The dual task paradigm: speech dominance or manual dominance?

Time estimation during motor activity

Several studies on time estimation showed that the estimation of temporal intervals is related to the amount of attention devoted to time. This is explained by the scalar timing theory, which assumes that attention alters the number of pulses transferred by our internal clock to an accumulator that keeps track of the elapsed time. In a previous study, it was found that time underestimation during cognitive-demanding tasks was more pronounced while walking than while sitting, whereas no clear motor-induced effects emerged without a concurrent cognitive task. What remains unclear then is the motor interference itself on time estimation. Here we aim to clarify how the estimation of time can be influenced by demanding motor mechanisms and how different motor activities interact with concurrent cognitive tasks during time estimation. To this purpose, we manipulated simultaneously the difficulty of the cognitive task (solving arithmetic operations) and the motor task. We used an automated body movement that should require no motor or mental effort, a more difficult movement that requires some motor control, and a highly demanding movement requiring motor coordination and attention. We compared the effects of these three types of walking on time estimation accuracy and uncertainty, arithmetic performance, and reaction times. Our findings confirm that time estimation is affected by the difficulty of the cognitive task whereas we did not find any evidence that time estimation changes with the complexity of our motor task, nor an interaction between walking and the concurrent cognitive tasks. We can conclude that walking, although highly demanding, does not have the same effects as other mental tasks on time estimation.

Speech Performance among Healthy Malay Female Speakers during Dual Tasks and Sentence Complexity

OBJECTIVES

This study aimed to identify how different dual tasks influence speech performance among young Malay speakers by manipulating the complexity of sentences. Fifteen female speakers (20-30 years old) were audio recorded while executing a series of simple and complex sentence repetition tasks.

METHODS

Hierarchical linear modeling was conducted to examine the effects of task (speech task only, speech + cognitive tasks, speech + linguistic tasks, and speech + manual tasks) and sentence complexity (simple vs. complex), as well as their interaction on 3 outcome measures (word accuracy rate [WAR], response reaction time [RRT], and duration of sentence [DoS]).

RESULTS

Significantly higher WAR were found when participants read the sentences only compared to when they completed another task simultaneously. The pairwise comparison further revealed that the conditions of speech + manual tasks produced the lowest WAR. For RRT, participants responded significantly faster when they only read the sentences (mean = 1.36, SE = 0.17) than when they read the sentences and conducted another task at the same time (mean = 1.66, SE = 0.14; p < 0.0001). A pairwise comparison showed that the conditions of speech + manual tasks produced the longest DoS. Neither the effect of sentence complexity nor the interaction between task and sentence complexity was statistically significant across WAR, RRT, and DoS.

CONCLUSIONS

The results augment what is known about interference between speech and other concurrent tasks and provides evidence that divided attention affects manual task performance more than other tasks. In addition, sentence complexity did not affect speech performance in healthy adults. This paradigm could be used in future studies to serve as a clinical marker that can potentially distinguish mild cognitive impairment from normal aging.

Estimates of functional cerebral hemispheric differences in monolingual and bilingual people who stutter: Dual-task paradigm

The inter-relationship of stuttering and bilingualism to functional cerebral hemispheric processing was examined on a dual-task paradigm. Eighty native German (L1) speakers, half of whom were sequential bilinguals (L2 = English), were recruited. The participants (mean age = 38.9 years) were organised into four different groups according to speech status and language ability: 20 bilinguals who stutter (BWS), 20 monolinguals who stutter (MWS), 20 bilinguals who do not stutter (BWNS), and 20 monolinguals who do not stutter (MWNS). All participants completed a dual-task paradigm involving simultaneous speaking and finger tapping. No performance differences between BWS and BWNS were found. In contrast, MWS showed greater dual-task interference compared to BWS and MWNS, as well as greater right- than left-hand disruption. A prevailing finding was that bilingualism seems to offset deficits in executive functioning associated with stuttering. Cognitive reserve may have been reflected in the present study, resulting in a bilingual advantage.

Interactions of Speech and Manual Movement in a Syncopated Task

The present study examined interactions of speech production and finger-tapping movement, using a syncopated motor task with two movements in 10 male right-handed undergraduate students ( M age = 21.0 yr.; SD =1.4). On the syncopated task, participants were required to produce one movement exactly midway between two other movements (target interresponse interval: 250 msec.). They were divided into two groups, the tap-preceding group and speech-preceding group. The author observed that the right hand showed a more variable peak force and intertap interval than the left hand in the speech-preceding group, indicating an asymmetrical interference of two movements. On the other hand, the mean differences between onsets of speech and tapping movement were shorter than 250 msec. over all conditions (the shortest mean difference was 50 msec.), suggesting a mutual entrainment of two movements. An asymmetry of entrainment was observed in the speech-preceding group, in which speech production was more strongly entrained with movements of the right hand than with those of the left hand.

Dual-Task Performance in Right- and Left-Handed Adults: A Finger-Tapping and Foot-Tapping Study

The dual-task paradigm has been used extensively to study laterality, with concurrent verbalization interfering with right finger-tapping in right handers. Only a few studies have used this paradigm to study interference patterns in left handers and have found inconsistent results. The dual-task paradigm has not been used to study interference effects with concurrent verbalization and foot-tapping. The objective of this study was to use this paradigm to assess whether verbal interference produces different effects on finger- and foot-tapping rate for right handers as compared to left handers. 12 right-handed and 12 left-handed men were studied, each with uncrossed hand and foot dominance, i.e., all individuals were either right handed and right footed or left handed and left footed. Subjects performed finger- and foot-tapping tasks with and without verbal interference. A significant relationship was found between handedness and finger and foot-tapping rate; individuals with a stronger right-hand preference tended to tap at a higher rate on the right side and vice-versa. Analogous relationships were not found when participants were tapping and speaking concurrently. With verbal interference, both right and left handers had a significant asymmetric effect with a decremental response in right finger-tapping rate and a facilitative effect on left finger-tapping rate. In contrast, there was a bilateral decremental response in foot-tapping with verbal interference in both right and left handers. The implications of these findings are discussed with reference to cerebral laterality of language systems and to the differential organization and integration of the motor representations of the hand and the foot.

Levodopa effects on hand and speech movements in patients with Parkinson's disease: a FMRI study

Levodopa (L-dopa) effects on the cardinal and axial symptoms of Parkinson’s disease (PD) differ greatly, leading to therapeutic challenges for managing the disabilities in this patient’s population. In this context, we studied the cerebral networks associated with the production of a unilateral hand movement, speech production, and a task combining both tasks in 12 individuals with PD, both off and on levodopa (L-dopa). Unilateral hand movements in the off medication state elicited brain activations in motor regions (primary motor cortex, supplementary motor area, premotor cortex, cerebellum), as well as additional areas (anterior cingulate, putamen, associative parietal areas); following L-dopa administration, the brain activation profile was globally reduced, highlighting activations in the parietal and posterior cingulate cortices. For the speech production task, brain activation patterns were similar with and without medication, including the orofacial primary motor cortex (M1), the primary somatosensory cortex and the cerebellar hemispheres bilaterally, as well as the left- premotor, anterior cingulate and supramarginal cortices. For the combined task off L-dopa, the cerebral activation profile was restricted to the right cerebellum (hand movement), reflecting the difficulty in performing two movements simultaneously in PD. Under L-dopa, the brain activation profile of the combined task involved a larger pattern, including additional fronto-parietal activations, without reaching the sum of the areas activated during the simple hand and speech tasks separately. Our results question both the role of the basal ganglia system in speech production and the modulation of task-dependent cerebral networks by dopaminergic treatment.

Hemispheric lateralization in aging: interest of the verbal-manual concurrency paradigm

INTRODUCTION

The aim of this study was to evaluate whether the classic asymmetry seen in hemispheric functioning is modified in older adults by using a verbal-manual concurrency task.

METHOD

Thirty-five right-handed participants divided into two groups according to age (15 older participants, mean age: 68 ? 8 years, without cognitive decline and 20 younger participants, mean age: 23 ? 2 years) had to perform a 30-second uni-manual tapping task, in both a single task (tapping alone) and dual task (tapping and performing a letter fluency task together) condition.

RESULTS

In younger participants, the letter fluency task disrupted the right hand more than the left hand whereas, in older participants, the letter fluency task disrupted both hands equally.

CONCLUSION

These results should be considered preliminary data using a behavioral dual task condition, which might be useful for studying lateralized hemispheric functioning and the processes of divided attention during aging.

Measuring familial sinistrality: Problems with dichotomous classification

Familial sinistrality (FS), or familial left-handedness, is usually measured as a presence/absence dichotomy (FS+/FS-). Measuring FS as a dichotomy is problematic in at least two ways: (1) magnitude data are discarded and (2) there is an increased risk of violating statistical assumptions. We hypothesised that using a non-dichotomised FS measure would allow more sensitive statistical tests than using a dichotomised measure. The non-dichotomised measure examined was proportion of family that is left-handed, or Proportional FS (FSP). FS measures of 60 healthy adults were consistent with our hypothesis: (1) The effect size was 2.3 times larger when FSP was used than when Dichotomised FS (FSD) was used; (2) the observed relationship was statistically significant when FSP was used but not when FSD was used; and (3) statistical assumptions were violated when FSD was used but not when FSP was used. When cases were weighted to account for an increased likelihood of FS in large families, results were similar. Next, bootstrapping methods were used to estimate empirically the long-term differences in power between analyses incorporating FSP and those incorporating FSD. Using FSP was associated with long-term increases in both power and effect size of over 50% relative to using FSD. We conclude that using a proportional rather than a dichotomous measure of FS will enhance the accuracy of observed effect sizes, increase the sensitivity of statistical tests, and reduce the likelihood of violating statistical assumptions.

The effects of divided attention on speech motor, verbal fluency, and manual task performance

PURPOSE

The goal of this study was to evaluate aspects of the functional distance hypothesis, which predicts that tasks regulated by brain networks in closer anatomic proximity will interfere more with each other than tasks controlled by spatially distant regions. Speech, verbal fluency, and manual motor tasks were examined to ascertain whether right-handed activity would interfere more with speech and language performance because of the presumed greater demands on the left hemisphere.

METHOD

Twenty young adults completed a speech task (repeating a sentence), a verbal fluency task (listing words beginning with the same letter), and right- and left-handed motor tasks (placing pegs and washers in a pegboard) in isolation and concurrently.

RESULTS

Speech kinematic data showed that during concurrent performance of manual tasks, lip displacement and peak velocity decreased, whereas sound pressure level increased. Spatiotemporal variability increased when the nondominant hand was used for a motor task. Manual motor scores significantly decreased when concurrently performed with the verbal fluency task but not with sentence repetition.

CONCLUSION

These findings suggest that the control of concurrent tasks may be more complex than is predicted by the functional distance hypothesis.

Dual-Task Interference in Right-and Left-Handers: Typical Laterality Patterns are Obtained Despite Reversal of Baseline Asymmetries

Lateralized interference between concurrent cognitive and manual activities is used to infer cerebral hemisphere specialization for the cognitive activity. However, some evidence indicates that lateralized interference depends largely on manual asymmetry in the single-task (baseline) condition. To test the competing explanations, we asked 40 right-handed (RH) and 40 left-handed (LH) adults to key press and calculate concurrently. Baseline manual asymmetries were manipulated by increasing the complexity of the task performed by the dominant hand. Nevertheless, in the dual-task conditions, RHs showed more overall interference when performing with the right hand and LHs showed a nonsignificant tendency in the opposite direction. The results indicate that differential interference patterns, similar to those previously reported for right- and left-handers, may be obtained even when the usual baseline manual asymmetries are reversed.

Concurrent Counting and Typing: Lateralized Interference Depends on a Difference Between the Hands in Motor Skill

In previous demonstrations of differences between left- and right-handers in dual-task performance, participants' hand preference has been confounded with asymmetry of manual skill. The present study was designed to disentangle those two factors as sources of lateralized interference in the concurrent-task paradigm. Forty-eight normal adults (24 females and 24 males) counted backward by ones or by twos while typing an easy or difficult sequence of letters with either hand. When participants were grouped according to self-reported hand preference, both groups showed bilaterally symmetric slowing, relative to single-task conditions. However, when the same participants were grouped according to manual asymmetry in the baseline condition, the cognitive task interfered significantly more with the faster hand than with the slower hand. Baseline typing rate, averaged across hands, did not influence dual-task interference. Both self-reported left-hand preference and left-hand superiority in baseline typing were associated with reduced interference on the cognitive task, and the reduced interference in those groups seemed to reflect relatively loose coupling between manual and cognitive tasks. The results support and extend Caroselli et al.'s (1997) findings regarding the effect of baseline manual asymmetry on the pattern of dual-task interference. Irrespective of the participant's hand preference, the presence or absence of baseline asymmetry may be sufficient to determine whether dual-task interference is lateralized.

Speech interactions with linguistic, cognitive, and visuomotor tasks

Lip movements were examined across several repetitive speaking conditions (speech alone and speaking concurrently with a linguistic, cognitive, or visuomotor challenge task) in 20 young adults. Performance in these nonspeech activities was also compared between isolated tasks and concurrent speech conditions. Linguistic challenges resulted in increased spatiotemporal variability of lip displacement across repetitions. Motor challenges led to more rapid speech with smaller lip displacement. These qualitatively different changes suggest that different aspects of attention are required for linguistic versus manual visuomotor activity. Vocal intensity increased for all concurrent task conditions compared with speech alone, suggesting increased effort compared to the control condition. Scores for linguistic performance decreased when utterance repetition occurred concurrently with the syntactic challenge. These findings reveal that speech motor activity can influence linguistic performance as well as be influenced by it. Although these data come from healthy speakers, they suggest that clinicians working with disordered speakers should not overlook the potential interactions among the demands of language formulation, cognitive activity, and speech motor performance.

Cervicobrachial muscle response to cognitive load in a dual-task scenario

People working in an office environment often have to deal with significant cognitive workload due to the coordination of multiple, simultaneous tasks. The objective of this research was to examine the impact of cognitive load in office-type tasks on physical-stress response, using a dual-task paradigm involving a primary cognitive task and secondary typing task. The central hypothesis of this research was that altering the demands of the cognitive task would lead to a difference in physical stress-level and performance. Cognitive load was manipulated by presenting participants with three different types of cognitive tasks described in taxonomy, including skill-, rule-, and knowledge-based tasks. Dependent variables examined in the study included: (1) electromyographic activity of the upper trapezius (pars descendens) and cervical erector spinae muscles, (2) performance in a secondary typing task, and (3) subjective measures of stress and cognitive workload. The results of this study revealed that the primary task causing the highest level of perceived workload also produced 61% higher muscle activity in the right trapezius, and 6 and 11% higher activity in the left and right cervical erector spinae, respectively, in comparison to muscle activity associated with the cognitive task causing the lowest perceived workload. With respect to performance, a 23% decrease was observed in typing productivity when the rule-based task was completed simultaneously vs. typing in the absence of any additional cognitive task (the baseline condition). This information may be used to better organize work activities in office environments to increase performance and reduce stress.

MOTOR PERFORMANCE AS A FUNCTION OF VERBAL, NONVERBAL INTERFERENCE AND HANDEDNESS

The effect of verbal and nonverbal interference on finger-tapping performance was analyzed in self-classified left (n = 15) and right-handed (n = 15) subjects. Data were analyzed with a Group (left hander, right hander) x Condition (with interference [verbal, nonverbal], without interference) x Hand (left hand, right hand) mixed factorial ANOVA with repeated measures in Condition and Hand factors. Verbal as well as nonverbal interference conditions, as compared to non-interference conditions, significantly impaired finger-tapping performance of the left relative to right handers.

Effects of concurrent motor, linguistic, or cognitive tasks on speech motor performance

This study examined the influence of 3 different types of concurrent tasks on speech motor performance. The goal was to uncover potential differences in speech movements relating to the nature of the secondary task. Twenty young adults repeated sentences either with or without simultaneous distractor activities. These distractions included a motor task (putting together washers, nuts, and bolts), a linguistic task (generating verbs from nouns), and a cognitive task (performing mental arithmetic). Lip movement data collected during the experimental conditions revealed decreases in displacement and velocity during the motor task. The linguistic and cognitive tasks were associated with increased spatiotemporal variability and increases in the strength of the negative correlations between upper and lower lip displacements. These findings show that distractor tasks during speech can have a significant influence on several labial kinematic measures. This suggests that the balance of neural resources allocated to different aspects of human communication may shift according to situational demands.

Asymmetries in finger-tapping interference produced by mental versus manual rotation of Shepard and Metzler type objects

Two experiments were conducted using dual-task finger-tapping procedures to examine cerebral hemisphere laterization for mental versus manual rotation. Actual three-dimensional block-designs based on Shepard and Metzler's (1971) abstract three-dimensional cubes were constructed. Forty-eight right-handed introductory psychology students participated in each study. The first experiment showed greater right-hand than left-hand interference for mental rotation implicating more left-hemisphere involvement. In contrast, more left-hand than right-hand finger-tapping disruption with manual rotation was observed suggesting more right-hemisphere involvement. A second experiment was conducted to determine if the right-hemisphere involvement found with manual rotation was due to the manual activity of handling and rotating the blocks. Results showed that dual-task interference produced by irrelevant manual rotation combined with mental rotation was not lateralized. Thus, the pattern of results indicate that the manipulospatial processing required in the first experiment was responsible for the asymmetry implicating right-hemisphere involvement.

Handedness and immune function

Geschwind, Galaburda, and Behan (GBG) have suggested that in utero levels of testosterone influence both cerebral and immune system developments (Geschwind and Behan, 1982; Geschwind and Galaburda, 1984; Geschwind and Galaburda, 1985). According to this theory, high levels of testosterone result in greater incidences of left-handedness, deviations from standard distribution of cerebral functions (known as anomalous dominance), and increased autoimmune dysfunction. While the original data supported these assertions, more recent tests of the hypothesis have been equivocal. One criticism of these studies is that the definition of both handedness and anomalous dominance are too vague. It was one of the aims of this project to investigate and clarify the GBG model by examining four different aspects of handedness as well as a more direct measure of anomalous dominance. In order to extend the GBG model, degree of left-handedness, general immune system functioning, and current testosterone levels were also examined. First, it was predicted and found that left handers had a higher incidence of autoimmune diseases in their immediate families than did right handers. Second, those left handers with an incidence of at least one autoimmune disease were more strongly left-handed than were those with no incidence of autoimmunity. Finally, it was observed that higher testos terone levels were supportive of general immunity. The present findings both support and expand the GBG model.

Hemispheric specialization for reading

Behavioral laterality tasks with linguistic stimuli were used to assess the differential processing efficiencies of the cerebral hemispheres in right- and left-handed adults. Findings from a lateralized lexical decision task with concrete nouns supported Zaidel's (1983) "direct access" model of hemispheric functioning. A dual task consisting of oral and silent reading indicated that the right hand was significantly more disrupted than the left during unimanual finger tapping; however, some bilateral interference was observed. Taken together the findings suggest that although the left hemisphere was relatively more efficient, the right hemisphere was dynamically involved in the reading process.

Developmental trends in right hemispheric participation in reading

Behavioral laterality tasks assessed the differential processing efficiencies of the cerebral hemispheres in younger and older reading-age children. Lateralized lexical decision task findings supported a "direct access" model of hemispheric processing for the younger children whereas the older children demonstrated a "callosal relay" pattern. A dual-task with oral and silent reading indicated that the right hand was significantly more disrupted than the left during unimanual finger tapping. The findings suggest that although the left hemisphere's involvement during reading is developmentally stable, the involvement of the right hemisphere appears to change dynamically as reading experience increases.

Alternate finger tapping test in patients with migraine

Migraine patients are thought to show some cognitive dysfunction and slight structural abnormalities in the white matter of the brain, whereas most patients with multiple sclerosis (MS) are known to have numerous white matter lesions, often affecting the corpus callosum. To demonstrate psychomotor dysfunction, an alternate finger tapping task (a-FTT) on a PC was administered to controls (n = 41), migraine patients (n = 25), and multiple sclerosis patients (n = 22). Five MS patients with secondary callosal atrophy detected by MRI were also investigated as a separate group. Significant slowing was demonstrated in migraine (P = 0.0005) and MS (P<0.0001). The poorest test results were found in patients with callosal atrophy. In summary, a-FTT on a PC is able to detect minimal psychomotor dysfunction in migraine.

Verbal-to-manual and manual-to-verbal dual-task interference in left-handed and right-handed adults

Verbal-manual interference was investigated with 80 students who were divided into four groups by sex and hand preference. Unilateral finger-tapping was measured during no-load conditions and during two concurrent tasks of word reading (aloud) and sentence reading (silent). During concurrent tasks, no selective interference effects for the preferred hand were found; however, when participants were classified according to consistent handedness instead of hand preference, consistent right-handers exhibited selective right-hand tapping interference during concurrent word reading, whereas consistent left-handers showed generalized interference. During concurrent sentence reading, men showed selective right-hand interference, irrespective of handedness. The influence of tapping on word reading was also examined. Concurrent tapping lowered word-reading performance substantially, showing that finger-tapping and word reading interfered reciprocally.

Webster's scattergram method: usefulness for assessing the asymmetry of interference between concurrent tasks

Webster (1988) described a means of using scattergrams to represent the manual and nonmanual data from dual-task laterality experiments in a composite analysis. We used Webster's method and the conventional approach (i.e., separate analyses) to analyze the results of an experiment in which 40 right-handed adults performed a verbal and a manual task concurrently. Whereas separate analyses yielded dissimilar outcomes for the two tasks, the scattergram analysis showed that overall interference was greater when the right hand performed the manual task than when the left hand performed the manual task. Advantages of the scattergram method for analyzing dual-task data are discussed.

Asymmetric interference between concurrent tasks: an evaluation of competing explanatory models

According to Kinsbourne's functional cerebral distance model, asymmetric interference between concurrent cognitive and manual tasks reflects the functional specialization of the cerebral hemispheres. However, alternative explanations include initial-values artifact (statistical bias model), motor asymmetry (manual dominance model), and a combination of functional cerebral distance and manual dominance (two-factor model). We evaluated the competing models in four experiments with right- and left-handed university students for whom manual dominance was in effect reversed by requiring the dominant hand to perform the more difficult manual task. The cognitive load of the nonmanual task was varied within each experiment. The results did not support any of the models but, instead, reflected only tradeoffs between manual and nonmanual performance. The primary implications for future research are that performance on both tasks must be measured, and each task must be sensitive to interference from the other task, if observed asymmetries are to be interpretable.

Tapping, talking and the thalamus: possible influence of the intralaminar nuclei on basal ganglia function

A patient with a discrete lesion of the left, intralaminar thalamic, nuclei exhibited a paradoxical finding with regard to finger-tapping. Normal subjects typically reduce their tapping rate when performing simultaneous verbal activity. Tapping was impaired in our patient's contralesional hand on baseline trials; however, performing the controlled oral word association (COWA) task, while finger-tapping, normalized her deficit. Subsequent experiments showed that motoric tasks rather than cognitive aspects of the COWA task were critical in potentiating finger-tapping performance. A SPECT study performed at rest revealed focal perfusion asymmetries in motor and premotor cortices. Because the caudal intralaminar nuclei project heavily to the striatum, striatal deafferentiation may account for these asymmetries. These observations provide some insight into the influences of the caudal intralaminar thalamic nuclei on basal ganglia function and the basal ganglia's influence on motor gating.

On the interaction of concurrent verbal and manual tasks: which initial task conditions produce interference?

The presence of interference (and whether it is generalized or lateralized) is highly dependent on the initial conditions of the experiment. Changes in task, instructions, and subject characteristics produce different interference outcomes. Clearly the present results are not predictable from the cerebral functional distance theory and support the idea that the theory has limited explanatory power depending on the specific tasks and conditions involved. Alternatively, the idea of entrainment among tasks can account for some of the results, particularly for the preferred-speed conditions. In other words, the interference may be interpreted in terms of a task integration of two motor responses into a common rate rather than in terms of structural interference effects (cf., Murphy & Peters, 1994). From this viewpoint, it would appear that much more could be gained by systematically changing the test conditions (task constraints) to determine the different sources of interference and/or by testing specific populations (e.g., Bathurst & Kee, 1994). In the meantime, if an experimenter is specifically looking for lateralized interference in a dual-task paradigm it might be better either not to choose unimanual tapping or to avoid the motor task of finger tapping altogether so that attentional capacity is not confounded by the effects of nonattentional, mutually interacting outcomes. In general, the results of this study reflect the sentiment of Abernathy (1988), who suggests that the application of the dual-task paradigm to problems in motor skills research requires careful consideration of the available constraints operating in specific conditions.

Progress in the measurement of laterality and implications for dyslexia research

Much of contemporary laterality research has been motivated by a need to increase the accuracy with which individuals can be classified as left- or right-hemisphere dominant for speech and language. Efforts to improve the classification accuracy of laterality methods have led not only to the refinement of laterality methods but also to the discovery of some of the mechanisms that contribute to asymmetric performance. Despite these advances, laterality methods still do not lead to definitive conclusions about hemispheric specialization in the individual case, and special caution must be used when interpreting results for individual dyslexic children. Event-related measurements of cerebral metabolism, still in the development phase, promise to complement but not replace behavioral laterality methods.

Interhemispheric control of manual motor activity

The interhemispheric control of manual motor processes is reviewed, focusing on the clinical evidence from patients with commissurotomies and with agenesis of the corpus callosum. There is little evidence for a role of the corpus callosum in transferring explicit motor commands. Rather, the corpus callosum seems important for transferring lateralised information (such as verbal or visuospatial activity) of the pre-motor variety. Also, the corpus callosum may become very significant when movement begins: there appears to be a transcallosal passage of corollary motor signals and feedback sensory signals that are used to control asychronous bimanual movements and to inhibit the opposite hemisphere from interfering when a simple unimanual movement is required.

Right-handers and left-handers show differences and important similarities in task integration when performing manual and vocal tasks concurrently

For a sample of 73 consistent left-handers, 46 inconsistent left-handers and 65 right-handers, reciprocal interactions between unimanual or bimanual tasks and concurrent reading tasks were shown to be a complex function of task characteristics and sample composition. The suggestion that subgrouping of left-handers [Hellige and Key, Cerebral Control of Speech and Limb Movements, North Holland, Amsterdam, 1990] influences task interactions in left-handers was borne out. Both manual and vocal performance were measured. The effects of the vocal task on the manual task were variable for groups and conditions, but an important common trend emerged: all handedness groups showed the same robust effect: vocal task performance was better when the right hand performed the manual task. Finally, speaking rate increased and tapping rate decreased during concurrent single hand tapping [cf. Hiscock, Brain Cognit. 1, 119-131, 1982]; and there was an opposite effect with concurrent bimanual tapping. The results favour a task integration model rather than an interference in cerebral functions space model.

Hemispheric language lateralization in seasonal affective disorder and light treatment

Thirty-seven patients with seasonal affective disorder (SAD) and 25 control subjects were given a concurrent task, the verbal-manual interference paradigm, as a measure of hemispheric language lateralization. In winter, depressed patients showed a symmetrical interference pattern and control subjects experienced greater overall disruption in right-hand tapping during concurrent verbalization. Patients showed a greater left-hand tapping decrement compared with control subjects. Light treatment decreased the decrement in tapping with the left but not with the right hand in patients. After treatment, patients did not differ from controls in dual task interference effects. Patients also showed no difference from controls in summer. The difference between depressed subjects and controls in performance measures of laterality suggests that winter depression is associated with a shift of laterality from the left to the right. The winter abnormalities appear to be normalized by bright light.

Dual task performance by patients with left or right speech dominance as determined by carotid amytal tests

Patients who had their speech dominance determined by carotid Amytal testing were evaluated with a dual task procedure consisting of reading and finger tapping. As expected, the asymmetry of interference between tasks varied with speech dominance. Patients with left hemisphere speech tended to show greater interference in the right hand whereas patients with right hemisphere speech showed greater interference in the left hand. Since the right hemisphere dominant patients were also right-handed, the results suggest that interference effects are more closely linked to speech than to motor dominance.

Clarification of the dual task dilemma: lateralized effects for perfunctory and purposeful tasks in left- and right-handed males

The hypothesis that behavioral asymmetries with the dual task paradigm represent manual dominance was investigated with right- and left-handed males performing verbal and spatial tasks ordered by complexity. Lateralization was assessed for nonideational (perfunctory) and ideational (purposeful) components of tasks with multivariate and ANCOVA procedures. The outcomes of prerequisite tests showed the assumptions for conducting ANCOVA procedures were not satisfied with different handedness groups in the same design. However, results of the multivariate analyses suggest lateralized effects are more likely to represent the cognitive task when interference is high and may represent manual dominance when interference is low.

Dual-task interference in left-handed subjects: hemispheric specialization vs manual dominance

Thirty-six left-handed subjects performed a dichotic listening task and two concurrent (verbal plus finger-tapping) tasks. Baseline tapping scores revealed both left- and right-hand dominance within left-handed subjects. Moreover when subjects were categorized as consistent or inconsistent for 'sidedness advantage' (e.g. consistent = same side for dominant hand and ear advantage), it was shown that the dual-task interference effects found on the concurrent tapping tasks were due to both hemispheric specialization and manual dominance.

Concurrent verbal interference of right and left proximal and distal upper extremity tapping

The purpose of this study was to investigate crossed and uncrossed control of the proximal (upper arm and shoulder) and distal (lower arm and hand) musculature of the arms using the dual-task paradigm. Forty-one strongly right-handed men performed a tapping task using primarily the musculature of the upper or lower arms, with and without concurrent verbal processing demands. The results showed that the left distal region was distinguished from the other three effector locations by its relative insensitivity to the demands of the dual-task (verbal processing) condition. Rapid alternating movements of the left arm were functionally independent from the left index finger location in response to dual-task demands. Dual verbal and tapping demands at this effector produced greater interference on both the primary and secondary task. The results preclude the attribution of interference effects to manual dominance factors alone. The results generally support anatomical accounts of increased ipsilateral control over left side arm but not hand movements. Neither the traditional cognitive hemispheric model nor the manual dominance hypothesis were adequate in accounting for the results. An alternative generalized capacity hypothesis was required to account for performance at the LE.

The lateralization of language comprehension using event-related potentials

Event-Related Potentials were recorded over occipital and parietal scalp from left- and right-handed adults presented with a language and a non-language visual stimulus using a divided field, "oddball" paradigm. The major finding of interest was that the P300 component was larger over the left than the right hemisphere of the right-handers when the language stimulus was presented to the left hemisphere; there were no hemispheric differences for the left-handers, regardless of field of presentation. These results are discussed in the context of developing noninvasive measures to lateralize language function.

Opposite musical-manual interference in young vs expert musicians

Cerebral lateralization for music has been studied through a music-manual interference paradigm (tapping) in a group of young musicians (seven males and seven females) attending the 1st and 3rd intermediate grades of Udine's "J. Tomadini" State Conservatory of Music and in a group of graduated expert musicians or higher course students during the execution of three distinct tasks (singing notes, whistling a melody and singing a melody). A significant superiority of the right hemisphere (greater degree of interference with the left hand) in these tasks has been found in young musicians, while an opposite left hemisphere superiority (greater degree of interference with the right hand) was evident in the expert musicians. Other differences between sexes and tasks were not significant. The modification of hemispheric specialization occurring during academical musical training are discussed in terms of the role of education in the cerebral organization of superior cognitive functions.

Rate and variability of finger tapping as measures of lateralized concurrent task effects

Using a sample of 48 normal right-handed adults, we assessed the effects of oral reading on concurrent unimanual finger tapping under all combinations of instructional set (speeded vs. consistent tapping), tapping movement (repetitive vs. alternating), task emphasis (reading emphasized vs. tapping emphasized), and tapping hand. Change in tapping rate and variability was measured relative to the corresponding single task control condition. Reading decreased the rate of speeded finger tapping but increased the rate of consistent tapping. In both instances, the right hand was affected more than the left hand. Asymmetries were comparable for repetitive and alternating tapping. When measured in terms of variability, however, effects were largely symmetric. The findings clarify the conditions under which lateralized concurrent task effects are most likely to occur and show that such effects are not statistical artifacts. It appears that subjects attempt to coordinate the timing of concurrent activities and that speech timing is more strongly linked to right-hand control than to left-hand control in right-handers.