Handedness and lexical skills in undergraduates.

The right shift (RS) theory of handedness and cerebral dominance (Annett, 1972, 1985) suggested that individual differences for patterns of cerebral dominance may be associated with different types of risk to cognitive functions. The higher prevalence of dextrality than sinistrality in humans depends on a single gene (RS+) which facilitates left hemisphere specialisation for speech but at the expense of some right hemisphere function (Annett, 1992b; Kilshaw and Annett, 1983). RS-- genotypes have random asymmetries for hand and brain while RS++ genotypes have strong right shift. Three samples of undergraduates were given tests of phonological processing and of real word homophone discrimination. As predicted, phonology was poor in comparison with homophone discrimination toward the left of the laterality continuum while the reverse was true to the right of the continuum. When groups with specific difficulty for each type of test in comparison with the other were distinguished, those with poor phonology were less dextral and those with poor homophone discrimination were more dextral for hand preference than the rest of the sample. Siblings differed for handedness also, consistent with the argument for a genetic influence.     

The theory of an agnosic right shift gene in schizophrenia and autism.

The right shift (RS) theory (Annett, M., 1972. The distribution of manual asymmetry. Br. J. Psychol. 63, 343-358; Annett, M., 1985. Left, Right, Hand and Brain: The Right Shift Theory. Lawrence Erlbaum, London) suggests that the typical pattern of human cerebral and manual asymmetries depends on a single gene (RS+) which impairs speech-related cortex of the right hemisphere. The theory offers solutions to several puzzles, including the distribution of handedness in families (Annett, M., 1978. A Single Gene Explanation of Right and Left Handedness and Brainedness. Lanchester Polytechnic, Coventry; Annett, M., 1996. In defense of the right shift theory. Percept. Motor Skills 82, 115-137), relations between handedness and cerebral speech laterality (Annett, M., 1975. Hand preference and the laterality of cerebral speech. Cortex 11, 305-328; Annett, M., Alexander, M.P., 1996. Atypical cerebral dominance: predictions and tests of the right shift theory. Neuropsychologia 34, 1215-1227) and handedness and dyslexia (Annett, M. et al., 1996. Types of dyslexia and the shift to dextrality. J. Child Psychol. Psychiatry 37, 167-180). If Crow's (Crow, T.J. et al., 1989. Schizophrenia as an anomaly of development of cerebral asymmetry. A postmortem study and a proposal concerning the genetic basis of the disease. Arch. Gen. Psychiatry 46, 1145-1150; Crow, T.J., 1997. Is schizophrenia the price that Homo sapiens pays for language? Schizophr. Res. 28, 127-141) theory that schizophrenia is due to an anomaly of cerebral dominance is correct, and if the RS theory is correct, schizophrenia could be due to an anomaly of the RS+ gene. If the RS+ gene were at risk for a mutation which caused a loss of directional coding, the mutant could be described as 'agnosic' for left and right. Such a gene would impair either hemisphere at random. When paired with another RS+ gene, both hemispheres would be impaired in 50% of cases. The other 50% and people in whom the agnosic gene is paired with an RS-allele (neutral for asymmetry and not giving hemisphere impairment) would have one unaffected hemisphere and, thus, normal development. Quantitative predictions based on the RS genetic theory as previously developed, plus an agnosic mutant with frequency required to give schizophrenia in 1% of the population, are consistent with estimates of concordance for schizophrenia in relatives. Homozygotes of the agnosic mutant would occur at about the rate estimated for autism.     

Hemispheric differences in split-brain monkeys learning sequential comparisons

Twelve split-brain rhesus monkeys were tested for differences in the abilities of their two cerebral hemispheres to learn discriminations based on the comparison of sequentially-presented visual stimuli. Overall there was no generalized advantage for either the left or right hemisphere. There was, however, a significant correlation between each monkey's handedness and the hemisphere that learned more readily; the more proficient hemisphere tended to be contralateral to the pre- operatively preferred hand. This result raises the possibility that handedness in monkeys may be more closely related to cognitive processing than is usually believed.     

Pathological left-handedness revisited: dichotic listening in children with left vs right congenital hemiplegia.

Thirty-one children with right (n = 18) and left (n = 13) congenital hemiplegia were compared for incidence of hand- and foot-preference, eye-dominance, and familial sinistrality. In addition, they were tested with dichotic listening for correct reports of consonant-vowel syllables. The two groups of children were closely matched on IQ and sensory functioning. Children with mental retardation, or epileptic seizures were not included. The results showed that 89% of the left hemisphere impaired (LHI) children were left-handed, all of them preferred the left foot, and 72% were left eye-dominant. In the right hemisphere impaired (RHI) group, everyone (100%) preferred the right hand and foot, and 62% were right eye dominant. The dichotic listening results showed a significant right ear advantage (REA) in the RHI-group, and a significant left ear advantage (LEA) in the LHI-group. The results are discussed in the framework of pathological handedness and shifts in hemisphere control of language in children with early brain injury. It is argued that the homogeneous samples, except for the site of lesion, provides an interesting possibility to compare cognitive effects of left and right hemisphere impairment in children.     

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.     

Cerebral lateralization and cognitive deficits after congenital hemiparesis

The purpose of this study was to investigate whether and how handedness is related to the processes of cerebral lateralization and cognitive performance in children with congenital insult. Fifty-six children (31 males and 25 females) with congenital hemiparesis and 14 control subjects were investigated. Of these children, 32 had a left hemisphere lesion, and 24 children had a right hemisphere lesion. There were 30 right-handed, 23 left-handed, and three ambidextrous children in the study group. The neuropsychologic assessment was performed using the NEPSY (a developmental neuropsychological assessment of child development) test battery. We found that 41% of the hemiparetic children and 72% of the children with a left hemisphere lesion were left-handed. In children contralateral to lesion handedness (no evidence of interhemispheric transfer of functions), we found diffuse cognitive deficits with impaired language abilities and poor visuomotor and narrative memory processing. In contrast, children with ipsilateral to brain lesion handedness (interhemispheric transfer of functions) demonstrated minimal or moderate side-specific cognitive dysfunction. Right-handed children with a right hemisphere lesion had attention, spatial, and short-term memory problems; left-handed children with a left hemisphere lesion had receptive language and visuomotor difficulties. Handedness combined with neuropsychologic assessment is a reliable indicator of the processes of cerebral reorganization after early brain insult.     

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.     

Left ear advantages in detecting emotional tones using dichotic listening task in an Arabic sample

Advantages associated with the left ear (right brain hemisphere) have been reported in some studies. Of these, some have specifically suggested that the left ear has a more heightened ability to detect emotional tones. Meanwhile others have pointed to factors such as age and gender as potentially leading to manifestations of human laterality. This study investigates which brain hemisphere is more involved in emotional processing of auditory information in Arab participants. We aimed to replicate the previous studies because no single study has been done in the Arabic region previously. Additionally, people in this region prefer to use the right side of their body, e.g., hand, ear, foot, etc., for most daily tasks. To acquire data a dichotic listening task (DLT) was administered to 28 male and 23 female (Edinburgh, UK) university students aged 19 to 38; 13 were left-handed and 38 were right-handed. The results showed a significant left ear advantage in the auditory processing of emotional information. There was a significant negative correlation between ear preference and handedness. Left ear advantage related only to handedness. Thus right-handed participants were more likely than left-handers to have a left ear advantage. The relationship between ear preference and gender was non-significant. The conclusion that might be drawn from this study is that the left ear (right hemisphere) is more involved in emotional processing than the right ear (left hemisphere), especially for right-handed people.     

Direction of hand preference: the connection with speech and the influence of familial handedness

Taking familial handedness into account, right- and left-handers with differing degrees of hand preference were monaurally presented with verbal stimuli (CVs) to which they responded using their right and left hands at separate times. This reaction time design was used to ascertain the relative cognitive functioning capacity of each hemisphere for verbal processing. However, the results disclosed more about the determination of direction of hand preference than about cognitive processing, per se. It was found that in both strong right-handers and strong left-handers with an incongruent hand preference (i.e., own handedness incongruent with family history of handedness) direction of hand preference is the result of suppression of the nonpreferred hand in the left hemisphere. Strong right- and strong left-handers with a congruent hand preference (i.e., hand preference congruent with family history of handedness) appear to have a more direct hand preference-left hemisphere mechanism. The findings of the present study are used to form part of a new theory of hand preference determination.     

Good phonetic errors in poor spellers are associated with right-handedness and possible weak utilisation of visuospatial abilities

Poor spellers in normal schools, who were not poor readers, were studied for handedness, visuospatial and other cognitive abilities in order to explore contrasts between poor spellers with and without good phonology. It was predicted by the right shift (RS) theory of handedness and cerebral dominance that those with good phonology would have strong bias to dextrality and relative weakness of the right hemisphere, while those without good phonology would have reduced bias to dextrality and relative weakness of the left hemisphere. Poor spellers with good phonetic equivalent spelling errors (GFEs) included fewer left-handers (2.4%) than poor spellers without GFEs (24.4%). Differences for hand skill were as predicted. Tests of visuospatial processing found no differences between the groups in levels of ability, but there was a marked difference in pattern of correlations between visuospatial test scores and homophonic word discrimination. Whereas good spellers (GS) and poor spellers without GFEs showed positive correlations between word discrimination and visuospatial ability, there were no significant correlations for poor spellers with GFEs. The differences for handedness and possibly for the utilisation of visuospatial skills suggest that surface dyslexics differ from phonological dyslexics in cerebral specialisation and perhaps in the quality of inter-hemispheric relations.     

DIRECTION OF HAND PREFERENCE: THE CONNECTION WITH SPEECH AND THE INFLUENCE OF FAMILIAL HANDEDNESS

Taking familial handedness into account, right- and left-handers with differing degrees of hand preference were monaurally presented with verbal stimuli (CVs) to which they responded using their right and left hands at separate times. This reaction time design was used to ascertain the relative cognitive functioning capacity of each hemisphere for verbal processing. However, the results disclosed more about the determination of direction of hand preference than about cognitive processing, per se. It was found that in both strong right-handers and strong left-handers with an incongruent hand preference (i.e., own handedness incongruent with family history of handedness) direction of hand preference is the result of suppression of the nonpreferred hand in the left hemisphere. Strong right- and strong left-handers with a congruent hand preference (i.e., hand preference congruent with family history of handed ness) appear to have a more direct hand preference-left hemisphere mechanism. The findings of the present study are used to form part of a new theory of hand preference determination.     

Hemispheric differences in dichaptic scanning of verbal and spatial material by adult males and females

An experimental design based on the combination of dichaptic presentation associated to the Posner’s paradigm was adopted to investigate laterality effects for verbal and spatial non-linguistic stimuli in male and female adult normal subjects. In a grapheme similarity judgment task based on “Name Identity” a right hand/left hemisphere advantage was found. Conversely, laterality effects were neither observed when the task involved “Perceptually Identical” or “Different” letter pairs. In a further experiment, the same methodology was adopted to verify hemispheric effects with spatial non-linguistic material, and a significant advantage for the left hand/right hemisphere was observed. Contrary to many previous studies, no gender or gender × task effects have been detected in both experiments. The present results suggest the existence, also in the tactile domain, of a direct link between input type and the linguistic or non-linguistic processing to which the two hemispheres are devoted. The overall pattern of data seriously hampers Witelson’s [Cortex 10 (1974) 3] original hypothesis that letter stimuli presented in the tactile modality are primarily processed as spatial stimuli, and are therefore dependent on the right hemisphere functioning.     

On the probability of pathological right-handedness

The increased incidence of manifest left-handedness in retardates and epileptics has been attributed to a shift in handedness following early brain injury. Predictive models using this hypothesis have presumed an equipotentiality for shifts in hemispheric dominance whether a left or a right hemisphere is the originally dominant, injured hemisphere. This note demonstrates that the presumption of equipotentiality may be relaxed without significant effect upon the conclusions reached using the predictive models.     

Handedness and lateralised tympanic membrane temperature in relation to approach–avoidance behaviour in Garnett's bushbaby (Otolemur garnettii)

Studies of handedness suggest a relationship between hemispheric specialisation and emotional processing. Recently measures of lateralised tympanic membrane temperature (TMT) have identified similar relationships (i.e., the left hemisphere is involved in approach behaviour and the right hemisphere avoidance behaviour). In the present study we examined lateralised changes in TMT in response to social interaction in 10 Garnett's bushbabies. Additionally, we examined whether handedness could be used as a predictor of approach–avoidance tendencies. We found a positive association between temperature change and both allogrooming and affiliative approach. Social behaviour did not differ between right- and left-handed bushbabies. These findings are discussed in terms of existing theories of asymmetric emotional processing. Overall, the data suggest that there is a left hemisphere specialisation for processing approach-related behaviours, which is consistent with existing models of lateralised emotional processing. Our data also indicate that TMT is a reliable, cost-effective measure of cerebral activation that is less invasive and more practical than alternative measures such as EEG, PET, and fMRI.     

Hemispheric differences in tactuo-spatial processing

A tactual recognition task employing six random forms presented in each of eight 45° relative disorientations was given to ten normal blindfolded subjects. A consistent right hemisphere advantage was obtained. Pattern of scores, however, was similar for both hemispheres, suggesting that: (a) the right hemisphere is doing the processing; (b) the left hemisphere has access to the results of this; (c) the right hemisphere has access to the information gathered by the left hemisphere. The results suggest that information loss occurs during hemispheric transfer.     

Cognitive asymmetries after wakings from REM and NONREM sleep in right-handed females

Eleven right-handed females were awakened from REM and NONREM sleep and tested on three tests designed to measure functions attributed to the left hemisphere and on three tests designed to measure right hemisphere function. A significant shift in cognitive profile was found in the direction of right hemisphere dominance after waking from REM sleep, and left hemisphere dominance after waking from NONREM sleep. Comparison of the cognitive shifts in females to that observed in males revealed a significant interaction of gender and awakening condition. Females showed a larger increase in right hemisphere performance following awakening from REM sleep relative to NONREM sleep, and a smaller increase in left hemisphere performance following awakening from NONREM sleep relative to REM sleep. The reverse trend was found for males.     

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.     

Does left-handedness affect the pattern of cerebral blood flow during cognitive activity?

In most people the left hemisphere of the brain is dominant for language functions. Because of an increased incidence of atypical language lateralisation in the right hemisphere of left-handed neurological patients, more systematic studies on handedness and cerebral asymmetry of language have been undertaken. The present study is aimed at clarification of the relationship between handedness and language dominance in healthy subjects. Lateralisation was measured directly using functional transcranial Doppler sonography in left-handed subjects. Twenty-six individuals participated in the study. Three kinds of tasks were used, differing in the material involved and in appropriate strategies to be employed by the subjects. Two important parameters of the MCA blood flow were analysed: mean relative increase in the blood flow velocity (MDV) and specific patterns of cognitive task performance (i.e. performance profiles). Our results indicate that the incidence of the right hemisphere dominance for language depends on the degree of handedness, since only in the group of consistent left-handers language was right lateralised. In other left-handers both cerebral hemispheres were functionally equivalent. The measurement of MCA blood flow velocity changes using transcranial Doppler ultrasonography seems to be a simple and non-invasive method of assessing the functioning of the two hemispheres of human brain.     

Aphasia in left-handers: lesion site, lesion side, and hemispheric asymmetries on CT

Language in left-handed aphasics with left (n = 27) or right (n = 4) hemisphere lesion was examined. Left occipital CT asymmetry was the most common asymmetry and could not be used to predict handedness or which hemisphere, if damaged, would produce aphasia. When left hemisphere lesion sites were matched between eight left-handed and eight right-handed aphasics, there were no significant group differences on language measures. Two nonfluent cases with good comprehension and large right frontal, parietal, and temporal lesions appeared to have "Broca's area" in the right hemisphere, but "Wernicke's area" in the left. Results suggest separate hemispheric loci for handedness, speech output, and/or comprehension dominance in some left-handers.     

Symmetry detection in typically and atypically speech lateralized individuals: A visual half-field study

Visuospatial functions are typically lateralized to the right cerebral hemisphere, giving rise to a left visual field advantage in visual half-field tasks. In a first study we investigated whether this is also true for symmetry detection off fixation. Twenty right-handed participants with left hemisphere speech dominance took part in a visual half-field experiment requiring them to judge the symmetry of 2-dimensional figures made by joining rectangles in symmetrical or asymmetrical ways. As expected, a significant left visual field advantage was observed for the symmetrical figures. In a second study, we replicated the study with 37 left-handed participants and left hemisphere speech dominance. We again found a left visual field advantage. Finally, in a third study, we included 17 participants with known right hemisphere dominance for speech (speech dominance had been identified with fMRI in an earlier study; Van der Haegen, Cai, Seurinck, & Brysbaert, 2011). Around half of these individuals showed a reversed pattern, i.e. a right visual half-field advantage for symmetric figures while the other half replicated the left visual-field advantage. These findings suggest that symmetry detection is indeed a cognitive function lateralized to the right hemisphere for the majority of the population. The data of the participants with atypical speech dominance are more in line with the idea that language and visuospatial functions are lateralized in opposite brain hemispheres than with the idea that different functions lateralize independently, although there seems to be more variability in this group.