Handedness measured by finger tapping: a continuous variable

Hand skill and hand preference in blind and sighted children

Congenitally blind and sighted blindfold children between the ages of 6 and 15 years were compared with each other for hand preferences and hand ability. All the children performed a 20-item hand preference test and every child performed three hand ability tasks: a sorting task, a finger dexterity task, and the Minnesota rate of manipulation task, each separately with the left and the right hand. Results indicated no differences between the hand preferences of the two groups. The sighted children were faster than the blind children on some of the hand ability tasks. There were no differences between the left and right hands for any of the tasks for either group. Results indicate an equipotentiality between the hands and suggest the possibility of training both hands during development on tasks that require tactile ability.

The accuracy of perception of a pinch grip force in older adults

The fact that humans can execute accurate movements and generate precise muscle forces is very important for hand function. Target-tracking tasks or target-matching tasks are often executed under combined visual and somatosensory feedback. When visual feedback is removed, subjects have to depend on their perception of force. The objective of the present study was to estimate the effects of aging on the perception of a pinch force produced by the thumb and index finger. In a first set of trials, young (n = 12, age = 25.3 +/- 2.4 years) and elderly (n = 12, age = 71.5 +/- 3.3 years) healthy individuals were asked to reproduce pinch forces which were equivalent to 5%, 20%, and 40% of their maximal pinch force (MPF). Prior to the execution of these trials, the subjects were familiarized with the force levels by matching targets displayed on a screen. They were then asked to reproduce each of these forces without any visual or verbal feedback. The results showed a larger error in the reproduced force for the elderly subjects when compared with the young adults. However, this larger error was mainly due to an initial overshoot in the force to be reproduced, followed by a gradual decrease towards the appropriate force. This transient overshoot was rarely seen in the performance of the younger subjects. In a second set of trials, the same subjects were asked to produce a pinch force of 5%, 20%, and 40% of MPF with 1 hand using visual feedback. They were also instructed to simultaneously apply a comparable pinch force with the other hand (without any feedback). For both young and older adults, the pinch forces produced by the 2 hands were the same. In addition, in both blocks of trials, hand dominance had no effects on the performance for all subjects. These results suggest that normal aging affects the production of force based on sensorimotor memory rather more than it affects comparative outputs from central descending commands.

Temporal regularity of tapping by the left and right hands in timed and untimed finger tapping

The temporal characteristics of repetitive finger tapping by the left and right hands were examined in two experiments. In the first experiment, interresponse intervals (IRIs) were recorded while right-handed male subjects tapped in synchrony with an auditory timing pulse (the synchronization phase) and then attempted to maintain the same tapping rate without the timing pulses (the continuation phase). The left and right hands performed separately, at four different rates (interpulse intervals of 250, 500, 750, and 1500 ms). There was no asymmetry of the asynchronies of the timing pulses and the associated responses in the synchronization phase or of the IRIs in either phase, but there was an asymmetry of chronization phase or of the IRIs in either phase, but there was an asymmetry in the temporal dispersion of the responses in both phases. in the second experiment, right-handed males tapped separately with each hand at three different speeds: as quickly as possible, at a fast but steady rate, and at a slow rhythmical rate. The speed asymmetry present when tapping as quickly as possible (with the preferred hand tapping more quickly ) was reduced when tapping at the fast steady rate and was absent when tapping at the slow rhythmical rate. The temporal dispersion of the IRIs produced by the nonpreferred hand was greater than the temporal dispersion of those produced by the preferred hand in all speed conditions. These results show smaller temporal dispersion of tapping by the preferred hand in right-handed males under different conditions, including submaximal speeds at which both hands respond at the same rate. This suggests that the motor system controlling the preferred hand in right-handers had more precise timing of response output than that controlling the nonpreferred hand.

Using a continuous index of laterality to determine how laterality is related to interhemispheric transfer and bimanual coordination in children

We sought to determine whether laterality is related to interhemispheric transfer and bimanual coordination during development. Children between 3 and 8 years of age were observed. In the first part of the experiment, we devised a continuous index to order the subjects according to their laterality. The laterality index included evaluation of hand and eye preference, and the right-left performance difference. In the second part of the experiment, we used this single index to determine whether laterality is related to interhemispheric transfer and bimanual coordination. Interhemispheric transfer was assessed by means of two tactile transfer tasks and one visuo-manual transfer task. We assessed bimanual coordination using the tapping task and the bimanual crank-rotation task. Results showed that right- and left-hand writers overlap on certain measures of laterality. They confirmed the improvement of interhemispheric transfer at around age 5 years, earlier progress in bimanual coordination with mirror than with parallel movements, and the existence of a relationship between visuo-manual interhemispheric transfer and bimanual coordination. The laterality index was not related to interhemispheric transfer, but it was related to the younger subjects' performance on the bimanual crank-rotation task: the less right handed, the better the bimanual coordination. In addition, on the same bimanual task, crossed hand-eye laterality was associated with better performance.

The relationship between handedness and fine motor performance

The aim of this study was to re-investigate the relationship between handedness and asymmetry in hand performance, and the sex difference in motor asymmetry. Three-hundred and ten medical students volunteered as subjects. Handedness was assessed by a 13-item questionnaire adapted from Chapman and Chapman. Fine motor performance was measured using a finger tapping task. In this task, subjects were required to tap as rapidly as possible with their index finger on a mouse button for a period of 10 s. There was a significant correlation between handedness scores and the scores of the finger tapping task. In the total sample, the correlation between hand speed and the handedness score indicated that the distribution of hand preference is associated with left hand speed, but not right hand speed. Results confirmed that right-handed females tend to have more asymmetric motor function than right-handed males.

Lateral preferences among indian school children

In the first study, 718 children from India, aged 4 to 11 years, were observed for their hand preference for ten common unimanual activities. The prevalence of left-handedness was found to be only 3.2 percent, i.e. one-third of that (9.6%) observed in a French study using a similar procedure. The degree (weak to strong) but not the direction of hand preference was found to be related to the children's age, with stronger preference among older children. The factor structure of handedness items was similar in France and India. In the second study, 400 schoolchildren from India, aged 6 to 18 years, were examined for handedness, footedness, eyedness, use of hand in space, and absolute and relative hand skill assessed by a peg-moving task and a dot-filling task. Prevalence of left-handedness was 4.2%. A sex difference was observed for handedness, footedness, use of hand in space. and relative hand skill, with higher proportions of right preferences and higher degree of lateralization (i.e., relative between hands asymmetry) in females. The degree of hand skill asymmetry increased with age. These results are discussed in relation to findings from previous studies in other countries using similar procedures.

Sidedness preference as an index of organization of laterality

A rightward bias to all forms of sidedness seems to be particularly human and perhaps characteristic of every human society. However, this phenomenon has been studied mainly in relation to lateralization of handedness and language. This study investigated the consistency of right- and nonright-handed individuals in prefer ring the same side for foot, eye, and ear using Coren's Lateral Preference Inventory. The sample of 194 students, 91 boys and 103 girls, ranging in age from 11.5-14.5 years (M age 13.2 yr.) was recruited from public junior high schools in a rural area in Greece on the island of Rhodes. Despite the dominance of the right side observed in handedness, footedness, eyedness, and earedness, this tendency was not equally strong in all four indexes of lateral preference, with handedness showing comparatively the strongest tendency and earedness showing comparatively the weakest tendency. Moreover, the strength of preference for the same side for foot, eye, and ear was significantly weaker in nonright-handed individuals compared to right handed individuals. These results agree with the hypothesis that nonright-handers may have reduced rath er than reversed asymmetry.

Predicting hand preference with performance on motor tasks

Handedness may be defined as preference or hand-differences in task performance. The strength and significance of the relationship between hand preference and hand performance asymmetries have been contested. To evaluate this relationship, we administered the Edinburgh Handedness Inventory and measured asymmetries in finger tapping, Purdue Pegboard, and grip strength in 30 subjects who prefer their right hand and 30 subjects who prefer their left hand. Hand asymmetries in finger tapping, Purdue Pegboard, and grip strength each predicted hand preference scores. However, a multiple regression equation best predicted hand preference by using performance of each task. Hand asymmetries in finger tapping correlated strongly with asymmetries in Purdue Pegboard performance, but neither of these asymmetries correlated strongly with asymmetries in grip strength. These findings indicate that hand preference and asymmetries in motor proficiency are strongly related, but suggest that preference and proficiency for different aspects of motor performance may be independently lateralized.

The effects of hand preference and gender on finger tapping performance asymmetry by the use of an infra-red light measurement device

We used an infra-red device to study the effects of gender and handwriting preference on manual asymmetry in tapping rate and intertap variability. Our sample (n=102) consisted of approximately equal number of subjects with respect to gender (52 women and 50 men) and handedness (52 right-handers and 50 left-handers). Data on overall performance indicated that men performed more quickly and regularly than women. The index used for measuring manual asymmetry was the difference between the hands as a proportion of the total. Therefore, the asymmetry index was adjusted to remove the influence of overall performance. The analyses based on asymmetry scores indicated a significant handedness effect: right-handers showed greater manual asymmetries than left-handers for both tapping rate and intertap variability. In addition, right handers exhibited a significant greater asymmetry for intertap variability than tapping rate. Taken together, these data may reflect greater hemispheric differences in right-handers, specially for intertap variability.

Hand preference and transcranial magnetic stimulation asymmetry of cortical motor representation

Human handedness may be associated with asymmetry in the corticospinal motor system. Previous studies measuring the threshold for eliciting motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) have provided evidence consistent with this hypothesis. However, TMS asymmetry observed in previous studies may have reflected cortical or spinal differences. We therefore undertook this investigation to test the hypothesis that handedness is associated with asymmetry in cortical motor representations. We used TMS to map contralateral cortical motor representations of the right and left abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscles in nine normal subjects (three left-handed). Using focal stimulation with a figure-of-8 shaped magnetic coil, we found no differences in MEP threshold or MEP size between the preferred and the nonpreferred hand. However, we observed that the number of scalp stimulation sites eliciting MEPs was statistically greater for APB and FCR muscles of the preferred limb. We found significant asymmetry between right-handed and left-handed subjects, such that in right-handers, the representation of the right APB was larger than that of the left APB, but in left-handers the representation of right APB was smaller than that of the left APB. These results suggest that handedness is associated with asymmetry in cortical motor representation.

Stability of rhythmic finger movement in children with a developmental coordination disorder

The stability of single and bimanual (i.e., in-phase and antiphase) rhythmic finger movements was studied in 24 children with a developmental coordination disorder (DCD) and 24 matched controls from a dynamic pattern perspective. Stability was assessed by applying perturbations and measuring the time the system needed to return to its initial stability (i.e., the relaxation time). In addition, fluctuations of the patterns were measured. For antiphase coordination patterns, the frequency at which loss of stability occurred was also determined. Children with DCD displayed less stable single and bimanual rhythmic coordination patterns than control children. Further, within the DCD group, 9 children were identified as having particularly poor bimanual coordination stability. Individual differences suggested that variability of individual limb oscillations might have contributed to this poorer interlimb coordination stability. Findings were discussed in relation to a previous study on DCD in which the Wing-Kristofferson timekeeper model was applied.

The relation of planum temporale asymmetry and morphology of the corpus callosum to handedness, gender, and dyslexia: a review of the evidence

Asymmetry of the planum temporale in relation to handedness, gender, and dyslexia is reviewed. The frequency of rightward asymmetry is rather higher than are estimates of the proportion of right hemisphere speech representation in the general population. Conversely, the frequency of leftward asymmetry is lower than the proportion of the population with left hemisphere speech. Neuro-anatomic asymmetry may relate more to handedness than to language lateralization. There are suggestions that neuroanatomic asymmetry is reduced in females compared to males but the data are inconclusive. Reports concerning handedness and gender differences in callosal structure are conflicting but, as with planum asymmetry, any effect of handedness is as likely to relate to degree as to direction of handedness. It has been reported that the plana are more often symmetrical in size or larger on the right side among dyslexics than controls but this has not always been found. However, greater frequency of atypical (a)symmetry of the planum in dyslexia would be consistent with the absence of a factor which, when present, biases the distribution of planum asymmetry toward the left (and handedness towards the right) as hypothesized by Annett (1985). Studies of the size of the corpus callosum in dyslexia have produced conflicting findings.

Transcranial magnetic stimulation reveals a hemispheric asymmetry correlate of intermanual differences in motor performance

Hemispheric asymmetries in the threshold for eliciting motor evoked potentials (MEPs) with transcranial magnetic stimulation (TMS) are associated with hand preference. We posited that hemispheric asymmetries in TMS thresholds may be strongly correlated with some hand-differences in motor performance. MEPs result from the activation of neuronal networks targeting large cortical motoneurons. Thus, MEP thresholds might reflect physiological features of the corticospinal motor system. Considering the role of corticospinal pathways in the control of independent finger movement, we hypothesized that MEP thresholds would better predict speed and dexterity than strength. In 30 right-handers and 30 left-handers, we correlated right and left hand-differences in the threshold for eliciting MEPs with hand-differences in the performance of three manual tasks: finger-tapping speed, pegboard dexterity, and grip strength. Correlations of hand-differences in TMS thresholds with hand-differences in performance indicated that a lower TMS threshold for one hand is strongly associated with greater ability with that hand. The correlations of hand-differences in TMS thresholds with hand-differences in finger-tapping and pegboard dexterity were significantly larger than the correlation of hand-differences in TMS thresholds with hand-differences in grip strength. Our results indicate that hemispheric asymmetries in MEP thresholds may have functional significance related to basic parameters of movement. These results are consistent with the critical role of the corticospinal motor system in the control of independent finger movement. Furthermore, they imply that asymmetry in the corticospinal motor system may be an important substrate for asymmetries in hand preference and performance.

Hand skill asymmetry in professional musicians

Hand skill asymmetry on two handedness tasks was examined in consistent right-handed musicians and nonmusicians as well as mixed-handed and consistent left-handed nonmusicians. Musicians, although demonstrating right-hand superiority, revealed a lesser degree of hand skill asymmetry than consistent right-handed nonmusicians. Increased left-hand skill in musicians accounted for their reduced asymmetry. Musicians predominantly playing keyboard instruments demonstrated superior tapping performance than musicians playing predominantly string instruments, although they did not differ with respect to hand skill asymmetry. Since the diminished tapping asymmetry in musicians was related to early commencement but not duration of musical training, results are interpreted as an adaptation process due to performance requirements interacting with cerebral maturation during childhood.

Hand, space and attentional asymmetries in goal-directed manual aiming

Two experiments were conducted to explore the interaction of the two cerebral hemispheres in motor control, by examining hand, space and attentional asymmetries in goal-directed aiming. In Experiment 1, right-handed subjects moved to targets more quickly with their right hand than their left hand. In addition, each hand was faster when moving in its own hemispace. Although in a control condition, movements were initiated more quickly with the left hand, visual distractors disrupted left hand performance more than right hand performance. For contralateral aiming, ipsilateral distractors caused the greatest interference. In Experiment 2, when targets and distractors were all presented at the midline, a right hand advantage was found for movement time along with a left hand advantage for reaction time, independent of target and distractor location. Our findings are discussed in terms of a right hemisphere role in movement preparation and the allocation of attention in space, and greater left hemisphere involvement in movement execution.

The hand performance test with a modified time limit instruction enables the examination of hand performance asymmetries in adults

Asymmetry of hand performance was investigated with the paper-and-pencil test of hand dominance originally developed by Steingruber in 1971. This test, originally constructed to measure asymmetry of hand performance in children, gave appropriate scores on asymmetry of hand performance in adults if a shorter time limit than originally proposed was applied. Asymmetry of hand performance in adults as measured with the new instruction showed reliable asymmetries in performance which are similar to those obtained on different widely accepted tests measuring asymmetries of hand performance.

Neuromuscular-skeletal constraints upon the dynamics of perception-action coupling

Four right-handed subjects performed rhythmic flexion and extension movements of the index finger in time with an auditory metronome. On each block of trials the forearm of the response hand was placed in a prone, neutral or supine position. In the flex-on-the-beat condition, subjects were instructed to coordinate maximum excursion in the direction of finger flexion with each beat of the metronome. In the extend-on-the-beat condition, subjects were instructed to coordinate maximum excursion in the direction of finger extension with each beat of the metronome. The frequency of the metronome was increased from 1.75 Hz to 3.50 Hz in eight steps (8-s plateaus) of 0.25 Hz. During trials prepared in the extend-on-the-beat pattern or to phase wandering often occurred, particularly at higher pacing frequencies. In marked constrast, during trials prepared in the flex-on-the-beat pattern such transitions were never present. Both the frequency and the alacrity of these transitions were greater when the forearm was in a prone or neutral position than when the forearm was in a supine position. These results are discussed with reference to the constraints imposed on the coordination dynamics by the intrinsic properties of the neuromuscular-skeletal system.

Physiological motor asymmetry in human handedness: evidence from transcranial magnetic stimulation

We hypothesized that human handedness might be associated with measurable differences in the excitability of the motor system. We compared the thresholds for electromyographic activation of the left and right abductor pollicis brevis (APB) and biceps muscles in 30 left-handers and 30 right-handers, by varying the direction of a brief monophasic pulse in a circular electromagnetic coil centered over the vertex of the scalp. In right-handers, we found that the threshold for activation of muscles in the right arm was lower than the threshold for activation of corresponding muscles in the left arm. In left-handers, the reverse was true. Threshold asymmetry was influenced significantly by the consistency with which each subject used the writing hand to perform other motor tasks, and was not significant between non-consistent left-handers and right-handers. Our results indicate that human handedness, and in particular, consistency of hand preference, are associated with lateralized differences in the excitability of motor system projections activated by transcranial magnetic stimulation. Our findings might reflect physiological differences in corticospinal tract function or cortical motor representation.

Hand preference consistency and fine motor performance in young children

Recent investigations suggest that children who establish handedness early are better coordinated than those who lack this characteristic. To test this assumption, 160 4- to 6-year-olds were classified by consistent (established) and inconsistent (mixed) hand preference behavior and tested for fine motor performance using a finger tapping task. Analysis of the data revealed no significant differences existed between groups in finger tapping performance in relation to total sample, age or sex. Thus, contrary to recent reports, children with consistent and inconsistent hand preferences do not appear to differ in motor behavior as revealed by performance on a fine motor task. Furthermore, it appears that the performance of those with inconsistent hand preference presents a significant challenge to current theoretical notions of the genesis of handedness.

Hand performance of French children on a finger-tapping test in relation to handedness, sex, and age

Hand performance and laterality scores on a unimanual tapping test were studied in relation to age, sex, and handedness in a sample of 209 French children. Each child performed three trials with each hand. Older children were faster, but differences between hands were not related to age. Right-handed girls were more lateralized than right-handed boys. Left- and right-handers could be differentiated more clearly by tapping speed than by variability of tapping. A subsample of 36 right-handers were retested to estimate reliability. The coefficients were high, especially when using the mean of trials as the dependent variable. Correlations between performance on the tapping task and another test of hand-efficiency (dot-filling task) were low.

Contributions of the right and left brains to manual asymmetry in hand skill in right-handed normal subjects

The relation of intermanual difference in hand skill to cerebral lateralization was studied in right-handed male and female subjects. Hand preference was assessed by the Edinburgh Handedness Inventory. Hand skill was measured by the peg moving task. In subjects with familial sinistrality (FS+), the mean right hand peg moving times (PMTs) were found to be significantly and negatively linearly correlated with the mean left minus right (L - R) hand PMTs in females (no correlation in males). Contrarily, there was a direct relationship between the mean L - R hand PMTs and the mean left hand PMTs in FS+ males (no correlation in FS+ females). Similar results were obtained with the FS- subjects. The correlations were modified by eye and foot preferences. The overall results suggested that generally the right brain in males and the left brain in females are of importance in determining the intermanual difference in hand skill; an insufficient right brain (a slower left hand) in males and a sufficient left brain (a faster right hand) in females would create a more asymmetrical organization in skill between hands.

The relation of hand preference to hand performance in left-handers: importance of the left brain

The associations among hand preference, hand skill (peg moving time, PMT) and hand performance (dot filling/20 s) were analyzed in left-handed subjects. In the total sample, the degree of left-hand preference linearly decreased from -100 to zero (Geschwind scores, GSs) as the number of dots placed by the right hand increased (direct correlation); the left hand did not show any significant relation to GSs. The right-hand PMT linearly decreased as the GSs decreased (negative linear correlation); the left hand skill did not exhibit any significant correlation with GSs. There was a negative linear correlation between right-hand PMT and dot-filling with right hand; the left-hand PMT was not related to dot-filling with left hand. In males, the right-hand PMT showed a negative linear correlation with GSs; no significant correlation between PMT and GSs was found for the left hand. There was a negative linear correlation between right minus left (R - L) PMT and GSs. Dot-filling with right hand was found to be positively linearly correlated with GSs (no correlation for the left hand). The L - R dot-filling showed a negative linear correlation with GSs. PMT for the right hand exhibited a significant negative linear relation to dot-filling only with right hand. Sex and familial sinistrality caused minor changes in these relationships. It was concluded that the left brain would constitute the main factor determining the degree of left-handedness.

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.

Relation of hand performance and preference in male and female left-handers to familial sinistrality and writing hand

The relations of hand performance to the degree of left-hand preference, and left- minus right-hand performance were studied in left-handed male and female subjects considering familial sinistrality and writing hand. Hand performance was assessed by a dot-filling test; hand preference was assessed by the Edinburgh Handedness Inventory. It was established that there were fundamental differences in relationships between performance and preference measures to sex, familial sinistrality, and writing hand, which also created different patterns in the relationships between hand performance and the difference in performance between hands.

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 Asymmetries in Interresponse Intervals During Rapid Repetitive Finger Tapping

Interresponse intervals (IRIs) were recorded as 8 right-handed male subjects tapped a key separately with the index finger of each hand as fast as possible for twenty 10-s runs. Frequency distributions of the IRIs produced by each hand showed that the shorter mean IRI that is usually reported for preferred-hand tapping is a result of a systematic production of shorter IRIs by the preferred hand. It is not secondary to inflation of the mean IRI of the non-preferred hand by the sporadic occurrence of long IRIs.

The dual task paradigm: speech dominance or manual dominance?

Two hundred and sixty monolinguals divided into subgroups based on gender, handedness, and familial left-handedness, were given a concurrent task, verbal-manual interference paradigm. The primary purpose of the study was to test whether the dual task interference effects were more related to hemispheric speech or manual dominance factors. The dominant hand, regardless of handedness, underwent relatively more tapping interference than the nondominant hand. The obtained results seriously question the basic assumptions underlying the interpretation of the dual task paradigm as an assessment index for hemispheric language lateralization. A possible explanation of this result that is in keeping with known hemispheric language representation proportions in left-handers is offered based on left hemisphere ipsilateral control of the left hand.

Differences between fingers and hands in tapping ability: dissociation between speed and regularity

The mean inter-tap interval (ITI) and the coefficient of variation of the ITI (ITIVAR) were measured in all five fingers of the preferred and non-preferred hand in two experiments. Subjects were right- or left-handed, males or females in experiments I and right-handed female typists, pianists, or controls in experiment II. Lack of consistent difference between right- and left-handers, and between those with and without special manual skills, suggested that hand differences in tapping are not a consequence of differential practise between hands. ITI showed differences both between fingers and between hands, whereas ITIVAR only showed differences between fingers. Separate mechanisms are inferred, and it is suggested that differences between fingers are a function of differential peripheral motor control, whereas differences between hands are a consequence of cerebral dominance of control mechanisms, and a model is presented.

Comparing different measures of laterality: do they relate to a single mechanism?

In the present study, a total of 126 adults were given three dichotic tasks, two half-field tasks, a peg-board task, several tapping tasks, the dynamometer, and a handedness questionnaire. A hierarchical cluster analysis was carried out on the matrix of product-moment correlations of laterality scores for all possible pairs of tasks. This analysis suggested that the laterality scores fall into three clusters: auditory (dichotic listening) scores, visual (half-field) scores, and manual (performance) scores. Canonical correlation analysis showed that these clusters must be considered to be independent of each other. The results are discussed with respect to the traditional interpretation of the concept of laterality. It is suggested that laterality phenomena may be dissociated and that this possibility deserves more attention.

Left-Handers and Right-Handers Compared on a Motor Task

Finger-tapping performance of 45 subjects of each sex and handedness combination, for a total of 180, was compared. Performance measures were speed (mean intertap interval) and regularity (standard deviation of intertap interval). Males tapped faster but not more regularly than females. The between-hand differences in performance were smaller for both measures in left-handers. When absolute magnitudes of between hand differences were compared, females showed smaller differences in regularity of tapping than males. Speed and regularity of tapping were statistically independent; both measures discriminated well between the preferred and nonpreferred hand of both handedness groups, but the differences in speed were more marked than the differences in regularity. Data on the performance of children on the same task are included for comparative purposes.