Callosal Afferents Research Articles

Altered distribution of inhibitory interneurons in polymicrogyria

There is a high incidence of epilepsy in patients with polymicrogyria; however, the epileptogenic mechanisms are largely unknown. The density of parvalbumin-immunoreactive interneurons was evaluated in an experimental model of polymicrogyria, in order to assess the potential changes in the development of one population of inhibitory interneurons. Newborn hamsters received an intracerebral injection of ibotenate, and all injected animals showed abnormal cortical layers characterized by one or two microgyrus in the fronto-parietal cortex. A quantitative analysis revealed that the ratios of parvalbumin-immunoreactive neurons in total neurons were significantly reduced in the medial paramicrogyral area, and in the medial and central parts of microgyrus in comparison to that in the lateral part of microgyrus (P<0.01). The lateral paramicrogyral area had the greatest number of parvalbumin-immunoreactive neurons, which was increased significantly in comparison to that in the control cortex (P<0.01). We suggest that the callosal, thalamic and intracortical afferents to the microgyrus and paramicrogyral area may induce a remarkable imbalance between the excitatory and inhibitory activities of the cortical structures, associated with the epileptogenic mechanism in polymicrogyria.

Central Auditory Impairment in Unilateral Diencephalic and Telencephalic Lesions

The extent of perceptual impairment following unilateral lesions in the auditory cortex, its thalamic or callosal afferents was studied with psychoacoustic tests. Thresholds for the discrimination of signal frequency, intensity and duration were acquired under three different conditions of headphone stimulation (‘monaural’, ‘interaural’, and ‘dichotic signal/noise tests’) using the three-alternative forced-choice procedure. The different test alternatives generated distinct auditory percepts, which is in accordance with the assumption of specific signal processing at the level of the auditory brainstem and at thalamocortical auditory areas. Twenty-one patients from neurology were studied who suffered from unilateral lesions in the auditory cortex, the auditory thalamus, or the acoustic radiation. Location and extent of the lesions were assessed by magnetic resonance imaging. Monaural tests of either ear revealed no deficits in auditory performance. The patients showed impaired discrimination of signal frequency, intensity and duration in the dichotic signal/noise tests, when the signals were presented to the ear contralateral and the noise ipsilateral to the lesion. With inverted signal and noise stimulation, however, the thresholds were in the range of age-matched controls. All patients were able to master the interaural tests, which indicates the preserved ability to lateralize sound sources to the left and to the right with either one of the auditory cortices left intact. Another 24 patients were studied who had lesions mostly close to but sparing the before-mentioned auditory structures. All of them showed unimpaired performance in all test alternatives. The results indicate the specificity of the dichotic signal/noise tests for the identification of unilateral lesions in thalamocortical auditory structures. In addition, the results also point to the capacity of each telencephalic hemisphere to process the full range of auditory lateralization from left to right.

Facilitation of motor skill learning by callosal denervation or forced forelimb use in adult rats

Unilateral forelimb sensorimotor cortex lesions in adult rats produce a compensatory hyper-reliance on the forelimb ipsilateral to the lesion and temporally related glial and neural plasticity in the contralateral homotopic cortex. Recently, we found that these lesions enhance acquisition of a motor skills task with the ipsilateral, non-impaired, forelimb in comparison to shams. This effect might be related to a denervation-induced facilitation of neuroplastic changes in the motor cortex opposite the lesion and/or to the lesion-induced hyper-reliance on the non-impaired forelimb. The present study assessed whether increased forelimb use, denervation of motor cortical callosal afferents, or a combination of the two influences acquisition of a skilled reaching task. Adult rats with partial corpus callosum transections or sham procedures were either forced to rely on one forelimb or permitted normal forelimb use for 8 days. Rats were then trained for 14 days with their previously non-preferred forelimb (and the forced-use limb) on a unilateral pellet retrieval task. Compared to shams, transections produced a greater acquisition rate and asymptotic performance level on the task. Forced-use improved reaching performance relative to controls, but this effect was less enduring than the improvements produced by transections alone. The addition of forced-use to transections did not further enhance performance. These findings suggest that denervation-induced changes are likely to be a major contributor to the enhanced learning observed after unilateral sensorimotor cortex lesions.

An fMRI study of the lateralization of motor cortex activation in acallosal patients.

Transcranial magnetic stimulation (TMS) studies have suggested that callosal afferents may mediate inhibition of the ipsilateral motor cortex (IMC) during unilateral hand movements. To test this concept, we used fMRI to determine whether acallosal patients have increased IMC activation with either complex or simple unilateral finger movements. Neither the localization of motor cortical regions activated, the volumes of activation, or the relative hemispheric lateralization of activations were different between the patients and normal controls. The potential callosal inhibitory pathway identified by TMS therefore does not appear to contribute to the interhemispheric suppression of physiologically relevant activations in the motor cortex as measured by fMRI.

Callosal terminals in the rat prefrontal cortex: synaptic targets and association with GABA-immunoreactive structures.

The callosal projections of the cerebral cortex play an important role in the functional integration of the two hemispheres, and the anatomy of these connections has been extensively studied in primary sensory and motor regions. In the present investigation, we examined the synaptic targets of callosal terminals in a limbic association area, the prefrontal cortex (PFC) in the rat. In addition, we examined the relationship of callosal afferents to GABA local circuit neurons within the PFC. Callosal terminals were labeled by either anterograde transport of Phaseolus vulgaris leucoagglutinin from superficial or deep layers or by anterograde degeneration following electrolytic lesion of the contralateral PFC. Callosal terminals in either the superficial or deep layers labeled by either method formed primarily asymmetric axo-spinous synapses (approximately 95%), while the remainder formed axo-dendritic synapses. Some of the dendrites postsynaptic to callosal terminals exhibited a morphology characteristic of local circuit neurons. This observation was confirmed in tissue immunolabeled for GABA, in which degenerating callosal terminals sometimes formed asymmetric synapses on GABA-labeled dendrites. In addition, GABA-labeled terminals and callosal afferents were sometimes observed to converge onto common postsynaptic dendritic shafts or spines within the PFC. These results indicate that callosal terminals in limbic association cortex, consistent with sensory and motor cortices, primarily target the spines of pyramidal neurons. In addition, the results suggest that callosal afferents to the PFC interact with GABA local circuit neurons at multiple levels. Specifically, a proportion of callosal terminals appear to provide excitatory drive to GABA cells, while GABA terminals may modulate the excitation from callosal inputs to the distal dendrites and spines of PFC pyramidal neurons.

Evidence for a facilitatory role of callosal afferents to the cat motor cortex in the initiation of conditioned bilateral movements.

The effects of selective transection of the rostralmost portion of the corpus callosum, which contains fibres interconnecting the motor cortices of the two hemispheres, on frequency of occurrence and latency of conditioned responses (CRs) in both eyes were examined in seven cats trained to blink in response to a 500-ms tone. A 100-ms air-puff delivered to one eye only (ipsilateral eye) 400 ms after tone onset was used as an unconditioned stimulus. Both before and after callosal lesion, bilateral CRs were the most frequent response pattern. Following callosal lesion, a statistically significant reduction in the percentage of CRs in at least one eye was observed in only two cats. In all seven animals, both before and after callosal lesion, the mean CR latencies of the ipsilateral eye were significantly shorter than those of the contralateral eye. Callosal lesion caused a significant increase in the mean CR latencies of both eyes in all subjects. These results provide evidence that the two hemispheres influence each other in controlling conditioned bilateral blinking by reciprocally exchanging facilitatory signals contributing to initiation of CRs in both eyes.

Laminar Termination Patterns of Thalamic, Callosal, and Association Afferents in the Primary Auditory Area of the Rhesus Monkey

Thalamic, callosal, and association afferents to the primary auditory koniocortex, area KA, were investigated in rhesus monkeys with the aid of anterograde tract tracing techniques (radioactively labeled amino acids, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), and the Fink-Heimer silver impregnation method). Different patterns of anterograde label in KA were produced by radioactively labeled amino acid injections in the contralateral primary auditory area, the ipsilateral medial geniculate body (MGB), or auditory association cortex of the superior temporal gyrus (STG). Thalamic afferents from the MGB were found throughout KA in layers IV and III where the density waxed and waned, forming regular patches of higher density label separated by areas of less dense label. Callosal afferents from the contralateral auditory cortices were found in layers IV, III, and II in larger, more spatially separated, and irregular patches. In contrast, association afferents from the auditory association cortex of the STG were found in a continuous band in layer I. To determine the extent to which the patches of callosal afferents overlap with or interdigitate between the high-density thalamic clusters, two types of double anterograde labeling experiments were undertaken. In one set of experiments isotope injections of the MGB were made in conjunction with transection of the corpus callosum in the same animal. In another set of experiments isotope injections of the MGB were made in one hemisphere and an injection of WGA-HRP was made in the opposite supratemporal plane, including KA, in the same animal. Both approaches demonstrate that the patches of callosal afferents in KA mainly occur between the high-density patches of thalamic afferents, although there is a variable degree of overlap at their periphery. Thus it seems that thalamic and callosal inputs to KA are organized in a largely complementary fashion while input from the lateral association cortex is continuous and overlaps both. When considered in light of pertinent anatomical and physiological studies in a variety of species, it appears that the areas mainly occupied by callosal or thalamic inputs may be differentially involved in frequency analysis and sound localization, respectively.

Development of callosal connections in the sensorimotor cortex of the hamster.

To investigate the development of corpus callosal connectivity in the hamster sensorimotor cortex, we have used the sensitive axonal tracer 1,1 dioctadecyl-3,3,3',3', tetramethylindocarbocyanine perchlorate (DiI), which was injected either in vivo or in fixed brains of animals 3-6 days postnatal. First, to study changes in the overall distribution of developing callosal afferents we made large injections of DiI into the corpus callosal tract. We found that the anterogradely labeled callosal axons formed a patchy distribution in the contralateral sensorimotor cortex, which was similar to the pattern of adult connectivity described in earlier studies of the rodent corpus callosum. This result stands in contrast to previous retrograde studies of developing callosal connectivity which showed that the distribution of callosal neurons early in development is homogeneous and that the mature, patchy distribution arises later, primarily as a result of the retraction of exuberant axons. The initial patchy distribution of callosal axon growth into the sensorimotor cortex described in the present study suggests that exuberant axons destined to be eliminated do not enter the cortex. In addition, small injections of DiI into developing cortex resulted in homotopic patterns of callosal topography in which reciprocal regions of sensorimotor cortex are connected, as has been shown in the adult. Second, to study the radial growth of callosal afferents we followed the extension of individual callosal axons into the developing cortex. We found that callosal axons began to invade the contralateral cortex on about postnatal day 3, with little or no waiting period in the callosal tract. Callosal afferents then advanced steadily through the cortex, never actually invading the cortical plate but extending into layers on the first day that they could be distinguished from the cortical plate. The majority of callosal axons grew radially through the cortex and did not exhibit substantial branching until postnatal day 8, the age when the cortical plate disappears and callosal afferents reach the outer layer of cortex. This mode of radial growth through cortex prior to axon branching could serve to align callosal afferents with their radial or columnar targets before arborizing laterally.

Symmetric synapses formed by callosal afferents in rat visual cortex

Following an electron microscopic examination of 437 degenerating terminals of callosally projecting axons in layer II/III at the 17/18a border, it has been found that some of these terminals form symmetric synapses with dendritic shafts (1.83%), pyramidal cell bodies (1.37%) and dendritic spines (0.46%). The remainder of the axon terminals (96.33%) form asymmetric synapses, mainly with dendritic spines although a few form synapses with dendrites. These results suggest that in the rat visual cortex the corpus callosum is a source of both inhibitory and excitatory input to the contralateral hemisphere.

Guidance of callosal axons by radial glia in the developing cerebral cortex

During development, columns of the mammalian cerebral cortex are formed by migration of neurons along fascicles of radial glia. Subsequently, axons of the corpus callosum connect reciprocal regions of each cerebral hemisphere. To determine whether the radial growth of callosal afferents through the developing cortex may be guided by particular cellular elements, we examined the ultrastructural relationship between callosal afferents and radial fibers in the early postnatal hamster sensorimotor cortex. Developing callosal axons and their growth cones were labeled with HRP injected into the cortex at 3 d postnatal when the growth cones have extended across the callosum and are just entering the contralateral cortex. An EM analysis of 30 HRP-labeled axons and their growth cones revealed that they extended upon fascicles of radial processes associated with migrating neurons. Reconstruction of seven of these growth cones, serially sectioned in their entirety, showed that growth cones were associated with the same radial fascicle as their axon. Growth cones also touched other cellular elements such as axons. However, the finding that callosal afferents, from the point at which they enter the cortex to their growth cones, were apposed to a continuous fascicle of radial fibers suggests that callosal axons are tracking along radial processes. We conclude that the majority of the radial processes within fascicles are likely to be glial, based on their relatively large diameters, electron-lucent cytoplasm with a regular array of microtubules, the presence of glycogen granules, occasional cytoplasmic protrusions lacking microtubules, and their consistent association with migrating neurons. We propose therefore that radial glia may serve a guidance function for growing callosal axons in their radial trajectory through the developing cerebral cortex.

Mechanisms underlying potentiation of synaptic transmission in rat anterior cingulate cortex in vitro.

1. The properties of the excitatory synapse made by callosal inputs onto layer V and layer VI cells in the anterior cingulate cortex were studied in an in vitro slice preparation with intracellular recording. 2. In the presence of picrotoxin, the excitatory postsynaptic potential (EPSP) had two components, a fast component blocked by the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and a slow component blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV). 3. Delivery of a brief tetanus to the afferent fibres led to a long-term potentiation (LTP) of the initial slope of the monosynaptic EPSP. The LTP displayed the property of co-operativity and could be blocked by APV or by buffering intracellular calcium. 4. Pairing of low frequency presynaptic activity or weak tetanic stimulation with postsynaptic depolarization failed to potentiate the EPSP. This suggests that postsynaptic depolarization alone is unable to explain the co-operativity. 5. It is concluded that the transmitter mediating the excitatory input between callosal afferents and layer V and layer VI pyramidal neurones is glutamate. Tetanic stimulation of these afferents leads to LTP which shares many but not all the properties of LTP seen in the CA1 region of the hippocampus.

Occipital cortex in man: Organization of callosal connections, related myelo‐ and cytoarchitecture, and putative boundaries of functional visual areas

Human area 17 is known to contain a single (the primary) visual area, whereas areas 18 and 19 are believed to contain multiple visual areas (defined as individual representations of the contralateral visual hemifield). This is known to be the case in monkeys, where several boundaries between visual areas are characterized by bands of callosal afferents and/or by changes in myeloarchitecture. We here describe the pattern of callosal afferents in (human) areas 17, 18, and 19 as well as their cortical architecture and we infer the position of some visual areas. Sections from occipital lobes of 6 human brains with unilateral occipital infarctions have been silver-impregnated for degenerating axons, thereby revealing callosal afferents to the intact occipital cortex. Their tangential distribution is discontinuous, even in cases with large lesions. A band of callosal afferents straddles the area 17/18 boundary, whereas the remainder of area 17 and a 15-45 mm wide stripe of area 18 adjacent to the callosal band along the 17/18 border are free of them. Patches of callosal afferents alternate with callosal-free regions more laterally in area 18 and in area 19. We conclude that, in man, a second visual area (analogue of V2) lies in area 18, horseshoe-shaped around area 17, and includes the inner part of the acallosal stripe adjacent to the callosal band along the 17/18 boundary. The outer part of this acallosal stripe belongs to a third visual area, which may contain dorsally the analogue of V3 and ventrally that of VP. Thus the lower parts of the second and third visual areas lie on the lingual gyrus, whereas the analogue of the macaque's fourth visual area probably lies on the fusiform gyrus. Although the proposed subdivision of the occipital cortex relies largely on the pattern of callosal afferents, some putative human visual areas appear to have distinct architectonic features. The analogue of V2 is rather heavily myelinated and its layer III contains large pyramidal neurons. Its upper part is not well delimited laterally since adjacent "V" has similar architecture. Its lower part, however, differs clearly from the adjacent "VP," which is lightly myelinated and lacks the large pyramids in layer III. The cortex lateral to "VP" is heavily myelinated and contains fairly large pyramids in layers III and V. The myeloarchitecture of the lateral part of the occipital cortex is not uniform; a very heavily myelinated region stands out in the lateral part of area 19, near the occipito-temporal junction.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic connections of callosal projection neurons in the vibrissal region of mouse primary motor cortex: an electron microscopic/horseradish peroxidase study.

Reciprocal axonal projections between homotypic areas of the vibrissal region of mouse primary motor cortex (MsI) (Porter and White: Neurosci. Lett. 47:37-40, '84) suggested the existence of reciprocal synaptic connections between callosal projection neurons and callosal afferents. In the present study, the retrograde transport of horseradish peroxidase (HRP) was combined with lesion-induced degeneration to identify synapses between callosal afferents and callosal neurons in the corresponding region of the contralateral cortex. The procedure was as follows: MsI was injected with HRP and aspirated on the following day. After 4 days, the animals were perfused and motor cortex was processed for HRP according to a variation of the Adams (Brain Res. 176:33-47,'77) technique, and postfixed in OsO4. The methods used consistently filled fine dendritic branches and spines with dense reaction product, thus allowing examination of synaptic contacts with these processes. All callosal projection neurons were identified as pyramidal neurons, having somata in cortical layers II/III and V. Labeled cells from each of the two levels were prepared for electron microscopy, and that part of each cell's apical dendrite that traversed the superficial cortical layers, where most callosal axons terminate, was cut in an unbroken series of thin sections. Micrographs were taken of all labeled profiles in each thin section, and tracings of the profiles were assembled to reconstruct the apical dendrites. Data on the distribution, type, and amount of callosal and other synapses with the shaft and spines of the apical dendrites were obtained by examining the reconstructions. In addition, profiles of basal dendrites of layer II/III cells were examined in thin sections to ascertain the numbers of callosal and other synapses formed with their shafts and spines. The proportion of synapses that each dendrite formed with callosal axon terminals was compared to the concentration of callosal afferents in the neuropil. Dendrites of both layer II/III and layer V pyramidal cells synapsed with callosal axon terminals. The apical and basal dendrites of layer II/III neurons formed a similar proportion of their synapses with callosal afferents, and this was similar to the concentration of callosal synapses in the surrounding neuropil. Segments of apical dendrites belonging to layer V and layer II/III neurons course through neuropil containing nearly the same concentration of degenerating callosal terminals, but the layer V cells form fewer callosal synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

The size of the zone of origin of callosal afferents projecting to the primary visual cortex contralateral to the remaining eye in rats monocularly enucleated at different postnatal ages.

The cortical zone from which callosal afferents projecting to the primary visual cortex (area Oc1) originate was studied in monocularly enucleated and normal rats. The extent of this cortical strip was determined by retrograde labeling with HRP and by measurement of its width in coronal sections. Albino rats were monocularly enucleated from the 23rd ontogenetic day to the 120th and iontophoretical injections into Oc1 contralateral to the remaining eye were done more than one year after enucleation. The width of the labeled strip of perikarya in the hemisphere ipsilateral to the remaining eye was largest in neonatally enucleated rats (about 1.1 mm) and declined with increasing age at which enucleation was performed. Additionally, the perikarya of callosal afferents in the hemisphere ipsilateral to the remaining eye in rats enucleated as young adults (90th and 120th ontogenetic day) were labeled in significantly wider strips (about 0.6 mm) than in unoperated control rats (about 0.4 mm).

The lamination and connectivity of embryonic cerebral cortex transplanted into newborn rat cortex.

Sheets of frontal or occipital cerebral cortex were taken from embryonic day (E) 15 rat embryos and placed in shallow depressions made in the occipitoparietal region of newborn rats. These transplants developed normal patterns of lamination, which could be in an inverted orientation if the transplant itself was placed upside down. Irrespective of the cortical area of origin of the grafted tissue, the transplants consistently received projections from those host thalamic nuclei that were normally found to innervate the adjacent host cortex. This indicates that immature cortical tissue, up to at least E15, may not contain the information necessary to define the specific thalamocortical connections characteristic of individual areas. On the contrary, the observed input pattern may be the result of sprouting of fibers that normally innervated host cortical regions adjacent to the transplant. Similarly, callosal afferents to transplants seemed to be a direct extension of the callosal input to the host cortex immediately beneath the transplant. Results from HRP studies of callosal connections indicated that transplant efferents to the contralateral cortex are smaller in magnitude than their afferents. This may be related to the superficial location of the transplants, which may limit the access transplant efferents have to the white matter. This study suggests that, while the cortical lamination is largely determined intrinsically, the innervation of the cortex is influenced by the context in which it develops.

Modification of callosal afferents of the primary visual cortex ipsilateral to the remaining eye in rats monocularly enucleated at different stages of ontogeny.

Callosal afferents to the primary visual cortex (area Oc1) mainly originate in the border region between the lateral portion of the primary visual cortex (area Oc1) and the laterally positioned secondary visual cortex (area Oc2L) of the contralateral hemisphere. The extent of this region has been determined by retrograde labeling with horseradish peroxidase (HRP). In normal rats the width of the retrogradely labeled cortical strip is about 0.3 mm. In rats monocularly enucleated from the 23rd up to the 44th ontogenetic day and subsequently injected as adults with HRP into Oc1 ipsilateral to the remaining eye, the perikarya of the callosal afferents from the opposite hemisphere are labeled in the form of significantly wider columns (about 0.8 mm) than in animals enucleated from the 50th ontogenetic day onwards. The latter do not differ from controls.

Proliferation of thalamic afferents in cerebral cortex altered by callosal deafferentation

In area 41, the auditory region of rat neocortex, callosal afferents project to layers I through III and thalamic afferents project to deep layer III through IV. Thus, these two extrinsic systems of afferents project simultaneously to only a narrow lamina in mid to low layer III. For this study, this narrow region of overlap is quantitatively examined to determine the distribution of callosal and thalamic afferents by observing degenerating terminals produced by separate callosal and thalamic lesions. The results show that of all asymmetric synapses observed in the neuropil of this narrow zone, 84% are dendritic spines and the balance are dendritic shafts. Although both callosal and thalamic afferents prefer to synapse with dendritic spines in the neuropil, 78% of the thalamic afferents synapse with dendritic spines while 93% of the callosal afferents synapse with dendritic spines. Vaughan & Foundas (1982) have shown that 3 months after callosal lesions in 1-month-old animals, additional thalamic axons have grown into, and proliferated in, this part of mid to low layer III. Quantitative analysis of the distribution of the degenerating thalamic axon terminals in these long-term callosally lesioned animals has been used to determine whether the proliferating thalamic afferents demonstrate any specificity in the pattern of synapses they make or whether the callosally deafferented neurons determine the pattern of synapses. The results indicate that thalamic axons do exhibit axon specificity, for after they have proliferated into the callosal domain, 80% of the degenerating terminals synapse with dendritic spines and 20% synapse with shafts. This distribution is most comparable to the normal distribution of thalamic axons in this region.

The organization of immature callosal connections

In newborn kittens, the anterograde transport of horseradish peroxidase, alone or bound to wheat-germ agglutinin, indicates that callosal axons have entered selectively the restricted portions of the neocortical gray matter (e.g., the area 17/18 border) which receive callosal afferents in adults. The callosal axons do also reach regions where they lack in the adult, but there they seem not to penetrate far into the gray matter. Neonatal injections of retrograde fluorescent tracers restricted to the gray matter in areas 17, 18, and posteromedial lateral suprasylvian area (PMLS) label neurons in the contralateral hemisphere only when the tracers were directed into regions known to receive callosal axons. In particular, injections near the 17/18 border label neurons in the contralateral hemisphere at the homologous site and at restricted, retinotopically corresponding locations in other visual areas: a pattern similar to the adult one. In contrast, an injection reaching the white matter of areas 17 or 18 labels a wider, continuous territory extending mediolaterally over most visual areas from 17 to posterolateral lateral suprasylvian area (PLLS) and including regions which later become acallosal; in addition, labeled neurons are found in the limbic cortex medial to area 17 and in the auditory cortex lateral to PLLS, none of which is known to project to either 17 or 18 in the adult. In flattened reconstructions of the cortex, the shape of the territory labeled by each of these injections is characteristically, although somewhat irregularly, crescent shaped; its rostrocaudal position varies with that of the injection. An injection extending into the white matter of more lateral visual areas (19, 21a, PMLS) labels callosal neurons over a similar territory, which extensively overlaps that labeled by the 17/18 border injections and likewise includes regions which are acallosal in the adult. In spite of the overlapping distribution of labeling obtained from separate injection sites, as in adults, each cytoarchitectonically (or retinotopically) defined area seems to receive from a different set of neurons, although a few neurons send bifurcating axons to more than one area. In conclusion, injections restricted to the cortical gray matter reveal a topographic organization of juvenile callosal connections similar to that of the adult. In contrast, injections extending into the white matter and adequate to reach the transitory callosal axons which appear to be confined there reveal what appears to be an earlier organization. These two organizations probably reflect different morphogenetic factors.

Long term effects of callosal lesions in the auditory cortex of rats of different ages

The corpus callosum was sectioned in groups of rats 3, 12, and 24 months of age, and the auditory cortex was examined three months later to determine whether there were age-related differences in the morphological response to the partial deafferentation. Material from the three groups of long-term callosally-lesioned rats were compared with three groups of age-matched control animals. Analysis focused on those cortical layers known to receive the heaviest callosal projection (layers II and III) and those neurons known to be postsynaptic to callosal afferents (layer V pyramidal neurons). There were no age-related changes in cortical thickness or in the relative thickness of the cortical layers in the control groups. However, the apical dendrites of layer V pyramidal neurons did lose dendritic spines and became thinner with age. In all three lesion groups, the cortex became thinner without altering the relative thickness of cortical layers; there was a decrease in the relative density of apical dendrite spines in layer III, but an increase in the density of these spines in layer IV. Both effects varied with age. Spine decreases in layer III were greatest in older animals and spine increases in layer IV were greatest in younger animals. The mean diameters of apical dendrites decreased in the youngest group of lesioned animals but increased in the oldest group. The results indicate that the effects of callosal deafferentation are age dependent.

Binocularity in the visual cortex of the adult cat does not depend on the integrity of the corpus callosum

The importance of the corpus callosum for binocular interaction in areas 17 and 18 of the adult cat is still a matter of controversy, since its specific role in integrating information from the two eyes has been suggested by some and questioned by others 14,16,31,32. We have reanalyzed the problem by assessing binocular interaction for single neurons in areas 17 and 18 of adult cats submitted to section of the posterior two-thirds of the corpus callosum. In 5 cats this interhemispheric disconnection was performed from 10 days to 7 weeks before the electrophysiological recordings; in another cat callosal afferents to the recording sites were at first partially eliminated by an acute lesion of corresponding cortical zones in the other hemisphere, and thereafter completely interrupted by a posterior callosal section performed in the same recording session. Recordings were mainly aimed at the callosal zone of areas 17 and 18, which coincides with the border between these two areas and corresponds to visual field regions bordering the vertical meridian. Electrophysiological recordings were carried out in awake, unanesthetized animals in which all nociceptive pathways were previously interrupted by a midpontine pretrigeminal transection 4. The results indicate that the interhemispheric disconnection, whether acute or chronic, does not disrupt binocularity in areas 17 and 18; moreover, the analysis of the ocular dominance for binocular neurons did not reveal any imbalance between the inputs from the two eyes, since at all levels of eccentricity the majority of binocular neurons was equally activated by both eyes. Since in previous experiments on anesthetized cats, section of the corpus callosum apparently reduced binocular interaction in areas 17 and 18, we suggest that such an effect, which was lacking in our unanesthetized cats, was probably due to an interaction or summation between callosotomy and anesthesia.