Asymmetries for hemiface size and mobility

Lateral dominance and asymmetries in hemiface size and mobility were examined in 42 right- and left-handed males and females. In photographs, right hemifaces were found to be larger than left hemifaces. In videotapes of subjects making unilateral facial movements, a significant majority were judged to have greater left-sided facial mobility. Hemiface size and mobility were independent of sex and handedness, and unrelated to each other.     

Left-sided oral asymmetries in spontaneous but not posed smiles

To investigate possible facial asymmetries during the production of posed and spontaneous smiles, the displacement of various reference points on the mouth were measured as subjects produced both kinds of smiles. Strobe cameras were used in combination with a computer-based analysis to record the smiles of left- and right-handed males and females. The analysis revealed that the left side of the mouth moved more than the right side during spontaneous but not posed smiles, supporting the notion that the right hemisphere may play a special role in emotional expression. This asymmetry was most apparent in left-handed females and right-handed males. These sex and handedness differences are discussed with reference to apparent inconsistencies in previous research on asymmetries in emotional expression.     

Effects of handedness on tactile temporal order judgment

We examined effects of handedness on the judgment of temporal order of successive taps delivered to both hands. When the subjects’ arms were uncrossed, the temporal resolution (84% correct responses) of right-handed subjects (52 ± 4 ms, n = 16) was significantly better than that of left-handed subjects (83 ± 9 ms, n = 16). When their arms were crossed, both groups tended to invert their judgment to a similar extent at intervals as long as 200–300 ms. In the arms crossed condition, right handed subjects inverted their judgment more often in response to left-hand-first stimuli than to right-hand-first stimuli, whereas left-handed subjects did not show the same asymmetry. We infer that hemispheric lateralization, which is generally stronger in right- than in left-handed subjects, contributes to the relatively better temporal resolution of right-handed subjects in the uncrossed condition, as well as to the asymmetric effect of stimulation order in the crossed condition.     

The relationship between right-handed children's assessed and familial handedness and lateral specialization

Eighty right-handed males (6–12 yr of age) stratified into age x degree of handedness x family history of handedness groups were administered three dichotic (digit, syllable, environmental sound) tests. Older subjects accurately identified more stimuli than the younger subjects across tasks. There were significant laterality differences for both verbal and nonverbal stimuli between family history of handedness groups regardless of the subjects' age and degree of handedness. Those subjects with familial sinistrality had attenuated right-side advantage for verbal and non-verbal stimuli and decreased nonverbal accuracy scores compared to the familial dextral subjects who evidenced a right-ear (left hemisphere) advantage for verbal stimuli and a left-ear (right hemisphere) advantage for nonverbal.     

Computed tomographic scan hemispheric asymmetries in right- and left-handed male and female subjects

Patterns of computed tomographic scan asymmetries in the frontal and occipital regions of the brain were examined in 172 right- and left-handed male and female subjects; these patterns were compared with those published previously to determine which asymmetries were reliable across studies. The patterns observed appeared to be unrelated to handedness or sex and suggested that, especially in the posterior region of the brain, the majority of individuals have left hemispheres longer and wider than their right hemispheres. If anatomic asymmetries are indeed substrates for functional asymmetries, it appears more likely that they are related to at least some aspects of language dominance than to cerebral dominance for handedness.     

Handedness and Spatial Ability

Data from a large-scale study of cognitive abilities were used to test the hypothesis that there are handedness-related differences in spatial visualization. The spatial factor score of the cognitive test battery provided a measure of spatial visualization. Analyses revealed a sex × handedness × ethnicity interaction on the spatial factor, and no significant handedness-related differences on verbal factor. In all ethnic groups strongly left-handed males had higher spatial scores than strongly righthanded males, whereas strongly left-handed females had lower spatial scores than strongly right-handed females. Among subjects of Japanese or Chinese ancestry, strongly left-handed subjects differed from ambidextrous subjects as well as from right-handed subjects, and these differences, too, were of opposite sign for males and females. It is suggested that the use of spatial ability measures which are relatively insensitive to differences in right hemisphere ability, as well as the failure to attend to degree of handedness, ethnicity, and particularly gender, may be responsible for the negative findings in this area of research.     

The effect of sex,laterality and familial handedness on intellectual abilities

This study assessed the relationship of sex, familial handedness and laterality to the intellectual abilities of 116 right-handed 12–14-yr-old subjects. Verbal scores were predicted by an interaction between familial handedness and the subject's own laterality. A strong right ear advantage for verbal stimuli and a large lateral difference between the ear advantage for verbal and nonverbal stimuli predicted high Verbal Intelligence scores for the strong right hand-eye preference subjects with family histories of dextrality. Conversely, attenuated ear advantage scores for verbal stimuli predicted high Verbal Intelligence scores for the strong right hand-eye familial sinistral subjects. Although the results were less consistent for Performance Intelligence scores, they were significantly and positively related to assessed hand-eye preference, and a significant interaction between familial handedness and the subject's own laterality was observed for the Object Assembly subtest. A strong left ear advantage for nonverbal stimuli predicted high scores for the strong right-preference familial dextral subjects and attenuated ear advantage for nonverbal stimuli predicted high scores for the strong right-preference familial sinistral subjects.     

Reaction time to word meaning and ink color of laterally-presented stroop stimuli: effects of handedness and sex

In two experiments, subjects pressed a key labeled Red or Green in response to a 100 msec duration stimulus presented to the left or right visual field. In Experiment I, subjects responded to the meaning of Stroop words; the stimulus was the word Red or Green printed in red, green, or white ink. In Experiment II, subjects responded to ink color; the stimulus was either the word Red or Green printed in red or green ink or a red or green color patch. In each experiment, there were 20 strongly right-handed subjects and 20 strongly left-handed subjects. Half the subjects in each handedness group were male and half were female. In both experiments, RT was faster when words were presented to the right visual field than to the left visual field, suggesting that both meaning and ink color of Stroop words were processed more quickly in the left hemisphere. Results of both experiments revealed faster reactions when meaning and ink color of the Stroop words were congruent than when they were not. A comparison with baseline trials indicated that the RT difference between responses to congruent and incongruent Stroop words was due to the incongruent cue interfering with information processing rather than to the congruent cue facilitating processing. Hypothesized interactions between stimulus position, congruence, handedness and sex were not significant.     

Can expectancy influence hemispheric asymmetries?

We carried out three experiments with the aim of verifying a critical assumption of Kinsbourne's (Acta Psychol., 33 (1970), 193-201; Attention and Performance V, London: Academic press, (1975), pp. 81-96) 'dynamic' attentional hypothesis of hemispheric asymmetries, namely, that asymmetries arise only when subjects know in advance what type of stimulus and/or cognitive mode they are about to be engaged with. We used a paradigm modified from Posner (J. Exp. Psychol., 109 (1980), 160-174) to study the effects of non-spatial 'cognitive' cueing on hemispheric asymmetries using a lexical decision and a visuo-spatial discrimination task (acute vs. obtuse angles). While we did not find significant overall hemispheric asymmetries with the spatial material, we found a consistent advantage of the left hemisphere in the lexical decision task. In Experiment 2 where the cue was presented in central vision and only the stimuli were lateralised and in Experiment 3 where both cue and stimuli were lateralised to the same hemisphere, the left hemisphere advantage did not interact with the effect of cueing. In contrast, in Experiment 4, where only the cue was lateralised and the stimuli were centrally presented, the left hemisphere advantage in the lexical decision task emerged only following invalid cueing. While the results of Experiments 2 and 3 are not in keeping with Kinsbourne's hypothesis, the result of Experiment 4 shows that some pre-exposural mechanisms may indeed affect the emergence of hemispheric asymmetries. A differential susceptibility in 'disengaging' from the processing mode induced by an invalid cue might represent another interesting example of hemispheric difference.     

Perceptual asymmetry on the dichotic fused words test and cerebral speech lateralization determined by the carotid sodium amytal test

The rhymed fused dichotic words test was administered to 61 epileptic patients whose lesions were atrophic and predominantly unilateral. Subjects were categorized according to the side of speech representation, as determined by intracarotid sodium Amytal injection (left-hemisphere representation, 35 subjects; right-hemisphere representation, 4 subjects; bilateral representation, 22 subjects). Results indicated that 33 of the patients with left-hemisphere speech representation obtained right-ear advantages, and all four of the patients with right-hemisphere speech representation obtained left-ear advantages. The distribution of scores for patients with bilateral speech straddled the zero ear-advantage point, but overlapped both of the other two groups to some extent. Neither handedness, familial handedness, sex, nor side of epileptogenic focus had any significant effect on the observed asymmetries. It was also found that stimulus dominance effects have an important influence on the results, and must be taken into account in the interpretation of dichotic listening asymmetries. It is concluded that this test yields valid estimates of speech lateralization; it is also apparent that the magnitude of ear advantage may be an important variable insofar as the more extreme asymmetries appear to be exclusively associated with speech dominance of the contralateral hemisphere, whereas small-magnitude asymmetries are often associated with bilateral speech representation.     

Verbal and nonverbal auditory processing among left- and right-handed good readers and reading-disabled children

Cerebral lateralization of left- and right-handed good readers and left- and right-handed reading disabled was examined with a sample of 60 children who ranged in age from 7-13 years via a dichotic selective attention task (free recall, directed left, directed right) using consonant-vowel (CV) and tonal stimuli. Several ANOVAs were conducted to evaluate gender, reader group, handedness, and stimuli effects of left- and right-ear reports across dichotic conditions. Results indicated males outperformed females across stimuli and conditions regardless of handedness and all subjects recalled more tonal stimuli than CV stimuli. More importantly, the expected REA (left hemisphere processing) was found for CV stimuli only by right-handed good readers across all three dichotic conditions. The left-handed good readers and left-handed reading-disabled children were left ear (LE) dominant in free recall and in the directed left condition, but were right ear (RE) dominant in the directed right condition. Conversely, right-handed reading-disabled children produced a REA during free recall and directed right conditions, but were LE dominant in the directed left condition. In contrast, a significant LEA (right hemisphere processing) was found for tonal stimuli across all dichotic conditions for all four groups. These findings lend support to the hypothesis that attentional factors have a greater influence on auditory processing of verbal than nonverbal stimuli for various groups of children and also suggest reversed or bilateralized processing abilities for language in strongly left-handed children with sinistral relatives.     

Right hemisphere language and cognitive deficit in sinistrals?

Ninety-six student subjects were studied in a strictly balanced design contrasting sex, handedness and familial dextrality/sinistrality. WAIS scores were obtained, and two tachistoscopic tasks (RT responses) administered, involving laterally presented verbal stimuli (word/non-word decisions with high frequency, concrete, imageable and low frequency, abstract non-imageable words), and a face discrimination task designed to generate right hemisphere processing. There was a striking WAIS Performance deficit among sinistrals, particularly familial; familial dextrals proved overall fastest; directional asymmetries were weakest for the sinistrals, particularly the non-familial; non-familial dextrals were no less lateralized than familial dextrals; parental handedness was a better predictor of performance than sibling handedness; and no differential pattern of field asymmetries appeared for the two classes of word stimuli. Some theoretical implications of these findings are discussed.     

Effect of handedness on the Stroop Colour Word Task

Handedness is associated with cerebral organisation, but its relationship with cognition remains unclear. Since the Stroop task is believed to measure aspects of executive control, this study aims to investigate the effect of handedness on Stroop interference. We used the Stroop task with 90 young adults with university education, of whom 47 (23 males) were right-handed and 43 (21 males) were left-handed. Main dependent variables were Stroop baseline (SB), Stroop incongruent (SI), and the proportional derivative Stroop reduction (SR) [SR=(SB ? SI)/SB×100%] (Bugg, Delosh, Davalos, & Davis, 2007; Graf, Uttl, &Tuokko, 1995) scores. The analysis revealed that SI is significantly affected by both handedness and the interaction of sex×handedness, whereas SR is only affected by handedness. After controlling for the effect of SB on SI, only the effect of handedness remained statistically significant [F(1, 83) = 6.44, p=.013]. Post-hoc comparisons showed that left-handed females performed significantly better than right-handed females on both SI (p=.003) and SR (p=.007). The data suggest that handedness is associated with cognitive function alterations, which lead to a smaller Stroop interference of left-handers irrespectively of sex, an effect that is more pronounced in the female subpopulation.     

Sex, handedness, and the morphometry of cerebral asymmetries on magnetic resonance imaging

Cerebral morphometry of linear measurements and hemispheric, frontal, temporal and parietal areas on MRI of 104 normal adults was analyzed for sex and handedness and right-left differences. The right hemisphere was significantly larger across all groups, but the left frontal width and area was larger in left handers. MANOVA showed separate effects of hand and sex for linear measures; right-handers had larger right anterior frontal, and left parietal and occipital widths. Ratios of L - R/L + R eliminated sex differences related to overall brain size, however, a significant sex-by-hand interaction was shown by the parietal area ratios, indicating larger left side in right-handed males and left-handed females. A 3-dimensional 'torque' was postulated, where the upper and rostral portions of the hemispheres were larger on the right and the lower and caudal parts equal, or slightly larger on the left. The relationship of anatomical asymmetries, sex and handedness is multi-dimensional, and may explain some of the variability of normal cognitive functions and deficits after brain damage.     

Choosing sides: On the variability of language lateralization in normal subjects

We report test-retest reliabilities and individual asymmetries for a lateralized lexical decision task. Although acceptable reliability was found for word recognition, most subjects did not show statistically significant asymmetries, despite a robust right visual field group advantage. Inter-subject variability was unrelated to sex, handedness, or familial sinistrality. We offer some suggestions as to why these differences are to be expected in the study of normal populations.     

Verbal dichotic listening in right and left-handed adults: laterality effects of directed attention

Dichotic listening performance of consonant vowel stimuli was studied in 51 adult right- and left-handers in three attention conditions: non-directed and directed to either the right or left ear. In the non-directed condition, a significant right-ear advantage was found in both handedness groups with a stronger asymmetry in right-handers. There are at least three explanations for this ear bias. The classic or structural hypothesis suggests that to the right ear projects more strongly to the language dominant left hemisphere. The callosal relay hypothesis is based on the influence of inhibitory connections via the corpus callosum. The attentional hypothesis suggests that each hemisphere primarily directs attention to contralateral space and because the left hemisphere is dominant for language in both groups, and is aroused by speech stimuli, attention is primarily directed to the right ear. Neither hypothesis can explain why greater than 95% of right-handers have left hemisphere language dominance, but only 70-80% have a right ear bias. Our results demonstrate that in the directed attention conditions both groups increased their lateral biases when directed to either the right or left. The classic or structural hypothesis cannot account for these changes, thereby providing support for the attentional hypothesis. In addition, the right-handed subjects exhibited a greater shift of bias than did the left-handed subjects, when directing their attention leftward. This finding suggests that right-handed people are better able to shift their attention than left-handed people.     

Handedness is modulated by sex and self-perception

The distribution of handedness is different for men and women. Less pronounced right- or left-handedness in men is well established and often attributed to direct effects of genetic factors. Many studies observing this sex difference assessed handedness via questionnaire. It may therefore be influenced by a sex-specific bias in self-perception. Permanent inadequate self-perceptions are key characteristics of hypochondriasis. The study therefore tested 1017 participants (614 females) on two standard questionnaires to assess handedness and hypochondriasis: the Edinburgh Inventory and the Whiteley Index. Effects of sex, hypochondriac traits and the direction of handedness (left-handers; right-handers) on the degree of handedness (DH; the strength of lateralisation) were tested with multivariate linear regression. In confirmation of previous results, the DH is lower (less pronounced) in right-handed males than right-handed females, but is similar across sexes in left-handers. Regression analyses showed that for right-handers, male sex and higher hypochondriac traits are independent predictors of lower DH. For left-handers, main effects of sex and hypochondriac traits are not significant whereas a significant interaction of these two factors indicates that in left-handed men higher hypochondriac traits are associated with a differentially stronger shift towards lower DH compared to left-handed women. In conclusion, the DH is modulated by sex-specific effects of self-perception for left-handers but not right-handers. The implications of this finding on current theories of the inheritance of handedness are discussed. The assessment of hypochondriac traits might be useful to control response bias in questionnaire-based studies of human handedness.     

Handedness is modulated by sex and self-perception

The distribution of handedness is different for men and women. Less pronounced right- or left-handedness in men is well established and often attributed to direct effects of genetic factors. Many studies observing this sex difference assessed handedness via questionnaire. It may therefore be influenced by a sex-specific bias in self-perception. Permanent inadequate self-perceptions are key characteristics of hypochondriasis. The study therefore tested 1017 participants (614 females) on two standard questionnaires to assess handedness and hypochondriasis: the Edinburgh Inventory and the Whiteley Index. Effects of sex, hypochondriac traits and the direction of handedness (left-handers; right-handers) on the degree of handedness (DH; the strength of lateralisation) were tested with multivariate linear regression. In confirmation of previous results, the DH is lower (less pronounced) in right-handed males than right-handed females, but is similar across sexes in left-handers. Regression analyses showed that for right-handers, male sex and higher hypochondriac traits are independent predictors of lower DH. For left-handers, main effects of sex and hypochondriac traits are not significant whereas a significant interaction of these two factors indicates that in left-handed men higher hypochondriac traits are associated with a differentially stronger shift towards lower DH compared to left-handed women. In conclusion, the DH is modulated by sex-specific effects of self-perception for left-handers but not right-handers. The implications of this finding on current theories of the inheritance of handedness are discussed. The assessment of hypochondriac traits might be useful to control response bias in questionnaire-based studies of human handedness.     

Sex differences in asymmetry of the planum parietale in chimpanzees (Pan troglodytes)

Magnetic resonance images were collected in 76 chimpanzees and the sylvian fissure was examined for the presence of a posterior bifurcation. A bilateral bifurcation of the sylvian fissure into an ascending and descending ramus was identified in 58 of the subjects. The posterior ascending ramus was measured in both hemispheres in order to evaluate the presence, magnitude, and direction of a planum parietale asymmetry. Statistical analysis revealed a main effect for sex. Specifically, females showed a significant rightward bias, whereas males did not. Moreover, an examination of posterior bifurcation patterns of the sylvian fissure revealed differences between the left and right hemispheres. In humans, subject handedness and sex have been found to have an effect on planum parietale asymmetry. To determine if this was also the case in our chimpanzee subjects, we evaluated whether or not planum parietale asymmetry was related to subject handedness. Although subject handedness was not directly related to planum parietale asymmetry quotients, whether or not the sylvian fissure bifurcated bilaterally at its posterior end was influenced by the handedness of the subjects. These results support the view that asymmetries in the perisylvian language areas were present in the common ancestor of humans and chimpanzees.     

Recognition of fused dichotic words: an examination of the effects of head-turn and perceived spatial position

Ear advantages for verbal stimuli were determined for male and female right-handed subjects using a fused dichotic words test. Subjects performed the word recognition task with their heads turned 90 degrees to the left, 90 degrees to the right, and straight ahead. In addition, subjects provided subjective judgements with regard to the perceived spatial position of the sound. A significant right ear advantage was evidenced under all three head turn conditions, although the magnitude of the REA was reduced in the right head turn condition. There was no significant effect of sex. Judgements of perceived spatial position indicated that subjects perceived the fused sound as coming from the centre of the head, regardless of the head's orientation with respect to the body. These findings suggest that ear of entry rather than the perceived position of a sound source is the major factor in determining the perceptual asymmetries observed with dichotic stimuli.