Sex and hemispheric differences in facial invariants extraction

This present study investigates sex differences in hemispheric cooperation during a facial identity matching task. The method used was a divided visual field paradigm in which the probe face was neutral or expressive and the target face was always neutral. Probe and target faces were presented both unilaterally and sequentially. A total of 28 right-handed women and 32 right-handed men participated in this study. The results confirm the women's advantage in face recognition and reveal symmetrical interhemispheric cooperation in women only. In men, processing time was faster when the probe face appeared in the left visual field-and encoded by the right hemisphere-and the target in the right visual field-projected to the left hemisphere-compared to the reverse direction. Interestingly, the data also show that women were not influenced by the expression of the probe face when matching identity, whereas men were always faster when the probe face was neutral, like the target, than when it was expressive. These results are discussed in light of Bruce and Young's (1986) model, and in terms of view-dependent and view-independent processes.     

Patterns of brain asymmetry in the perception of positive and negative facial expressions

The divided visual field technique was used to investigate the pattern of brain asymmetry in the perception of positive/approach and negative/withdrawal facial expressions. A total of 80 undergraduate students (65 female, 15 male) were distributed in five experimental groups in order to investigate separately the perception of expressions of happiness, surprise, fear, sadness, and the neutral face. In each trial a target and a distractor expression were presented simultaneously in a computer screen for 150 ms and participants had to determine the side (left or right) on which the target expression was presented. Results indicated that expressions of happiness and fear were identified faster when presented in the left visual field, suggesting an advantage of the right hemisphere in the perception of these expressions. Fewer judgement errors and faster reaction times were also observed for the matching condition in which emotional faces were presented in the left visual field and neutral faces in the right visual field. Other results indicated that positive expressions (happiness and surprise) were perceived faster and more accurately than negative ones (sadness and fear). Main results tend to support the right hemisphere hypothesis, which predicts a better performance of the right hemisphere to perceive emotions, as opposed to the approach–withdrawal hypothesis.     

Identity–expression interaction in face perception: Sex,visual field,and psychophysical factors

We investigated the psychophysical factors underlying the identity–emotion interaction in face perception. Visual field and sex were also taken into account. Participants had to judge whether a probe face, presented in either the left or the right visual field, and a central target face belonging to same person while emotional expression varied (Experiment 1) or to judge whether probe and target faces expressed the same emotion while identity was manipulated (Experiment 2). For accuracy we replicated the mutual facilitation effect between identity and emotion; no sex or hemispheric differences were found. Processing speed measurements, however, showed a lesser degree of interference in women than in men, especially for matching identity when faces expressed different emotions after a left visual presentation probe face. Psychophysical indices can be used to determine whether these effects are perceptual (A’) or instead arise at a post-perceptual decision-making stage (B”). The influence of identity on the processing of facial emotion seems to be due to perceptual factors, whereas the influence of emotion changes on identity processing seems to be related to decisional factors. In addition, men seem to be more “conservative” after a LVF/RH probe-face presentation when processing identity. Women seem to benefit from better abilities to extract facial invariant aspects relative to identity.     

Patterns of brain asymmetry in the perception of positive and negative facial expressions

The divided visual field technique was used to investigate the pattern of brain asymmetry in the perception of positive/approach and negative/withdrawal facial expressions. A total of 80 undergraduate students (65 female, 15 male) were distributed in five experimental groups in order to investigate separately the perception of expressions of happiness, surprise, fear, sadness, and the neutral face. In each trial a target and a distractor expression were presented simultaneously in a computer screen for 150 ms and participants had to determine the side (left or right) on which the target expression was presented. Results indicated that expressions of happiness and fear were identified faster when presented in the left visual field, suggesting an advantage of the right hemisphere in the perception of these expressions. Fewer judgement errors and faster reaction times were also observed for the matching condition in which emotional faces were presented in the left visual field and neutral faces in the right visual field. Other results indicated that positive expressions (happiness and surprise) were perceived faster and more accurately than negative ones (sadness and fear). Main results tend to support the right hemisphere hypothesis, which predicts a better performance of the right hemisphere to perceive emotions, as opposed to the approach-withdrawal hypothesis.     

Sex hormonal modulation of hemispheric asymmetries in the attentional blink.

The present study examines differences in functional cerebral asymmetries modulated by gonadal steroid hormones during the menstrual cycle in women. Twenty-one right-handed women with regular menstrual cycles performed a double-stream rapid serial visual presentation (RSVP) task, with one stream in each visual field, during the low steroid menses and the high steroid midluteal phase. They were required to detect a target item, and then a probe item, each of which could appear in either stream. If the probe item appeared 200 ms after the target, detection of the probe was impaired-a phenomenon known as the "attentional blink." This occurred in both streams in the midluteal phase, but only in the right visual field during menses. Thus low steroid levels appeared to restrict the attentional blink to the left hemisphere, while high levels of estradiol and progesterone in the midluteal phase appeared to reduce functional asymmetries by selectively increasing the attentional blink in the right hemisphere. This effect appears to be mediated by estradiol rather than progesterone, and it is compatible with the assumption of a hormone-related suppression of right hemisphere functions during the midluteal phase.     

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.     

Representations of facial identity in the left hemisphere require right hemisphere processing

A quintessential example of hemispheric specialization in the human brain is that the right hemisphere is specialized for face perception. However, because the visual system is organized contralaterally, what happens when faces appear in the right visual field and are projected to the nonspecialized left hemisphere? We used divided field presentation and fMRI adaptation to test the hypothesis that the left hemisphere can recognize faces, but only with support from the right hemisphere. Consistent with this hypothesis, facial identity adaptation was observed in the left fusiform face area when a face had previously been processed by the right hemisphere, but not when it had only been processed by the left hemisphere. These results imply that facial identity information is transferred from the right hemisphere to the left hemisphere, and that the left hemisphere can represent facial identity but is less efficient at extracting this information by itself.     

Asymmetries in spontaneous facial expressions and their possible relation to hemispheric specialization

Unilateral expressive facial movements, as documented by field observation and by careful analysis of video recordings in the laboratory, occur more on the left side of the face than on the right side in right-handed people. In one of our studies, but not in others, a larger proportion of females than males showed this bias. Left-handers show no consistent asymmetries. It is suggested that facial expressions are mediated more by the right hemisphere than by the left.     

Emotional attention in acquired prosopagnosia

The present study investigated whether emotionally expressive faces guide attention and modulate fMRI activity in fusiform gyrus in acquired prosopagnosia. Patient PS, a pure case of acquired prosopagnosia with intact right middle fusiform gyrus, performed two behavioral experiments and a functional imaging experiment to address these questions. In a visual search task involving face stimuli, PS was faster to select the target face when it was expressing fear or happiness as compared to when it was emotionally neutral. In a change detection task, PS detected significantly more changes when the changed face was fearful as compared to when it was neutral. Finally, an fMRI experiment showed enhanced activation to emotionally expressive faces and bodies in right fusiform gyrus. In addition, PS showed normal body-selective activation in right fusiform gyrus, partially overlapping the fusiform face area. Together these behavioral and neuroimaging results show that attention was preferentially allocated to emotional faces in patient PS, as observed in healthy subjects. We conclude that systems involved in the emotional guidance of attention by facial expression can function normally in acquired prosopagnosia, and can thus be dissociated from systems involved in face identification.     

Psychophysics reveals a right hemispheric contribution to body image distortions in women but not men

We tested the hypothesis that the right cerebral hemisphere contributes to the enhanced body image distortions seen in women when compared to men. Using classical psychophysics, 60 right-handed healthy participants (30 women) were briefly presented with size-distorted pictures of themselves, another person (an experimenter), and a non-corporal, familiar object (a coke bottle) to the central, right, and left visual field. Participants had to decide whether the presented stimulus was fatter or thinner than the real body/object, and thus compare the presented picture with the stored representation of the stimulus from memory. From these data we extracted the amount of image distortion at which participants judged the various stimuli to be veridical. We found that right visual field presentations (initial left hemisphere processing) revealed a general "fatter" bias, which was more evident for bodies than for objects. Crucially, a "fatter" bias with own body presentations in the left visual field (initial right hemisphere processing) was only found for women. Our findings suggest that right visual field presentation results in a general size overestimation, and that this overestimation is more pronounced for bodies than for objects. Moreover, the particular "fatter" bias after own body presentations to the left visual field in women supports the notion of a specific role of the right hemisphere in sex-specific body image distortion.     

Evidence for sex differences in right-hemisphere dominance for emotions

Discriminative reaction times to the six emotions that can more reliably be recognised in the human face were measured in 24 normal subjects (12 males and 12 females). Stimuli appeared to the right or the left of fixation point and the subject discriminated target from non-target emotions by a motor response. In the case of female subjects responses were faster with left visual field presentation, indicating a right hemisphere superiority in the discrimination. In the case of male subjects no consistent lateral asymmetry was found.     

Hemispheric differences for visual search: Serial vs parallel processing revisited

Subjects were tachistoscopically presented with arrays of two, three or four stimuli to the right or left hemisphere and judged whether all of the items were the same or whether one was physically different from the rest. Separate groups of right-handed subjects viewed letters of featurally similar symbols as stimulus items. Faster and more accurate responding was obtained for left hemisphere presentations for both same and different response judgments. Response time was independent of array size, with same judgments made faster than different judgments for both visual field conditions. Extensive practice shortened reaction time and decreased error rate, but did not change the pattern of hemispheric or judgment effects. Virtually identical results were observed for both stimulus conditions. These findings suggest that the left hemisphere can process information in parallel when the task situation requires featural analysis of stimulus materials.     

Reaction-times of normal subjects to monaurally presented verbal and tonal stimuli

Sixteen right-handed subjects responded to monaurally presented verbal and tonal stimuli with their right and left hands at separate times. Subjects' reaction-times to speech were significantly faster when they heard words in their right ears, while their reaction-times to tones proved significantly faster after left-ear stimulations. Reaction-times to speech were significantly faster when subjects responded with their right hands, but after tonal stimulations, left-handed responses were significantly faster. These results were interpreted as demonstrating consistent laterality effects—the faster response occuring after intrahemispheric processing. Furthermore, the interactions between ear stimulated and hand of response were suggestive of minor hemisphere processing.     

Hand preference and sex shape the architecture of language networks

In right-handed subjects, language processing relies predominantly on left hemisphere networks, more so in men than in women, and in right- versus left-handers. Using DT-MRI tractography, we have shown that right-handed men are massively interconnected between the left-hemisphere language areas, whereas the homologous in the right hemisphere are sparse; interhemispheric connections between the language areas and their contralateral homologues are relatively strong. Women and left-handed men have equally strong intrahemispheric connections in both hemispheres, but women have a higher density of interhemispheric connections.     

Processing of facial blends of emotion: support for right hemisphere cognitive aging

Clinical research on facial emotions has focused primarily on differences between right and left hemiface. Social psychology, however, has suggested that differences between upper versus lower facial displays may be more important, especially during social interactions. We demonstrated previously that upper facial displays are perceived preferentially by the right hemisphere, while lower facial displays are perceived preferentially by the left hemisphere. A marginal age-related effect was observed. The current research expands our original cohort to include 26 elderly individuals over age 62. Fifty-six, strongly right-handed, healthy, adult volunteers were tested tachistoscopically by flashing randomized facial displays of emotion to the right and left visual fields. The stimuli consisted of line drawings displaying various combinations of emotions on the upper and lower face. The subjects were tested under two conditions: without attend instruction and with instructions to attend to the upper face. Based on linear regression and discriminant analyses modeling age, subject performance could be divided into two distinct groups: Young (< 62 years) and Old (> 62 years). Without attend instructions, both groups robustly identified the emotion displayed on the lower face, regardless of visual field presentation. With instructions to attend to the upper face, the Old group demonstrated a markedly decreased ability to identify upper facial displays, compared to the Young group. The most significant difference was noted in the left visual field/right hemisphere. Our results demonstrate a significant decline in the processing of upper facial emotions by the right hemisphere in older individuals, thus providing partial support for the right hemisphere hypothesis of cognitive aging. The decreased ability to perceive upper facial displays coupled with age-related deficits in processing affective prosody may well cause impaired psychosocial competency in the elderly.     

Neural substrates of facial emotion processing using fMRI

We identified human brain regions involved in the perception of sad, frightened, happy, angry, and neutral facial expressions using functional magnetic resonance imaging (fMRI). Twenty-one healthy right-handed adult volunteers (11 men, 10 women; aged 18-45; mean age 21.6 years) participated in four separate runs, one for each of the four emotions. Participants viewed blocks of emotionally expressive faces alternating with blocks of neutral faces and scrambled images. In comparison with scrambled images, neutral faces activated the fusiform gyri, the right lateral occipital gyrus, the right superior temporal sulcus, the inferior frontal gyri, and the amygdala/entorhinal cortex. In comparisons of emotional and neutral faces, we found that (1) emotional faces elicit increased activation in a subset of cortical regions involved in neutral face processing and in areas not activated by neutral faces; (2) differences in activation as a function of emotion category were most evident in the frontal lobes; (3) men showed a differential neural response depending upon the emotion expressed but women did not.     

Hemispheric asymmetry and interhemispheric transfer in reaching programming.

The purpose of this study was to explore the intrahemispheric processes and the interhemispheric transfer that occur during the programming of a pointing movement. Twenty five subjects participated in this experiment: 12 were right-handed (Rhr), 12 left-handed (Lhr), and 1 was left-handed with a posterior callosal lesion. The task consisted in producing an open loop pointing response toward a visual target appearing briefly on the right or the left of a central fixation point. Reaction times (RTs) were shorter for the Rhrs when reaching with the left hand than with the right hand. No such hand-related difference was observed in the Lhrs. The left hand advantage indicates that one process was faster in the right hemisphere of Rhrs. This faster process appears not to be visual but motor or visuomotor. For either hand, responses were faster when the target appeared in the visual field homolateral to the pointing hand (uncrossed condition) than when it appeared contralaterally to the hand (crossed condition). The crossed vs uncrossed difference did not vary between Rhrs and Lhrs or between the hands. The transfer time between the hemispheres was symmetrical whatever its direction. The partially callosotomized left-handed subject was two-fold slower than the control Lhrs. His uncrossed responses were faster than the crossed ones, but his interhemispheric transfer time was very asymmetrical: it was normal from right to left hemisphere but was highly increased in the opposite direction. An attempt at modelling the RT data is proposed and the possibility of different callosal locations for the interhemispheric transfer is discussed.     

Neural correlates of conscious and unconscious vision in parietal extinction

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.     

Neural Correlates of Conscious and Unconscious Vision in Parietal Extinction

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.