was used to assess the structure of lexical knowledge in anterior and posterior aphasics. In general responses were faster for typical category members than for atypical ones and slower for semantically related nonmembers than for unrelated ones. Both groups performed at a high level of accuracy when classifying typical category members and semantically unrelated nonmembers. Their performance diverged at the category boundary. Anterior aphasics maintained relatively high levels of accuracy when classifying atypical category members and semantically related nonmembers while posterior aphasics did not. These results point to differences in the status of lexical knowledge for anterior and posterior aphasics.     

Concept formation in non-verbal categorization tasks in brain-damaged patients with and without aphasia

The study of thinking in patients with aphasic language impairments has long since been considered as means for learning about neuropsychological processes. In the present investigation, a non-verbal categorization task was used, requiring the subjects to sort several common objects according to different principles. The experimental group consisted of aphasic patients with motor and sensory aphasia. Two control groups were used, one consisting of normal subjects and the other of non-aphasic patients with injuries in the right hemisphere of the brain. The sorting behaviour was analysed according to several aspects. The final product of the categorization was assessed, as well as the behavioural processes observed during the sorting. The results were expressed in terms of different conceptual levels achieved. The results of the investigation showed that the aphasics were significantly inferior to normal controls both quantitatively and qualitatively in all sorting aspects. Possible mechanisms underlying the deficiencies in the sorting behaviour of the two aphasic groups were discussed.     

The neurological substrates for prosodic aspects of speech

The ability to comprehend and produce the stress contrast between noun compounds and noun phrases (e.g., greenhouse vs. green house) was examined for 8 nonfluent aphasics, 7 fluent aphasics, 7 right hemisphere damaged (RHD) patients, and 22 normal controls. The aphasics performed worse than normal controls on the comprehension task, and the RHD group performed as well as normals. The ability to produce stress contrasts was tested with a sentence-reading task; acoustic measurements revealed that no nonfluent aphasic used pitch to distinguish noun compounds from phrases, but two used duration. All but one of the RHD patients and all but one of the normals produced pitch and/or duration cues. These results suggest that linguistic prosody is processed by the left hemisphere and that with brain damage the ability to produce pitch and duration cues may be dissociated at the lexical level.     

Sentence understanding and knowledge of the world: Evidences from a sentence-picture matching task performed by aphasic patients

The abilities of aphasic patients to perform a sentence-picture matching task were investigated on simple declarative reversible sentences. Two factors were found to differentially affect Broca's versus Wernicke's aphasics' performance: sentence plausibility and spatial arrangement. Sentence plausibility, that is to say, the liability of two given nouns in a sentence to act as agent and recipient according to normal subjects' expectancies, had a significant effect on the frequency of correct responses of Broca's aphasics but not on those of Wernicke's. Conversely, the latter were found to be more sensitive to a match or mismatch in the left-to-right spatial arrangement of the grammatical subject and object in a sentence on the one hand, and the order of the corresponding personages in the picture on the other. The results are discussed in terms of “frame selection” and the handling of ordered structure strategies contributing to normal system, but selectively disturbed in the case of brain damage.     

Analytical competence and language impairment in aphasia

Matched groups of Broca's and Wernicke's aphasics, brain-damaged patients without aphasia and chronic schizophrenics were tested in a nonverbal matching task where the subject had to indicate which of two pictures was more closely linked to a clue picture. Eight additional verbal and nonverbal reference tasks were administered. Both aphasic groups performed worse than brain-damaged controls when the identification of individual attributes or actions shared by clue and referent was required, but were unimpaired when the two had a set of referential-situational associations in common. Factor analyses resulted for both groups in two factors, one of which represents general Language Impairment. For the Broca's aphasics this factor was closely related to general organic deficit as measured by the Trail Making Test; for the Wernicke's aphasics it was associated with tasks which might be considered illustrative of analytical competence in isolating and comparing individual features of objects or concepts.     

Hemispheric differences in semantic processing: Category matching is not the same as category membership

Two closely related semantic processing tasks were studied under identical procedural conditions in order to examine lateral visual field effects on reaction times. In Experiment 1, reaction times did not differ as a function of visual field when subjects decided whether a lateral word was a member of a foveally presented category word (category membership task). On the other hand, reaction times were faster for right than for left visual field stimulus presentations when subjects decided whether two words, one lateral and one foveal, belonged to the same category (category matching task), although this advantage did not occur immediately. In Experiment 2, the laterality effect in the category matching task was studied as a function of word familiarity. Two groups of subjects performed the matching task for two blocks of trials, one group receiving the same word list in each block and the other receiving different lists. No visual field differences in reaction times were observed for either group during the first block of trials, but a distinct right field advantage appeared for both during the second block. The data from these experiments suggest that category matching strategies rely upon structures or processes localized in the left hemisphere, although their influence is not immediate. Category membership strategies, on the other hand, do not depend upon such localized structures.     

Strategies of semantic categorization in the cerebral hemispheres

Strategies of semantic categorization in intact cerebral hemispheres were studied in two experiments by presenting names of typical and atypical category instances to the left visual field (LVF) (right hemisphere) or to the right visual field (RVF) (left hemisphere). The results revealed that the typicality of instances had a large effect on categorization times in the LVF in both experiments, suggesting that the right hemisphere relies strongly on a holistic, similarity-based comparison strategy. In Experiment 1, the typicality effect was weaker in the RVF than in the LVF. In Experiment 2, a typicality effect in the RVF was observed for the "four-footed animal" category but not for the "bird" category. The hypothesis that the left hemisphere employs a strategy based on defining or necessary features is not supported by the observed typicality effect in the "four-footed animal" category. Instead, it is suggested that the left hemisphere may be able to categorize on the basis of prestored instance-category knowledge. When such knowledge is not available (e.g., as for four-footed animals), a similarity-based comparison strategy is employed by the left hemisphere.     

Pronoun comprehension in aphasia: a comparison of three languages

The research reported here investigated the effect of phonological and syntactic factors on the processing of pronouns by aphasics. The comprehension of these "closed-class" elements was studied in three different languages: French, Dutch, and German. The cross-linguistic design made it possible to vary phonological status (clitic/nonclitic) and phrasal category (noun phrase/prepositional phrase) as well as grammatical relation (direct/indirect object) while keeping class membership (closed class) and meaning constant. A sentence-picture matching task was given to 20 German-speaking, 16 Dutch-speaking, and 14 French-speaking aphasics, half of each language group being classified as agrammatic Broca's and half as paragrammatic Wernicke's aphasics. The results suggest that Broca's aphasics' limitations in retrieving pronouns, and therefore other closed-class elements, are not a function of either phonological status, phrasal category, or grammatical relation. These subjects' observed high level of performance on pronouns in language comprehension appears due to the kind of semantic and syntactic information they encode. Our findings indicate that a more refined distinction than closed class vs. open class is necessary.     

The acquisition of automatic response biases through categorization

Participants categorized left- and right-pointing line drawings presented together with pictures, whose pointing direction (to the left or right) was ambiguous (spatially ambiguous pictures; Experiments 1 and 2) or that pointed neither to the left nor to the right (spatially neutral pictures; Experiment 3). Subsequently, the spatially ambiguous and neutral pictures were used in a Simon task, wherein participants made left and right keypresses on the basis of the color of the pictures, while ignoring the object that they depicted. In all three experiments, performance was facilitated when the response required by the color matched the pointing direction of the line drawings with which the picture had been previously paired. Performance was impeded when the response required by the color did not match the pointing direction of the line drawings with which the picture had been previously paired. Thus, responses indirectly associated with pictures by category membership were automatically triggered even when the responses were inappropriate.     

The game of the name: An examination of linguistic reference after brain damage

Two aspects of naming have generally been examined in aphasics: the appreciation of a single property of a concept and the ability to use the names of fully elaborated concepts. In applying to aphasics a recently proposed model of semantic organization—one which allows the simultaneous examination of these two aspects—two studies were conducted: Anterior and posterior aphasics were asked (1) to name members of superordinate categories, their responses being converted into prototypicality scores (the degree to which a member of a category resembles an ideal member of that category) and (2) to differentiate between more or less prototypical members of a category and their superordinates. The results revealed that, while anterior and posterior aphasics differ in their use of the names representing fully elaborated concepts, these groups show no differences in terms of the appreciation of a single property of a concept. The implications of these findings for reference were discussed.     

Perceptual fluency can be used as a cue for categorization decisions

Learning in the prototype distortion task is thought to involve perceptual learning in which category members experience an enhanced visual response (Ashby & Maddox. Annual Review of Psychology, 56, 149-178, 2005). This response likely leads to more-efficient processing, which in turn may result in a feeling of perceptual fluency for category members. We examined the perceptual-fluency hypothesis by manipulating fluency independently from category membership. We predicted that when perceptual fluency was induced using subliminal priming, this fluency would be misattributed to category membership and would affect categorization decisions. In a prototype distortion task, the participants were more likely to judge stimuli that were not members of the category as category members when the nonmembers were made perceptually fluent with a matching subliminal prime. This result suggests that perceptual fluency can be used as a cue during some categorization decisions. In addition, the results provided converging evidence that some types of categorization are based on perceptual learning.     

Nonverbal categorization in fluent and nonfluent anomic aphasics

This study aimed to demonstrate that the naming difficulties of a particular group of aphasics, namely, fluent aphasics, are related to an underlying inability to organize feature set information. In order to test this hypothesis, the performance of fluent aphasics, nonfluent aphasics, and a nonaphasic brain-injured control group, was examined on a nonverbal categorization task, which was carefully structured in terms of instance typicality. Scores of visuoperceptual and naming tests were correlated with categorization task errors. As predicted, fluent aphasics showed a significant deficit in performance on the categorization task in comparison with other subjects. Differences in the nature of the errors the fluent aphasics made suggested that their problems were related to difficulties in abstracting the prototype for each category and in sorting category members with reference to these prototypes. For fluent aphasics, but not other subjects, a significant correlation was found between categorization task performance and naming ability.     

The effect of pictorial context on sentence memory in Broca's and Wernicke's aphasia

Eighteen aphasic patients (8 Broca's and 10 Wernicke's aphasics), 11 right hemisphere damaged patients and 12 normal subjects were tested to assess the effect of pictorial context on verbal memory with a sentence recognition task. The subjects were read aloud a stimulus sentence describing a simple event and simultaneously shown a picture congruent or incongruent with the sentence. Immediately following or after an interval of 30 sec, the subjects were read aloud a second sentence and asked to judge whether this sentence was the same or different from the stimulus sentence. The results indicated that verbal memory was better retained in supportive situations than in distracting situations, and that this contextual effect was greater in aphasic patients than in any of the other groups. Verbal memory declined rapidly after an interval in aphasic patients, but not in normal subjects. The Broca's and the Wernicke's aphasics demonstrated different performance patterns when semantic analysis of sentences was critical.     

Perceptual differentiation as a source of category effects in object processing: Evidence from naming and object decision

The locus of category effects in picture recognition and naming was examined in two experiments with normal subjects. Subjects carried out object decision (deciding whether the stimulus is a “real” object or not) and naming tasks with pictures of clothing, furniture, fruit, and vegetables. These categories are distinguished by containing either relatively many exemplars with similar perceptual structures (fruit and vegetables;structurally similar categories), or relatively few exemplars with similar perceptual structures (clothing and furniture;structurally dissimilar categories). In Experiment 1, responses to the stimuli from the structurally similar categories were slower than responses to stimuli from the structurally dissimilar categories, and this effect was larger in the naming than in the object decision task. Further, prior object decisions to stimuli from structurally similar categories facilitated their subsequent naming. In Experiment 2, we orthogonally manipulated object decision and naming as prime and target tasks, again with stimuli from the four categories. Category effects, with responses slower to objects from structurally similar categories, were again larger in naming than in object decision, and these category effects in naming were reduced by priming with both naming and object decision. We interpret the data to indicate that category effects in object naming can reflect visually based competition which is reduced by the preactivation of stored structural knowledge for objects.     

Apparent shift in visual field preference after unilateral stroke

Patients with either a left- or a right-hemisphere stroke lesion scored higher in tasks of word-picture matching and of nonverbal shape matching when information was presented tachistoscopically (120 msec) to the visual field (VF) projecting to their undamaged hemisphere. Left-hemisphere stroke patients (n = 13) were dissociated from right-hemisphere stroke patients (n = 15) by low word recognition from memory and by low right VF but nearly normal left VF accuracy in word-picture matching or shape matching; the former appeared to rely upon processing of word meaning by the right hemisphere. In contrast, right-stroke patients had higher right than left VF scores in both tasks, and their discrimination of nonverbal shapes via the right VF was not different from that of controls (n = 15). Preferred processing by the VF projecting to the undamaged hemisphere appeared as a shift in perceptual asymmetry but may indicate, in support of a "direct access" model, that each hemisphere responds more or less efficiently to word and to nonverbal shape discriminations.     

Left hemispheric interference with nonverbal performance in aphasics: comparison with data from split-brain studies

Two nonverbal tasks (Nebes' Figural Unification Test and Arc-Circle Matching Task) were given to 10 aphasic patients and 10 normal controls. Observations in split-brain patients have shown that the right hemisphere performs almost normally on these tasks while the left hemisphere fails completely. Left and right hand performances of aphasics and controls were compared with those of split-brain patients as reported in the literature. In the Figural Unification Test the aphasics' left hand performance was significantly poorer than that of split-brain patients. This is interpreted as an effect of left hemispheric interference with right hemispheric nonverbal performance. The lack of a similar impairment of left hand performance in the Arc-Circle Matching Task suggests that this interference is only provoked by tasks which, due to the possible verbalization of stimulus material, tend to stimulate left hemispheric activity.