The parallel brain: the cognitive neuroscience of the corpus callosum
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It also provides a comprehensive introduction to cognitive neuroscience. Focusing on the central problem of simple reaction time, it examines the most basic possible sequence of perception-decision-action. The task is to press a button with one hand as soon as a patch of light is detected in the peripheral visual field. When the patch appears in the visual field opposite the responding hand, there must be interhemispheric transfer prior to response. But transfer of what—a visual input code?
A cognitive decision code? A motor response code? Combining animal models, normal human studies, and clinical evidence, the authors apply anatomical, physiological, and behavioral perspectives to this question. The emerging view is that the corpus callosum consists of many parallel interhemispheric channels for communication and control, and that every transfer channel is context-dependent and modulated by attention.
After the great excitement that followed the work of Roger Sperry on split-brain patients, corpus callosum research progressively faded away as a hot topic in cognitive neuroscience. Zaidel and Iacoboni's book will give a great boost in reinstating the problem of hemispheric specialization as a central one in cognitive neuroscience.
It points out a variety of interesting problems that the corpus callosum approach allows one to address, and summarizes many of the most important contributions in this field. The commentaries following the main articles are especially valuable. They give the reader a sense of how lively and interesting are the issues covered in the book, and one hopes that they will be the catalyst for additional research in the field. How the two hemispheres of the brain communicate and interact and the dysfunctions that occur in interhemispheric communication with specific psychiatric and cognitive abnormalities are crucial topics for understanding higher brain function.
The Parallel Brain provides the most comprehensive review of this literature to date and is a must-read for anyone interested in brain asymmetry and interhemispheric interaction. This volume will be indispensable to students and scholars for years to come. Terry E. 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.
The concept of a distributed neural circuitry has inspired investigations into how brain connectivity enables hemispheric specialization for mental processes, which range from basic perception Nassi and Callaway to higher cognitive functions and consciousness Galaburda and Geschwind ; Zaidel and Iacoboni A common theme of these investigations is how perceptions, thoughts and actions emerge from the integration of sensory, cognitive and motor inputs received or processed in separate cerebral hemispheres.
Cortico-cortical communication within and between hemispheres relies on a widespread white matter network that also connects to subcortical gray matter. The white matter tracts can be divided in association pathways that are intrahemispheric fibers connecting cortices of the frontal, parietal, temporal and occipital lobes and commissural pathways that are interhemispheric fibers connecting the cortical areas of both hemispheres Aralasmak et al.
Several cortico-cortical association and cortico-subcortical projection pathways ensure communication within each hemisphere. Superior longitudinal fasciculus SLF fibers, for example, connect temporo-parieto-occipital regions with the frontal lobe. While left-hemispheric SLF fibers have been related to speech Duffau et al.
The inferior longitudinal fasciculi connect temporal to occipital lobes in each hemisphere and have been associated with higher-order visual functions such as object recognition left-lateralized; Salvan et al. The inferior fronto-occipital fasciculi connect visual association areas with the frontal eye fields, and there is some evidence that they mediate simultaneous perception intrahemipherically needed for selecting visual information in crowded scenes right-lateralized or bilateral; Battelli et al.
The cingulum is a large fiber bundle of the limbic system connecting cingulate and hippocampal gyri with amygdala, nucleus accumbens, thalamus and frontal cortices, areas involved in emotion, reward and executive control. Thus, each hemisphere has extensive cortico-cortical and cortico-subcortical connections Mori et al. The myriad information transferred via these fiber tracts and processed in each hemisphere is also available for integration between the two hemispheres via commissural pathways to form a unified perception. The major commissural pathway is the corpus callosum CC , a thick band of white matter fibers consisting of more than million axons connecting the right and left cerebral hemispheres of the brain and playing an important role in interhemispheric transfer of information Aboitiz et al.
Although callosal fibers mostly project to homotopic regions of the hemispheres e. Clarke and Zaidel ; Cook ; Rakic and Yakovlev The CC has no macroscopic anatomical landmarks that delimit anatomical and functionally distinct regions Hofer and Frahm It has been geometrically divided into three regions Duara et al. Although our understanding of the organization of CC white matter pathways has considerably increased Poupon et al. Sagittal, axial and coronal section of the human brain The SRI24 multichannel atlas of normal adult human brain structure; Rohlfing et al.
Other illustrated interhemispheric connections are the anterior commissure AC and the superior colliculi SC. In humans, primary visual, auditory and motor cortex functions are highly lateralized with the left cerebral hemisphere processing right-sided sensory input and motor output and the right cerebral hemisphere processing left-sided sensory input and motor output.
Input from both hemispheres needs to be integrated and selected for further higher-order cognitive processing and for a unified motor response Stephan et al. From an evolutionary standpoint the CC is assumed to play an integral role in the development of higher cognitive processes and hemispheric specialization Doron and Gazzaniga During human evolution, brain and CC size increased considerably.
Some studies suggest that the communication speed between the two hemispheres is faster with increased brain size Caminiti et al. Interestingly, recent animal studies have suggested that more lateralized brains are better to act directly on many sources of information at the same time Magat and Brown For example, chimpanzees that use one hand to fish termites out of their mounds are more efficient than chimpanzees that are ambidextrous Marchant and McGrew Although the CC is the largest interhemispheric commissure, other commissural pathways include the anterior, hippocampal, posterior, habenuar, and tectal commissures and also the two sides of the brainstem reticular formation have extensive connections Aralasmak et al.
The superior colliculus and the tectal commissure are thought to play a role in visual transfer and to maintain interhemispheric transfer of basic visual information in patients with callosal agenesis, callosotomy and commissurotomy Tardif and Clarke Nevertheless, studies in acallosal and split-brain patients state quite clearly that absence or loss of CC integrity contributes to impairment in sensory and cognitive integration Fabri et al.
In fact, many years of research have demonstrated a relationship between callosal connectivity and prolonged interhemispheric transfer time in split-brain patients and in acallosal patients Corballis ; Iacoboni et al. Less radical disruption of callosal fiber integrity such as seen in patients with multiple sclerosis Warlop et al. Different callosal functions have been proposed for interhemispheric communication.
For example, the hemispheric insulation view states that the CC acts as a shield to reduce interference and to prevent potentially harmful intrusions from the opposite hemisphere Kinsbourne and Hicks ; Liederman and Meehan , whereas the callosal inhibition view assumes that stimulus-driven activity in one hemisphere suppresses activity in the other, thereby, for instance, causing a shift in attention to the contralateral side of the more activated hemisphere Chiarello and Maxfield Callosal reciprocal inhibition may further equilibrate activation and prevent attention from being skewed to the contralateral side when one hemisphere is disproportionately active Kinsbourne Interhemispheric cooperation between the hemispheres has been assumed to be more advantageous than within hemispheric processing in situations of high processing demand Banich and Belger Interhemispheric sharing of information and the need for inhibiton or isolation from each other are not mutually exclusive and may depend on the kind of information processed.
It is, for example, well established that the two hemispheres have different functions, with a left-hemispheric superiority in processing speech and language and a right-hemispheric superiority for certain visuospatial functions. Thus, when one hemisphere is superior to the other in processing specific information, intrahemispheric processing may be more advantageous, and callosal isolation or inhibition would increase processing efficiency and performance.
By contrast, when both hemispheres are qualified to process the same information, sharing of hemispheric resources may be more advantageous and require callosal interhemispheric cooperation Bloom and Hynd First, we will provide an overview of the role of the CC in lower-level processes and interhemispheric visuomotor integration as documented in recent behavioral and neuroimaging studies on the crossed-uncrossed difference, an estimation of interhemispheric transfer time Poffenberger , and on redundancy gain, a measure of visuomotor integration Miller ; Mordkoff and Yantis , Secondly, we will explore the contribution of the CC to higher-order cognitive functions, such as attention and executive control, and to integrating the left hemisphere, specialized for processing local details and language, and right hemisphere, specialized for processing holistic, global forms and spatial attention functions Aziz-Zadeh et al.
Approaches describing and analyzing interhemispheric integration of information in humans include simple reaction time RT tasks in which targets are presented in the same uncrossed or opposite crossed visual field in relation to the responding hand Poffenberger ; Zaidel and Iacoboni In normal subjects the CUD is approximately 4—6 ms Marzi et al. Hanajima et al. There is an ongoing debate whether the CUD is based solely on the anatomy of callosal fibers or whether the CUD may also be mediated by attentional components Mooshagian et al. DTI yields estimates of fractional anisotropy FA that indexes white matter structural integrity and measures the orientational displacement and distribution of water molecules in vivo across tissue components Pierpaoli and Basser FA is typically higher in fibers with a homogeneous or linear structure than in tissue with an inhomogenous structure, such as areas with pathology Lansberg et al.
DTI studies have revealed correlations between subtle variations in regional white matter callosal microstructure and behavioral measures of IHTT and cognitive ability Muetzel et al. In adolescence 9—23 years , for example, age-related increases in callosal FA were associated with better bimanual coordination using an alternating finger-tapping task Muetzel et al. In normal aging, Schulte et al. By contrast, Westerhausen et al. Methodological variables such as stimulus parameters eccentricity and luminance, fixation control, forehead-chin rest Schiefer et al.
There is also evidence that attention influences CUD measures. In a functional magnetic resonance imaging fMRI study, Weber et al. They found that crossed relative to uncrossed trials CUD activated anterior callosal area, but when attentionally manipulated, activated superior colliculi and right superior parietal cortex, a region typically associated with attentional shifts.
This is consistent with findings of Iacoboni and Zaidel suggesting that shifts in spatial attention Corbetta et al. Activations of premotor and parietal cortex regions observed with fMRI also map to fibers linking through the mid-body and anterior regions of the CC Iacoboni and Zaidel ; Schulte et al. Recently, Mooshagian and colleagues examined the influence of spatial attentional shifts on CUD in patients with agenesis of the CC and complete commissurotomy and in healthy subjects by alternating between natural arms uncrossed and unnatural arm positions arms crossed.
The CUD was smaller with arms crossed unnatural arms position than uncrossed natural arms position in patients as well as normal controls indicating that the relative arm position is sufficient to modulate the CUD even in the absence of the CC. In a recent event related potential ERP experiment, Moes et al. The reduced asymmetry in women is mainly due to shorter left-to-right IHTT in women than men. Different assumptions have been made to explain this asymmetry in IHTT such as a smaller average size in the right occipital lobe resulting in fewer callosal fibers projecting from left-to-right posterior areas Saron and Davidson , faster axonal conduction in the right hemisphere relative to the left Barnett and Corballis , or the degree of hemispheric specialization as suggested by a reversal of asymmetric transfer times for verbal and non-verbal stimuli Nowicka et al.
Alternatively, differences in the distribution of gray and white matter in the two cerebral hemispheres have been proposed to reflect neuronal organization promoting inter-or intrahemispheric processing with more gray matter relative to white matter in the left hemisphere than in the right, particularly in the frontal regions, emphasizing parallel intrahemispheric processing rather than interhemispheric transfer Gur et al. Current volumetric magnetic resonance imaging Pujol et al. To explore our visual environment, we integrate visual information from each visual hemifield as it reaches the contralateral cerebral hemisphere.
Because of the anatomical arrangement of the human visual system that projects visual inputs from each visual hemifield to the contralateral visual cortex, the independent contributions from each hemisphere and the role of the corpus callosum for the integration of parallel visuomotor processes can be readily assessed by comparing paired with single targets presented to one or both visual hemifields. Typically, responses are faster to stimulus pairs than single stimuli, a phenomenon called the redundant targets effect RTE , or summation effect Miller ; Mordkoff and Yantis ; Roser and Corballis The RTE is usually tested with brief light flashes presented for less than ms to prevent eye movements that would shift the visual field Reuter-Lorenz et al.
Statistically, the probability of eliciting a fast response is twice as high when two stimuli are presented simultaneously than when only one is presented. Here, co-activation from otherwise independent processing channels may occur to increase signal strength of redundant targets and produce a speeded response Miller Recent studies have associated enhanced redundancy gain co-activation model with neural correlates and implied a possible role of the CC Bucur et al.
For example, studies on split brain patients and on normal subjects testing redundancy gain for paired targets within the same visual hemifield, i. However, split-brain and acallosal patients typically show abnormally enhanced bilateral RTEs compared to healthy adults, which implies that other than callosal commissures can mediate interhemispheric processing advantages Aglioti et al.
It has been argued that healthy individual may not benefit as much from a bilateral presentation as do the split-brain individuals because an intact corpus callosum assures fast transfer of information between hemispheres even when only one hemisphere is stimulated directly by visual input, i.
Epub The Parallel Brain : The Cognitive Neuroscience Of The Corpus Callosum
But how can enhanced RTEs in the split-brain depend upon convergence of information from separated hemispheres when at the same time prolonged CUDs in split-brain patients reflect slow subcortical transmission? To explicate this paradox in the split-brain, Reuter-Lorenz et al.
Corballis et al. Even though the CC is the largest commissure at the cortical level, there are other pathways connecting the hemispheres: the anterior and posterior commissures, and subcortical projections through the hippocampal, habenular, and intercollicular brain systems Bayard et al. Activation of the superior colliculi, part of the retino-tectal pathway within the visual brain system, has been associated with the initiation of fast responses e.
Iacoboni et al.
In split-brain patients with long interhemispheric transfer times IHTT as measured with the CUD, feedback to the superior colliculi would be asynchronous and sum up over time to be larger than in normal subjects with short IHTT and synchronous superior colliculus input. The larger superior colliculus activation would feed a stronger signal back to the extrastriate cortices, which in turn send stronger activation to the premotor cortex, speeding the response. Consistent with his model, Iacoboni et al. Further evidence for an involvement of subcortical structures, such as the superior colliculus, or cortico-subcortical interactions Roser and Corballis comes from split-brain studies Corballis ; Savazzi and Marzi demonstrating reduced RTEs with equiluminant stimuli that are invisible to the superior colliculi and thus restrict processing to cortical pathways of the retino-geniculate parvocellular system Livingstone and Hubel However, despite converging evidence for a perceptual explanation for the RTE Corballis et al.
This normal variability in RTE suggests involvement of different neural loci extrastriate, premotor or frontal cortical areas depending on strategy selection, task demands, and stimulus characteristics. Whereas all individuals showed bilateral activation of extrastriate cortices to paired in contrast to summed single stimulus conditions, individuals with small RTEs activated frontal and premotor areas more than those with large RTEs Schulte et al.
Modified from Schulte et al. Our analysis was between individuals, whereas that of Mooshagian et al. This finding is of particular interest because the temporo-parietal junction has been implicated in spatial attention and, when damaged, in the occurrence of visuo-spatial neglect Lux et al.
In this case, the contralesional stimulus is extinguished from awareness Meister et al. In patients with unilateral left extinction after right temporo-parietal damage, enhanced redundancy gains have been observed even when one stimulus in a pair was presented to the left hemifield and therefore was not perceived Marzi et al. Thus, despite extinction of left-sided stimuli in paired trials, neglect patients showed unconscious parallel processing of contralesional stimuli RTE. This is supported by surprising results from Marzi et al. They speculated that this laterality difference might be related to a left—right asymmetry in callosal projections with callosally projecting neurons being more numerous in the right than the left hemisphere Marzi et al.
Consequently, they argued, it would be reasonable to assume that unilateral right-hemispheric lesions will cause a greater loss of callosal fibers than similar left-hemispheric lesions. That RTEs after right temporo-parietal damage were not abolished due to reduced or delayed visual input from the contralesional visual hemifield Marzi et al. It further supports the assumption that subcortical interhemispheric pathways play a role in redundancy gain and that cortical interhemispheric pathways mediate visual awareness Marzi et al.
In addition, current research indicates that these neural co-activation systems of bilateral sensorimotor integration as measured by redundancy gain further depend on cognitive processes engaged by task characteristics Reinholz and Pollmann , and individual differences in strategy selection Schulte et al. For example, using functional MRI in healthy subjects, Reinholz and Pollmann studied redundancy gain for higher-level visual identification and categorization processes using combinations of pictures of faces and houses presented simultaneously to both visual hemifields.
They observed fusiform face area FFA activation for redundant pictures of faces and parahippocampal place areas PPAs activation for redundant pictures of buildings suggesting involvement of task-specific higher-order visual object-selective areas in visuomotor integration of redundant information.
Despite the ongoing debate over the cortical locus of co-activation, results from split-brain research clearly demonstrate that interhemispheric integration of simple percepts can occur without the presence of the CC through subcortical transmission of information that is projected to cortical sites in both hemispheres. In the split-brain, enhanced RTEs have been interpreted as an absence of callosal inhibition Corballis et al.
Yet, in healthy individuals, a recent study showed greater RTEs for equiluminant than high-contrast stimuli implying cortical rather than subcortical integration mechanisms Schulte et al. Furthermore, callosal degradation without disconnection, e. These results imply a cooperative role of the CC and that such cooperation is reduced by subtle degradation of callosal integrity.
In addition, data in healthy subjects with an intact CC showed bilateral extrastriate activation for RTEs Schulte et al. Subtle degradation of callosal structure also occurs in healthy aging Bartzokis et al. In summary, the role of the corpus callosum for the integration of lower-level visuomotor functions is controversial. While split-brain research indicates that the CC acts in an inhibitory fashion within a subcortico-cortical network Corballis et al. The review of the few studies on interhemispheric communication emanating from the right temporo-parietal junction further suggests a special role of the corpus callosum in conscious perception Marzi et al.
Activation of the right temporo-parietal junction appears to mediate co-activation for visuomotor integration in conjunction with homologous temporo-parietal regions in the contralateral hemisphere Mooshagian et al. Hemispheric specialization for higher-order cognition such as the left-hemispheric dominance for language and right-hemispheric dominance for visuospatial attention Damasio and Damasio ; Gazzaniga has been assumed to be a consequence of interhemispheric connectivity, i. Although the functions of the CC are more associated with transferring sensory information and visuomotor integration Banich recent studies highlight the pivotal role of CC for higher-order cognitive functions such as identification of complex stimuli Banich et al.
Studying neuropsychiatric diseases, such as alcoholism, that are marked by subtle white matter neuropathology, particularly of the CC Harper and Kril , ; Harper and Matsumoto ; Pfefferbaum and Sullivan , ; Pfefferbaum et al. It has been assumed that both hemispheres interact with each other in a dynamic push-pull fashion to equalize the direction of visuospatial attention Kinsbourne , Kinsbourne and Bruce ; Reuter-Lorenz et al. In this activation-orienting model it is postulated that visuospatial attention is biased in the direction contralateral to the more activated hemisphere Schulte et al.
Based on the assumption that dynamic interactions between the hemispheres via the CC affect attentional processing Banich , we recently tested whether the integrity of the CC in alcoholics predicts the functioning of neural systems known to underlie higher-order component processes of selective attention and conflict processing. To identify the role of hemispheric lateralization and interhemispheric transfer within the fronto-parietal attention system, we developed a lateralized Stroop paradigm Schulte et al. For nearly 75 years, the Stroop color-word effect has provided a rich paradigm for parsing and manipulating processes of attention and conflict in the context of stimuli naturally compelling because of their semantic content Stroop ; MacLeod Processing words at their semantic level is involuntary and automatic and naturally overrides perceptual tags, such as the color of the ink of a written word that has no intrinsic value Carter et al.
The Stroop effect can be conceptually considered as a left hemisphere lateralized task with greater interference arising from the semantic content of the word MacLeod Because of its preferentially processing of verbal information e. This laterality effect was demonstrated in a study Weekes and Zaidel , which used separate color words and color patches and found greater Stroop interference when color words were presented to the left than right hemisphere independent of the location of the color patch i.
In our lateralized Stroop match-to-sample task, we analyzed hemispheric preference for Stroop word-color information by comparing reaction times to right visual field trials preferred left cerebral hemisphere for word processing with reaction times to left visual field trials non-preferred right hemisphere. We speculated that controls were more successful than alcoholics in disengaging their attention from the invalidly cued color to correctly process the color of the Stroop stimulus in nonmatch color trials. Subsequent disengagement and shift of attention away from the invalidly cued color to the correct target color of the Stroop stimulus is associated with right-hemisphere functions Blumstein and Cooper ; Christman ; Hartje et al.
This left visual field advantage in nonmatch trials was correlated with a larger callosal splenium area in controls but not alcoholics indicating that information presented to the non-preferred hemisphere is transmitted via the splenium to the hemisphere that is specialized for efficient processing. This processing route for visual hemifield information was disrupted in alcoholics, possibly as a result of callosal thinning. Furthermore, our visual world can be decomposed into component features and then integrated into more complex stimuli, objects and scenes—a concept originated from investigations of the visual cortex Felleman et al.
Thus, whole-part perception is based on the extraction of higher-order or global features such as the silhouette of a face, or street scene and details on the local level e. A widely used paradigm to study whole-part perception is a target detection task that uses hierarchical letters, where a large global letter is made of smaller local letters, modeling the hierarchical structure of visual world scenes Fink et al. Thus, the study of local-global perception may contribute to our understanding of the role of the CC for higher-order visual integration.
Although RT studies using lateralized hemifield presentations have not always shown local-global hemispheric differences Blanca et al. Together, these studies suggest that, in principle, each hemisphere can process local and global information; however, hemispheric processing advantages emerge when local and global information is processed simultaneously in both hemispheres.
The competition hypothesis Han et al. With lateralized presentations, the hemisphere contralateral to the stimulated hemifield receives the visual information prior to the ipsilateral hemisphere that receives information after interhemispheric transfer through the CC. The resulting time difference in processing between hemispheres may eliminate interhemispheric competition, and also hemispheric differences associated with local and global feature processing.
Results from an ERP study Han et al. Model of whole-part perception: Parallel intrahemispheric and transcallosal interhemispheric processing of local and global features. With central stimulation, the specialized hemisphere assigns more resources to a given local or global target level promoting hemispheric differences with global features e.
Local-global paradigms have yielded two measures: a the difference in response times to local and global targets, or precedence effect, which indexes parallel local and global processes assuming independent processing channels for local and global features, and b the difference in response times to incongruent e.
Several studies indicate a global-over-local precedence effect, i. Up to date, only a few studies have investigated the role of the CC in local-global precedence and interference. In , Robertson et al. However, Weekes et al. Thus, in contrast to Robertson et al. Rather, because each hemisphere can process local and global information, interference effects occur within one hemisphere in the absence of the CC. The different results from these two studies may be due to slight differences in stimulus characteristics and presentation: Robertson et al.
Thus neither study provides clear evidence for callosal involvement in local-global interference. In addition to precedence and interference, we also tested the relation between the CC and local-global facilitation.
Local-global facilitation effects have been found previously for attentional cueing Lamb and Yund ; Robertson et al. Together, these findings suggest that the CC mediates lateralized higher-order cognition such as whole-part perceptual integration. Specifically, anterior callosal integrity appears to mediate local-global interference callosal inhibition , whereas posterior callosal integrity seems to mediate local-global facilitation callosal cooperation.
Thus, component processes of visuospatial perception and attention are attributable, at least in part, to the integrity of callosal pathways, relevant for the integration of lateralized brain functions. Numerous studies have since contributed to our current understanding of interhemispheric sharing of information from early visual input to complex decision-making. Many of these studies have focused on split-brain patients whose cerebral commissures had been surgically disconnected to improve intractable epilepsy for reviews see Gazzaniga , ; Lassonde and Ouimet The recent advance of neuroimaging techniques such as DTI and fMRI have provided the opportunity to study CC function in situations of callosal degradation without disconnection.
These studies have documented a central role of the CC for interhemispheric visuomotor integration and for higher-order cognitive functions, and have shown that even subtle degradation of the CC in neurologically impaired patients can be related to deficits in the transfer of information between the hemispheres. For visuomotor integration, split-brain research suggests that the CC exerts an inhibitory function on the hemispheres and that absence of callosal inhibition results in enhanced bilateral processing advantages. Recent DTI and fMRI studies that tested bihemispheric visuomotor integration in the connected brain, however, provide evidence for callosal cooperation.
Whether the specific role of the CC for visuomotor integration is inhibitory or cooperative remains unclear. At this point, it appears plausible to assume that callosal inhibition and cooperation are not mutually exclusive but depend on complex interactions within a subcortico-cortical network that probably equilibrate hemispheric activation according to task demands. A further aim of this review was to focus on the role of the CC for higher-order lateralized cognitive functions. Together, the reviewed research provides clear indication that the CC contributes to the integration of perception and action, promoting a unified experience of the way we perceive the visual world and prepare our actions.
Of particular interest is emerging evidence from neuroimaging that the CC employs a differentiated role with callosal areas transmitting different types of information depending on the cortical destination of connecting fibers. Studies on patients with callosal degradation indicated that anterior callosal fibers linking frontal and premotor areas of the two hemispheres were associated with inhibitory functions in situations of semantic competition Stroop and local-global interference, whereas posterior callosal areas connecting temporo-parietal and occipital cortical regions were related with facilitation from redundant targets and local-global features.
Thus, to achieve an interhemispheric balance between component brain functions, the CC appears to exert both functional inhibition and excitation Bloom and Hynd Future directions should include new imaging techniques such as fiber tractography in combination with functional neuroimaging and electrophysiological methods to explore the specific role of regional callosal connectivity for brain functions in the healthy brain and callosal degradation. We thank Margaret J. Rosenbloom for comments on the manuscript. Disclosures The authors declare that no conflicts of interest are associated with the preparation of this article.
Anatomy-Based Visualizations of Diffusion Tensor Images of Brain White Matter
Corresponding author. Tilman Schulte: moc. Copyright notice. The publisher's final edited version of this article is available at Neuropsychol Rev. See other articles in PMC that cite the published article. Abstract 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.
Pathways Subserving Intra-and Interhemispheric Communication Cortico-cortical communication within and between hemispheres relies on a widespread white matter network that also connects to subcortical gray matter. Intrahemispheric Communication Several cortico-cortical association and cortico-subcortical projection pathways ensure communication within each hemisphere.
Interhemispheric Communication The major commissural pathway is the corpus callosum CC , a thick band of white matter fibers consisting of more than million axons connecting the right and left cerebral hemispheres of the brain and playing an important role in interhemispheric transfer of information Aboitiz et al. Open in a separate window. Callosal Function in Interhemispheric Communication Different callosal functions have been proposed for interhemispheric communication.
Interhemispheric Communication for Visuomotor and Cognitive Functions First, we will provide an overview of the role of the CC in lower-level processes and interhemispheric visuomotor integration as documented in recent behavioral and neuroimaging studies on the crossed-uncrossed difference, an estimation of interhemispheric transfer time Poffenberger , and on redundancy gain, a measure of visuomotor integration Miller ; Mordkoff and Yantis , The Role of the CC for the Integration of Lower-Level Functions Interhemispheric Transfer Time and the Corpus Callosum Approaches describing and analyzing interhemispheric integration of information in humans include simple reaction time RT tasks in which targets are presented in the same uncrossed or opposite crossed visual field in relation to the responding hand Poffenberger ; Zaidel and Iacoboni Parallel Processing, Redundancy Gain and the Corpus Callosum To explore our visual environment, we integrate visual information from each visual hemifield as it reaches the contralateral cerebral hemisphere.
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The Role of the CC for the Integration of Higher Cognitive Functions Hemispheric specialization for higher-order cognition such as the left-hemispheric dominance for language and right-hemispheric dominance for visuospatial attention Damasio and Damasio ; Gazzaniga has been assumed to be a consequence of interhemispheric connectivity, i.
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Parsing executive processes: strategic vs. The rises and falls of disconnection syndromes. Varieties of interhemispheric inhibition, or how to keep a good hemisphere down. Individual differences in stroop and local-global processing: a possible role of interhemispheric interaction. Simple reaction times to lateralized light flashes. Varieties of interhemispheric communication routes. Anatomical-behavioral relationships: corpus callosum morphometry and hemispheric specialization. Behavioural Brain Research. Callosal inhibition: the key to the brain code. Behavioral Science. Interhemispheric neural summation in the absence of the corpus callosum.
Hemispheric interactions in simple reaction time. Paradoxical interhemispheric summation in the split brain. Redundancy gain in simple reaction time following partial and complete callosotomy. Superior parietal cortex activation during spatial attention shifts and visual feature conjunction. A functional MRI study of preparatory signals for spatial location and objects. Sensory-motor and cognitive functions of the human posterior parietal cortex involved in manual actions.
Neurobiology of Learning and Memory. New York: Freeman; White matter dis connections and gray matter dys functions in visual neglect: gaining insights into the brain networks of spatial awareness. Event or emergency? Two response systems in the mammalian superior colliculus. Trends in Neurosciences. Hemispheric specialization of memory for visual hierarchical stimuli. Visuospatial dysfunction following unilateral brain damage: dissociations in hierarchical and hemispatial analysis. Journal of Clinical and Experimental Neuropsychology. Intersensory facilitation in the motor component?
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A reaction time analysis. Psychological Research. Doricchi F, Tomaiuolo F.