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PubMed1983

The postnatal development of post-activation potentiation in the rat neocortex.

The postnatal development of short-term potentiation (STP) and long-term potentiation (LTP) of the transcallosal response (TCR) was examined in anesthetized rats, aged 7 days (PN7) to adult (greater than or equal to PN180). Stimulation of callosal fibers produced a biphasic, positive-negative TCR recorded at the surface of the anterior neocortex in all age groups. The TCR showed a decrease in threshold, latency and halfwidth, and an increase in peak amplitude with age. High frequency callosal stimulation produced marked STP and LTP of the mature TCR. STP in young animals, however, could not be reliably detected until after PN16. LTP was not detected until PN18. STP and LTP approached adult levels rapidly after their initial appearance. The correlations between STP/LTP onset and development and known structural developments such as initial synaptogenesis, spine formation and myelinogenesis were not particularly strong. The implications of these results for specific hypotheses of STP/LTP mechanisms were discussed.

Animals↗
PubMed1984

Axon strata of the cerebral wall in embryonic mice.

The stratification of principal fiber systems affiliated with the developing neocortex has been analyzed by means of HRP tracing methods, monoamine histofluorescence and silver impregnations in mouse embryos ranging from the 15th to 16th embryonic day (E15/16) to the end of gestation (E19 = the day of birth). As early as E15/16 a fiber stratum divides the subplate and marks the inferior boundary of the developing cortex. Axons coursing in this fiber plane, termed the external sagittal stratum (ESS), include components of at least 5 identifiable systems: thalamocortical, corticothalamic, ipsilateral corticocortical, callosal and monoaminergic. The neocortical afferents of extrinsic origin, i.e., the thalamocortical, callosal and monoaminergic systems, cross the intermediate zone from their separate directions and converge upon the ESS. After a variable course through this stratum, single fibers ascend from their parent fascicles to ramify densely in the cortical subplate (CSB). Fibers of each of the extrinsic afferent systems mingle with each other and with locally arising axons within the CSB. Axons of the monoaminergic projection as well as fibers of the thalamic projection cross the cortical plate to ramify in the marginal zone. Other axons apparently of local intracortical origin course tangentially through the cortical plate. Otherwise, the cortical plate is devoid of proliferating axons at this early developmental stage. The set of observations illustrates the existence of sharply defined boundaries between axon-rich and axon-poor strata of the developing neocortex. These boundaries also compartmentalize postmigratory neurons with respect to their state of differentiation.

Animals↗
PubMed1996

Absence of postnatal testosterone fails to demasculinize the male rat's corpus callosum.

We had previously shown that elimination of testosterone from embryonic day 17 through adulthood reduced the midsagittal area of the male rat corpus callosum (CC). However, day 1 castration, performed after the 2-h post-birth testosterone surge, was without effect. To elucidate the contribution of this surge on the CC, male rats were delivered by cesarean section and castrated within 20 min. This procedure eliminated the 2-h postnatal rise in testosterone levels. The prenatal surge in testosterone, which occurs on embryonic day 18, remained intact. In adulthood, callosal area was examined in castrate males, sham males, and intact females. Castrate males and sham males had significantly larger CCs as compared to females. The two male groups did not differ from each other. Body weight was significantly higher in sham versus castrate males, establishing the effectiveness of the castration. These results show that hormonal organization of the CC in the male is the result of the independent action of prenatal testicular androgens, and suggest that the end of this period marks the end of callosal sensitivity to testicular hormone influence. In addition, this report documents sexual dimorphism of the CC in a third rat strain.

Analysis of Variance↗
PubMed1995

Exuberant development of connections, and its possible permissive role in cortical evolution.

The callosal visual connections of the cat provide a model for studying the phenotypes of cortical axons and their differentiation. The terminal arbor of a callosal axon develops in several successive stages. At each stage, the arbor approximates the adult phenotype more closely. This is achieved through two mechanisms: (1) exuberant, but increasingly constrained, growth and (2) partial deletion of previously generated parts of the arbor. This differentiation is controlled by interactions of the axon with its cellular environment, and by visual experience. It might have played a permissive role in the evolution of the cerebral cortex by enabling adjustments of cortical connectivity to changes in the number, size, internal organization and cellular composition of cortical areas.

Animals↗
PubMed1982

The contribution of the corpus callosum to receptive fields in the lateral suprasylvian visual areas of the cat.

In both ordinary cats and 'Boston' Siamese cats the visual areas in the lateral parts of the middle and posterior suprasylvian gyri (LSA) contain an extensive representation of the ipsilateral half of the visual field. In addition, in both groups of cats the overwhelming majority of neurons in LSA can be driven from both eyes. In Siamese cats this binocular interaction is in marked contrast with what is found in area 17 where neurons are almost exclusively activated through the contralateral eye. Transection of the posterior 1/3 to 1/2 of the corpus callosum had a different effect on the physiological organization of LSA in the two types of cats. In ordinary cats it caused the loss of the ipsilateral hemifield representation in the eye ipsilateral to the side of recording and reduced this representation in the other eye. However, after the section of the corpus callosum LSA neurons remained binocular. In Siamese cats the callosal transection left the representation of the ipsilateral hemifield in LSA unaffected, both totally abolished the input from the ipsilateral eye. These findings suggest that the visual callosal input to LSA has a different functional significance in ordinary and Siamese cats. In the former cats it may be related to perceptual equivalence across the vertical meridian of the visual field, whereas in the latter cats it may subserve interocular equivalence.

Animals↗
PubMed1984

Perfect interocular transfer of visual pattern discriminations in split-chiasm cats trained with fading.

It has been reported that the interocular transfer of visual pattern discrimination is imperfect in split-chiasm cats. Perhaps the efficiency of callosal transfer is to some extent determined by the training procedure employed. To test that possibility, we trained 3 split-chiasm cats with a Classical Procedure used in interocular transfer studies (CP cats) and 4 split-chiasm cats with Fading (Fading cats). The Fading procedure consisted of the presentation of two stimuli which differed maximally at the beginning of training; through a series of gradual steps their difference was reduced to a minimal value required to control the desired discriminative behavior. Confirming previous studies, CP cats showed a drop in the discriminative performance when the viewing eye was changed from trained to untrained and they needed additional trials to reattain criterion with the untrained eye. Fading cats, however, showed no difference when the viewing eye was changed; criterion was immediately reached with the untrained eye. It is suggested that the perfect interocular transfer shown by the Fading paradigm represents efficient callosal transmission and stabilized, well organized memory traces.

Animals↗
PubMed1990

The ocular dominance and receptive field properties of visual cortex cells of cats following long-term transection of the optic chiasm and monocular deprivation during adulthood.

Plasticity-induced interhemispheric transfer of visual information to cortical cells was studied in adult cats. The direct contralateral visual pathway was surgically eliminated permitting binocularity only by callosal transfer. In order to enhance the interhemispheric transfer, one hemisphere was made less visually active by depriving it chronically from visual input. Single cell recording was made in areas 17-18 boundary, the callosal projection zone, of operated (OC), operated and deprived (OCMD), and normal control cats. In the OCMD cats, greater than 90% of the cells in each hemisphere reacted ipsilaterally to the deprived or non-deprived eye. Only 3.1% of the cells in both hemispheres of the OCMD cats and 3.9% in the OC cats had contralateral input via the corpus callosum. The two hemispheres were similar in the selectivity of their cells to stimulus orientation and direction. The average receptive field area of the OCMD cats was also similar for the ipsilaterally driven cells in the two hemispheres; it was 1.2 degrees 2 for the deprived eye and 1.1 degrees 2 for the normal eye. The receptive fields (greater than 95%) of both eyes of the OCMD cats were found in the nasal visual hemifields and greater than 70% of them were at eccentricities of less than 5 degrees from the vertical meridian. The disappearance of the temporal (contralateral) hemifields in these cats and the physiological properties of their cortical cells were determined merely by the chiasm transection which had thus induced nearly complete interhemispheric separation. No effect of the monocular deprivation, in normal adult cats or in cats with chiasm transection was found, even after long periods (greater than 7 months). Therefore, plasticity-induced interhemispheric transfer of visual information was not found during adulthood.

Animals↗
PubMed1994

Some new trends in the study of the corpus callosum.

In recent years the corpus callosum has provided a model for the study of cortical connections in the adult and developing brain. In particular, aspects of development originally described in the corpus callosum could be generalized to other cortical connections. New frontiers include the analysis of the human corpus callosum, studies of callosal connections at the cellular level and the analysis of dynamic interactions between the hemispheres. Gross morphological parameters of the human corpus callosum have been measured and related to gender, handedness etc. The detailed dendritic and axonal morphology of individual callosal neurons and their development is being defined. Electrophysiological investigations and computer stimulations are stressing temporal aspects of the interactions between the hemispheres.

Animals↗
PubMed1994

The spatial relationship between the cerebral cortex and fiber trajectory through the corpus callosum of the cat.

We related fiber trajectory through the feline corpus callosum to the site of fiber origin in the cortical mantle and to functional modality. The cortical fields which contribute axons to the different portions of the corpus callosum were revealed by applying horseradish peroxidase (HRP) to the cut ends of selected groups of callosal axons in twelve adult cats. Overall, the application of HRP at progressively more caudal positions in the corpus callosum labels fields of neurons at successively more caudal positions in the cerebral cortex. Comparison of these data to functionally distinct cortical zones shows that the callosal body conveys a mixture of fibers arising from functionally diverse regions of the cerebrum, whereas portions of the rostral and caudal ends appear to be essentially unimodal, conveying motor and visual signals, respectively.

Animals↗
PubMed1994

Neuronal interactions in cat visual cortex mediated by the corpus callosum.

The article summarizes three sets of physiological and anatomical studies carried out to investigate the structural basis of the functional interactions between visual cortical areas 17 and 18 in the two cerebral hemispheres of cats. (1) The visual field representations in the transcallosal sending and receiving zones are defined. (2) The consequences of severing callosal fibers on the visual field representation at the area 17/18 border are described. (3) Lastly, experiments using cooling to reversibly inactive transcallosal inputs are reported. The observations on the transcallosal sending and receiving zones show that callosal connections of area 17 are concerned with a vertical hour-glass shaped region of the visual field centered on the midline, and this region is doubly represented, once in each hemisphere. The zone represents azimuths within +/- 4 degrees of the midline at the 0 degree horizontal meridian, and azimuths out to +/- 15 to +/- 25 degrees at positions distant from the horizontal meridian. The observations suggest that, in addition to interactions between neurons concerned with positions immediately adjacent to the midline, there are positions, especially high and low in the visual field, where interactions can occur between neurons which have receptive fields displaced some distance from the midline. The extent of this double representation is reduced by approximately 2/3 when the corpus callosum is cut. The retention of some bilateral representation in these animals suggests that there are alternate routes for across-the-midline transmission of visual signals. Or, more likely, there are ganglion cells in temporal retina with crossed projections that make significant contributions to the remaining double representation of the visual field. Lastly, the results obtained using cooling inactivation of transcallosal fibers show that many excitatory and inhibitory circuits are under the direct control of transcallosal fibers in the normally functioning brain. These connections appear to be no different from intrinsic connections of area 17, and they undoubtedly contribute to the binding of the two half-field representations, one in each hemisphere, and perceptual unity across the midline.

Animals↗
PubMed1994

Schizophrenia and the corpus callosum: developmental, structural and functional relationships.

Several empirical and theoretical connections exist between schizophrenia and the corpus callosum: (1) disconnection symptoms resemble certain psychotic phenomena; (2) abnormal interhemispheric transmission could explain typically schizophrenic phenomena; (3) cases of psychosis have been found in association with complete and partial agenesis of the callosum; (4) experimental neuropsychology with schizophrenic patients has revealed abnormal patterns of interhemispheric transfer; (5) studies using magnetic resonance imaging have shown abnormal callosal dimensions in schizophrenic patients. The evidence in support of these links is discussed critically. Novel neuropsychological approaches in the study of information transfer in the visual modality between the cerebral hemispheres, consistent with callosal hyperconnectivity in schizophrenic patients but not matched psychiatric controls are highlighted. Some suggestions for further work including integrating functional and structural measures are offered.

Agenesis of Corpus Callosum↗
PubMed1994

Topography of commissural fibers in the corpus callosum of the cat: a study using WGA-HRP method.

The topography of the commissural fibers in the corpus callosum (CC) of the cat was systematically investigated using the WGA-HRP method. WGA-HRP was injected into various parts of the cerebral cortex and locations of WGA-HRP-stained commissural fibers in the CC were examined. Commissural fibers were arranged in a topological fashion in the CC. Cortical areas rostral to the cruciate sulcus (CrS), corresponding to motor or premotor cortices, projected fibers into the genu of the CC, while fibers from the cortex caudal to the CrS passed through the CC slightly caudal to the genu. When WGA-HRP was injected into the lateral gyrus (LG), it was observed that fibers from the anterior LG passed through the anterior one-third of the CC, whereas those from the posterior LG passed through or near the splenium, and fibers from the middle LG passed between those from the anterior and posterior LG. Similarly, the suprasylvian gyrus (SSG) projected commissural fibers in the CC in a rostrocaudal topological manner. Fibers from the anterior SSG passed through the anterior one-third of the CC, and those from the middle SSG through the middle one-third of the CC and upper part of the splenium. Injection into the most posterior part of the middle SSG revealed fibers passing through the caudal end of the splenium. Callosal fibers from the anterior SSG were focused on in this study, because this area (area 2v) is considered one of the vestibular projection cortices and is an area of special interest to the authors. Callosal fibers from the anterior SSG were observed to pass through the anterior one-third of the body of the CC. When WGA-HRP was injected into auditory areas, fibers from the anterior and middle ectosylvian gyri (ESG) were observed to pass through the posterior one-third of the body of the CC or through the splenium, while fibers from the posterior ESG passed through the splenium. WGA-HRP was also injected into the cingulate gyrus (CiG). Fibers from the anterior CiG (area 24) passed through the anterior portion of the CC while those from the posterior CiG (area 23) passed through the posterior portion of the CC.

Animals↗
PubMed1991

Effects of handedness and sex on the morphology of the corpus callosum: a study with brain magnetic resonance imaging.

In view of conflicting data in the existing literature, we examined 53 normal subjects using a handedness questionnaire and callosal area measurements obtained from midsagittal MRI images. The callosum was found to be significantly larger in nonconsistent right-handers (NCRH), especially in its anterior half and especially for males. A significant hand x sex interaction, favoring consistent right-handed (CRH) females, was also found for the posterior midbody, a region known to house interhemispheric fibers connecting the right and left posterior association cortices. These results (1) confirm Witelson's (1985) first findings on postmortem specimens; (2) validate a dichotomy between CRH and NCRH rather than simply considering the writing hand, as was the case in most other similar studies; and (3) suggest that at least two different sex-related--probably hormonal--factors may be acting during the callosal development, one explaining the larger anterior half in NCRH males and the other the larger posterior midbody in CRH females.

Adolescent↗
PubMed1984

Commissural afferents to the cortex surrounding the posterior part of the superior temporal sulcus in the monkey.

Commissural afferents to the cortex surrounding the posterior part of the superior temporal sulcus were studied in Japanese monkeys by the horseradish peroxidase method. After injection of the enzyme, many callosal neurons were labeled contralaterally in the cortical area corresponding to the injection site (homotopical area) and in other regions (heterotopical area). Most of the callosal neurons were triangular in shape, occurring for the most part in layer III of both homotopical and heterotopical areas (about 75-90% of the total number of labeled cells). Mean diameters of the cell bodies were about 11-13 micron.

Animals↗
PubMed1988

Plasticity of claustroneocortical connections via the corpus callosum in the cat.

The cat's distribution of claustral cells that project to the contralateral visual cortex via the corpus callosum was examined. Horseradish peroxidase (HRP) was applied to severed callosal axons to label a heterogeneous population of callosal connections. Cats reared with optically induced strabismus, and Siamese cats, had HRP-filled cells extending more ventrally in the claustrum than in controls. In these groups the compaction of labeled cells was higher than in controls and the amount of increased labeled area was not dependent on the resulting eye alignment. This indicates possible plasticity of visual claustrocallosal connectivity in cats.

Animals↗
PubMed1989

Interhemispheric connections differ between symmetrical and asymmetrical brain regions.

Coronal sections from the brains of male Wistar rats that underwent corpus-callosectomy in adulthood were stained with Cresyl Violet for Nissl substance or by the Fink-Heimer method for terminal axonal degeneration. Measurements of volumetric asymmetry of neocortical region SM-I were made, and the per cent of terminal degeneration computed. As in previous studies, there was a negative correlation between asymmetry coefficient and total (right plus left) architectonic volume, indicating that symmetrical brain regions are larger than the average of the corresponding regions in asymmetrical brains. It was also found that as volumetric asymmetry increased, the per cent of axonal termination decreased, partly as a result of a decrease in the number of patches of callosal axonal termination. These results are interpreted in the light of what is known about the ontogenesis of callosal connectivity, and mechanisms for the development of architectonic asymmetry in the cerebral cortex are postulated.

Animals↗
PubMed1989

Commissural size in neonatal rats: effects of sex and prenatal alcohol exposure.

Sex differences have been reported in the size of the adult corpus callosum in both humans and rodents. This experiment investigated whether sex and/or different prenatal treatment conditions would influence commissural size at birth. Male and female 3-day-old Long Evans rats were selected from one of three prenatal treatment histories: prenatal alcohol-exposed (35% ethanol-derived calories, 35% EDC), nutritional control (0% ethanol-derived calories, 0% EDC) or standard control (lab chow). Midline sagittal areas of the corpus callosum and the anterior commissure were determined for these subjects. Male control subjects had significantly larger callosal areas than females. Prenatal alcohol exposure significantly abolished this sexual dimorphism, with 35% EDC males having a significantly smaller callosal area than males from both control groups. This effect was independent of prenatal treatment differences in body or brain size. There were no significant sex differences in the midline sagittal area of the anterior commissure, nor were there any apparent effects of prenatal treatment on this measure. These results indicate that sex differences in the size of the corpus callosum are present at birth. Since a difference in myelination cannot account for this difference in area, there may be a sex difference in the number of fibers or in the average fiber size. Additionally, the effects of prenatal alcohol exposure on male, but not female, offspring suggest that this alcohol-related birth defect is hormonally mediated.

Animals↗
PubMed1991

Corpus callosum: demasculinization via perinatal anti-androgen.

The male rat's corpus callosum is significantly larger than the female's. This dimorphism depends in part on the early presence of testosterone, since postnatal administration of testosterone to female pups enlarges their callosa in adulthood to the size of males. However, castrating males on day 1 is ineffective in reducing (demasculinizing) the size of their callosa as adults. We then addressed the question as to whether testosterone acts prior to day 1 to enlarge the callosa of males. To investigate this hypothesis pregnant rats were administered a non-steroidal androgen blocker, flutamide, during the last 5 days of pregnancy, while controls received vehicle only. Male pups from flutamide litters were castrated on day 3 to prevent postnatal recovery following clearance of flutamide, while others received sham surgery. Callosal sex differences were found between males and females of control litters, but not between males and females from flutamide litters. The absence of sex effects among flutamide litters was a consequence of small callosal size in flutamide-castrated males as compared to control males. We concluded that the prenatal production of testosterone in the male rat pup contributes to sexual dimorphism in the callosa of adult rats.

Androgen Antagonists↗