A rule-based categorization deficit in Alzheimer's disease?

This study examined the categorization processes that Alzheimer's disease (AD) patients use during assessments of semantic memory. Rule-based categorization involves the careful, analytic processing of strict criteria to determine category membership, particularly for items from graded categories with ambiguous category membership; similarity-based categorization requires an overall comparison of a test stimulus with a prototype or remembered exemplar of the category and is relatively effective for the rapid categorization of items with unambiguous category membership. To assess these processes in AD, patients were asked to decide the category membership of test stimuli for categories with poorly defined or fuzzy boundaries (e.g., VEGETABLE) and for categories with well-defined boundaries (e.g., FEMALE) and then to judge the representativeness of the test stimulus for its chosen category. A subgroup of AD patients demonstrated a typical pattern of impaired semantic memory compared to healthy control subjects; that is, difficulty deciding the category membership of test items from fuzzy categories. Among these patients, we found no deficit in category membership decisions about items taken from well-defined categories. We also found that AD patients and healthy controls do not differ in their representativeness judgments of items within a correctly judged category. These findings are most consistent with the hypothesis that rule-based categorization difficulty limits semantic memory in AD.     

Perceptual biases in processing facial identity and emotion

Normal observers demonstrate a bias to process the left sides of faces during perceptual judgments about identity or emotion. This effect suggests a right cerebral hemisphere processing bias. To test the role of the right hemisphere and the involvement of configural processing underlying this effect, young and older control observers and patients with right hemisphere damage completed two chimeric faces tasks (emotion judgment and face identity matching) with both upright and inverted faces. For control observers, the emotion judgment task elicited a strong left-sided perceptual bias that was reduced in young controls and eliminated in older controls by face inversion. Right hemisphere damage reversed the bias, suggesting the right hemisphere was dominant for this task, but that the left hemisphere could be flexibly recruited when right hemisphere mechanisms are not available or dominant. In contrast, face identity judgments were associated most clearly with a vertical bias favouring the uppermost stimuli that was eliminated by face inversion and right hemisphere lesions. The results suggest these tasks involve different neurocognitive mechanisms. The role of the right hemisphere and ventral cortical stream involvement with configural processes in face processing is discussed.     

Contextual mood priming following left and right hemisphere damage

This research examined the influence of mood-congruent and mood-incongruent contexts on recognizing affective prosody after brain damage. Predictions stemmed from an associative network theory of learning and memory. Thirty-three male subjects, 11 each in right hemisphere damaged (RHD), left hemisphere damaged (LHD), and normal control groups judged moods from the prosody of semantically neutral phrases. In one task, the prosodic stimulus phrases were judged in isolation. In another task, the phrases were preceded by short paragraphs which were either congruent or incongruent in emotional tone with the prosodic stimuli. These paragraphs were designed to prime specific mood choices. As anticipated, LHD subjects' prosodic mood recognition was more accurate when given congruent rather than incongruent affective contexts. Congruent contexts facilitated, and incongruent contexts disrupted, their prosodic mood judgments to the same extent as normals. RHD subjects showed a partial context decrement. They were less accurate than normal or LHD subjects in the congruent condition, and were unaffected by incongruent contexts. When given congruent biasing paragraphs, however, RHD subjects did experience facilitation on a par with that found for the other groups, indicating spared sensitivity to certain contextual factors. The distinction between automatic and effortful processes is offered as a potential explanation for the RHD group's pattern of performance.     

Interhemispheric dissociation of expressive and receptive language functions in patients with complex-partial seizures: An amobarbital study

Bilateral intracarotid amobarbital procedures (IAP) were performed in 144 patients with medically intractable complex-partial seizures. As a result of language testing, 29 patients (20.1%) were found to have bilateral language representation to different degrees. In four (2.8%) of these patients--all right-handers with early onset of epilepsy and/or evidence of early brain damage--there was strong evidence of an interhemispheric dissociation of expressive and receptive language functions. Two of these patients had circumscribed temporal foci (one left, one right), and receptive language functions were represented in the hemisphere contralateral to the focus. One patient with a right frontal focus showed left-hemisphere dominance for expressive functions, while the fourth patient exhibited left-hemisphere dominance for receptive functions associated with a right temporo-parietal focus. It is argued that in these four cases the circumscribed functional and/or structural impairments have led to a shift of the anatomically associated language functions to the opposite hemisphere (rather than to neighboring regions of the same hemisphere). These findings substantiate the hypothesis that in special circumstances the anterior (expressive) language area can be located in one hemisphere and the posterior (receptive) area in the other.     

Does Global Context Modulate Cerebral Asymmetries? A Review and New Evidence on Word Imageability Effects

In this article we examine whether the distribution of function across the right and left cerebral hemispheres for lexical processing is influenced by the global context within which words are presented. A review of previously published studies indicates that the ubiquitous right visual field (RVF)/left hemisphere advantage for word recognition may be reduced or eliminated for nouns, content words, or high image words, but only when such items are presented along with verbs, function words, or low image words. However, paradoxically, when the former items are presented in more homogeneous contexts, the RVF advantage is uniformly observed. We propose that the processing efficiency of a hemisphere for a given stimulus depends on that item's relation to the other stimuli provided, that is, the global context. This was examined in a visual half-field experiment that varied whether high and low image nouns were presented in homogeneous (blocked lists) or heterogeneous (mixed lists) contexts. An unvarying RVF advantage was observed for high image words in homogeneous contexts, but this advantage was eliminated when the same items were presented in heterogeneous contexts. We suggest that stimulus heterogeneity maximizes reliance on differing, but complementary, computational biases across hemispheres. Hence, the extent to which the left and right hemispheres are recruited for the recognition of individual word types can vary dynamically with variation in the stimulus environment.     

Can semi-supervised learning explain incorrect beliefs about categories?

Three experiments with 88 college-aged participants explored how unlabeled experiences—learning episodes in which people encounter objects without information about their category membership—influence beliefs about category structure. Participants performed a simple one-dimensional categorization task in a brief supervised learning phase, then made a large number of unsupervised categorization decisions about new items. In all three experiments, the unsupervised experience altered participants’ implicit and explicit mental category boundaries, their explicit beliefs about the most representative members of each category, and even their memory for the items encountered during the supervised learning phase. These changes were influenced by both the range and frequency distribution of the unlabeled stimuli: mental category boundaries shifted toward the middle of the range and toward the trough of the bimodal distribution of unlabeled items, whereas beliefs about the most representative category members shifted toward the modes of the unlabeled distribution. One consequence of this shift in representations is a false-consensus effect (Experiment 3) where participants, despite receiving very disparate training experiences, show strong agreement in judgments about representativeness and boundary location following unsupervised category judgments.     

Language localization in cases of left temporal lobe arachnoid cyst: evidence against interhemispheric reorganization

We investigated whether left-hemisphere arachnoid cysts lead to reorganization of the language function using PET. A group analysis demonstrated that patients showed no more right-hemisphere activation than a matched control group. Several patients had clear language localizations in the left hemisphere during language comprehension; none of the patients showed right-hemisphere activation. We conclude that left-hemisphere tissue must suffer considerable compromise before reorganization of language into the right hemisphere becomes necessary. Language activations within the left hemisphere are clearly displaced. This is consistent with mere physical displacement in some patients rather than reorganization within the left hemisphere; in others intrahemispheric reorganization cannot be excluded.     

Differential hemispheric mediation of nonverbal visual stimuli.

Studies employing schematic faces of three features explored the role of the left hemisphere as an analytic processor in difficult discriminations and the role of the right hemisphere in easy gestalt matches. In a discrimination task, the successively presented members of a stimulus pair differed in all three features. When both stimuli went to the same visual field there was a Visual Field X Judgments interaction; judgments same were processed faster in the left visual field. When a test stimulus differing on all three features from a previously memorized target face was matched to the latter, both judgments same and different were made faster for stimuli in the left visual field. However, when the test stimulus and the memory items differed on only one feature the right field proved superior for both judgments same and different.     

Trial-by-trial adjustments in control triggered by incidentally encoded semantic cues

Cognitive control mechanisms provide the flexibility to rapidly adapt to contextual demands. These contexts can be defined by top-down goals—but also by bottom-up perceptual factors, such as the location at which a visual stimulus appears. There are now several experiments reporting contextual control effects. Such experiments establish that contexts defined by low-level perceptual cues such as the location of a visual stimulus can lead to context-specific control, suggesting a relatively early focus for cognitive control. The current set of experiments involved a word–word interference task designed to assess whether a high-level cue, the semantic category to which a word belongs, can also facilitate contextual control. Indeed, participants exhibit a larger Flanker effect to items pertaining to a semantic category in which 75% of stimuli are incongruent than in response to items pertaining to a category in which 25% of stimuli are incongruent. Thus, both low-level and high-level stimulus features can affect the bottom-up engagement of cognitive control. The implications for current models of cognitive control are discussed.     

Left anterior lobectomy and category-specific naming

Damasio and colleagues (1996) have proposed that the left anterior temporal region supports knowledge pertaining to living objects, whereas more posterior temporal regions play a critical role in naming nonliving things. Accordingly, one might expect that left-sided anterior temporal lobectomy should have a more profound effect on the naming of living as opposed to nonliving things. As part of a multicenter collaborative project, seventy-nine patients (all left-hemisphere speech dominant) were tested pre- and post-left-temporal lobectomy on a task that required naming of living and nonliving items equated for name frequency, familiarity, and visual complexity. Consistent with the proposals of Damasio et al. (1996), left temporal lobectomy impaired naming ability, particularly for living things. When individual outcomes were considered, twice as many patients showed a relative decline in naming living as opposed to nonliving things.     

Processing of perceptual information is more robust than processing of conceptual information in preschool-age children: evidence from costs of switching

Is processing of conceptual information as robust as processing of perceptual information early in development? Existing empirical evidence is insufficient to answer this question. To examine this issue, 3- to 5-year-old children were presented with a flexible categorization task, in which target items (e.g., an open red umbrella) shared category membership with one test item (e.g., a folded umbrella) and perceptual characteristics with another test item (e.g., a red mushroom). Participants were instructed to either categorize stimuli by the same dimension (i.e., perceptual similarity or category membership) in both phases of the task, or switch from categorizing by one dimension to categorizing by the other dimension. Results pointed to a strong asymmetry in switch costs: conceptual switch costs were higher than perceptual switch costs. These results suggest that processing of perceptual information remains more robust than processing of conceptual information at least until 5 years of age.     

Asynchrony of lateral onset as a factor in difference in visual field.

An experiment in matching judgments was designed to examine a role of perceptual process in apparent asymmetry. Recognition of Hirakana letters (Japanese letters) was required. The experimental condition in which stimuli were presented to the left visual field first and to the right visual field second produced more errors for all stimulus intervals (0 to 60 msec.) than experimental conditions where stimuli were presented to the right visual field first and to the left one second. Especially, superiority of the latter condition was marked with the longest stimulus interval employed. These results indicate superiority of the left hemisphere function for recognizing Hirakana letters and suggest that not only memory but also perceptual process contributes to this laterality effect.     

How does reading direction modulate perceptual asymmetry effects?

Left-side bias effects refer to a bias towards the left side of the stimulus/space in perceptual/visuospatial judgments, and are argued to reflect dominance of right hemisphere processing. It remains unclear whether reading direction can also account for the bias effect. Previous studies comparing readers of languages read from left to right with those read from right to left (e.g., French vs. Hebrew) have obtained inconsistent results. As a language that can be read from left to right or from right to left, Chinese provides a unique opportunity for a within-culture examination of reading direction effects. Chinese participants performed a perceptual judgment task (with both face and Chinese character stimuli; Experiment 1) and two visuospatial attention tasks (the greyscales and line bisection tasks; Experiment 2) once before and once after a reading task, in which they read Chinese passages either from left to right or from right to left for about 20 min. After reading from right to left, participants showed significantly reduced left-side bias in Chinese character perceptual judgments but not in the other three tasks. This effect suggests that the role of reading direction on different forms of left-side bias may differ, and its modulation may be stimulus-specific.     

Drawing deficits in brain-damaged patients' freehand pictures

This study examines errors in brain-damaged patients' attempts to produce freehand drawings of common objects in response to a linguistic target (e.g., "fruit"). The pictures were carefully analyzed according to the features which dictionaries claim should vary necessarily from item to item, including color, shape, relative size, and special features which help distinguish between similar items. A "recognizability score" was created to quantify patients' overall drawing capacity. The findings revealed that subjects with right hemispheric insult are significantly more impaired overall than left hemisphere-damaged patients in producing freehand pictures. However, freehand drawing skills do not break down in an undifferentiated fashion after right hemispheric insult: Group and individual data reveal that patients with right central (primarily parietal) insult are most impaired at expressing shape attributes in their pictures, right posterior patients with primarily temporoparietal insult experience the greatest difficulty attributing color to their pictures, but patients with right anterior insult are impaired at expressing color, shape, and relative size in their pictures. All patients with right hemisphere insult often attribute specific features indiscriminantly to any member of the same category, resulting in anomalous pictures like a "potato bush." One patient with left posterior insult seemed to encounter difficulty generating the mental images necessary to serve as the basis for his pictures. These findings are discussed with reference to an image-based model for freehand drawing.     

Effects of concurrent verbal memory on recognition of stimuli from the left and right visual fields.

Two experiments examined the effect of concurrently holding 0, 2, 4, or 6 nouns in memory on the recognition of visual stimuli briefly presented to the left or right visual fields. When stimuli to be visually recognized were complex visuospatial forms it was found that a relatively easy memroy load of 2 or 4 nouns improved visual recognition accuracy on right visual field (left-hemisphere) trials relative to the no-memory condition; however, a more difficult memory load of 6 nouns decreased visual recognition accuracy to a level slightly below the no-memory condition. There were no effects of concurrent verbal memroy on visual form recognition on left visual field (right-hemisphere) trials. When the stimuli to be visually recognized were words it was found that a relatively easy memroy load of 2 or 4 nouns improved visual recognition accuracy and a more difficult load of 6 nouns decreased visual recognition accuracy on both left and right visual field trials. The complete pattern of results indicates that several factors including cerebral hemisphere specialization, stimulus codability, selective perceptual orientation, and selective cerebral hemisphere interference interact in systematic ways to produce overall visual laterality effects.     

Effects of categorical labels on similarity judgments: a critical analysis of similarity-based approaches

Our goal in the present study was to evaluate the claim that category labels affect children's judgments of visual similarity. We presented preschool children with discriminable and identical sets of animal pictures and asked them to make perceptual judgments in the presence or absence of labels. Our findings indicate that children who are asked to make perceptual judgments about identical items judge discriminable items less accurately when making subsequent similarity judgments. Thus, labels do not generally affect children's perceptual similarity judgments; rather, children's reliance on labels to make similarity judgments appears to be attributable to flaws in the methodological approaches used in prior studies. These results have implications for the role of perceptual and conceptual information in children's categorization and induction.     

Asymmetrical hemisphere activation enhances global–local processing

Decades of research focusing on the neurophysiological underpinnings related to global–local processing of hierarchical stimuli have associated global processing with the right hemisphere and local processing with the left hemisphere. The current experiment sought to expand this research by testing the causal contributions of hemisphere activation to global–local processing. To manipulate hemisphere activation, participants engaged in contralateral hand contractions. Then, EEG activity and attentional scope were measured. Right-hand contractions caused greater relative left-cortical activity than left-hand contractions. Participants were more narrowly focused after left-hemisphere activation than after right-hemisphere activation. Moreover, N1 amplitudes to local targets in the left hemisphere were larger after left-hemisphere activation than after right-hemisphere activation. Consistent with past research investigating hemispheric asymmetry and attentional scope, the current results suggest that manipulating left (right) hemisphere activity enhanced local (global) attentional processing.     

Asymmetrical hemisphere activation enhances global–local processing

Decades of research focusing on the neurophysiological underpinnings related to global–local processing of hierarchical stimuli have associated global processing with the right hemisphere and local processing with the left hemisphere. The current experiment sought to expand this research by testing the causal contributions of hemisphere activation to global–local processing. To manipulate hemisphere activation, participants engaged in contralateral hand contractions. Then, EEG activity and attentional scope were measured. Right-hand contractions caused greater relative left-cortical activity than left-hand contractions. Participants were more narrowly focused after left-hemisphere activation than after right-hemisphere activation. Moreover, N1 amplitudes to local targets in the left hemisphere were larger after left-hemisphere activation than after right-hemisphere activation. Consistent with past research investigating hemispheric asymmetry and attentional scope, the current results suggest that manipulating left (right) hemisphere activity enhanced local (global) attentional processing.     

Laterality of lesions and trait-anxiety on working memory performance

An asymmetry of anterior cerebral activation favoring the right hemisphere has been associated with dispositional negative affect including trait-anxiety, while the opposite appears true of cerebral asymmetry favoring the left hemisphere. It was hypothesized that an asymmetry of cerebral activation, as defined by scores on a measure of trait-anxiety, ipsilateral to the side of an anterior brain lesion would be associated with less efficient cognitive processing than greater activation in the hemisphere contralateral to the lesion. Patients with anterior left (n = 16) or right (n = 15) hemisphere lesions completed the State-Trait Anxiety Inventory and several neurocognitive tasks. Of the abilities tested, only Digit Span scores showed an interaction between side of lesion and presumed activation asymmetry. Patients with right- but not with left-hemisphere damage showed significant differences in working memory performance depending on the presumed direction of asymmetry of the two hemispheres, supporting the dual roles of the right hemisphere in affective processing and directed attention.     

Measuring the influence of similarity on category-specific effects

There is evidence for both advantages and disadvantages in normal recognition of living over nonliving things. This paradox has been attributed to high levels of perceptual similarity within living categories having a different effect on performance in different contexts. However, since living things are intrinsically more similar to each other, previous studies could not determine whether the various category effects were due to perceptual similarity, or to other characteristics of living things. We used novel animal and vehicle stimuli that were matched for similarity to measure the influence of perceptual similarity in different contexts. We found that displaying highly similar objects in blocked sets reduced their perceived similarity, eliminating the detrimental effect on naming performance. Experiment 1 demonstrated a disadvantage for highly similar objects in name learning and name verification using mixed groups of similar and dissimilar animals and vehicles. Experiment 2 demonstrated no disadvantage for the same highly similar objects when they were blocked, e.g., similar animals presented alone. Thus, perceptual similarity, rather than other characteristics particular to living things, is affected by context, and could create apparent category effects under certain testing conditions.