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Corpus callosum and cerebral laterality in a modular brain model
Medical Hypotheses (2000) 55(2), 177–182 © 2000 Harcourt Publishers Ltd DOI: 10.1054/mehy.1999.0934, available online at http://www.idealibrary.com on
Corpus callosum and cerebral laterality in a modular brain model C. W. Wong Division of Neurosurgery, Department of Surgery, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong, China
Summary This paper elaborates the function of corpus callosum in the brain model that contains encoding and modulating axons: the former encode data as presynaptic axonal ‘on-off’ patterns, and the latter help the former convert data into long-term memory through the development of long-term potentiation and depression. It is hypothesized that callosal axons transfer data codes as interhemispheric memory. Bisection of corpus callosum cuts off interhemispheric data transfer and results in strange-hand syndrome, decreased attention and difficulties in acquiring new interdependent bimanual skills. While uniting two hemispheres for a unitary consciousness, corpus callosum contributes to two similar sets of integrated abstract memory, one in each hemisphere. Therefore, it takes bilateral cerebral lesions to manifest a failure of converting short-term memory into long-term memory. The asymmetric callosal data transfer may correlate with cerebral laterality where a cerebral function, such as language, is conducted mainly in one hemisphere for the benefit of less interhemispheric data-traffic. Complete lateralization of a cerebral function is the rare occasion when the specialized neuron groups (modules) for that function all reside in one hemisphere. It is possible that many cerebral functions including language are incompletely lateralized, and corpus callosum links the non-lateralized modules with the lateralized ones. The more the cerebral lateralization, the fewer the non-lateralized modules to be linked, and the smaller the corpus callosum. © 2000 Harcourt Publishers Ltd
INTRODUCTION The human corpus callosum appears at the fetal age of 11–12 weeks and matures in adulthood (1). Its function is more than holding the cerebral hemispheres together, or spreading seizures from one hemisphere to the other (2). This paper aims to elaborate the role of corpus callosum in the brain model that contains encoding and modulating axons: the former encode data as presynaptic axonal ‘on-off’ patterns, and the latter help the former convert data into long-term memory through the development of
Received 15 February 1999 Accepted 4 June 1999 Correspondence to: Dr. Cheuk-Wah Wong, Flat B, 4/F Chiat Hing Building, 213–221 Yu Chau Street, Kowloon, Hong Kong, China. Fax: + 852 2691 2272; E-mail: [email protected]
long-term potentiation and depression (3–5). In the brain model, a functional area of Brodmann has many modules (neuronal chips) and each module supports a facet of a cerebral function (3). Cerebral module was first evident in the somatosensory cortex of the cats where a vertical column of neurons responded to a single stimulation of certain modality with nearly identical latencies, indicating that these neurons function as a unit (6). Modules have been found in the auditory, visual, and association cortex, the latter of which occupies 95% of the cortical surface and is involved in higher cortical function (7). Functional imaging studies have provided enormous supports to the concept of modules (8). For example, there is a lexicon for spoken-word recognition in the middle part of the left superior and middle temporal gyri, and a lexicon for written word in the posterior part of the left middle temporal gyri (9). 177
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Fig. 1(A) The schematic cytoarchitecture of a module (between the broken lines) in the left hemisphere (L), and a module in the right hemisphere (R). Apart from sending axons to the modules of the contralateral hemisphere, callosal neurons (black) may send collateral axons to other modules of the ipsilateral hemisphere. The neurons of cortical layer I (hatched) send axons to non-callosal neurons (white) which, by definition, do not send axons to the contralateral hemisphere. Here the modulating axons are omitted, the axonal signal of 0 and 1 arbitrarily given, and the interhemispheric data codes underlined. The combined interhemispheric and intrahemispheric data codes are: 01011 in L, 00111 in R, and 0101100111 in L and R, the latter of which may represent part of a memory such as an interdependent bimanual skill. (B) When the corpus callosum is bisected, the callosal signals of 0 and 1 are not conveyed across hemispheres L and R. The intrahemispheric data codes, 010 in L and 111 in R, can still be converted into long-term memory by the memory-conversion circuits in either hemisphere. When these intrahemispheric data codes, 010 and 111, are reactivated during the process of memory retrieval, they cannot trigger the action of the interdependent bimanual skill originally triggered by the code of 0101100111 (Fig 1A). However, these intrahemispheric codes, 010 in L and 111 in R respectively, may serve as memory codes for a skill of alternating manual control.
Callosal cytoarchitecture A cerebral module is about 0.25 mm across and 2–3 mm long (7). Figure 1A shows two simplified modules that contain callosal and non-callosal neurons (7,10,11). Residing in layers II–VI (12), callosal neurons send axons to non-callosal neurons in the corresponding layers of the opposite cortex. Across the hemispheres, there is no reciprocal connection of callosal neurons (12,13). Within a module, there is no direct connection between callosal neurons and neurons of layer I (14). Afferent axons from thalamus and elsewhere provide data codes to the modules where the layer-I neurons send their output data codes to the non-callosal neurons (15), and the non-callosal neurons send data to the callosal neurons (Fig. 1A). It is the non-callosal neurons that send axons to the subcortical telencephalic centers, brainstem, and spinal cord (13,16). In rats, the ipsilateral collateral axons of callosal neurons survived even 20 weeks after callosotomy (17).
body (20). Electrophysiological studies reveal multiple channels of interhemispheric transfer in corpus callosum: different transfer properties for different functional modalities (21–23). These callosal interhemispheric transfers can be asymmetric, for example, more transfers from the right hemisphere to the left (24,25). Corpus callosum and cerebral laterality It is well known that the left hemisphere specializes in language, the right one in spatial perception, and the left dominates the right in daily life (2,26,27). Such a cerebral laterality correlates with the cerebral asymmetry that results from underdeveloped planum temporale of the right hemisphere (28). For example, women and lefthanded males have a lesser degree of cerebral asymmetry because they have better developed right hemispheres in comparison to right-handed males (29). In general, the more the cerebral asymmetry, the more the cerebral lateralization, and the smaller the corpus callosum (30).
Functional channels in corpus callosum A human corpus callosum contains 200 million myelinated and unmyelinated axons (18). In the genu, and the anterior and mid splenium of the corpus callosum, there are thin axons that connect the association areas of prefrontal, temporal and parietal lobes (18,19). The thick callosal axons connect the somatosensory, visual, and perhaps auditory and motor cortices (18). For example, tactile transfer is localized to the mid-posterior callosal Medical Hypotheses (2000) 55(2), 177–182
A HYPOTHESIS OF CORPUS CALLOSUM Corpus callosum contains encoding axons that transfer interhemispheric data codes as series of axonal ‘on-off’ patterns across the cerebral hemispheres. These interhemispheric data codes act in concert with the intrahemispheric data codes of non-callosal axons as content of consciousness (Fig. 1A). Both interhemispheric and intrahemispheric data codes can be converted into © 2000 Harcourt Publishers Ltd
One brain, two sets of abstract memory
long-term memory by modulating axons of the memoryconversion circuits, A and B, in each hemisphere (3,4). DISCUSSION The prefrontal lobe is an important source of modulating axons which, by definition, do not convey data codes (3). Corpus callosum may contain modulating axons as circuit C for conversion of interhemispheric data codes into long-term memory. However, it seems possible to explain the memory disturbances of the callosotomized patients without a circuit C. Memory disturbances in callosotomized patients Callosotomy, surgical bisection of the corpus callosum, is an effective treatment reserved for intractable epilepsy. Callosotomized patients may lead an apparently normal life (2), although they have exceptional difficulties in learning new skills of interdependent bimanual movements, such as those involved in using the toy called ‘etch-a-sketch’ (31). These patients can retain the interdependent bimanual skills acquired before callosotomy, and they can even learn new bimanual skills consisting of parallel or alternate control (31). Zaidal believes that this results from missing the outputs of the right hemisphere, or the normal interaction between left and right. Some verbal memory, especially newly-learned material, is not up to the level preceding callosotomy because there is rudimentary verbal function in the right hemisphere (31). According to the brain model (3,4), acquirement of an interdependent bimanual skill involves conversion of interhemispheric and intrahemispheric data codes into long-term memory (Fig. 1A). The interhemispheric and intrahemispheric data codes exist mainly in the cortical areas of the trunk and the proximal limbs, because the cortical areas of the distal limbs are almost devoid of callosal axons (32,33). To execute an interdependent bimanual skill, the interhemispheric and intrahemispheric data codes may need to initiate the corresponding action data codes in the cortical areas of the distal limbs. Once converted into long-term memory (3,4), these corresponding action data codes for the distal limbs may survive callosotomy and be reactivated by the remaining intrahemispheric data codes through the mechanism of data convergence where parts of the original memory codes lead to the converged, abstract memory (5). This may be one reason why callosotomized patients can retain a previously acquired interdependent bimanual skill in the presence of degenerated callosal axons and decayed interhemispheric data codes (31). The difficulties of the callosotomized patients in acquiring a new interdependent bimanual skill may result from the lack of interhemispheric data codes to © 2000 Harcourt Publishers Ltd
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initiate the corresponding action data codes in the cortical areas of the distal limbs (Fig. 1B). Non-existent action data codes cannot be converted into long-term memory as interdependent bimanual skill. However, the remaining intrahemispheric data codes after callosotomy may still establish action codes in the cortical areas of the distal limbs for a new bimanual skill of parallel or alternate control (Fig. 1B). Two sets of abstract memory Normally, each hemisphere receives similar inputs from the sensory organs. Parts of the sensory data are stored in the ipsilateral hemisphere and parts of them are transferred to the contralateral hemisphere by the corpus callosum. These callosally transferred interhemispheric data codes are integrated with the intrahemispheric data codes of the contralateral hemisphere, both of which can be converted into long-term memory by the memoryconversion circuits (3,4). Hence, there are two sets of integrated memory, one in each hemisphere (Fig. 2A). When a hemisphere ceases to receive ipsilateral inputs, the contralateral sensory organs convey inputs to the unaffected hemisphere whose callosal axons transfer the computed data codes to the affected hemisphere. These callosally transferred data may be stored as memory in different tiers of synapses of the affected hemisphere (Fig. 1A). This results in two sets of semi-integrated memory, one in the unaffected hemisphere and the other in the affected one (Fig. 2B). The two sets of memory, integrated and semiintegrated, are converged, abstract memories because neither hemisphere directly shares the primary (sensory) inputs with the other. This may be the reason why it takes bilateral lesions in the memory conversion-circuits to manifest a failure of consolidating short-term memory into long-term memory (3,4). Unitary consciousness, non-unitary consciousness, attention In the brain model, the content of consciousness is determined by series of presynaptic axonal ‘on-off’ patterns (data codes) along the neural circuits (3). These codes result from computation of both the interhemispheric and intrahemispheric data, and serve as the integrated content of a unitary consciousness. Callosotomy cuts off interhemispheric transfer and splits the content of consciousness into two separate sets of intrahemispheric data codes (Fig. 1B). Each hemisphere uses its own set of intrahemispheric data codes to communicate with the environment. When language is the tool of communication, the left hemisphere speaks with its data codes and the content of consciousness is languageMedical Hypotheses (2000) 55(2), 177–182
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Fig. 2
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(A) Two sets of integrated memory; (B) Two sets of semi-integrated memory.
dominant. When a non-language means is the tool of communication, the right hemisphere expresses with its data codes and the content of consciousness is non-languagedominant (2,26). Such a consciousness is regarded as nonunitary (2,26), and its content non-integrated. Although callosotomized patients have few problems of recent memory (34), they may manifest decreased attention (24,35–38). According to the brain model, attention is a state of consciousness whose content is uninterruptedly displayed as series of interrelated data codes in groups of synapses for a period of time (3). This requires smooth and continuous computation of the interrelated data codes among many parts of the brain, including corpus callosum, frontal, posterior parietal, and anterior cingulate cortex (38). After callosotomy, the brain may fail to process the data codes that require integrated, interhemispheric computation. This interrupts the display of the content of consciousness, allows other data codes displaying a different content of consciousness to set in, and manifests distraction, or decreased attention (3,35–38).
Strange-hand syndrome Callosal transfer of data codes keeps either hemisphere informed of the existence of the other. Callosotomy cuts off the interhemispheric data-flow and creates mutual neglect between the hemispheres. This may result in strange-hand syndrome, such as neglect of a limb, interhand interference, and inability to name certain parts of the body (31,39). Strange-hand syndrome is more prominent in the left hand or the parts of the body connected to the right hemisphere, because the left hemisphere dominates the right one in language and motor control (25). When the residual callosal neurons subsequently establish a rudimentary circuit, or a compensatory subcortical, Medical Hypotheses (2000) 55(2), 177–182
extracallosal connection is activated (40), the strangehand syndrome may improve (31,39). It is believed that the lack of disconnection syndrome in persons with agenesis of corpus callosum results from compensatory development of the uncrossed input pathways, such as those of the spinothalamic tract (41). Like decreased attention, strange-hand syndrome and visuospatial neglect may develop with lesions in the cortical areas that cease to send callosal signals to inform the other cerebral hemisphere (8,42).
Cerebral lateralization, cerebral modularization The human fetus starts to demonstrate cerebral asymmetry at the age of 29–31 weeks (43), which may be driven by genes (44), testosterone (45), or both. With cerebral laterality accompanying specialization of neuron groups, a cerebral function such as language can be conducted mainly in one hemisphere for the benefit of less interhemispheric data-traffic. The asymmetric, right-to-left shift of callosal transfers may correlate with cerebral laterality: the rudimentary language and manual data codes in the right hemisphere being mobilized to the highly specialized left hemisphere for integrated computation (24,25,32,33). Cerebral lateralization may be incomplete because different neuronal groups for different components of a cerebral function, such as language and motor skill, can reside in different hemispheres of the same person (42,46,47). These different neuronal groups can be regarded as modules (6–11), or neuronal chips (3). Cerebral modularization may be induced by the benefit of division of labor: a facet of cerebral function being better conducted with anatomically related neurons than the same amount of anatomically unrelated neurons. © 2000 Harcourt Publishers Ltd
One brain, two sets of abstract memory
Theoretically, complete cerebral lateralization is a special case of cerebral modularization, when the modules of a cerebral function all reside in one hemisphere. Although there are two sets of converged (5), abstract memory in the brain, only the dominant modules can make full use of those memories involved in the lateralized cerebral function, such as speech (20,27,46,47). Within a cerebral hemisphere, the long association fascicles may transfer data codes from one module to the others. Corpus callosum may not be essential for cerebral lateralization because cerebral laterality has been demonstrated in persons with agenesis of corpus callosum (48,49). However, it is possible that corpus callosum links the non-lateralized modules with the lateralized ones. The excessive callosal and non-callosal synapses in early life may facilitate the linking capacity of corpus callosum (50,51). Once the interhemispheric channels are established, or the critical period has passed (41), the redundant callosal synapses disappear (51,52), including loss of contact with the neurons of layer I as in the cortex of the rats (14,15). Hence, the more complete the cerebral lateralization, the fewer the non-lateralized modules to be linked, and the smaller the corpus callosum is (30). To take over the function of a disordered module in the early life, the excessive non-callosal and callosal synapses (51,52) may allow the brain to establish new connection with other neuron groups in the ipsilateral and the contralateral hemisphere. This is a possible mechanism for younger children to develop compensatory functional modules after brain damage (41), and for those with atopic functional modules (42,46,47).
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Corpus callosum and cerebral laterality in a modular brain model C. W. Wong Division of Neurosurgery, Department of Surgery, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong, China
Summary This paper elaborates the function of corpus callosum in the brain model that contains encoding and modulating axons: the former encode data as presynaptic axonal ‘on-off’ patterns, and the latter help the former convert data into long-term memory through the development of long-term potentiation and depression. It is hypothesized that callosal axons transfer data codes as interhemispheric memory. Bisection of corpus callosum cuts off interhemispheric data transfer and results in strange-hand syndrome, decreased attention and difficulties in acquiring new interdependent bimanual skills. While uniting two hemispheres for a unitary consciousness, corpus callosum contributes to two similar sets of integrated abstract memory, one in each hemisphere. Therefore, it takes bilateral cerebral lesions to manifest a failure of converting short-term memory into long-term memory. The asymmetric callosal data transfer may correlate with cerebral laterality where a cerebral function, such as language, is conducted mainly in one hemisphere for the benefit of less interhemispheric data-traffic. Complete lateralization of a cerebral function is the rare occasion when the specialized neuron groups (modules) for that function all reside in one hemisphere. It is possible that many cerebral functions including language are incompletely lateralized, and corpus callosum links the non-lateralized modules with the lateralized ones. The more the cerebral lateralization, the fewer the non-lateralized modules to be linked, and the smaller the corpus callosum. © 2000 Harcourt Publishers Ltd
INTRODUCTION The human corpus callosum appears at the fetal age of 11–12 weeks and matures in adulthood (1). Its function is more than holding the cerebral hemispheres together, or spreading seizures from one hemisphere to the other (2). This paper aims to elaborate the role of corpus callosum in the brain model that contains encoding and modulating axons: the former encode data as presynaptic axonal ‘on-off’ patterns, and the latter help the former convert data into long-term memory through the development of
Received 15 February 1999 Accepted 4 June 1999 Correspondence to: Dr. Cheuk-Wah Wong, Flat B, 4/F Chiat Hing Building, 213–221 Yu Chau Street, Kowloon, Hong Kong, China. Fax: + 852 2691 2272; E-mail: [email protected]
long-term potentiation and depression (3–5). In the brain model, a functional area of Brodmann has many modules (neuronal chips) and each module supports a facet of a cerebral function (3). Cerebral module was first evident in the somatosensory cortex of the cats where a vertical column of neurons responded to a single stimulation of certain modality with nearly identical latencies, indicating that these neurons function as a unit (6). Modules have been found in the auditory, visual, and association cortex, the latter of which occupies 95% of the cortical surface and is involved in higher cortical function (7). Functional imaging studies have provided enormous supports to the concept of modules (8). For example, there is a lexicon for spoken-word recognition in the middle part of the left superior and middle temporal gyri, and a lexicon for written word in the posterior part of the left middle temporal gyri (9). 177
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Fig. 1(A) The schematic cytoarchitecture of a module (between the broken lines) in the left hemisphere (L), and a module in the right hemisphere (R). Apart from sending axons to the modules of the contralateral hemisphere, callosal neurons (black) may send collateral axons to other modules of the ipsilateral hemisphere. The neurons of cortical layer I (hatched) send axons to non-callosal neurons (white) which, by definition, do not send axons to the contralateral hemisphere. Here the modulating axons are omitted, the axonal signal of 0 and 1 arbitrarily given, and the interhemispheric data codes underlined. The combined interhemispheric and intrahemispheric data codes are: 01011 in L, 00111 in R, and 0101100111 in L and R, the latter of which may represent part of a memory such as an interdependent bimanual skill. (B) When the corpus callosum is bisected, the callosal signals of 0 and 1 are not conveyed across hemispheres L and R. The intrahemispheric data codes, 010 in L and 111 in R, can still be converted into long-term memory by the memory-conversion circuits in either hemisphere. When these intrahemispheric data codes, 010 and 111, are reactivated during the process of memory retrieval, they cannot trigger the action of the interdependent bimanual skill originally triggered by the code of 0101100111 (Fig 1A). However, these intrahemispheric codes, 010 in L and 111 in R respectively, may serve as memory codes for a skill of alternating manual control.
Callosal cytoarchitecture A cerebral module is about 0.25 mm across and 2–3 mm long (7). Figure 1A shows two simplified modules that contain callosal and non-callosal neurons (7,10,11). Residing in layers II–VI (12), callosal neurons send axons to non-callosal neurons in the corresponding layers of the opposite cortex. Across the hemispheres, there is no reciprocal connection of callosal neurons (12,13). Within a module, there is no direct connection between callosal neurons and neurons of layer I (14). Afferent axons from thalamus and elsewhere provide data codes to the modules where the layer-I neurons send their output data codes to the non-callosal neurons (15), and the non-callosal neurons send data to the callosal neurons (Fig. 1A). It is the non-callosal neurons that send axons to the subcortical telencephalic centers, brainstem, and spinal cord (13,16). In rats, the ipsilateral collateral axons of callosal neurons survived even 20 weeks after callosotomy (17).
body (20). Electrophysiological studies reveal multiple channels of interhemispheric transfer in corpus callosum: different transfer properties for different functional modalities (21–23). These callosal interhemispheric transfers can be asymmetric, for example, more transfers from the right hemisphere to the left (24,25). Corpus callosum and cerebral laterality It is well known that the left hemisphere specializes in language, the right one in spatial perception, and the left dominates the right in daily life (2,26,27). Such a cerebral laterality correlates with the cerebral asymmetry that results from underdeveloped planum temporale of the right hemisphere (28). For example, women and lefthanded males have a lesser degree of cerebral asymmetry because they have better developed right hemispheres in comparison to right-handed males (29). In general, the more the cerebral asymmetry, the more the cerebral lateralization, and the smaller the corpus callosum (30).
Functional channels in corpus callosum A human corpus callosum contains 200 million myelinated and unmyelinated axons (18). In the genu, and the anterior and mid splenium of the corpus callosum, there are thin axons that connect the association areas of prefrontal, temporal and parietal lobes (18,19). The thick callosal axons connect the somatosensory, visual, and perhaps auditory and motor cortices (18). For example, tactile transfer is localized to the mid-posterior callosal Medical Hypotheses (2000) 55(2), 177–182
A HYPOTHESIS OF CORPUS CALLOSUM Corpus callosum contains encoding axons that transfer interhemispheric data codes as series of axonal ‘on-off’ patterns across the cerebral hemispheres. These interhemispheric data codes act in concert with the intrahemispheric data codes of non-callosal axons as content of consciousness (Fig. 1A). Both interhemispheric and intrahemispheric data codes can be converted into © 2000 Harcourt Publishers Ltd
One brain, two sets of abstract memory
long-term memory by modulating axons of the memoryconversion circuits, A and B, in each hemisphere (3,4). DISCUSSION The prefrontal lobe is an important source of modulating axons which, by definition, do not convey data codes (3). Corpus callosum may contain modulating axons as circuit C for conversion of interhemispheric data codes into long-term memory. However, it seems possible to explain the memory disturbances of the callosotomized patients without a circuit C. Memory disturbances in callosotomized patients Callosotomy, surgical bisection of the corpus callosum, is an effective treatment reserved for intractable epilepsy. Callosotomized patients may lead an apparently normal life (2), although they have exceptional difficulties in learning new skills of interdependent bimanual movements, such as those involved in using the toy called ‘etch-a-sketch’ (31). These patients can retain the interdependent bimanual skills acquired before callosotomy, and they can even learn new bimanual skills consisting of parallel or alternate control (31). Zaidal believes that this results from missing the outputs of the right hemisphere, or the normal interaction between left and right. Some verbal memory, especially newly-learned material, is not up to the level preceding callosotomy because there is rudimentary verbal function in the right hemisphere (31). According to the brain model (3,4), acquirement of an interdependent bimanual skill involves conversion of interhemispheric and intrahemispheric data codes into long-term memory (Fig. 1A). The interhemispheric and intrahemispheric data codes exist mainly in the cortical areas of the trunk and the proximal limbs, because the cortical areas of the distal limbs are almost devoid of callosal axons (32,33). To execute an interdependent bimanual skill, the interhemispheric and intrahemispheric data codes may need to initiate the corresponding action data codes in the cortical areas of the distal limbs. Once converted into long-term memory (3,4), these corresponding action data codes for the distal limbs may survive callosotomy and be reactivated by the remaining intrahemispheric data codes through the mechanism of data convergence where parts of the original memory codes lead to the converged, abstract memory (5). This may be one reason why callosotomized patients can retain a previously acquired interdependent bimanual skill in the presence of degenerated callosal axons and decayed interhemispheric data codes (31). The difficulties of the callosotomized patients in acquiring a new interdependent bimanual skill may result from the lack of interhemispheric data codes to © 2000 Harcourt Publishers Ltd
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initiate the corresponding action data codes in the cortical areas of the distal limbs (Fig. 1B). Non-existent action data codes cannot be converted into long-term memory as interdependent bimanual skill. However, the remaining intrahemispheric data codes after callosotomy may still establish action codes in the cortical areas of the distal limbs for a new bimanual skill of parallel or alternate control (Fig. 1B). Two sets of abstract memory Normally, each hemisphere receives similar inputs from the sensory organs. Parts of the sensory data are stored in the ipsilateral hemisphere and parts of them are transferred to the contralateral hemisphere by the corpus callosum. These callosally transferred interhemispheric data codes are integrated with the intrahemispheric data codes of the contralateral hemisphere, both of which can be converted into long-term memory by the memoryconversion circuits (3,4). Hence, there are two sets of integrated memory, one in each hemisphere (Fig. 2A). When a hemisphere ceases to receive ipsilateral inputs, the contralateral sensory organs convey inputs to the unaffected hemisphere whose callosal axons transfer the computed data codes to the affected hemisphere. These callosally transferred data may be stored as memory in different tiers of synapses of the affected hemisphere (Fig. 1A). This results in two sets of semi-integrated memory, one in the unaffected hemisphere and the other in the affected one (Fig. 2B). The two sets of memory, integrated and semiintegrated, are converged, abstract memories because neither hemisphere directly shares the primary (sensory) inputs with the other. This may be the reason why it takes bilateral lesions in the memory conversion-circuits to manifest a failure of consolidating short-term memory into long-term memory (3,4). Unitary consciousness, non-unitary consciousness, attention In the brain model, the content of consciousness is determined by series of presynaptic axonal ‘on-off’ patterns (data codes) along the neural circuits (3). These codes result from computation of both the interhemispheric and intrahemispheric data, and serve as the integrated content of a unitary consciousness. Callosotomy cuts off interhemispheric transfer and splits the content of consciousness into two separate sets of intrahemispheric data codes (Fig. 1B). Each hemisphere uses its own set of intrahemispheric data codes to communicate with the environment. When language is the tool of communication, the left hemisphere speaks with its data codes and the content of consciousness is languageMedical Hypotheses (2000) 55(2), 177–182
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Fig. 2
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(A) Two sets of integrated memory; (B) Two sets of semi-integrated memory.
dominant. When a non-language means is the tool of communication, the right hemisphere expresses with its data codes and the content of consciousness is non-languagedominant (2,26). Such a consciousness is regarded as nonunitary (2,26), and its content non-integrated. Although callosotomized patients have few problems of recent memory (34), they may manifest decreased attention (24,35–38). According to the brain model, attention is a state of consciousness whose content is uninterruptedly displayed as series of interrelated data codes in groups of synapses for a period of time (3). This requires smooth and continuous computation of the interrelated data codes among many parts of the brain, including corpus callosum, frontal, posterior parietal, and anterior cingulate cortex (38). After callosotomy, the brain may fail to process the data codes that require integrated, interhemispheric computation. This interrupts the display of the content of consciousness, allows other data codes displaying a different content of consciousness to set in, and manifests distraction, or decreased attention (3,35–38).
Strange-hand syndrome Callosal transfer of data codes keeps either hemisphere informed of the existence of the other. Callosotomy cuts off the interhemispheric data-flow and creates mutual neglect between the hemispheres. This may result in strange-hand syndrome, such as neglect of a limb, interhand interference, and inability to name certain parts of the body (31,39). Strange-hand syndrome is more prominent in the left hand or the parts of the body connected to the right hemisphere, because the left hemisphere dominates the right one in language and motor control (25). When the residual callosal neurons subsequently establish a rudimentary circuit, or a compensatory subcortical, Medical Hypotheses (2000) 55(2), 177–182
extracallosal connection is activated (40), the strangehand syndrome may improve (31,39). It is believed that the lack of disconnection syndrome in persons with agenesis of corpus callosum results from compensatory development of the uncrossed input pathways, such as those of the spinothalamic tract (41). Like decreased attention, strange-hand syndrome and visuospatial neglect may develop with lesions in the cortical areas that cease to send callosal signals to inform the other cerebral hemisphere (8,42).
Cerebral lateralization, cerebral modularization The human fetus starts to demonstrate cerebral asymmetry at the age of 29–31 weeks (43), which may be driven by genes (44), testosterone (45), or both. With cerebral laterality accompanying specialization of neuron groups, a cerebral function such as language can be conducted mainly in one hemisphere for the benefit of less interhemispheric data-traffic. The asymmetric, right-to-left shift of callosal transfers may correlate with cerebral laterality: the rudimentary language and manual data codes in the right hemisphere being mobilized to the highly specialized left hemisphere for integrated computation (24,25,32,33). Cerebral lateralization may be incomplete because different neuronal groups for different components of a cerebral function, such as language and motor skill, can reside in different hemispheres of the same person (42,46,47). These different neuronal groups can be regarded as modules (6–11), or neuronal chips (3). Cerebral modularization may be induced by the benefit of division of labor: a facet of cerebral function being better conducted with anatomically related neurons than the same amount of anatomically unrelated neurons. © 2000 Harcourt Publishers Ltd
One brain, two sets of abstract memory
Theoretically, complete cerebral lateralization is a special case of cerebral modularization, when the modules of a cerebral function all reside in one hemisphere. Although there are two sets of converged (5), abstract memory in the brain, only the dominant modules can make full use of those memories involved in the lateralized cerebral function, such as speech (20,27,46,47). Within a cerebral hemisphere, the long association fascicles may transfer data codes from one module to the others. Corpus callosum may not be essential for cerebral lateralization because cerebral laterality has been demonstrated in persons with agenesis of corpus callosum (48,49). However, it is possible that corpus callosum links the non-lateralized modules with the lateralized ones. The excessive callosal and non-callosal synapses in early life may facilitate the linking capacity of corpus callosum (50,51). Once the interhemispheric channels are established, or the critical period has passed (41), the redundant callosal synapses disappear (51,52), including loss of contact with the neurons of layer I as in the cortex of the rats (14,15). Hence, the more complete the cerebral lateralization, the fewer the non-lateralized modules to be linked, and the smaller the corpus callosum is (30). To take over the function of a disordered module in the early life, the excessive non-callosal and callosal synapses (51,52) may allow the brain to establish new connection with other neuron groups in the ipsilateral and the contralateral hemisphere. This is a possible mechanism for younger children to develop compensatory functional modules after brain damage (41), and for those with atopic functional modules (42,46,47).
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