S. V. Alekseenko

Neuronal connection of the cortex and reconstruction of the visual space.

The distributions of retrograde labeled cells in fields 17 and 18 and the fields 17/18 transitional zone were studied in both hemispheres of cats after microiontophoretic administration of horseradish peroxidase into individual cortical columns in fields 17, 18, 19, and 21a. The clustered organization of the internal connections of the cortical fields, the asymmetrical locations of labeled callosal cells relative to the injected columns, and the defined distribution of labeled cells in layers A of the lateral geniculate body suggested that eye-specific neuronal connections support “binding” of the visual hemifields separately for each eye. Application of marker to columns in fields 19 or 21a demonstrated disparate inputs from fields 17 and 18 and the fields 17/18 transitional zone. It is suggested that these connections may support the extraction of loci and stereoscopic boundaries located in the central sectors of the visual space.

Neuronal connections of eye-dominance columns in the cat cerebral cortex after monocular deprivation.

Plastic changes in intrahemisphere neuronal connections of the eye-dominance columns of cortical fields 17 and 18 were studied in monocularly deprived cats. The methodology consisted of microintophoretic administration of horseradish peroxidase into cortical columns and three-dimensional reconstruction of the areas of retrograde labeled cells. The eye dominance of columns was established, as were their coordinates in the projection of the visual field. In field 17, the horizontal connections of columns receiving inputs from the non-deprived eye via the crossed-over visual tracts were longer than the connections of the “non-crossed” columns of this eye and were longer than in normal conditions; the connections of the columns of the deprived eye were significantly reduced. Changes in the spatial organization of horizontal connections in field 17 were seen for the columns of the non-deprived eye (areas of labeled cells were rounder and the density of labeled cells in these areas were non-uniform). The longest horizontal connections in deprived cats were no longer than the lengths of these connections in cats with strabismus. It is suggested that the axon length of cells giving rise to the horizontal connections of cortical columns has a limit which is independent of visual stimulation during the critical period of development of the visual system.

Interhemisphere connections of the visual cortex in cats with bilateral strabismus

The distribution of retrograde labeled callosal cells after microiontophoretic application of horseradish peroxidase into individual cortical columns in fields 17 and 18 was studied in cats reared with bilateral strabismus (with an angle of eye deviation of 10–35°). The area containing labeled cells was located asymmetrically in relation to the position of the injected column in the opposite hemisphere. Some of the cells were located in those parts of the transitional zone between fields 17 and 18 whose retinotopic coordinates corresponded to the column coordinates (as in intact cats). Other labeled cells were located in fields 17 and 18 and were grouped into clusters located at distances of about 1000 µm from the marginal clusters of the transitional zone. The locations of labeled cells in the lateral geniculate body showed that most columns receive inputs from the ipsilateral eye. Evidence for eye specificity of these monosynaptic interhemisphere connections is presented. The functional significance of changes in these connections in bilateral strabismus is discussed.

Effects of Divergent Strabismus on the Horizontal Connections of Neurons in the Cat Visual Cortex

Neuron connections of eye dominance columns in the visual cortex were studied in kittens at age 5–6 months with experimental strabismus induced on days 13–16 of life (by tenotomy of the medial ocular muscle). A photographic method was used to assess the angle between the visual axes of the eyes. The operated oculomotor muscle was found to undergo partial reattachment to the eyeball. In some kittens, the operated eye returned to the normal position, while others showed persisting deviation of the eyes by 5–15°. Loss of the binocular properties of cortical neurons have been demonstrated in such animals (Sireteanu et al., 1993). Differences were seen in the extents of connections of cortical eye dominance columns between these groups of cats. Elimination of diplopia and non-correspondence of the two retinal images in kittens of these groups is suggested to be mediated by different mechanisms: alternation of the eyes on fixation or suppression of the activity of the deviant eye.

Interhemisphere Connections of Eye Dominance Columns in the Cat Visual Cortex in Conditions of Impaired Binocular Vision

Data from studies of interhemisphere connections in fields 17 and 18 of cats reared in conditions of impaired binocular vision (monocular deprivation, uni- and bilateral strabismus) are presented. Monosynaptic connections between neurons were studied by microiontophoretic application of horseradish peroxidase into cortical eye dominance columns and the distributions of retrograde labeled callosal cells were analyzed. Spatial asymmetry and eye-specific interhemisphere neuron connections persisted in conditions of monocular deprivation and strabismus. Quantitative changes in connections were less marked in monocular deprivation than strabismus. In cats with impaired binocular vision, as in intact animals, the widths of callosal-receiving zones were greater than the widths of the callosal cell zones, which is evidence for the non-reciprocity of interhemisphere connections in cortical areas distant from the projection of the vertical meridian. Morphofunctional differences between cells mediating connections in the opposite directions are proposed.

Structure of internal interneuronal connections in field 17 of the cat cerebral cortex.

The spatial distribution of horizontal internal connections in field 17 of the cat cortex was studied after microiontophoretic application of horseradish peroxidase to individual cortical columns. Cluster analysis of the distribution of labeled cells in the superficial layers in the tangential plane of the cortex was performed. Field 17 included 7 ± 1 clusters of up to five cells. Clusters were distributed into two layers, separated by 1.2 ± 0.3 mm. The distance between the centers of the clusters forming rows was 0.8 ± 0.3 mm. The spatial characteristics of the grouping of cells sending axons into the cortical column were compared with published data based on optical visualization of the activity of neurons in orientational and eye-dominant columns of the visual cortex. It is suggested that columns in field 17 are associated with 6–8 hypercolumns, though only with a single type of neuron within these hypercolumns, in terms of eye dominance and orientational preference.

Layerwise Organization of Neurons Providing Interhemisphere Connections in the Visual Cortex of Cats with Impaired Binocular Vision

The distribution of callosal neurons in the cortical layers was studied in intact (n = 7) and experimental cats with strabismus (n = 10) or monocular deprivation (n = 5) after microiontophoretic administration of horseradish peroxidase into the eye dominance columns of fields 17 and 18 and the fields 17/18 transitional zone. Callosal neurons in cats with impairments to binocular vision were located more deeply in layers II/III and more shallowly in layer IV than in intact cats. In addition, the proportion of callosal neurons in layer IV increased and the proportion in layer II/III decreased in cats with impaired binocular vision, as compared with intact cats, and these changes were more marked in monocular deprivation. These data point to the important role of sensory information in the formation of the laminar distribution of callosal neurons.

Effects of Strabismus and Monocular Deprivation on the Sizes of Callosal Cells in Cortical Fields 17 and 18 in the Cat Brain

Structural changes in the visual cortex were studied in conditions of deranged binocular experience by assessing the sizes (body areas) of callosal cells in fields 17 and 18 in monocularly deprived cats and in cats with convergent strabismus. These cells were detected by injection of horseradish peroxidase into columns in cortical fields 17 and 18 and the fields 17/18 transitional zone. In both groups, the mean size of callosal cells in field 17 was greater than normal, though this difference in field 18 was seen only in monocularly deprived cats. Differences in the mean sizes of field 17 and 18 cells in cats of the study groups were found to be due to the number of large cells. In cats with strabismus, callosal cells of size greater than 200xa0μm2 accounted for 58% of cells in field 17 and 8% in field 18. In monocularly deprived cats, there was no difference in the proportions of large callosal cells in these fields (28% and 26%, respectively). These data provide evidence that cytoarchitectonic changes occurred in layers of the visual cortex, serving as sources of interhemisphere connections, in conditions of early derangement of binocular experience.

The Sizes of Cells Providing Interhemisphere and Intrahemisphere Connections in the Cat Visual Cortex after Lesioning of Binocular Vision

The sizes (body areas) of 658 retrograde labeled cells in layers 2/3 of cortical fields 17 and 18 in both hemispheres were measured in intact cats, monocularly deprived cats, and cats with bilateral convergent strabismus after injection of horseradish peroxidase into the eye dominance columns of these fields. The mean size of callosal cells in all groups of cats was significantly greater than the mean size of cells providing intrahemisphere connections. The mean sizes of callosal cells in monocularly deprived cats and cats with strabismus were significantly greater than the mean size of callosal cells in intact cats, by 26.6% and 20.2%, respectively. The mean size of intrahemisphere cells in monocularly deprived cats was not different from the mean sizes of interhemisphere cells in cats with strabismus and intact cats. These data lead to the conclusion that impairments to binocular vision lead to increases in the sizes of callosal cells in the visual cortex.

Changes in the structure of neuronal connections in the visual cortex of cats with experimentally induced bilateral strabismus.

The spatial distribution of neuronal connections in cortical field 17 was studied in cats with experimentally induced bilateral convergent strabismus on postnatal days 10–14. Horseradish peroxidase was applied microiontophoretically to individual columns of neurons in fields 17 and 18 and retrograde-labeled cells were identified in both hemispheres. Increases and decreases in the extent of intrahemisphere connections were seen in the mediolateral direction (projections of the horizontal meridian of the visual field). Most columns showed increases in interhemisphere connections in this same direction, which may support the more reliable unification of the two visual hemifields. In addition, some columns showed increases in intra-and interhemisphere connections in the rostrocaudal direction (projections of the vertical meridian). Thus, bilateral strabismus induced during the critical period of development leads to changes in the structure of both intrahemisphere and interhemisphere connections of individual cortical columns in fields 17 and 18.