Is interhemispheric transfer of visuomotor information asymmetric? Evidence from a meta-analysis

Ageing and midline crossing inhibition

Age-related deficits of lower extremity lateral movements were investigated to identify a specific age range for the reappearance of midline crossing inhibition (MCI) along a developmental continuum. Ten individuals (five men and five women) representing each decade between the ages of 40 and 89 (five age groups) performed 108 seated trials on a lower extremity apparatus that measured choice reaction time and movement time. Midline crossing inhibition was operationally defined as statistically slower contralateral reaction times when compared to ipsilateral reaction times. The two eldest age groups (70- and 80-year-olds) exhibited MCI only on the first day of testing. Practice may facilitate cross lateral integration in normal ageing adults, even those of advanced age. There were no significant differences for movement times across age groups.

Attention and interhemispheric transfer: a behavioral and fMRI study

When both detections and responses to visual stimuli are performed within one and the same hemisphere, manual reaction times (RTs) are faster than when the two operations are carried out in different hemispheres. A widely accepted explanation for this difference is that it reflects the time lost in callosal transmission. Interhemispheric transfer time can be estimated by subtracting RTs for uncrossed from RTs for crossed responses (crossed-uncrossed difference, or CUD). In the present study, we wanted to ascertain the role of spatial attention in affecting the CUD and to chart the brain areas whose activity is related to these attentional effects on interhemispheric transfer. To accomplish this, we varied the proportion of crossed and uncrossed trials in different blocks. With this paradigm subjects are likely to focus attention either on the hemifield contralateral to the responding hand (blocks with 80% crossed trials) or on the ipsilateral hemifield (blocks with 80% uncrossed trials). We found an inverse correlation between the proportion of crossed trials in a block and the CUD and this effect can be attributed to spatial attention. As to the imaging results, we found that in the crossed minus uncrossed subtraction, an operation that highlights the neural processes underlying interhemispheric transfer, there was an activation of the genu of the corpus callosum as well as of a series of cortical areas. In a further commonality analysis, we assessed those areas which were activated specifically during focusing of attention onto one hemifield either contra- or ipsilateral to the responding hand. We found an activation of a number of cortical and subcortical areas, notably, parietal area BA 7 and the superior colliculi. We believe that the main thrust of the present study is to have teased apart areas important in interhemispheric transmission from those involved in spatial attention.

Lack of asymmetrical transfer for linguistic stimuli in schizophrenia: an ERP study

OBJECTIVE

To assess the mechanisms underlying lack of speeded information transfer asymmetry (faster right to left) for verbal information in schizophrenia.

METHODS

Interhemispheric transfer times (IHTT) between the hemispheres were assessed using a lateralized lexical-decision task in males with schizophrenia (N = 12) and matched controls (N = 12). Words were presented to the left visual field (LVF), right visual field (RVF), or bilaterally (BVF) while 128-channel EEG was recorded continuously. A direct measure of IHTT in each direction was obtained by comparing the latencies of the N160 evoked potential (EP) component in the hemispheres contralateral and ipsilateral to stimulation.

RESULTS

Controls showed faster information transfer from the right to left hemisphere (R-to-L) for linguistic stimuli. The two groups did not differ for IHTTs L-to-R. Lack of IHTT asymmetry in the schizophrenia groups was associated with an overall concomitant decrease in the amplitude of the N160 in the right hemisphere.

CONCLUSIONS

Differences in IHTT asymmetry may be attributed to lack of right hemisphere activation and not callosal dysfunction as has been previously suggested.

SIGNIFICANCE

It is suggested that a relative excess of myelinated axons in the right hemisphere speeds IHTT faster R-to-L, findings are discussed with reference to differences in right hemisphere white matter connectivity in schizophrenia.

Contribution of posterior corpus callosum to the interhemispheric transfer of tactile information

Three total and three partial callosotomy patients underwent neuropsychological testing to evaluate interhemispheric transfer of tactile information. Tactile transfer is required to name objects presented to the left hand, to compare objects held in either hand, and to transfer topological information between hands. Tactile Naming, Same-Different Recognition, and Tactile Finger Localization Tests (intra- and intermanual tasks) were administered as specific tools. Results were compared with previous fMRI data from the same subjects and with the performance of a control group (20 age-matched subjects). Total callosotomy patients performed modestly: mean correct responses were 93% and 30% (right and left hand, respectively) in Tactile Naming; 68% in Same-Different Recognition; 84% and 76% (right and left hand stimulation, respectively) in intermanual Tactile Finger Localization, and 100% in the intramanual task. Partial callosotomy patients achieved 93-100% accuracy: all have an intact splenium, and one, and possibly all, also an intact posterior callosal body. Controls scored 99% in Tactile Naming, both hands, and Same-Different Recognition; 100% in intramanual Tactile Finger Localization; and 96% and 95%, with right and left hand stimulation, respectively, in the intermanual task. Differences between the two callosotomy groups were significant, as were those between total callosotomy patients and controls. The partial callosotomy group scored like the control subjects. Neuropsychological data agree with previous functional findings, further demonstrating that interhemispheric tactile transfer requires posterior corpus callosum integrity.

Corpus callosal microstructural integrity influences interhemispheric processing: a diffusion tensor imaging study

Normal aging and chronic alcoholism result in disruption of brain white matter microstructure that does not typically cause complete lesions but may underlie degradation of functions requiring interhemispheric information transfer. We examined whether the microstructural integrity of the corpus callosum assessed with diffusion tensor imaging (DTI) would relate to interhemispheric processing speed. DTI yields estimates of fractional anisotropy (FA), a measure of orientation and intravoxel coherence of water diffusion usually in white matter fibers, and diffusivity (<D>), a measure of the amount of intracellular and extracellular fluid diffusion. We tested the hypothesis that FA and <D> would be correlated with (i) the crossed-uncrossed difference (CUD), testing visuomotor interhemispheric transfer; and (ii) the redundant targets effect (RTE), testing parallel processing of visual information presented to each cerebral hemisphere. FA was lower and <D> higher in alcoholics than in controls. In controls but not alcoholics, large CUDs correlated with low FA and high <D> in total corpus callosum and regionally in the genu and splenium. In alcoholics but not controls, small RTEs, elicited with equiluminant stimuli, correlated with low FA in genu and splenium and high <D> in the callosal body. The results provide in vivo evidence for disruption of corpus callosum microstructure in normal aging and alcoholism that has functional ramifications for efficiency in interhemispheric processing.

A split-brain model of Alzheimer's disease?

It has been proposed that features of Alzheimer-type dementia (AD) reflect a breakdown in cortical connectivity that can be likened to a disconnection syndrome. One hypothesized consequence of this pathology is that AD patients should be disproportionally impaired on measures of interhemispheric transfer. However, there is a paucity of studies bearing on this prediction. We report the results from two measures of interhemispheric interaction obtained from healthy younger and older adults, and older adults with probable AD. One measure examined speeded simple manual responses to a lateralized light flash (i.e., the Poffenberger task) and the other examined the interhemispheric coordination of computational resources using within and across hemifield variants of visual letter-matching tasks. AD patients show an overall impairment of performance on both intra and interhemispheric conditions in all tasks. However, there is no indication of disproportionate alteration of interhemispheric processes mediating either visuomotor transfer or visual letter-matching and the allocation of computational resources. The results, therefore, call into question the appropriateness of a "split-brain" model for AD, at least in the domain of visual processing. Although the results are not specifically diagnostic of a disconnection syndrome, they are consistent with the possibility of a breakdown of cortico-cortical connectivity both within and between the hemispheres in AD.

The effects of redundant stimuli on visuospatial processing in developmental dyslexia

The interhemispheric deficit theory of dyslexia postulates that reading difficulties can arise from abnormal communication/collaboration between the cerebral hemispheres. A currently popular way to gather information about interhemispheric processing and integration is with the redundant stimuli task, where participants respond to stimuli presented to the left visual field, right visual field, or both visual fields simultaneously. In neurologically normal individuals, response times to bilateral simple stimulus presentations are faster than response times to a single stimulus in either visual field alone (referred to as redundancy gain). In contrast, individuals with no corpus callosum exhibit greater redundancy gains than would be expected by probability summation. In the present study, 11 children with phonological dyslexia showed a similar "over violation" of the probability (race) model when responding with the left but not the right hand. This asymmetry was not found in age- and IQ-matched control children. The results are at least partially consistent with the notion of phonological dyslexia involving deficits in the transfer of information across the corpus callosum.

Magnetic stimulation and the crossed?uncrossed difference (CUD) paradigm: selective effects in the ipsilateral and contralateral hemispheres

When a visual target is presented to one hemifield, manual responses made to the target using the ipsilateral hand (uncrossed responses) are faster than responses using the contralateral hand (crossed response), because there is no need for visuomotor information to be transferred between the hemispheres. This difference in response times is termed the crossed-uncrossed difference (CUD) and is a valuable means of estimating interhemispheric transfer time. We aimed to investigate the CUD by applying repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lateralized target-detection task. Eleven neurologically healthy subjects, all right-handed, participated in the study. Relative to sham TMS we increased the CUD, by inhibiting the crossed latencies, but only when rTMS was applied to the hemisphere receiving visual information. These results replicate and extend previous findings and suggest the inhibitory rTMS effect under the crossed condition might be because the weak visual output is unable to activate the crossed pathway.

Interhemispheric Transmission of Visuomotor Information for Motor Implementation

Using transcranial magnetic stimulation (TMS), we addressed the contribution of both hemispheres to the visuomotor control of each hand. The subjects had to press one of two buttons as quickly as possible after the go-signal. A precue preceding this conveyed full, partial or no advance information (hand and/or button), such that reaction time (RT) shortened with increasing amount of information. We gave TMS over each hemisphere at various time intervals (100-350 ms) after the go-signal and before the expected onset of response, and measured its effect on RT, movement time (MT) and error rate. At short intervals (100-200 ms), left hemisphere TMS delayed RT and prolonged MT of both hands, while right hemisphere TMS delayed RT only of the right hand, without affecting error rates. At long intervals (250-350 ms), TMS produced slightly more pronounced RT delays of the contralateral hand. RT was delayed more if the precues were less informative. The results suggest the importance of interhemispheric transmission of visuomotor information for motor implementation. The right hemisphere may play a role mainly in calculating target and effector information, determining RT, while the left hemisphere may play a role in elaborating the motor program and determining MT.

Inter-limb coupling in bimanual rhythmic coordination in Parkinson’s disease

Recently, it has been shown that rhythmic inter-limb coordination is disturbed in patients with Parkinson's disease (PD). The present study aims to investigate whether this coordination deficit is primarily the result of an impaired coupling, related to hypoactivation of the supplementary motor area (SMA), or primarily the indirect result of an asymmetrical distribution of PD symptoms over the left and right limbs (a peripheral process). Thirty PD patients and 30 matched control participants tapped with the index fingers anti-phase and left and right leading gallop patterns in four visual feedback conditions. Symmetrically affected participants performed significantly worse than asymmetrically affected and control participants in the gallop patterns. This result suggested that the central deficit has a stronger effect on inter-limb coupling in PD than the neuromuscular and biomechanical asymmetry between the limbs. Detailed analysis of inter-tap intervals (variability and correlation) suggested that this deficit leads to a compensatory asymmetrical inter-limb coupling in the primarily right-affected patient group, and under specific circumstances also in the primarily left-affected patient group. The difference in coordination strategy between left- and right-affected patients suggested that pre-morbid hand preference is an important structural constraint on the coupling strategies available to the participants.

Interlimb transfer of novel inertial dynamics is asymmetrical

Mechanisms underlying interlimb transfer of adaptation to visuomotor rotations have recently been explored in depth. However, little data are available regarding interlimb transfer of adaptation to novel inertial dynamics. The present study thus investigated interlimb transfer of dynamics by examining the effect of initial training with one arm on subsequent performance with the other in adaptation to a 1.5-kg mass attached eccentrically to the forearm. Using inverse dynamic analysis, we examined the changes in torque strategies associated with adaptation to the extra mass, and with interlimb transfer of that adaptation. Following initial training with the dominant arm, nondominant arm performance improved substantially in terms of linearity and initial direction control as compared with naïve performance. However, initial training with the nondominant arm had no effect on subsequent performance with the dominant arm. Inverse dynamic analysis revealed that improvements in kinematics were implemented by increasing flexor muscle torques at the elbow to counter load-induced increases in extensor interaction torques as well as increasing flexor muscle torques at the shoulder to counter the extensor actions of elbow muscle torque. Following opposite arm adaptation, the nondominant arm adopted this dynamic strategy early in adaptation. These findings suggest that dominant arm adaptation to novel inertial dynamics leads to information that can be accessed and utilized by the opposite arm controller, but not vice versa. When compared with our previous findings on interlimb transfer of visuomotor rotations, our current findings suggest that adaptations to visuomotor and dynamic transformations are mediated by distinct neural mechanisms.

Inter- and intra-hemispheric processing of visual event-related potentials in the absence of the corpus callosum

Interhemispheric differences of the N100 latency in visual evoked potentials have been used to estimate interhemispheric transfer time (e.g., Saron & Davidson, 1989). Recent work has also suggested that the P300 component could reflect the efficacy of interhemispheric transmission (Polich & Hoffman, 1998). The purpose of the present study was to study the differential role of the corpus callosum (CC) and anterior commissure (AC) in the interhemispheric propagation of these two electrophysiological components. Thus, the amplitude and latency distribution of the N100 and P300 components were analyzed using high-density electrical mapping in a subject with agenesis of CC but preservation of AC, a subject with agenesis of both CC and AC, and 10 neurologically intact control subjects. The task consisted of a modified visual oddball paradigm comprising one frequent and two rare stimuli, one presented on the same and the other on the opposite side of the frequent stimulus. Interhemispheric differences in latency were found for the N100 component in controls. However, in the acallosal subjects, this component was not identifiable in the indirectly stimulated hemisphere. In controls, no interhemispheric differences were observed in the distribution of the P300 latency and amplitude to rare and frequent stimuli. The distribution of the P300 amplitude in the acallosal subject with an AC was identical to that of the controls, whereas in the acallosal subject lacking the AC, the amplitude was greater in the hemisphere receiving the frequent stimuli, regardless of the visual hemifield in which the rare stimuli were presented. In both acallosal subjects, hemispheric differences in the P300 latency were observed, the latencies being shorter in the hemisphere directly stimulated for all categories of stimuli. These results suggest that the interhemispheric transfer of both the N100 and P300 components relies on the integrity of cortical commissures. Possible P300 generator sources are discussed.

Experimental disentangling of spatial-compatibility and interhemispheric-relay effects in simple reaction time (Poffenberger paradigm)

Spatial-compatibility effects can be obtained in simple reaction time (SRT) provided that spatially distinct responses are frequently required. Since this effect is limited to trials with relatively long reaction times (RTs), Hommel (1996b) proposed that if the response does not occur shortly after stimulus detection, then the spatial code of the stimulus can interfere with that of the response. A series of experiments is reported showing that (a) spatial compatibility in SRT to lateralized stimuli is not an alternative, but rather a complementary, explanation to interhemispheric transfer time (contrary to what Hommel surmised), and (b) the spatial compatibility component is essentially limited to the first trial after shifting response preparation from one-half of the visual fields to the other, suggesting a mechanism akin to an orienting response.

Perceptual interactions between bilaterally presented words: what you get is often not what you see

Bilateral presentations of words, 1 in the left visual hemifield (LVF) and 1 in the right (RVF), are used widely in studies of hemispheric asymmetry. However, although words shown centrally (i.e., nonlaterally) produce perceptual interactions in which 1 word alters the perceived identity of the other, perceptual interactions between bilaterally presented words have never been reported. To investigate this issue, the authors used brief, bilateral displays of words (e.g., romp-ramp) presented simultaneously. An eye tracker and forced-choice task ensured appropriate presentation and testing. Report accuracy was greatest for RVF words. However, this was accompanied by perceptual interactions that occurred almost exclusively in responses to LVF words, indicating that RVF words often altered the perceived identity of LVF words but not vice versa.

The superior colliculus subserves interhemispheric neural summation in both normals and patients with a total section or agenesis of the corpus callosum

To verify the possibility that the superior colliculus (SC) subserves interhemispheric neural summation, we presented single or double white visual targets to one or both hemifields in normal participants and in patients lacking the corpus callosum (one with total callosotomy and one with callosal agenesis). Simple reaction time was typically faster with double than single stimuli, a phenomenon known as the redundant target effect (RTE); moreover, confirming previous results, we found a larger RTE in patients without callosum than in normals. In both groups, the redundancy gain was related to neural coactivation rather than to probability summation. The novel finding was that, when using monochromatic purple stimuli that are invisible to the SC, we found a similar redundancy gain in both groups; moreover, this redundancy gain was probabilistic rather than neural. Control experiments with monochromatic red stimuli yielded a RTE of the neural type similar to that with white stimuli and this confirmed that the probabilistic RTE found was specific for purple stimuli. In conclusion, visual input to the SC is necessary for interhemispheric neural summation in both normals and in individuals without the corpus callosum while probabilistic summation can occur without a collicular contribution.

Callosal lesions and behavior: history and modern concepts

Callosotomy has played a unique role in the treatment of epilepsy and in the understanding of human brain function. The pioneering work of Dejerine and Liepmann presenting the first findings of callosal lesion pathology at the turn of the 20th century was accepted but then quickly forgotten. Two schools resurrected the phoenix of callosal syndromes: Roger Sperry and Michael Gazzaniga leading in experimental neuroscience, and Norman Geschwind leading in clinical neurology. Callosotomy remains an effective technique to treat atonic, tonic, and tonic-clonic seizures, especially in patients with symptomatic generalized epilepsies such as Lennox-Gastaut syndrome. Neurologic, cognitive, and behavioral complications limit its use given that precise characterization of these complications as well as their frequency is difficult. The high frequencies of developmental delays, severe seizures, head injuries, antiepileptic drug burden, and other factors limit the ability to attribute a specific change to surgical intervention, since surgery can change multiple factors. For example, subtle behavioral changes in executive function and personality are difficult to delineate in a population with preexisting neurologic and psychiatric disorders. Despite this, a clearer picture of the effects of callosotomy, as defined by clinical neurology and neuropsychology as well as cognitive neuroscience, is emerging.

Callosum and movement control: case reports

This article explores the role of directionality of callosal traffic (codified as handedness), based on personal clinical observations and a critical review of the literature. Based on this evidence, a technical definition of handedness is offered as opposed to the behavioral method in use until now. In the vast majority of right-handers neural and behavioral handedness match. The situation is the opposite in left-handers where two thirds of them are wired to be right-handers, causing the well-known heterogeneity seen in left-handed cohorts. The callosum-length proximity of the dominant side of the body to the command center in the major hemisphere is the source of its neurophysiological superiority compared to the nondominant side. Clinical syndromes in which the new scheme are manifested are reviewed, indicating the existence of an excitatory influence by the neuronal aggregate devoted to voluntary actions, housed in the major hemisphere, on their counterparts in the minor hemisphere. The latter is exclusively devoted to volitional movements occurring on the nondominant side. Thus, it is the directionality of callosal traffic that is responsible for cerebral asymmetries seen in the motor realm.

Redundancy gain in the acallosal brain

The authors measured simple reaction time (RT) to visual stimuli, presented either singly to 1 or the other visual field or in bilaterally presented pairs, to 2 women with callosal agenesis. The stimuli were either white against a black background or gray against an equiluminant yellow background. RTs to bilateral pairs were decreased beyond predictions based on a simple race between independent unilateral processes, implying interhemispheric neural summation. This effect was enhanced under equiluminance in the participant M.M. whose anterior commissure was within normal limits, but not in the participant J.P. whose anterior commissure was enlarged. The anterior commissure may act, relative to its size, to inhibit cortical activation to bilateral pairs, which then acts to decrease subcortical neural summation.

Mechanisms underlying interlimb transfer of visuomotor rotations

We previously reported that opposite arm training improved the initial direction of dominant arm movements, whereas it only improved the final position accuracy of non-dominant arm movements. We now ask whether each controller accesses common, or separate, short-term memory resources. To address this question, we investigated interlimb transfer of learning for visuomotor rotations that were directed oppositely [clockwise (CW)/counterclockwise (CCW)] for the two arms. We expected that if information obtained by initial training was stored in the same short-term memory space for both arms, opposite arm training of a CW rotation would interfere with subsequent adaptation to a CCW rotation. All subjects first adapted to a 30 degrees rotation (CW) in the visual display during reaching movements. Following this, they adapted to a 30 degrees rotation in the opposite direction (CCW) with the other arm. In contrast to our previous findings for interlimb transfer of same direction rotations (CCW/CCW), no effects of opposite arm adaptation were indicated in the initial trials performed. This indicates that interlimb transfer is not obligatory, and suggests that short-term memory resources for the two limbs are independent. Through single trial analysis, we found that the direction and final position errors of the first trial of movement, following opposite arm training, were always the same as those of naive performance. This was true whether the opposite arm was trained with the same or the opposing rotation. When trained with the same rotation, transfer of learning did not occur until the second trial. These findings suggest that the selective use of opposite arm information is dependent on the first trial to probe current movement conditions. Interestingly, the final extent of adaptation appeared to be reduced by opposite arm training of opposing rotations. Thus, the extent of adaptation, but not initial information transfer, appears obligatorily affected by prior opposite arm adaptation. According to our findings, it is plausible that the initiation and the final extent of adaptation involve two independent neural processes. Theoretical implications of these findings are discussed.

Differential effects of low-frequency rTMS at the occipital pole on visual-induced alpha desynchronization and visual-evoked potentials

Visual-induced alpha desynchronization (VID) and visual-evoked potentials (VEPs) characterize occipital activation in response to visual stimulation but their exact relationship is unclear. Here, we tested the hypothesis that VID and VEPs reflect different aspects of cortical activation. For this purpose, we determined whether VID and VEPs are differentially modulated by low-frequency repetitive transcranial magnetic stimulation (rTMS) over the occipital pole. Scalp EEG responses to visual stimuli (flashed either to the left or to the right visual field) were recorded for 8 min in six healthy subjects (1) before, (2) immediately following, and (3) 20 min after left occipital rTMS (1 Hz, 10 min). The parameters aimed to reduce cortical excitability beyond the end of the TMS train. In addition, simple reaction times to visual stimulation were recorded (left or right hand in separate blocks). In all subjects, VID was significantly and prominently reduced by rTMS (P = 0.0001). In contrast, rTMS failed to modulate early VEP components (P1/N1). A moderate effect was found on a late VEP component close to manual response onset (P = 0.014) but this effect was in the opposite direction to the VID change. All changes were restricted to the targeted left occipital cortex. The effects were present only after right visual field stimulation when a right hand response was required, were associated with a behavioral effect, and had washed out 20 min after rTMS. We conclude that VID and early VEPs represent different aspects of cortical activation. The findings that rTMS did not change early VEPs and selectively affected VID and late VEPs in conditions where the visual input must be transferred intrahemispherically for visuomotor integration (right visual field/right hand) are suggestive of rTMS interference with higher-order visual functions beyond visual input. This is consistent with the idea that alpha desynchronization serves an integrative role through a corticocortical "gating function."

Egocentric body-centered coordinates modulate visuomotor performance

Parietal damage has been hypothesized to distort the body-centered coordinate frame and produce the ipsilesional spatial bias characteristic of unilateral neglect. The present studies investigated the role of the egocentric frame in normal visuomotor performance by manipulating the alignment of the body midline in neurologically-intact adults. The results from two experiments indicate that: (1) rightward rotation causes a right visual field advantage in detection times for lateralized targets; (2) rightward rotation evokes an increase in visual sensitivity to right visual field targets and a decrease in sensitivity to left visual field targets; and (3) leftward rotation does not affect response latency or sensitivity, however, response criterion is mildly affected. By demonstrating that the alignment of the body-centered frame can induce neglect-like asymmetries in visuomotor performance in neurologically-intact subjects, the results support a role for an altered body-centered representation in clinical neglect.

Interhemispheric transmission of visuomotor information in humans: fMRI evidence

Normal human subjects underwent functional magnetic resonance imaging (fMRI) while performing a simple visual manual reaction-time (RT) task with lateralized brief stimuli, the so-called Poffenberger's paradigm. This paradigm was employed to measure interhemispheric transmission (IT) time by subtracting mean RT for the uncrossed hemifield-hand conditions, that is, those conditions not requiring an IT, from the crossed hemifield-hand conditions, that is, those conditions requiring an IT to relay visual information from the hemisphere of entry to the hemisphere subserving the response. The obtained difference is widely believed to reflect callosal conduction time, but so far there is no direct physiological evidence in humans. The aim of our experiment was twofold: first, to test the hypothesis that IT of visuomotor information requires the corpus callosum and to identify the cortical areas specifically activated during IT. Second, we sought to discover whether IT occurs mainly at premotor or perceptual stages of information processing. We found significant activations in a number of frontal, parietal, and temporal cortical areas and in the genu of the corpus callosum. These activations were present only in the crossed conditions and therefore were specifically related to IT. No selective activation was present in the uncrossed conditions. The location of the activated callosal and cortical areas suggests that IT occurs mainly, but not exclusively, at premotor level. These results provide clear cut evidence in favor of the hypothesis that the crossed-uncrossed difference in the Poffenberger paradigm depends on IT rather than on a differential hemispheric activation.

Age- and sex-related differences in temporal judgments to visual stimuli: support for hemispheric equivalence

Light-emitting diodes in avisual half-field display were employed to examine hemispheric asymmetries in temporal resolution among young and older adults. Participants judged whether pairs of spatially separated diodes were illuminated simultaneously. No visual field threshold differences emerged for either age group, thus supporting hemispheric equivalence. Older adults had significantly higher thresholds than did younger adults, regardless of spatial location. The results further revealed that older females had significantly higher thresholds than did older males, younger males, and younger females. The results further revealed sex differences, favoring females, when interhemispheric transfer times (IHTTs) were examined for a central bilateral presentation. However, sex effects were not revealed when IHTTs were examined for a peripheral bilatepresentation, indicating a disadvantage forolder females.

Orienting visuospatial attention generates manual reaction time asymmetries in target detection and pointing

Right-handers exhibit a left hand advantage in response preparation when pointing to targets. These manual asymmetries are generally attributed to a right hemisphere specialization for spatial processing. More precisely, the left hand reaction time (RT) advantage was recently supposed to reflect specifically the right hemisphere superiority for movement planning. This study proposes to investigate a possible attentional origin for manual RT asymmetries. In a first experiment, we used the covert orienting of attention paradigm to measure subjects' RTs when reaching at targets (pointing task) both in valid, neutral and invalid conditions, either in the left or in the right visual fields and with the left and the right hand. In a second experiment, we applied the same paradigm to a detection task (key-pressing). Results revealed that orienting of attention to spatial locations was more time consuming when responding with the right than with the left hand, whether movement planning was required or not. It is suggested that the right hemisphere dominance for orienting of visuospatial attention account, partly at least, for the RT asymmetries classically observed in manual aiming.

Auditory information is beneficial for adults with Down syndrome in a continuous bimanual task

Much recent research using discrete unimanual tasks has indicated that individuals with Down syndrome (DS) have more difficulty performing verbal-motor tasks as compared to visual-motor tasks (see Perceptual-Motor Behavior in Down Syndrome, Human Kinetics, Champaign, IL, 2000, p. 305 for a review). In continuous tasks, however, individuals with DS perform better when movement is guided by auditory information compared to visual information (Downs Syndr.: Res. Prac. 4 (1996) 25; J. Sport Exercise Psy. 22 (2000) S90). The aim of the present study was to investigate if there are any differences for adults with DS between visual, auditory and verbal guidance in a continuous bimanual task. Ten adults with DS, 10 adults without DS and 10 typically developing children drew lines bimanually towards the body (down) and away from the body (up) following three different guidance conditions: visual (flashing line), auditory (high tone, low tone), and verbal ("up", "down"). All participants produced mostly in-phase movements and were close to the 1000 ms target time for all guidance conditions. The adults with DS, however, displayed greater variability in their movement time, movement amplitude and bimanual coordination than adults without DS. For all groups, the left hand was slower and more variable in producing the lateral movements than the right hand. The results regarding guidance information suggest that auditory information is beneficial for repetitive bimanual tasks for adults with DS. Possible mechanisms that cause these results will be discussed.

Interhemispheric transfer and laterality effects in simple visual reaction time in schizophrenics

INTRODUCTION

There is evidence that schizophrenics have an abnormal corpus callosum and an abnormal pattern of cerebral asymmetries. We investigated whether there are corresponding functional abnormalities in interhemispheric transfer (IT) and laterality effects.

METHODS

Medicated schizophrenic patients and matched controls were tested in the Poffenberger paradigm, that is, a simple manual reaction time (RT) paradigm with laterally presented visual stimuli designed to provide a behavioural estimate of IT. By subtracting RT averaged across the uncrossed hand-hemifield conditions, from RT averaged across the crossed hand-hemifield conditions, one can obtain an estimate of IT time.

RESULTS

In schizophrenic patients the difference between crossed and uncrossed conditions was 0 because of an unusually prolonged RT in the uncrossed condition right hand/ right field. A broadly similar result has been obtained previously in the tactile modality (Ditchfield & Hemsley, 1990) and is consistent with a left hemisphere impairment. This effect was still present when the patients were retested about 2 years later.

CONCLUSIONS

These results demonstrate the existence in schizophrenic patients of a consistent slowing down of simple visuomotor responses subserved by the left hemisphere.

Reduced interhemispheric transmission in schizophrenia patients: evidence from event-related potentials

Interhemispheric transfer was investigated in 14 schizophrenia patients and 15 age- and sex-matched healthy controls in a lateralized lexical decision task. Words and pseudowords were tachistoscopically presented either to the left or to the right visual hemifield. Event-related potentials were determined from a 65-channel electroencephalogram. Information transfer between hemispheres was assessed by the interhemispheric transmission time (IHTT), the N1-latency difference between ipsilateral and contralateral hemisphere.Controls, but not schizophrenia patients showed significantly faster IHTT from the right to the left hemisphere for words, while IHTT from the left to the right hemisphere did not differ between groups and stimuli. These findings are interpreted in terms of a deficit in schizophrenia to transfer verbal information from the right to the left hemisphere via the corpus callosum.

Manual reaction time asymmetries in human subjects: the role of movement planning and attention

Hemispheric asymmetries in spatial processing are generally considered to be responsible for the shorter reaction time (RT) of the left hand classically observed for right-handers when pointing at targets. Surprisingly, despite the special role which the right cerebral hemisphere is known to play in visual attention, the attentional hypothesis for hand movement preparation asymmetries is currently rejected. This study aims to test the respective roles of visual attention and movement planning in the left hand RT advantage for goal-directed movements. Two experiments were conducted with the same subjects, a simple visual detection task and a classical pointing task, using the same lateralized stimuli. Subjects used the left hand and the right hand alternatively in order to react to the stimuli. In the detection task, the reaction consisted of simply releasing a switch as quickly as possible after the appearance of a target, whereas in the pointing task, it consisted of performing lateralized reaching movements towards the same target. The main results of this study revealed left hand shorter RTs for both tasks, emphasizing the role which right hemisphere dominance for visuospatial attention plays in manual aiming asymmetries. Moreover, a direct comparison of the RTs obtained in both experiments showed that the specific cost of movement planning was lower when using the left hand, therefore also revealing right hemisphere dominance for movement planning.

Electric source imaging of human brain functions

We review recent methodological advances in electromagnetic source imaging and present EEG data from our laboratory obtained by application of these methods. There are two principal steps in our analysis of multichannel electromagnetic recordings: (i) the determination of functionally relevant time periods in the ongoing electric activity and (ii) the localization of the sources in the brain that generate these activities recorded on the scalp. We propose a temporal segmentation of the time-varying activity, which is based on determination of changes in the topography of the electric fields, as an approach to the first step, and a distributed linear inverse solution based on realistic head models as an approach to the second step. Data from studies of visual motion perception, visuo-motor transfer, mental imagery, semantic decision, and cognitive interference illustrate that this analysis allows us to define the patterns of electric activity that are present at given time periods after stimulus presentation, as well as those time periods where significantly different patterns appear between different stimuli and tasks. The presented data show rapid and parallel activation of different areas within complex neuronal networks, including early activity of brain regions remote from the primary sensory areas. In addition, the data indicate information exchange between homologous areas of the two hemispheres in cases where unilateral stimulus presentation requires interhemispheric transfer.

Interhemispheric transfer time in a patient with a partial lesion of the corpus callosum

The interhemispheric transfer time (ITT) of basic visuo-motor integration was investigated in a patient who had a lesion of the corpus callosum that spared the splenium and rostrum. Overall, 4291 simple reaction times were collected during unimanual responses to tachistoscopically presented lateralized simple visual stimuli at 4 degrees, 6 degrees and 10 degrees. Despite retaining some abilities that typically require the integration of information between hemispheres (e.g. haptic naming, tachistoscopic lateralized consonant reading) the patient performed similarly to completely callosotomized patients in a basic visuo-motor ITT task (overall 25.5 ms) at any eccentricity. These findings suggest that specific callosal channels mediate the basic visuo-motor ITT and these do not include the rostrum and/or the splenium of the corpus callosum.

Differential effects of aging on the functions of the corpus callosum

Several structural imaging studies have revealed atrophy in regions of the corpus callosum due to normal aging. We examined the performance of young and senior adults on 2 behavioral measures of interhemispheric interactions to test for possible functional consequences of callosal decline. In a simple reaction time task, the efficiency of sensorimotor transfer was assessed by comparing response times from conditions that required interhemispheric transfer (e.g., the left hand responding to a right visual field light) to those from conditions that did not require transfer (e.g., the left hand responding to a left visual field light). Older adults were selectively slower when interhemispheric transfer was required. In the second task, participants matched letters presented within the same visual field or in opposite visual fields. This task is thought to index attentional functions of the corpus callosum, in particular, callosal contributions to resource allocation (Banich, 1998). For more difficult tasks, older participants showed a performance advantage on bilateral conditions requiring transfer compared to unilateral conditions that did not require transfer. This advantage equaled or exceeded that observed in younger adults. Together these results suggest that age does not have uniform effects on callosal function. Whereas sensorimotor functions show age-related decline, attentional functions of the corpus callosum may be relatively preserved and assume a more prominent role in the aging brain.

Relationship between extremities in motor performance

The purpose of the study was to examine the relationship between the extremities in different aspects of motor performance, including simple reaction time, choice reaction time, speed of movement, tapping speed, and coordination, i.e., speed of movement/accuracy. 186 healthy randomly selected right-handed subjects (93 men, 93 women; aged 21-70 years) were measured on a test battery of five tests for both hands and feet. Correlations between the extremities in motor performance were highest between hands (r=.71-.80, p<.001) or between feet (r=.57-.88, p<.001) depending on the complexity of response for all subjects. These relations remained at almost the same magnitude even when age was eliminated. Correlations between the upper and lower extremities were lower (r = .40-.62). Correlations between extremities were lower for one age group (21-30 years) than for all subjects (21-70 years). These results showed that the motor performance for the hands is not a very good indicator of the motor performance of the lower extremities, especially in a homogeneous age group, and it seems that the lower extremities should be studied with specific motor performance tests.

Hemispheric asymmetry and interhemispheric transfer in pointing depend on the spatial components of the movement

The purpose of the present study was to compare the asymmetry and transfer in 3 pointing movements with increasing spatial requirements. The triggering signal was one of four visual targets appearing on the right or left of a central fixation point (FP). The first task consisted in simply removing the arm from the starting platform; the second was a pointing movement towards the FP, and the third was a classical pointing task towards one of the four lateral targets. 20 right-handers (Rhrs) and 20 left-handers (Lhrs) participated in this experiment. In the classical pointing task (task 3), the reaction times were shorter in the Rhrs using their left hand. No such hand-related difference was observed in the Lhrs. No hand asymmetry was observed in the other tasks. In addition, the responses were faster in the uncrossed than in the crossed conditions, in task 3 only. It was concluded that in pointing tasks, both the hemispheric asymmetry and the interhemispheric transfer depend on the spatial requirements of the movement.

Movement observation affects movement execution in a simple response task

The present study was designed to examine the hypothesis that stimulus-response arrangements with high ideomotor compatibility lead to substantial compatibility effects even in simple response tasks. In Experiment 1, participants executed pre-instructed finger movements in response to compatible and incompatible finger movements. A pronounced reaction time advantage was found for compatible as compared to incompatible trials. Experiment 2 revealed a much smaller compatibility effect for less ideomotor-compatible object movements compared to finger movements. Experiment 3 presented normal stimuli (hand upright) and flipped stimuli (hand upside-down). Two components were found to contribute to the compatibility effect, a dynamic spatial compatibility component (related to movement directions) and an ideomotor component (related to movement types). The implications of these results for theories about stimulus-response compatibility (SRC) as well as for theories about imitation are discussed.

Lateralized interhemispheric transfer of color cues: evidence for dynamic coding principles of visual lateralization in pigeons

Visual feature discrimination tasks in pigeons reveal a right eye/left hemisphere dominance at the population level. Anatomical studies and lesion data show this visual lateralization to be related to asymmetries of the tectofugal system, which ascends from the tectum over the n. rotundus to the forebrain. Anatomically, this system is characterized by numerous morphological and connectional asymmetries which result in a bilateral visual representation in the dominant left hemisphere and a mostly contralateral representation in the subdominant right hemisphere. Ontogenetically, visual lateralization starts with an asymmetrical embryonic position within the egg, which leads to asymmetries of light stimulation. Differences in exposure to light stimulation between the eyes result in activity differences between the ascending tectofugal pathways of the left and the right hemisphere, which are transcribed during a critical time span into morphological asymmetries. The asymmetries established after this transient period finally start to determine the lateralized processes of the visual system for the entire life span of the individual. We now can show that these anatomical lateralizations are accompanied by asymmetries of interocular transfer, which enable a faster shift of learned color cues from the dominant right to the left eye than vice versa. In summary, our data provide evidence that cerebral asymmetries are based both on "static" anatomical and on "dynamic" process-dependent principles.

Internally driven vs. externally cued movement selection: a study on the timing of brain activity

Brain imaging studies in man and single cell recordings in monkey have suggested that medial supplementary motor areas (SMA) and lateral pre-motor areas (PMA) are functionally dissociated concerning their involvement in internally driven and externally cued movements. This dichotomy, however, seems to be relative rather than absolute. Here, we searched for further evidence of relative differences and aimed to determine by what aspect of brain activity (duration, strength, or both) these might be accounted for. Event-related potentials (ERPs) were recorded while healthy, right-handed subjects selected one of three possible right hand digit movements based either on 'internal' choice or 'external' cues. The results obtained from ERP mapping suggest that movement selection evokes the same electrical brain activity patterns in terms of surface potential configurations in the same order and at the same strength independent of the selection mode. These identical configurations, however, differed in their duration. Combined with the results of a distributed source localization procedure, our data are suggestive of longer lasting activity in SMA during the 'internal' and longer lasting activity in PMA during the 'external' condition. Our results confirm previous findings in showing that SMA and PMA are distinctively involved in the two tasks and that this functional dichotomy is relative rather than absolute but indicate that such a dissociation can result from differences in duration rather than pure strength of activation.

Maturation of the mismatch negativity: effects of profound deafness and cochlear implant use

The use of cochlear implants to restore auditory sensation in deaf children is increasing, with a trend toward earlier implantation. However, little is known about how auditory deprivation and subsequent cochlear implant use affect the maturing human central auditory system. Our previous studies have demonstrated that the obligatory auditory evoked potentials (AEPs) of implanted children are very different from those of normal-hearing children. Unlike the obligatory potentials, which primarily reflect neural responses to stimulus onset, the mismatch negativity (MMN) provides a neurophysiological measure of auditory short-term memory and discrimination processes. The purpose of this investigation is to review our studies of the effects of auditory deprivation due to profound deafness and cochlear implant use on the maturation of the MMN in children, placed in the context of overall age-related changes in the AEPs. The development and application of a statistical technique to assess the MMN in individuals is also reviewed. Results show that although the morphology of the obligatory AEPs is substantially altered by the absence of a normal N(1) peak, the MMN is robustly present in a group of implanted children who have good spoken language perception through their device. Differences exist in the scalp distribution of the MMN between implanted and normal-hearing children. Specifically, the MMN appears to be more symmetrical in amplitude over both hemispheres, whereas it is initially much larger over the contralateral hemisphere in normal-hearing children. These findings suggest that, compared to N(1), the MMN is a better measure of basic auditory processes necessary for the development of spoken language perception skills in profoundly deaf children and adults who use a cochlear implant.

Crossed-uncrossed difference in simple reaction times to lateralized flashes: between- and within-subjects variability

In unimanual reaction times (RT) to lateralized flashes, contralateral responses tend to be slower than ipsilateral responses. This has been called Crossed-Uncrossed Difference (CUD). The CUD tends to show variability across subjects and across studies, but until now the stability of the CUD in an individual subject has not been investigated. To address the role of inter- and intra-subject variability in the CUD, three normal right handers were tested over 50 experimental sessions of 800 trials each, for a total of 40,000 trials of simple reaction times to lateralized flashes. In each subject, CUDs were computed for each session, over two, three, or more sessions, and over the entire dataset. These CUDs were then compared to the CUDs obtained in a group of 15 normal right handers, each tested once in a single session. Results show that: (i) CUD variability across several sessions in a single subject mimics the variability observed in a sample of subjects tested in a single session; (ii) this variability is considerably reduced when the CUD is computed over at least 2400 trials per subject; (iii) CUDs computed over 2400 and up to 12,000 of trials tend to be extremely similar ( approximately 2 ms) across the three subjects tested here; (iv) when reaction times are ordered from the fastest to the slowest and divided into bins, the CUD is remarkably stable over the entire reaction time distribution; and (v) in contrast to the variability of the CUD, the variability for crossed and uncrossed responses across several sessions in a single subject is small and does not mimic the variability observed in a sample of subjects tested in a single session. Taken together, these data suggest that the intersubject variability in the CUD observed in single experimental sessions does not represent a reliable intersubject difference and that the CUD computed over thousands of trials reflects hard-wired mechanisms of callosal transmission.

Evidence for physiological asymmetries in the intertectal connections of the pigeon (Columba livia) and their potential role in brain lateralisation

In pigeons, visual object processing is lateralised with a dominance of the left tectofugal system. To test the hypothesis, that avian visual lateralisation may arise, at least in part, from asymmetric interhemispheric inhibition, the intertectal modulation was quantified in 19 pigeons. Field potentials were recorded from intratectal electrodes in response to a stroboscope flash to the contralateral eye. Electrical stimulation of the contralateral tectum changed these flash-evoked potentials. This change was taken as a measure of intertectal modulation. It was found that the left-to-right tectotectal modulation was more pronounced than vice versa, supporting the hypothesis of an asymmetric modulation between the tecta of both hemispheres. It is conceivable that this lateralised interhemispheric crosstalk could constitute an important component of asymmetric visual processing.

Interhemispheric transfer of colour and shape information in the presence and absence of the corpus callosum

Two split-brained subjects, one (L.B.) with full forebrain commissurotomy and one (R.B.) with callosal agenesis, and a group of twenty neurologically intact subjects were tested in three discrimination tasks: a go-no go task, a two-choice task, and a three-choice task. The discriminations were based on colour in Experiment 1, and on shape in Experiment 2. The stimuli were presented in one or other visual field, and the subjects responded with the fingers of one or other hand, allowing the differences in reaction time between crossed and uncrossed responses (CUD) to be calculated. For the normal subjects the CUD tended to diminish with the complexity of the tasks, suggesting that both hemispheres were increasingly involved. Unlike R.B. and the normal controls, who made virtually no errors, L.B. had increasing difficulty as task complexity increased. He was better able to transfer information from the right to the left hemisphere than vice versa, but an analysis of his accuracy under the crossed conditions showed that the amount transferred was always well under one bit. This confirms previous evidence that L.B. has very limited subcortical transfer of either colour or shape.

VEP-interhemispheric transfer time in 20-32 Hz band in man

This research aimed to investigate callosal transfer in the different frequency bands of VEP to lateralized reversal of checkerboard pattern as stimuli. The chosen band pass filters (4-8 Hz, 8-15 Hz, 15-20 Hz, 20-32 Hz) were applied to the VEPs of subjects, and four different components for each VEP were obtained. Latency differences between hemispheres for digitally unfiltered and filtered VEPs were computed to estimate IHTT. Different IHTTs in the theta (17 ms), alpha (9 ms) and beta1 (7 ms) bands from right to left and from left to right were estimated, supporting the previous report. Furthermore, a transfer within 4.5 ms from right to left in the 20-32 Hz band for the occipital lobe was found. In conclusion, our findings suggest that the frequency analysis of VEP to lateralized stimuli give us additional information, relating to the types of callosal fibers.

Spatiotemporal modulation of attention during smooth pursuit eye movements

The present investigation examined how attention is distributed across both space and time during smooth pursuit eye movements. This was accomplished by measuring manual button pressing latencies to the sudden appearance of a peripheral target during the onset, maintenance, or offset of the pursuit response to a step-ramp target motion. The results showed that manual response latencies were shorter for stimuli flashed ahead of the pursuit target than for those presented in its wake. In addition, the latencies to the peripheral target appearance were shorter overall during pursuit maintenance compared to pursuit onset or offset. Taken together, these results indicate that attention is significantly modulated both in space and time during smooth pursuit eye movements.

Right hemispheric contribution to motor programming of simultaneous bilateral response

The purpose of this study was to verify the hypothesis that the motor program which integrates the left- and right-hand responses in the bilateral reaction time (RT) task is located in the right hemisphere. 8 female and 50 male students performed bilateral simultaneous RT tasks to lateralized light stimuli. Uncrossed RT based on the relationship between the unifying center in the right hemisphere and hemifield stimulus was shorter than crossed RT for the bilateral response. Therefore, the hypothesis was supported. Some areas of the right hemisphere contributed to unification of the movements of the right and left hands for bilateral movements in reaction time.

Visual evoked potential interhemispheric transfer time in different frequency bands

OBJECTIVE

Visual evoked potentials (VEP) have been used to estimate interhemispheric transfer time (IHTT). However, the complex wave of VEP is most probably formed by different generators of neural populations that act through different frequency channels. If the main peaks of VEP are established by different types of generators, which can also be connected to each other by a different type of callosal fibres, we would be able to estimate a wide range of various IHTT by measuring the latency between time-locked peaks of narrow band-pass filtered VEP. This research aimed to test this hypothesis.

METHODS

Nine right-handed men were presented with a reversal of a checkerboard pattern as stimuli at RVF or LVF, and EEG was recorded at O1, O2, P3, P4. The grand-averaged VEPs were transformed to the frequency domain by means of the fast Fourier transform to obtain the amplitude frequency characteristics. Band-pass filters were chosen adequately, according to tuning frequencies indicated by clear peaks in the amplitude frequency characteristics. The chosen band pass filters (4-8 Hz, 8-15 Hz, 15-20 Hz, 20-32 Hz) were applied to the VEP of the subjects, and 4 different components of VEPs for each VEP were obtained. The latency of P100 and N160 of unfiltered VEP was measured. In the band-pass digital filter applied VEPs, positive and negative peaks, which are consistent with P100 and N160, were measured for each subject. Latency differences between hemispheres for digitally unfiltered and filtered VEPs were computed to estimate IHTT.

RESULTS

In the different frequency bands, different IHTTs were estimated, ranging from 3 ms to 30 ms. Approximately 16 ms for theta band, 11 ms for alpha band, 6 ms for 15-20 Hz and 3 ms for 20-32 Hz bands were found.

CONCLUSIONS

Our findings support the hypothesis which states that unfiltered VEPs provide us with only a rough estimation of IHTT. Also, they are consistent with anatomical findings that describe callosal fibres of varying dimensions, predicting various velocities between hemispheres.