Differential impact of posterior lesions in the left and right hemisphere on visual category learning and generalization to contrast reversal

Hemispheric differences in the learning and generalization of pattern categories were explored in two experiments involving sixteen patients with unilateral posterior, cerebral lesions in the left (LH) or right (RH) hemisphere. In each experiment participants were first trained to criterion in a supervised learning paradigm to categorize a set of patterns that either consisted of simple geometric forms (Experiment 1) or unfamiliar grey-level images (Experiment 2). They were then tested for their ability to generalize acquired categorical knowledge to contrast-reversed versions of the learning patterns. The results showed that RH lesions impeded category learning of unfamiliar grey-level images more severely than LH lesions, whereas this relationship appeared reversed for categories defined by simple geometric forms. With regard to generalization to contrast reversal, categorization performance of LH and RH patients was unaffected in the case of simple geometric forms. However, generalization to contrast-reversed grey-level images distinctly deteriorated for patients with LH lesions relative to those with RH lesions, with the latter (but not the former) being consistently unable to identify the pattern manipulation. These findings suggest a differential use of contrast information in the representation of pattern categories in the two hemispheres. Such specialization appears in line with previous distinctions between a predominantly lefthemispheric, abstract-analytical and a righthemispheric, specific-holistic representation of object categories, and their prediction of a mandatory representation of contrast polarity in the RH. Some implications for the well-established dissociation of visual disorders for the recognition of faces and letters are discussed.     

A left hemisphere contribution to visuospatial processing

Men with chronic, focal, unilateral missile injures of the brain--twenty-five with left hemisphere lesions (LH group) and twenty with right hemisphere lesions (RH groups)--and twenty-two control subjects were given two visuoperceptual and two visuospatial tests. The LH group was significantly impaired in relation to the control group on both the spatial tasks. A different pattern of dissociable perceptual and spatial deficits was found in the experimental groups: better preserved perceptual than spatial performance was observed more frequently in the LH group whereas the converse--relatively better spatial than perceptual performance--was more evident in the RH group. Double dissociations in performance on the two spatial tasks were found predominantly in the LH group. These findings suggest an important left hemisphere contribution to visuospatial processing and the possibility of a more focal representation of spatial abilities in the left hemisphere than in the right.     

Canonical views of faces and the cerebral hemispheres

Evidence is given for a special, canonical, status of one specific view in the identification of familiar faces. In the first experiment, subjects identified by name the fully frontal or profile poses of briefly familiarised individuals less efficiently than an intermediate pose. In addition, in a matching experiment using faces seen in different poses, it was found that one specific intermediate pose (corresponding to 22.5 degrees of angle from the full frontal view) was matched more efficiently in the right visual field (RVF) than in the left visual field (LVF). This finding supports the hypothesis of a superiority of the left hemisphere (LH) over the right hemisphere (RH) in processing a familiar face's canonical view. The other tested "noncanonical" views (i.e., full frontal, 45 degrees, and profile) of these same familiar faces were better matched in the LVF (i.e., the RH); especially at low levels of familiarity. We conclude that, for each familiar face, a viewer-centred representation of the canonical (22.5 degrees ) view is stored in the LH's memory system, whereas multiple views of familiar faces are stored in a memory system of the RH. With increasing levels of familiarity other views are increasingly more efficiently encoded by the LH, and in fact for facial self-recognition the full-front view is superior to any of the other tested views. These findings taken together suggest that complementary lateralised memory subsystems in the two cerebral hemispheres store different sets, only partially overlapping, of view-centred face representations.     

Alterations of the sense of "humanness" in right hemisphere predominant frontotemporal dementia patients.

OBJECTIVE: To evaluate the sense of "humanness" in frontotemporal dementia (FTD) patients with right hemispheric involvement. BACKGROUND: Early in the course, FTD is often asymmetric, and those with predominant right frontotemporal disease have disproportionate disturbances in social behavior and empathy. A disruption in a sense of humanness may underlie these behavioral disturbances. METHOD: Sixteen patients with asymmetric FTD on functional neuroimaging underwent recognition tests of facial masking, human-animal morphing, and facial distortion. Additional tests evaluated facial discrimination and the recognition of famous faces, facial emotions, and animate-inanimate differences. RESULTS: On the distorted and morphed face tasks, 8 FTD patients with predominant right hemisphere involvement were significantly more likely to call morphed and distorted faces "human" as compared with both 8 FTD patients with predominant left hemisphere involvement and normal controls. The FTD groups did not differ on thresholds for recognizing masked faces or on other face recognition measures. CONCLUSIONS: In FTD, right hemispheric involvement may alter the threshold for judging someone as human independent of the recognition of faces or facial affect. These results suggest that a specific sense of humanness facilitates a person recognition network in the right frontotemporal region of the brain.     

Evoked potential correlates of right hemisphere involvement in language recovery following stroke

A probe evoked potentials procedure was used to assess the relative engagement of both cerebral hemispheres during a language task in the following four groups of dextral adults: left hemisphere (LH)-damaged aphasics recovering from stroke, dysarthrics, right hemisphere (RH)-damaged nonaphasic patients, and normal control subjects. In agreement with previous findings using the probe procedure, the present results indicate greater task-specific RH activation in recovering aphasics and, to a lesser degree, dysarthric patients and greater LH activation in both nonaphasic, RH-damaged patients and normal control subjects. On the basis of these data, we suggest that increasing participation of the nondominant hemisphere may subserve restitution of language in adults sustaining LH lesions.     

Sex related effect of unilateral brain lesions on the perception of the Mueller-Lyer illusion.

The aim of this study was to investigate the effect of unilateral brain lesions on Mueller-Lyer (M-L) illusion in the two sexes. Patients with left hemisphere (LH) and right hemisphere (RH) damage and control subjects participated in the experiment. They inspected series of M-L patterns in which the shaft with out-going fins was gradually shortened until it induced a perception opposite to the original illusion, that is, the shaft with out-going fins appeared to be shorter than the shaft with in-going fins. The subjects' task was to decide, on each trial, whether the variable shaft was longer or shorter than the other one. The point where the judgements changed from one category to the other was established using the Spearman distribution method for determining psychophysical thresholds, and was considered the measure of the strength of the illusion. The higher the value of the threshold, the stronger the illusion. Our results showed sex-related hemispheric asymmetry in subjects' susceptibility to the M-L illusion, i.e., both LH and RH lesions in females, but only RH lesions in males resulted in an increase of the strength of illusion. Moreover, males with LH lesion as well as controls partially corrected the illusory perception with practice, while both LH and RH damaged females and RH damaged males did not show this learning effect.     

Enhanced right hemisphere involvement during cognitive processing may relate to intellectual precocity

In the present study, intellectually precocious and average ability youths performed a dichotic listening task (Experiment 1) and a free-vision chimeric face task (Experiment 2). Patterns of hemispheric lateralization and the relative involvement of the left and right hemispheres during cognitive processing were assessed. In Experiment 1, the average ability youths demonstrated a right ear/left hemisphere (re/LH) superiority for identification of CV syllables, while the gifted subjects failed to show any ear/hemisphere advantage. In Experiment 2, members of both groups tended to judge the leftside smile/rightside neutral half-faces as "happier", a pattern indicative of enhanced right hemisphere (RH) arousal. Notably, the degree of RH involvement was significantly greater in the gifted as compared to average ability youths. Moreover, laterality scores of the precocious on the chimeric face task correlated with their performance on the College Board Scholastic Aptitude test (SAT), i.e. the greater the leftward bias, the higher the SAT score. These findings, taken in composite, suggest that a high level of RH involvement during cognitive processing may be related to intellectual precocity.     

Sex differences in face processing are mediated by handedness and sexual orientation

Previous research has demonstrated sex differences in face processing at both neural and behavioural levels. The present study examined the role of handedness and sexual orientation as mediators of this effect. We compared the performance of LH (left-handed) and RH (right-handed) heterosexual and homosexual male and female participants on a face recognition memory task. Our main findings were that homosexual males have better face recognition memory than both heterosexual males and homosexual women. We also demonstrate better face processing in women than in men. Finally, LH heterosexual participants had better face recognition than LH homosexual participants and also tended to be better than RH heterosexual participants. These findings are consistent with differences in the organisation and laterality of face-processing mechanisms as a function of sex, handedness, and sexual orientation.     

Interaction between local and global visual orientation signals in subjects with unilateral brain lesions

Two groups of right handed, male stroke patients with lesions confined to the left (LH, n=10) or right (RH, n=10) cerebral hemisphere were tested on visual vertical judgements with isolated line stimuli and lines presented in the context of a tilted frame. The psychometric functions indicate no reduction in the precision of orientation judgements among the brain injured subjects when compared with age-matched controls (n=6) with cardiovascular disease, but the systematic shift in perceived vertical induced by a tilted visual frame was significantly larger for RH-subjects than for LH-subjects or controls (mean illusion 6 and 3° respectively). The results are interpreted within the “two visual systems”-theory of the rod-and-frame effect and it is suggested that the right hemisphere is dominantly involved in the integration of visual and vestibular input.     

Sex differences in callosal transfer and hemispheric specialization for face coding

Previous studies have shown a reduced lateralization of brain functions in women compared with men. Similarly, some studies have shown that the inter-hemispheric transfer (IHTT) of information is asymmetric in men, with faster latencies in the RH→LH compared with the LH→RH direction, and symmetric in women. The aim of the present study was to investigate IHTT and hemispheric lateralization during face processing in the two sexes. Event-related potentials (ERPs) were recorded in strictly right-handed people (16 men and 17 women) engaged in a face-sex categorization task. Occipital P1 and occipito/temporal N170 were left lateralized in women and bilateral in men. Overall the data suggest a certain involvement of the LH in face feature analysis (possibly related to sex-coding) in both sexes. N170 to contralateral stimuli was larger over the RH in men and the LH in women. IHTT was approximately 4 ms at the P1 level and approximately 8 ms at the N170 level. It was asymmetric in men, with faster latencies in the left visual field (LVF)/RH→LH (170 ms) direction than in the right-visual field (RVF)/LH→RH (185 ms) direction and symmetric in women. These findings suggest that the asymmetry in callosal transfer times might be due to faster transmission times of face-related information via fibers departing from the more efficient to the less efficient hemisphere. Overall, our findings also support the notion that the transfer time of visual inputs might be more rapid and symmetric in women than in men.     

Handedness is mainly associated with an asymmetry of corticospinal excitability and not of transcallosal inhibition.

OBJECTIVE: The study aims to compare transcallosal inhibition (TI), as assessed by the paired-pulse transcranial magnetic stimulation (TMS) technique, in a sample of right-handed subjects (RH) and left-handed subjects (LH). Motor thresholds (MTs) and motor evoked potential (MEP) amplitudes were also measured in the two groups, as an index of corticospinal activity. METHODS: Thirty-two normal subjects (16 RH and 16 LH) were recorded with a paired-pulse TMS paradigm (intensity of both pulses=120% of MT). The inter-stimulus intervals (ISIs) were 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 ms for both motor cortices, and MEP responses were recorded from the abductor digiti minimi muscles. RESULTS: Both groups showed a clear TI centred around the 12 ms ISI, but no difference was found as a function of handedness or of hemisphere. On the other hand, the two groups differed in terms of corticospinal activity, since the hand motor dominant hemisphere had lower MTs than the non-dominant one in LH, and larger MEP amplitudes for the right hand were found in RH. CONCLUSIONS: Results point to a functional asymmetry of the motor cortex on the hand-dominant versus the non-dominant hemisphere, while handedness does not seem associated with functional differences in callosal inhibition, as measured by the inter-hemispheric paired-pulse TMS technique.     

Schizophrenia and affective disorder show different visual scanning behavior for faces: a trait versus state-based distinction?

BACKGROUND: Abnormal visual scanpaths to faces and facial expressions in schizophrenia may underlie schizophrenic subjects' disturbed interpersonal communication. This study is the first to examine the specificity of these impairments to schizophrenia, by including an affective disorder psychiatric control group. METHODS: The visual scanpath performance of 65 schizophrenia, 52 affective disordered, and 61 control subjects were compared in two experiments. In the "face recognition" experiment, subjects viewed four identifiable (non-degraded) neutral faces versus four matched non-identifiable (degraded) control faces. In the "facial affect recognition" experiment, subjects viewed positive (happy), negative (sad), and neutral (control) facial emotion stimuli. Concurrent behavioral tasks were face matching (face recognition) and expression matching (facial affect recognition), each under two multiple-choice conditions (7 or 3 options). RESULTS: Scanpath disturbances were most apparent in schizophrenia subjects, who maintained a comparatively "restricted" scanpath style to all face stimuli. Schizophrenics subjects also showed the greatest recognition difficulties, particularly for neutral and happy faces. Scanpath parameters for affective disorder subjects differed only from the schizophrenia (but not the control) group, except for attention to facial features where they generally avoided facial features in all expressions and showed the greatest attentional problems of all groups for degraded faces. CONCLUSIONS: Our results suggest that a global restriction of visual scanpaths is specific to schizophrenic psychosis and might be a trait marker for this disorder, whereas scanpath abnormalities in affective disorder might instead reflect severe state-based (or discrete) attentional disturbances.     

The influence of face similarity in the case of the perception of morphed self-face

The purpose of the study was to examine if differences in behavioral effects in terms of facial self-recognition, compared to other identity recognition (familiar, strange) exist. Morphed versions of three facial identities were used in the experiment. The subject's own face was morphed with an unknown identity. A face of a highly familar person and of a stranger were also morphed in the same manner. This morphing procedure was repeated six times for each identity, but with six different unknown faces, in which three of the unknown faces were rated as being similar and the other three as dissimilar. The reaction times and categorical boundaries were then measured. The major finding of the study was that there were significant delayed mean reaction times for the morphed images of version "self versus similarly rated unknown faces" in contrast to the images of "self versus dissimilarly rated unknown faces" only. No significant differences were found in any of the other morphed versions. The authors interpret this result as being uniquely associated to the facial self-recognition and suggest that this association might be related to the "self effect."     

The influence of face similarity in the case of the perception of morphed self-face

The purpose of the study was to examine if differences in behavioral effects in terms of facial self-recognition, compared to other identity recognition (familiar, strange) exist. Morphed versions of three facial identities were used in the experiment. The subject's own face was morphed with an unknown identity. A face of a highly familar person and of a stranger were also morphed in the same manner. This morphing procedure was repeated six times for each identity, but with six different unknown faces, in which three of the unknown faces were rated as being similar and the other three as dissimilar. The reaction times and categorical boundaries were then measured. The major finding of the study was that there were significant delayed mean reaction times for the morphed imges of version "self versus similarly rated unknown faces" in contrast to the images of "self versus dissimilarly rated unknown faces" only. No significant differences were found in any of the other morphed versions. The authors interpret this result as being uniquely associated to the facial self-recognition and suggest that this association might be related to the "self effect."     

FMRI correlates of execution and observation of foot movements in left-handers

While several studies have assessed the brain patterns of cortical activations following executed and observed movements of the dominant and non-dominant lower limbs in right-handed (RH) subjects, the functional correlates of foot movement in left-handed (LH) subjects have not been investigated, yet. We investigated brain function lateralization during action execution and observation with the dominant and non-dominant feet in 8 left-handers (LH). Thirteen right-handers (RH) were also studied while performing the same tasks with their right-foot.Compared to left-foot movement, during right-foot movement, LH had greater activations of the left primary sensorimotor cortex (SMC) and the right cerebellum. Compared to right-foot movement, during left-foot movement, LH subjects activated areas of the sensorimotor network, the mirror-neurons system (MNS) and the visual network lateralized to the contralateral hemisphere. During right-foot movement no between-group difference was found.LH had a pattern of activations lateralized to the right hemisphere during right-foot observation and to the left hemisphere during left-foot observation. Compared to left-foot observation, during right-foot observation, LH had greater activations of frontal and parietal regions and visual areas. The opposite contrast showed higher activation of the right lateral occipito-temporal cortex in LH during left-foot observation. During right-foot observation, compared to RH, LH had greater activations of the bilateral primary SMC and of MNS and visual system regions.In LH, the performance of simple motor acts with the dominant lower limb might be achieved through a complex adaptation and interaction between different neuronal pathways and the daily-life environment.     

Categorical-semantic and spatial-imagery judgements of non-verbal stimuli in the cerebral hemispheres.

A divided visual field task was given to two groups of normal subjects to investigate hemisphere differences in the processing of standardised pictorial stimuli. There were two conditions: subjects were asked to decide whether an entity represented by a picture was living or non-living, a task involving a categorical judgement based on semantic information, or, in the second condition, whether these depictions represented entities which were bigger or smaller than a cat. This latter task, it is suggested, requires visual imagery to compare spatial dimensions. The first, categorical task produced an LH advantage in reaction time. The second, imagery task, produced an RH advantage provided the comparison involved items whose sizes were relatively close to that of a cat. Furthermore, the size difference was inversely related to reaction time, only when items were presented to the RH. The data obtained are consistent with the notion that there are at least two systems for processing visual information, one specializing in categorical and semantic distinctions related to LH functions, and the other, specialized in spatial coordinates, an aspect of visual imagery, related to the RH.     

A tale of two recognition systems: implications of the fusiform face area and the visual word form area for lateralized object recognition models

Two areas of current intense interest in the neuroimaging literature are that of the visual word form area (VWFA) and of the fusiform face area (FFA) and their roles in word and face perception, respectively. These two areas are of particular relevance to laterality research because visual word identification and face identification have long been shown to be especially lateralized to the left hemisphere and the right hemisphere, respectively. This review therefore seeks to evaluate their significance for the broader understanding of lateralization of object recognition. A multi-level model of lateralized object recognition is proposed based on a combination of behavioral and neuroimaging findings. Rather than seek to characterize hemispheric asymmetries according to a single principle (e.g., serial-parallel), it is suggested that current observations can be understood in terms of three asymmetric levels of processing, using the framework of the Janus model of hemispheric function. It is suggested that the left hemisphere represents features using an abstract-category code whereas the RH utilizes a specific-exemplar code. The relationships between these features are also coded asymmetrically, with the LH relying on associative co-occurrence values and the RH relying on spatial metrics. Finally, the LH controlled selection system focuses on isolating features and the RH focuses on conjoining features. It is suggested that each hemisphere utilizes efficient (apparently parallel) processing when stimuli are congruent with its preferred processing style and inefficient (apparently serial) processing when they are not, resulting in the typical left-lateralization for orthographic analysis and right-lateralization for face analysis.     

Semantically convergent and semantically divergent priming in the cerebral hemispheres: lexical decision and semantic judgment

The effects of semantically divergent and semantically convergent priming on the processing of lexical ambiguity by the two cerebral hemispheres were examined in two visual hemifield experiments. The experiments investigated the ability of the right hemisphere (RH) and the left hemisphere (LH) to summate activation from two single word primes followed by a laterally presented ambiguous target word. In a lexical decision task (experiment 1), the two priming words were either both related to the dominant meaning of the target (new, fresh-NOVEL), or to the subordinate meaning (story, book-NOVEL), or to one dominant and one subordinate meaning (new, story-NOVEL). Results indicated that the LH benefited most from semantically convergent primes that converged onto the dominant meaning of the ambiguous target word, whereas the RH benefited most from semantically mixed (divergent) primes, that diverge on alternate meanings of the ambiguous target word. We used the same stimuli in a semantic relatedness judgment task (experiment 2), and found that the facilitation in the RH was significantly larger when the primes were mixed than when both primes converged on a single (i.e. either dominant or subordinate) meaning of the to-be-presented target word. In contrast, the only facilitation found in the LH was when the two primes were associated with a single meaning (either dominant or subordinate) of the to-be-presented target word. When the primes were mixed, there was no facilitation in the LH. These results support previous findings indicating that during word recognition, the RH activates a broader range of related meanings than the LH, including alternate meanings of ambiguous words. Thus, by summating activation for seemingly incongruous elements, the RH may be critically involved in at least one important component of verbal creativity.     

Reading disorders and weak Verbal IQ following left hemisphere stroke in children: No evidence of compensation

Children who suffer a left-hemisphere (LH) stroke do not appear to experience the verbal deficits that are apparent in adults with similar lesions. To explain these findings it has been suggested that the plasticity of the developing brain enables homologous areas in the right hemisphere (RH) to compensate for LH weakness. In this paper we find evidence against this theory from investigations into the effect of LH and RH lesions on intelligence and literacy skills in a group of children with unilateral postnatal stroke. At group level, there was no significant effect of LH stroke on Verbal IQ (VIQ) or upon reading ability, when compared with RH stroke. But at the individual level, the majority of children with LH stroke achieved standard scores below average, with some below −2 SD. The standard scores of children with RH lesions were normally distributed, showing no detrimental effect that would indicate compensation. Large discrepancies between VIQ and Performance IQ (PIQ), in both directions, were also a characteristic of the LH group. Using normative data that we collected on the reading of irregular and regular words and nonwords, we identified patterns of phonological and surface dyslexia in both lesioned groups. We argue that LH stroke reduces the efficiency of verbally-related skills, but that the processes available to children with postnatal lesions are those available to normally developing children, and that these can entail processes in either hemisphere.     

Unattended facial expressions asymmetrically bias the concurrent processing of nonemotional information

Planned and reflexive behaviors often occur in the presence of emotional stimuli and within the context of an individual's acute emotional state. Therefore, determining the manner in which emotion and attention interact is an important step toward understanding how we function in the real world. Participants in the current investigation viewed centrally displayed, task-irrelevant, face distractors (angry, neutral, happy) while performing a lateralized go/no-go continuous performance task. Lateralized go targets and no-go lures that did not spatially overlap with the faces were employed to differentially probe processing in the left (LH) and right (RH) cerebral hemispheres. There was a significant interaction between expression and hemisphere, with an overall pattern such that angry distractors were associated with relatively more RH inhibitory errors than neutral or happy distractors and happy distractors with relatively more LH inhibitory errors than angry or neutral distractors. Simple effects analyses confirmed that angry faces differentially interfered with RH relative to LH inhibition and with inhibition in the RH relative to happy faces. A significant three-way interaction further revealed that state anxiety moderated relations between emotional expression and hemisphere. Under conditions of low cognitive load, more intense anxiety was associated with relatively greater RH than LH impairment in the presence of both happy and threatening distractors. By contrast, under high load, only angry distractors produced greater RH than LH interference as a function of anxiety.