Further studies on the size specificity of commissural projecting neurons of layer III in areas 17,18,19 and the lateral suprasylvian area of the cat's visual cortex.

Abstract

Summary.After injection of horseradish peroxidase (HRP) into the commissural area in the border region between areas 17 and 18, the lateral part of area 19 and the lateral Suprasylvian area (LS) of cat's visual cortex, HRP-labeled neurons were distributed preferentially in layer III in each cortical region of the other hemisphere. Layer I was free from labeling in each cortical region.There were some areal differences in the distribution of labeled neurons in other layers.In the area 17/18 border region, a significant population of labeled neurons was found in the upper layer IV. The labeling of neurons in deeper layers (V and VI) increased in area 19 and further in the LS. There was no great difference in the laminar distribution between neurons which appeared after HRP injections into the homotopical region and those after heterotopical injections on the other side.The commissural area, 17/18 border region and the LS, contained in layer III a larger number of medium-sized to large neurons than acommissural area 17 on the splenial gyrus, where layer III was filled with densely packed small neurons.The size distribution of layer III was unimodal with a marked skew towards the larger value in each cortical region.About 50% of labeled neurons in layer III were distributed in a narrow range at the border of two categories of cell sizes, viz small and medium-sized to large. Among the labeled neurons in layer III, 34.2-47.1% had larger cell sizes. Some of the smallest neurons in layer III were also labeled, however, they appeared only after HRP injections into the homotopical region of the cerebral cortex on the opposite side.

Visual-field map in the transcallosal sending zone of area 17 in the cat

“…The aims of the present study were to define the zone of cortex along the full length of area 17 that contains neurons which project to the opposite hemisphere and to define the representation of the visual field in this zone. In agreement with previous studies (Innocenti, 1980;Jouandet et al, 1985;Segraves & Rosenquist, 1982;Shoumura, 1979Shoumura, , 1981, the callosal sending zone of the marginal gyrus, as demarcated by cells retrogradely labeled from the opposite hemisphere, has been shown to occupy a zone in area 17 3-4 mm wide adjacent to the border with area 18, and typically the zone extends as far ventrally as the suprasplenial sulcus. What is new in this study is the description of this zone in area 17 as it extends into the posterolateral gyrus.…”

Visual-field map in the callosal recipient zone at the border between areas 17 and 18 in the cat

“…2. The differential laminar distribution of transcallosal projections primarily to layers II and III with lesser projections to layers I, V, and VI within these fields is compatible with the observations of others (Berman & Payne, 1983;Fisken et al, 1975;Garey et al, 1968;Segraves & Rosenquist, 1982;Shatz, 1977;Shoumura, 1981).…”

“…It has long been held that the corpus callosum is responsible for the blending together of the two visual-hemifield representations to permit a single percept of the visual field (Choudhury et al 1965;Hubel & Wiesel, 1967). To this end, callosal axons arise in area 17 from neurons in a 3-4 mm wide swath of cortex alongside the border with area 18 (Innocenti, 1980(Innocenti, , 1986Payne, 1986Payne, , 1991Segraves & Rosenquist, 1982;Shatz, 1977;Shoumura, 1981), and terminate in a narrow strip of cortex at the border between areas 17 and 18 in the opposite hemisphere (Ebner & Myers, 1965;Fisken et al, 1975;Garey et al, 1968;Innocenti, 1980Innocenti, , 1986Payne, 1986;Sanides, 1978;Shatz, 1977;Segraves & Rosenquist, 1982;Shoumura, 1981;Voigt et al, 1988). When this pattern of connectivity is compared to the visual-field maps compiled for this region of cortex by Albus and Beckmann (1980) and Tusa et al (1978), several features about the representation of the visual field in the callosal fiber recipient zone can be educed:…”