The H-reflex recovery curve from the wrist flexors: lateralization of motoneuronal excitability in relation to handedness in normal subjects

Spinal motor asymmetry was studied in relation to handedness. Hand preference was assessed by Oldfield's Questionnaire and Geschwind scores. hand skill was evaluated by a peg moving task. The motoneuronal excitability was assessed by the size and recovery curve of the H-reflex elicited by stimulation of the right and left median nerves. H-reflex was recorded by cup electrodes placed over the wrist flexors. The mean reflex latencies from the right and left sides were 23.4 and 23.2 ms, respectively. The mean amplitude of the maximum H reflex was significantly higher on the right side than the left side in right-handers without familial sinistrality (FS). There were no significant side differences in the amplitudes of H-reflexes of the right-handers with FS. A genetic factor was suggested to involve this left shift of spinal motor lateralization in right-handers with FS. The recovery curve studies showed that the motoneuronal excitability was higher on the right than the left in right-handers without FS. There was no excitability difference between the right and left sides of the right-handers with FS. In left-handers, the motoneuronal excitability was significantly higher on the left than the right side. Voluntary flexions of the wrist increased the H-reflex especially on the right side of the right-handers. There was no relationship between this corticospinal facilitation and baseline EMG activity. The H-reflex amplitude was found to be inversely correlated with hand skill in right-handers. It was concluded that motoneuronal excitability is associated with handedness and also depends on FS. It was suggested that small reflexes are especially suitable in fine motor control of rapid aimed-movements, as the well-established relation between the size of motor units and fine motor control.     

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.     

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.     

Temporal integration of form as a function of subject handedness and retinal locus of presentation

Two experiments are reported examining differences in perceptual processing as a function of subject handedness. Experiment 1 compared performance in the left vs right visual fields of right- vs left-handed subjects. Only right-handed subjects exhibited an interaction of visual field with spatial frequency (as mediated by interflash interval), suggesting that left-handers are not differentially lateralized for high vs low spatial frequency processing. Experiment 2 examined foveal performance in left-handers vs right-handers with or without familial sinistrality. Subjects with personal or familial sinistrality exhibited superior performance when the processing of low frequencies was required, suggesting that sinistrality may confer an advantage in the processing of lower spatial frequencies.     

The left brain determines the degree of left-handedness

The contribution of right- and left-hand skills to left-handedness was studied in 42 left-handed male subjects. Hand preference was assessed by the Edinburgh Handedness Inventory. Hand skill was assessed by a peg-moving task; 10 trials were given to each hand. Peg-moving times decreased linearly with each trial (visuomotor learning). Both hands exhibited equal learning capacities. The learning curves were the same for the left-hands of left-handers with and without familial sinistrality (FS). The right-hand of left-handers with FS was found to be slower than that without FS. The right- and left-hand skills and their learning curves were about the same in left-handers with right-hand writing, exhibiting no difference from the left-hand skill and learning curve of left-handers with left-hand writing. Right-hand skill decreased linearly as left-hand preference increased from -40 to -100; left-hand skill was not related to hand preference. Right-minus left (R-L) time for peg moving increased linearly with hand preference from -40 to -100. R-L time for peg moving linearly decreased as the right-hand skill increased; the left-hand skill was not associated with R-L time for peg moving. It was concluded that the right hand (left brain) determines left-handedness; the neural structures only on the left side exhibit pronounced plastic changes to genetic and environmental influences in left-handers.     

Intermanual coordination in relation to handedness, familial sinistrality and lateral preferences

Intermanual coordination assessed by alternating finger tapping and finger-tapping asymmetry were investigated in 105 healthy right- and 105 left-handers and related to handedness, familial sinistrality and lateral preferences (in hand-clasping, arm-folding and eyedness). Compared to right-handers, left-handers with less pronounced left-hand preferences (Subgroup B) showed higher values in intermanual coordination and lower values in finger-tapping asymmetry. Moreover, familial sinistrality and eyedness interacted with handedness effects. While in right-handers intermanual coordination was significantly higher in subjects with dominant left eye, in left-handers of the Subgroup B it was somewhat higher in those with dominant right eye. Higher values in intermanual coordination and reduced asymmetry in finger tapping may be associated with a greater bihemispheric control and better performance in fast bimanual movements.     

CORRELATIONS BETWEEN NONVERBAL INTELLIGENCE AND NERVE CONDUCTION VELOCITIES IN RIGHT-HANDED MALE AND FEMALE SUBJECTS

A neurological theory of intelligence suggesting a direct correlation between nerve conduction velocity and psychometric intelligence was tested. Cattell's Culture Fair Intelligence Test was used to asses the nonverbal intelligence (IQ) of subjects. The motor median nerve conduction velocity from right hand of males was positively correlated with IQ. In subjects with no familial sinistrality (FS–), the motor ulnar-nerve conduction velocity from the right and left hands of males negatively correlated with IQ; there were inverse correlations between IQ and nerve conduction velocity (motor median nerve from right, sensory median nerve from right and left) in females. In subjects with familial sinistrality (FS+), IQ directly correlated with nerve conduction velocity from motor median (right and left), sensory median (right), and motor ulnar (right) nerves, but only in males. The speed hypothesis and neurological theory of intelligence were not supported by these results, which, in contrast, emphasized the importance of sex and familial sinistrality in any theory of intelligence.     

Patterns of hand preference and unintentional injuries among Indian attempted hand switchers and hand non-switchers

This study examines the patterns of hand preference and unintentional injuries of attempted hand switchers and hand non-switchers. Data were collected from 3698 participants in Kharagpur, India, on measures of hand preference, hand switching, and unintentional injuries. The direction of left- or right-handedness was on the basis of hand used for the item “writing on paper” and the degree of handedness was based on the average score of remaining items in the handedness inventory. Results reveal that, among attempted hand-switchers, learned right-handers were not right-sided in hand continuum as the natural right-handers, but left-handers were left-sided as natural left-handers. With increasing age the learned right-handers become less right-sided and natural right-handers become more right-sided. Females (males) are found to be more right-handed than males (females) among learned right-handers (natural right-handers). On the direction of handedness, the learned right-handers have more than twice the risk of unintentional injuries than the natural right- and left-handers. On degree of handedness, the use of inconsistent left and both hands among natural left-handers, the use of inconsistent right and both hands among natural right-handers, and the use of weak right hand among learned right-handers increase their vulnerability to unintentional injuries. Any deviation from the genetic make-up in hand use elevates the risk of unintentional injuries, suggesting that one should not change the biological hand.     

The relationship between nonverbal intelligence, familial sinistrality and Geschwind scores in right-handed female subjects

I proposed that there might be a strong relationship between the psychological and motor systems, and argued that hand preference could be related to intelligence; higher IQs are to be expected in right-handers with familial sinistrality (FS) than without FS (Tan, in press). This hypothesis was tested in this work. Cattle's Culture Fair Intelligence Test was used to assess the ability of spatial reasoning in right-handed females. Hand preference was assessed by the Edinburgh Handedness Questionnaire; a laterality score (Geschwind score) was calculated for each subject. The sample from the Nursery High-school had a significantly lower mean IQ than that from the Medical Faculty. The incidence for the consistent right-handers was significantly higher in the sample with lower mean IQ than that with higher mean IQ. The incidence for the weak right-handers was significantly higher in the sample with higher mean IQ than that with lower mean IQ. The incidence for familial sinistrality was significantly higher in the sample with higher mean IQ than that with lower mean IQ. It was concluded that handedness, familial sinistrality, and intelligence are interrelated traits; an attenuation in cerebral asymmetry as a result of an increase in the right hemisphere's mental abilities, reflecting itself in weak right-handedness in conjunction with FS, could be a prerequisite for well-developed nonverbal intelligence.     

Correlations between nonverbal intelligence and nerve conduction velocities in right-handed male and female subjects

A neurological theory of intelligence suggesting a direct correlation between nerve conduction velocity and psychometric intelligence was tested. Cattell's Culture Fair Intelligence Test was used to asses the nonverbal intelligence (IQ) of subjects. The motor median nerve conduction velocity from right hand of males was positively correlated with IQ. In subjects with no familial sinistrality (FS-), the motor ulnar-nerve conduction velocity from the right and left hands of males negatively correlated with IQ; there were inverse correlations between IQ and nerve conduction velocity (motor median nerve from right, sensory median nerve from right and left) in females. In subjects with familial sinistrality (FS+), IQ directly correlated with nerve conduction velocity from motor median (right and left), sensory median (right), and motor ulnar (right) nerves, but only in males. The speed hypothesis and neurological theory of intelligence were not supported by these results, which, in contrast, emphasized the importance of sex and familial sinistrality in any theory of intelligence.     

Left-handers' smiles: asymmetries in the projection of a posed expression

Asymmetries in the expression of a posed smile and in a relaxed facial expression were observed in 24 left-handers. Neither writing position nor familial sinistrality predicted the variance of the results. Left-handers were found to smile more with their left than right side of face; an asymmetry which had previously been observed in right-handers. If anything, left-handers' smiles were more asymmetric, though in the same direction, than right-handers. When relaxed, however, the left-handers' face was judged more unhappy on its right than left side--a reversal of the direction of asymmetry previously noted in right-handers' relaxed expressions. No single neurological or psychological theory accounts for these results; it is suggested that hand preference may exert a myotonic effect which is reflected in judgements of relaxed facial expressions.     

DEVELOPMENT OF MOTOR CO-ORDINATION BY NORMAL LEFT-HANDED CHILDREN

Ninety-six normal left-handed children were tested for motor co-ordination on a series of timed repetitive tasks, alternating left and right hands, feet and fingers. Preferred hand advantage could be demonstrated to the same degree for the left hand of these children as it had been for the preferred right hand in previously studied groups of the same age (Denckla 1973, 1974). The only test (finger to thumb successive opposition) that failed to show a preferred hand advantage in left-handers also had failed to show such an advantage among right-handers. This same test was performed more rapidly by left-handed girls than boys, as it had been by right-handed girls. However, left-foot superiority appeared to be less well established among left-handers than superiority of the right foot among right-handers, and at a later age. Left-handed children were less likely than their right-handed counterparts to choose their preferred hand when trying out a new task. Writing posture, i.e. whether inverted or straight, could not be established unambiguously for the left-handed children up to the age of 10, and often differed for writing and printing.     

Reaction time differences between left- and right-handers during mental rotation of hand pictures

During mental rotation tasks using hand pictures, right-handers make left–right judgements by mentally rotating their own hand to an orientation of the presented hand image. Although strategy difference for the tasks between left- and right-handers has been suggested, the strategy of left-handers has been incompletely understood. In the present study we compared differences in reaction times between 15 left-handed and 16 right-handed normal individuals during a mental rotation task using simple hand pictures. Participants were required to identify pictures of a hand presented in four orientations (upright, counterclockwise rotated, clockwise rotated, and inverted) as either a right or a left hand. Right-handers recognised a right hand faster than a left hand, whereas no significant difference was seen for left-handers. Both left- and right-handers recognised a right hand faster than a left hand in counterclockwise-rotated images, and recognised a left hand faster than a right hand in clockwise-rotated images. The findings suggest that the differences in the reaction times between left- and right-handers depend on a laterality balance of hand motor skills. During mental rotation task using simple hand pictures, left-handers may mentally simulate their own hand to match the stimulus image as similar to right-handers.     

Cerebral organization of verbal and motor functions in left-handed and right-handed adults: effects of concurrent verbal tasks on unimanual tapping performance

Two experiments were conducted to study the interference effects of different concurrent verbal tasks on unimanual single-finger tapping and unimanual sequential finger tapping. Experiment 1 involved 24 right-handed university students. In the dual-task conditions, right-handers showed a greater right-hand than left-hand performance reduction for single-finger tapping, and an equal right-hand and left-hand reduction for sequential tapping. Experiment 2 involved 60 left-handed university students, divided into four groups according to familial sinistrality and writing hand posture. In the dual-task conditions, left-handers showed a greater left-hand than right-hand performance reduction for single-finger tapping, and an equal left-hand and right-hand reduction for sequential finger tapping. A dichotic listening task revealed a left hemispheric dominance for auditory linguistic functioning in most of the left-handers. Familial sinistrality and hand posture, on the whole, did not influence tapping performances. However, these factors influenced the ear asymmetries on the dichotic listening task. It is speculated that, with single-finger tapping, interference only takes place beyond the point of language motor programming, that is to say, at the motor areas and the supplementary motor areas of the cortex.     

Hand dominance for signing: clues to brain lateralization of language

Virtually all right-handed individuals are left hemisphere dominant for language. Sign languages of the deaf provide an unusual vehicle for exploring the link between handedness and hemispheric specialization for language since in sign language the hands themselves are the language articulators. Performance of the right and left hand was examined in deaf native users of American Sign Language (ASL) for speeded production of one-handed signs and for shadowing of signed discourse. Opposite patterns of asymmetries in hand performance were found in right- and left-handers. However, left-handers were more flexible than right-handers in signing with their non-preferred hand. Furthermore, unusual patterns of hand use for sign were found in a deaf signer with a left hemisphere lesion, possibly indexing increased mediation of the intact hemisphere. Implications for brain organization of language in a visual-gestural mode are discussed.     

Do environmental and hereditary factors affect the psychophysiology and left-right shift in left-handers?

The principal objective of this study was to investigate the reality of sinistrality in left-handed subjects. The subjects were assessed by a 20-item questionnaire with two groups taken from Oldfield's and Yetkin questionnaires. The relation of different effects on left-hand preference was studied in men and women considering familial information and writing hand. The degree of hand preference was determined by Geshwind scores (GSs). The GS degrees of hand preferences were divided into weak and strong left-hand ranging from -20% to -100%. After assessing the GSs we asked the subjects to answer 22 questions with different aims, which were especially written for only left-handers. Some of them were based on familial sinistrality and hereditary relation to left-handers. The others concerned hand, foot, eye, shoulder, and ear preferences; the psychology of left-handedness; the knowledge concerned with left-handedness; the success, ability, and interest education and skill, and morphological differences in hand and foot sizes. In the present study, it was found that the subjects have had at least one left-handed person in their family or among their relatives. Some subjects had shifted their left hand preference in favor of their right hands. Nevertheless, the left-handers have been found as a presence with their own peculiarities.     

Abstracts of platform and poster presentations

Abstract

Two experiments were conducted to study the interference effects of different concurrent verbal tasks on unimanual single-finger tapping and unimanual sequential finger tapping. Experiment 1 involved 24 right-handed university students. In the dual-task conditions, right-handers showed a greater right-hand than left-hand performance reduction for single-finger tapping, and an equal right-hand and left-hand reduction for sequential tapping. Experiment 2 involved 60 left-handed university students, divided into four groups according to familial sinistrality and writing hand posture. In the dual-task conditions, left-handers showed a greater left-hand than right-hand performance reduction for single-finger tapping, and an equal left-hand and right-hand reduction for sequential finger tapping. A dichotic listening task revealed a left hemispheric dominance for auditory linguistic functioning in most of the left-handers. Familial sinistrality and hand posture, on the whole, did not influence tapping performances. However, these factors influenced the ear asymmetries on the dichotic listening task. It is speculated that, with single-finger tapping, interference only takes place beyond the point of language motor programming, that is to say, at the motor areas and the supplementary motor areas of the cortex.     

A close relationship between hand skill and the excitability of motor neurons innervating the postural soleus muscle in right-handed female subjects

The claim that there is not a consistent inhibition of the H-reflex from the dominant leg was examined and rejected. Hand preference was assessed by the Edinburgh Inventory, and hand skill by the peg moving task. All the subjects were right-handed in preference and skill. It was re-established that there is an inverse relationship between hand skill and the excitability of motoneurons innervating the postural soleus muscle in right-handed female subjects without familial sinistrality. There was no significant difference between the recovery curves from the right and left sides in subjects with familial sinistrality. The H reflex from right leg was different between these right-handers. There was a positive linear correlation between the asymmetry index of hand skill greater than zero (right-hand dominance) and the asymmetry index for H-reflex recovery curve greater than zero (left dominance in reflex activity). It was concluded that there is a spinal motor asymmetry in postural leg muscles to handedness.     

The influences of handedness, sex, familial sinistrality and androgyny on language laterality, verbal ability, and spatial ability

This study assessed possible influences of handedness, sex, familial sinistrality (FS), and self-rated androgyny on language laterality and on spatial and verbal test performances of 225 right-handers and 134 left-handers. Left-handers and right-handers performed differently on all tasks. Right-handers were significantly more lateralized on the language laterality task and were superior to left-handers on the spatial visualization task. Left-handers obtained slightly, but significantly, higher vocabulary scores. Androgyny had a significant main effect on spatial ability, the more androgynous subjects being superior. Androgyny also interacted with sex and FS factors on the spatial task. All in all, present result are consistent with the conclusion that each of the subject factors studied may prove to be of some relevance for a complete understanding of laterality and cognitive ability patterns.     

Hemispheric dominance for speech and calculation: electrophysiologic correlates of left dominance in left handedness

Slow brain potentials were recorded in left-handers and right-handers during: (i) processing of language and mental arithmetic tasks, without vocalization, and (ii) subsequent writing down of the answers with either the right or left hand. Left-sided laterality of negative potentials was taken as evidence of hemispheric dominance. It appeared during the processing of words and numbers in 26 of the 30 left-handers and was localized mainly in the left frontal and temporal parietal regions. Similar results were found with the right-handers. This electrophysiological evidence indicates that the left hemisphere is dominant for language and calculation in the vast majority of left-handers. Only when writing with either their left or right hand do left-handers show less left-sided laterality than right-handers.