Interhemispheric neural summation in the split brain with symmetrical and asymmetrical displays

Interhemispheric Integration after Callosotomy: A Meta-Analysis of Poffenberger and Redundant-Target Paradigms

The central role of the corpus callosum in integrating perception and cognition across the cerebral hemispheres makes it highly desirable for clinical and basic research to have a repertoire of experimental paradigms assessing callosal functioning. Here, the objective was to assess the validity of two such paradigms (Poffenberger, redundant-target paradigms) by conducting single-step meta-analyses on individual case data of callosotomy patients. Studies were identified by systematic literature search (source: Pubmed and WebOfKnowledge, date: 07.03.2022) and all studies were included that reported callosotomy case data for either paradigm. Twenty-two studies (38 unique cases) provided 116 observations of the crossed-uncrossed difference (CUD) for the Poffenberger paradigm, while ten studies (22 cases, 103 observations) provided bilateral redundancy gain (bRG) measures. Using linear-mixed models with "individual" and "experiment" as random-effects variable, the mean CUD was estimated at 60.6 ms (CI95%: 45.3; 75.9) for commissurotomy, 43.5 ms (26.7; 60.2) for complete callosotomy, and 8.8 ms (1.1; 16.6) for partial anterior-medial callosotomy patients. The estimates of commissurotomy/callosotomy patients differed significantly from patients with partial callosotomy and healthy controls. The mean bRGmin (minimum unilateral reference) was estimated at 42.8 ms (27.1;58.4) for patients with complete and 30.8 ms (16.8; 44.7) for patients with partial callosotomy, both differing significantly from controls. One limitation was that different formulas for bRG were used, making it necessary to split the sample and reducing test power of some analyses. Nevertheless, the present findings suggest that both paradigms assess interhemispheric callosal integration, confirming their construct validity, but likely test distinct callosal functions.

The redundant target paradigm and its use as a blindsight-test: A meta-analytic study

The redundant target effect (RTE) is the well-known effect whereby a single target is detected faster when a second, redundant target is presented simultaneously. The RTE was shown in different experimental designs and applied in various clinical contexts. However, there are also studies showing non-effects or effects in the opposite direction. Our meta-analysis aims to investigate the replicability of the RTE. Herein, we focused on the clinical context within which the RTE has been applied most often and for which it gained particular prominence: The research on blindsight and other forms of residual vision in patients with damage to the neuronal visual system. The application of the RTE in clinical contexts assumes that whenever vision is present, an RTE will be found. Put differently, the RTE as a tool to uncover residual vision presumes that the RTE is a consistent feature of vision in the healthy population. We found a significant summary effect size of the RTE in healthy participants. The effect size depended on certain experimental features: task type, target configuration in the redundant condition, and how reaction times were computed in the single condition. A specific feature combination is typically used in blindsight research. Analyzing studies with this feature combination revealed a significant summary effect size in healthy participants predicting positive RTEs for future studies. A power-analysis revealed a required sample size of 14 participants to obtain an RTE with high reliability. However, the required sample size is rarely reached in blindsight research. Rather, blindsight research is mostly based on single-case studies. In summary, the RTE is a robust effect on group level but does not occur in every single individual. This means failure to obtain an RTE in a single patient should not be interpreted as evidence for the absence of residual vision in this patient.

Interhemispheric transfer of visual information: Meaningfulness and response formation

We examined whether Redundancy Gain (RG) can be dissociated from the response stage of a go/nogo paradigm, and whether the meaningfulness of a stimulus modulates the stage at which interhemispheric transfer occurs. Experiment 1 used a lateralized match-to-category paradigm, taken from categories with varying meaningfulness. Experiment 2 presented a novel design, which separates the perceptual stage from response formation, in examination of RG. A sequence of two stimuli was presented. Participants responded by matching the category of the second stimulus to that of the first. The redundant stimulus could appear at the first or the second stage, thus redundancy gain could be separated from the response. Experiment 1 revealed that redundancy gain occurs earlier in the process of stimulus identification for highly meaningful stimuli than for less meaningful stimuli. The results of Experiment 2 support the hypothesis that redundancy gain results from interhemispheric integration of perceptual information, rather than response-formation. Results from both experiments suggest that redundancy gain arises from interhemispheric integration in the perceptual stage, and the efficiency of this integration depends on the meaningfulness of the stimulus. These results are relevant to current hypotheses about the physiological mechanisms underlying RG.

Absence of distracting information explains the redundant signals effect for a centrally presented categorization task

The redundant signals effect, a speed-up in response times with multiple targets compared to a single target in one display, is well-documented, with some evidence suggesting that it can occur even in conceptual processing when targets are presented bilaterally. The current study was designed to determine whether or not category-based redundant signals can speed up processing even without bilateral presentation. Toward that end, participants performed a go/no-go visual task in which they responded only to members of the target category (i.e., they responded only to numbers and did not respond to letters). Numbers and letters were presented along an imaginary vertical line in the center of the visual field. When the single signal trials contained a nontarget letter (Experiment 1), there was a significant redundant signals effect. The effect was not significant when the single-signal trials did not contain a nontarget letter (Experiments 2 and 3). The results indicate that, when targets are defined categorically and not presented bilaterally, the redundant signals effect may be an effect of reducing the presence of information that draws attention away from the target. This suggests that redundant signals may not speed up conceptual processing when interhemispheric presentation is not available.

Enhancing the ecological validity of tests of lateralization and hemispheric interaction: Evidence from fixated displays of letters or symbols of varying complexity

Two experiments expand upon behavioural evidence of interactions among lateralization, hemispheric interaction, and task complexity with findings from an ecologically valid procedure. Target displays of letters or symbols were presented at fixation in go/no-go matching tasks of physical or categorical identity. Simultaneously with the target, a distractor appeared in the left visual field or right visual field to weight processing of the target to the hemisphere ipsilateral to the distractor, or the distractor did not appear at all. Comparison of the respective distractor-present trials with distractor-absent trials measures the relative costs or benefits of hemispheric interaction. Both experiments found that 3-item displays were processed faster and more accurately than displays of 5 items, suggesting they are relatively simple. Accuracy to the simple tasks showed left-hemisphere lateralization in the lexical task, right-hemisphere lateralization in the spatial task, a cost of hemispheric interaction compared to the advantaged hemisphere, and a benefit of hemispheric interaction compared to the less-advantaged hemisphere, suggesting that the contributions of the less-advantaged hemisphere interfere with processing, and that the advantaged hemisphere controls the lion's share. In contrast, 5-item displays for physical match in both experiments showed a significant benefit to accuracy of hemispheric interaction compared to the left hemisphere, an insignificant benefit compared to the right hemisphere, no lateralization, no cost of hemispheric interaction, and a consequence to performance that was more costly to the hemisphere that had been advantaged in simple tasks, suggesting that the advantaged hemisphere relinquishes control as tasks become more complex and complementary processing results from both increased collaboration and decreased lateralization between the hemispheres. The findings expand upon behavioural evidence, converge with imaging evidence, and suggest future directions for brain mapping.

Abnormal asymmetry of brain connectivity in schizophrenia

Recently, a growing body of data has revealed that beyond a dysfunction of connectivity among different brain areas in schizophrenia patients (SCZ), there is also an abnormal asymmetry of functional connectivity compared with healthy subjects. The loss of the cerebral torque and the abnormalities of gyrification, with an increased or more complex cortical folding in the right hemisphere may provide an anatomical basis for such aberrant connectivity in SCZ. Furthermore, diffusion tensor imaging studies have shown a significant reduction of leftward asymmetry in some key white-matter tracts in SCZ. In this paper, we review the studies that investigated both structural brain asymmetry and asymmetry of functional connectivity in healthy subjects and SCZ. From an analysis of the existing literature on this topic, we can hypothesize an overall generally attenuated asymmetry of functional connectivity in SCZ compared to healthy controls. Such attenuated asymmetry increases with the duration of the disease and correlates with psychotic symptoms. Finally, we hypothesize that structural deficits across the corpus callosum may contribute to the abnormal asymmetry of intra-hemispheric connectivity in schizophrenia.

Redundancy Gain in Binocular Rivalry

A behavioural advantage is found across a wide range of stimuli when two targets are presented in opposite hemifields compared with those targets being presented together in one hemifield, or one target being presented alone. This advantage for responses to multiple targets versus a single target is often termed redundancy gain. Here we report on the findings of two experiments investigating redundancy gain in binocular rivalry. Experiment 1 presented a rival pair in one hemifield with an additional image presented to both eyes in the opposite hemifield. There was a weak effect of this stable image on the perceived dominance of the images within the rival pair. Experiment 2 presented a second rival pair in either the same or opposite hemifield and showed that instances of joint predominance were greater when the two pairs were presented in opposite hemifields than within the same hemifield. Therefore, the findings suggest that redundancy gain may be extended to stimuli presented under binocular rivalry conditions.

Fiber tract-driven topographical mapping (FTTM) reveals microstructural relevance for interhemispheric visuomotor function in the aging brain

We present a novel approach - DTI-based fiber tract-driven topographical mapping (FTTM) - to map and measure the influence of age on the integrity of interhemispheric fibers and challenge their selective functions with measures of interhemispheric integration of lateralized information. This approach enabled identification of spatially specific topographical maps of scalar diffusion measures and their relation to measures of visuomotor performance. Relative to younger adults, older adults showed lower fiber integrity indices in anterior than posterior callosal fibers. FTTM analysis identified a dissociation in the microstructural-function associates between age groups: in younger adults, genu fiber integrity correlated with interhemispheric transfer time, whereas in older adults, body fiber integrity was correlated with interhemispheric transfer time with topographical specificity along left-lateralized callosal fiber trajectories. Neural co-activation from redundant targets was evidenced by fMRI-derived bilateral extrastriate cortex activation in both groups, and a group difference emerged for a pontine activation cluster that was differently modulated by response hand in older than younger adults. Bilateral processing advantages in older but not younger adults further correlated with fiber integrity in transverse pontine fibers that branch into the right cerebellar cortex, thereby supporting a role for the pons in interhemispheric facilitation. In conclusion, in the face of compromised anterior callosal fibers, older adults appear to use alternative pathways to accomplish visuomotor interhemispheric information transfer and integration for lateralized processing. This shift from youthful associations may indicate recruitment of compensatory mechanisms involving medial corpus callosum fibers and subcortical pathways.

Visual interhemispheric communication and callosal connections of the occipital lobes

Callosal connections of the occipital lobes, coursing in the splenium of the corpus callosum, have long been thought to be crucial for interactions between the cerebral hemispheres in vision in both experimental animals and humans. Yet the callosal connections of the temporal and parietal lobes appear to have more important roles than those of the occipital callosal connections in at least some high-order interhemispheric visual functions. The partial intermixing and overlap of temporal, parietal and occipital callosal connections within the splenium has made it difficult to attribute the effects of splenial pathological lesions or experimental sections to splenial components specifically related to select cortical areas. The present review describes some current contributions from the modern techniques for the tracking of commissural fibers within the living human brain to the tentative assignation of specific visual functions to specific callosal tracts, either occipital or extraoccipital.

Bilateral redundancy gain and callosal integrity in a man with callosal lipoma: a diffusion-tensor imaging study

We investigated whether abnormalities in the structural organization of the corpus callosum in the presence of curvilinear lipoma are associated with increased facilitation of response time to bilateral stimuli, an effect known as the redundancy gain (RG). A patient (A.J.) with a curvilinear lipoma of the corpus callosum, his genetically-identical twin, and age-matched control participants made speeded responses to luminant stimuli. Structural organization of callosal regions was assessed with diffusion-tensor imaging. A.J. was found to have reduced structural integrity in the splenium of the corpus callosum and produced a large RG suggestive of neural summation.

Are the basal ganglia critical in producing redundancy gain effects on simple sensorimotor responses? An investigation on the effects of Parkinson's disease

Redundancy gain (RG) is a well-known effect in the experimental psychology literature which is thought to reflect integration of sensory information. RG is a facilitation in speed of responding on a detection task when two stimuli are presented at once compared to when one stimulus is presented alone. Even though sensorimotor tasks involving integration of sensory information form the basis of a large repertoire of human skilled actions, the neural basis of reliable effects such as RG remains elusive. The present study examines whether the integrity of the basal ganglia system is likely to be critical for RG effects to occur. Based on a thorough analysis of performance on a standard paradigm of RG (and on the related paradigm of crossed-uncrossed differences: CUDs) in patients with mild to moderate Parkinson's disease and matched controls, we found virtually no differences between groups. We conclude that normal RG and CUD effects are not likely to rely critically on the BG.

Contribution of callosal connections to the interhemispheric integration of visuomotor and cognitive processes

In recent years, cognitive neuroscience has been concerned with the role of the corpus callosum and interhemispheric communication for lower-level processes and higher-order cognitive functions. There is empirical evidence that not only callosal disconnection but also subtle degradation of the corpus callosum can influence the transfer of information and integration between the hemispheres. The reviewed studies on patients with callosal degradation with and without disconnection indicate a dissociation of callosal functions: while anterior callosal regions were associated with interhemispheric inhibition in situations of semantic (Stroop) and visuospatial (hierarchical letters) competition, posterior callosal areas were associated with interhemispheric facilitation from redundant information at visuomotor and cognitive levels. Together, the reviewed research on selective cognitive functions provides evidence that the corpus callosum contributes to the integration of perception and action within a subcortico-cortical network promoting a unified experience of the way we perceive the visual world and prepare our actions.

Parallel interhemispheric processing in hemineglect: relation to visual field defects

Parallel interhemispheric processing is required to explore our visual environment and to integrate visual information from both hemifields simultaneously. Damage to the right temporo-parietal cortex can disrupt such parallel processes and result in neglect and visual extinction of stimuli in the left contralesional visual space. Neglected or extinguished stimuli can still be processed, yet without reaching the patient's awareness. Such unconscious processing has been attributed to structurally intact primary visual areas in neglect. To study whether unconscious parallel processing depends on visual functional integrity, we compared the performance of neglect patients with visual field defects (VFDs) (n=11) and hemianopic patients with partial or complete blindness of one visual hemifield (n=11) on redundant targets effects (RTE). The RTE manifests as faster reaction times to redundant paired (two stimuli, one in each hemifield) than single stimulation (in one hemifield). We found RTEs, i.e., unconscious processing, in neglect patients but not in hemianopic patients. Furthermore, neglect patients showed large crossed-uncrossed differences (CUDs), i.e., faster response times to ipsi- than contralesional hemifield stimulation, reflecting a difference in processing speed for single stimuli in the two hemispheres that were correlated with VFDs and visual extinction. The finding that extinction, but not RTE, was correlated with the CUD suggests that under competitive bilateral stimulus conditions the delayed contralesional visual field input may not be detected by the intact left hemisphere, which presumably mediates the task given the impairment of the right hemisphere. By contrast, unconscious parallel processing of contralesional stimuli (RTE) occurred even when contralesional visual field input is lacking (VFD) or delayed (CUD) and is possibly mediated via subcortical visual pathways.

Fast visuomotor processing of redundant targets: the role of the right temporo-parietal junction

Parallel processing of multiple sensory stimuli is critical for efficient, successful interaction with the environment. An experimental approach to studying parallel processing in sensorimotor integration is to examine reaction times to multiple copies of the same stimulus. Reaction times to bilateral copies of light flashes are faster than to single, unilateral light flashes. These faster responses may be due to ‘statistical facilitation’ between independent processing streams engaged by the two copies of the light flash. On some trials, however, reaction times are faster than predicted by statistical facilitation. This indicates that a neural ‘coactivation’ of the two processing streams must have occurred. Here we use fMRI to investigate the neural locus of this coactivation. Subjects responded manually to the detection of unilateral light flashes presented to the left or right visual hemifield, and to the detection of bilateral light flashes. We compared the bilateral trials where subjects' reaction times exceeded the limit predicted by statistical facilitation to bilateral trials that did not exceed the limit. Activity in the right temporo-parietal junction was higher in those bilateral trials that showed coactivation than in those that did not. These results suggest the neural coactivation observed in visuomotor integration occurs at a cognitive rather than sensory or motor stage of processing.

Special Section: Towards an Embodiment of Goals

This paper discusses both a dissociation view and a dynamic view with respect to the study of voluntary, goal-directed behavior. The dissociation view builds on the recently reintroduced ideomotor principle, and conceives of clearly dissociated and hierarchical roles for the planning and control of action. The dynamic view has a more integral and dynamic conception of how planning, control, and timing merge in the guidance of behavior. This view, however, lacks a clear way of encompassing the goaldirectedness of behavior. For behavior to be effective and efficient, sensory information has to play an equally important role in guiding action as goal-related information does. As a third view, a dynamic action-selection approach is introduced by combining aspects of the former two. This model is able to merge ideomotor and sensorimotor processes continuously and in real time. In discussing the action-selection approach, a special emphasis is given to the role of long-term influences like preferences and goals.

Action planning in young children's tool use

Tool use consists of at least two coupled phases of activities, involving multi-step problem solving. It therefore provides an interesting window on the development of planning in goal-directed behavior. This study investigated 2-year-olds' and 3-year-olds' hand use in picking up and subsequently using a tool for displacing a target-object towards a specified goal location. The children had to use a stick (Experiment 1; N = 41 in total) or a cane (hooked stick) that was lying in varying starting orientations (Experiment 2; N = 32 in total). Age differences were found in the way goal-related information in combination with tool-related information influenced the choice of which hand to use in different phases of the task. A view on action planning is developed as a dynamic action-selection process for which actions to take. This process integrates factors that are internal to the child's action system (e.g. motor preferences and dexterity) with external (i.e. sensory) sources of information.

Redundancy gain with static versus moving hands: a test of the hemispheric coactivation model

A simple reaction time (RT) experiment was conducted to test the hypothesis that redundancy gain arises partly because of hemispheric coactivation. Stimuli were presented to the left or right of fixation, or redundantly to both, and the participants had to make keypress responses as rapidly as possible to stimulus onset. A "static" condition, in which the participants held their hands at rest, was compared with a "dynamic" condition, in which they moved their hands back and forth in an oscillating motion prior to stimulus onset. As predicted from the hypothesis of hemispheric coactivation, redundancy gain, measured as the decrease in mean RT to redundant stimuli as compared with single stimuli, was smaller in the dynamic condition than in the static one.

On the relationship between the redundant signals effect and temporal order judgments: Parametric data and a new model

Paradigms used to study the time course of the redundant signals effect (RSE; J. O. Miller, 1986) and temporal order judgments (TOJs) share many important similarities and address related questions concerning the time course of sensory processing. The author of this article proposes and tests a new aggregate diffusion-based model to quantitatively explain both the RSE and TOJs and the relationship between them. Parametric data (13 stimulus onset asynchronies) from an experiment with pairs of visual stimuli (626-nm LEDs) confirm that, relative to central signals (3 degrees), peripheral signals (35 degrees) yield slower reaction times, more strongly modulated RSE time-course functions, and flatter TOJ psychometric functions. All of these qualitative features are well captured, even in quantitative detail, by the aggregate diffusion model.

Functional asymmetry and interhemispheric cooperation in the perception of emotions from facial expressions

The present study used the redundant target paradigm on healthy subjects to investigate functional hemispheric asymmetries and interhemispheric cooperation in the perception of emotions from faces. In Experiment 1 participants responded to checkerboards presented either unilaterally to the left (LVF) or right visual half field (RVF), or simultaneously to both hemifields (BVF), while performing a pointing task for the control of eye movements. As previously reported (Miniussi et al. in J Cogn Neurosci 10:216-230, 1998), redundant stimulation led to shorter latencies for stimulus detection (bilateral gain or redundant target effect, RTE) that exceeded the limit for a probabilistic interpretation, thereby validating the pointing procedure and supporting interhemispheric cooperation. In Experiment 2 the same pointing procedure was used in a go/no-go task requiring subjects to respond when seeing a target emotional expression (happy or fearful, counterbalanced between blocks). Faster reaction times to unilateral LVF than RVF emotions, regardless of valence, indicate that the perception of positive and negative emotional faces is lateralized toward the right hemisphere. Simultaneous presentation of two congruent emotional faces, either happy or fearful, produced an RTE that cannot be explained by probability summation and suggests interhemispheric cooperation and neural summation. No such effect was present with BVF incongruent facial expressions. In Experiment 3 we studied whether the RTE for emotional faces depends on the physical identity between BVF stimuli, and we set a second BVF congruent condition in which there was only emotional but not physical or gender identity between stimuli (i.e. two different faces expressing the same emotion). The RTE and interhemispheric cooperation were present also in this second BVF congruent condition. This shows that emotional congruency is the sufficient condition for the RTE to take place in the intact brain and that the cerebral hemispheres can interact in spite of physical differences between stimuli.

fMRI evidence for individual differences in premotor modulation of extrastriatal visual-perceptual processing of redundant targets

To perceive the vast array of stimuli in the world around us, the visual system employs parallel processing mechanisms that ensure efficiency in perceiving multiple objects in a scene. A way to test this efficiency is to measure reaction time (RT) to pairs of identical stimuli, presented singly or as doublets; typically, the resulting phenomenon is the redundant targets effect (RTE), which manifests as faster RTs to paired than singly presented stimuli. It is controversial, however, whether the neural locus of the parallel processing mechanisms invoked to produce the RTE is perceptual or motor and why some studies observe a substantial RTE and others do not. To resolve these two issues, we measured the RTE in young adults while undergoing functional MRI. Regarding the question of a perceptual or motor basis for the RTE, we observed that bilateral activation of extrastriate cortex was prominent in paired vs. the sum of the two single stimulus conditions, indicating that the RTE invoked perceptual mechanisms; by contrast, the motor cortex was not disproportionately activated in this comparison. Regarding the magnitude of the RTE, we compared activation patterns in individuals with small vs. large RTEs and observed that frontal and premotor areas were activated with small RTEs. These data indicate that the primary processing level of response facilitation, observed as the RTE, is perceptual, but the modulation of the RTE magnitude is premotor and associated with basic aspects of response selection and preparation.

Multisensory processing in the redundant-target effect: a behavioral and event-related potential study

Participants respond more quickly to two simultaneously presented target stimuli of two different modalities (redundant targets) than would be predicted from their reaction times to the unimodal targets. To examine the neural correlates of this redundant-target effect, event-related potentials (ERPs) were recorded to auditory, visual, and bimodal standard and target stimuli presented at two locations (left and right of central fixation). Bimodal stimuli were combinations of two standards, two targets, or a standard and a target, presented either from the same or from different locations. Responses generally were faster for bimodal stimuli than for unimodal stimuli and were faster for spatially congruent than for spatially incongruent bimodal events. ERPs to spatially congruent and spatially incongruent bimodal stimuli started to differ over the parietal cortex as early as 160 msec after stimulus onset. The present study suggests that hearing and seeing interact at sensory-processing stages by matching spatial information across modalities.

Now you see it, now you don't: Variable hemineglect in a commissurotomized man

We describe the case of a callosotomized man, D.D.V., who shows unusual neglect of stimuli in the left visual field (LVF). This is manifest in simple reaction time (RT) to stimuli flashed in the LVF and in judging whether pairs of filled circles in the LVF are of the same or different color. It may reflect strong left-hemispheric control and consequent attention restricted to the right side of space. It is not evident in simple RT when there are continuous markers in the visual fields to indicate the locations of the stimuli. In this condition, his RTs are actually faster to LVF than to right visual field (RVF) stimuli, suggesting a switch to right-hemispheric control that eliminates the hemineglect. Neglect is also not evident when D.D.V. responds by pointing to or touching the locations of the stimuli, perhaps because these responses are controlled by the dorsal rather than the ventral visual system. Despite his atypical manifestations of hemineglect, D.D.V. showed evidence of functional disconnection typical of split-brained subjects, including prolonged crossed-uncrossed different in simple reaction time, inability to match colors between visual fields, and enhanced redundancy gain in simple RT to bilateral stimuli even when the stimulus in the LVF was neglected.

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.

Early Visual Evoked Potentials in Callosal Agenesis

Three participants with callosal agenesis and 12 neurologically normal participants were tested on a simple reaction time task, with visual evoked potentials collected using a high-density 128-channel system. Independent-components analyses were performed on the averaged visual evoked potentials to isolate the components of interest. Contrary to previous research with acallosals, evidence of ipsilateral activation was present in all 3 participants. Although ipsilateral visual components were present in all 4 unilateral conditions in the 2 related acallosal participants, in the 3rd, these were present only in the crossed visual field-hand conditions and not in the uncrossed conditions. Suggestions are made as to why these results differ from earlier findings and as to the neural mechanisms facilitating this ipsilateral activation.

Exaggerated redundancy gain in the split brain: A hemispheric coactivation account

Recent studies of redundancy gain indicate that it is especially large when redundant stimuli are presented to different hemispheres of an individual without a functioning corpus callosum. This suggests the hypothesis that responses to redundant stimuli are speeded partly because both hemispheres are involved in the activation of the response. A simple formal model incorporating this idea is developed and then elaborated to account for additional related findings. Predictions of the latter model are in good qualitative agreement with data from a number of sources, and there is neuroanatomic and psychophysiological support for its underlying structure.

Redundancy gain in simple reaction time following partial and complete callosotomy

Four subjects with partial or complete section of the corpus callosum were tested on simple reaction time (RT) to visual stimuli presented either singly in one or other visual field, or simultaneously in both visual fields. The subject with posterior callosal section showed evidence of redundancy gain with bilateral stimuli beyond that attributable to probability summation ("enhanced" redundancy gain), and prolonged interhemispheric transfer. One of the two subjects with anterior section, like normals, showed little evidence of enhanced redundancy gain, and no evidence of prolonged interhemispheric transfer. The other did show some enhanced redundancy gain at the fast end of the RT distribution. These and other results suggest that the posterior corpus callosum provides the principal route or routes of interhemispheric transfer of the information required for simple visuomotor responses, and is also responsible for the much reduced redundancy gain in normal subjects relative to that in split-brained subjects. The subject with complete callosal section was unusual in that he responded only very rarely to stimuli in the left visual field (LVF), yet he showed markedly reduced RTs to bilateral relative to right visual field (RVF) stimuli.

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.

Interhemispheric neural summation in the split brain: effects of stimulus colour and task

Four split-brained subjects, two subjects with agenesis of the corpus callosum, and 14 normal subjects performed two tasks requiring responses to red or green disks, briefly presented either singly in the left visual field, singly in the right visual field, or simultaneously in both visual fields. In Experiment 1, simple reaction times to these stimuli, regardless of colour, were recorded (the Go-Both Task), and found to be faster to bilateral-redundant stimulus pairs, than to single stimuli. This so-called "redundancy gain" was much larger for acallosal or split-brained subjects than for normal subjects and exceeded the predictions of a race model, implying neural summation. Experiment 2 used the same stimuli, but subjects were required to respond only to stimuli of a designated colour (the Go/No-Go Task). Redundant target stimuli produced neural summation, while stimuli pairs that included a non-target stimulus did not. These results suggest that neural summation in the acallosal or split brain involves the convergence of response-associated activation, and that redundant sensory processes are not sufficient.

Interhemispheric visuo-motor integration in humans: the effect of redundant targets

We used event-related functional magnetic resonance imaging (fMRI) to investigate the functional locus of response facilitation during parallel visuo-motor processing. In a simple reaction-time task, subjects typically respond faster to two copies of the same stimulus than to a single copy. This facilitation, called the redundant-target effect, can occur at three functional levels: perceptual, 'cognitive' or motor. Normal right handers were studied while performing a simple reaction-time task to unilateral (left or right) and bilateral light flashes. Subjects were instructed to respond with their right index finger. Reaction times were faster to bilateral light flashes than to unilateral ones, even right flashes. Greater fMRI signal for bilateral stimuli compared to unilateral ones was observed in the left precentral and postcentral gyrus, and in the right precentral gyrus. A greater fMRI signal for bilateral and for unilateral left stimuli, compared to unilateral right stimuli, was observed in an area of the right intraparietal sulcus. These results support the hypothesis that the functional locus of response facilitation during parallel visuo-motor processing is premotor.

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.

Paradoxical interhemispheric summation in the split brain

We measured simple reaction time (RT) to light flashes, presented either singly or in pairs, in two people who had undergone callosotomy, one person with agenesis of the corpus callosum, and 17 normal subjects. The three split-brained subjects' RTs were decreased to bilateral pairs beyond predictions based on a simple race between independent unilateral processes, while those of the normal subjects were actually longer than predicted by the race model. This effect was present whether the bilateral pairs were in mirror-image locations or not, but was not present when the pairs were presented unilaterally. Since summation does not depend on close spatial correspondence, and also occurs when inputs are staggered in time, we suggest that it is due to cortical projection to a subcortical arousal system, and is normally inhibited by the corpus callosum.