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PubMed1979

Commissural columns in the sensory-motor cortex of monkeys.

Callosally projecting cells and the terminal ramifications of their axons were identified in the monkey sensory-motor cortex by retrograde and anterograde labeling techniques, often by double labeling cells and axons in the same animal. Bundles of callosal fibers terminate in small column-like zones 0.5-1 mm wide in the motor cortex (area 4) and in the first (SI) and second (SII) somatic sensory areas. Such columns are aligned in register to form elongated strips extending mediolaterally in the long axes of the pre- and postcentral gyri. Significant portions of area 4, SI and SII, in regions corresponding to the representations of the hand and foot, are not callosally connected. The cells of origin of callosal fibers in SI are largely confined to layer IIIB and form columns and strips corresponding to the above. In connected zones of SI, the callosal connection is reciprocal and precisely point-to point. This and the laminar distribution of the terminal ramifications of callosal fibers (to layers I-IV) suggest that callosal fibers may arise from the terminate upon exactly homotopic, column-like groups of layer IIIB pyramidal cells. Commissurally projecting cells and their terminal ramifications are not limited to particular architectonic fields or particular parts of fields in SI. All architectonic fields of SI project heterotopically to the contralateral SII.

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PubMed1976

Interhemispheric neocortical connections of the corpus callosum in the reeler mutant mouse: a study based on anterograde and retrograde methods.

The tangential organization of the callosal system of interhemispheric connections, as judged by the distribution of axon terminals as well as by the distribution of cells of origin of callosal axons, is normal in the reeler mutant mouse. As in the normal animal connections between the two cerebral hemispheres are homotopic. In the reeler, as in the normal animal, medium-sized pyramidal cells are, numerically speaking, the principal cells of origin of the callosal system. These lie superficially in the cortex of the normal animal but deep within the cortex of reeler. Callosal terminals are most densely concentrated at the cortical level of the small and medium-sized pyramids in both reeler and normal animals. It is probable, therefore, that the same classes of neurons are interconnected by the callosal system in the normal and reeler mouse despite malposition of neurons in reeler. The patterns of intracortical distribution of terminals of callosal axons is evidently governed by the positions of their target cells.

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PubMed1978

The third visual complex of rhesus monkey prestriate cortex.

1. Two independent but neighbouring visual areas, V3 and V3A, sharing a common cytoarchitectural plan, but in each one of which the visual fields are separately represented, have been studied anatomically, functionally, and in combined anatomico-physiological experiments. 2. The properties of single cells in the two areas are so similar, judged by the techniques used in this study, that it is often impossible to tell whether any one penetration was sampling from cells in V3 or V3A. This is especially so if the cells have receptive fields in the lower hemi-quadrants, since the vertical meridian of the lower visual fields is represented along the V3-V3A boundary and since a transition from V3 to V3A along this border is not accompanied by a shift in receptive field positions of cells. 3. Since the visual fields, including the vertical meridian, are separately represented in these two areas, and since regions of vertical meridian representation are callosally connected, a simple and certain method of specifying the boundary between V3 and V3A is to examine the degeneration following section of the callosal splenium. A heavy patch of degeneration then marks the V3-V3A boundary. Within this patch, however, is a sub-patch containing fewer callosal fibres, or none at all. The boundary between V3 and V3A was taken to be at this subpatch. 4. Since the horizontal meridian is represented at the V2-V3 boundary, and since V1 projects to both these areas, sending coarse fibres to V3 and fine fibres to V2, it was found that the boundary between V2 and V3 could be precisely drawn by making a lesion in the horizontal meridian representation in V1 and noting where, in the prestriate cortex, fine fibres give way to coarse ones, without an intervening gap. 5. Double tracer anatomical experiments, in which tritiated proline was injected into V1 of animals whose callosal splenium had been sectioned, showed that whereas V3 receives a direct input from V1, V3A does not. V3A, instead, was found to receive an input from V3. Double tracer anatomical experiments were undertaken to study a possible input from V2 to V3A. Although such experiments did not reveal a direct input from V2 to V3A, they were not entirely conclusive. 6. The vast majority of cells in V3 and V3A were binocularly driven, without obvious monocular preferences. Some cells, however, though responding to stimulation of the individual eyes, summated their responses to binocular stimulation. Others responded only when both eyes were simulataneously stimulated. In any oblique penetration, cells preferring binocular stimulation only occurred either singly or in groups. 7. In an oblique penetration, the shift from a cell responding to binocular stimulation only to one responding equally well to stimulation of either eye was not necessarily accompanied by a shift in orientational preferences, shifts in the former...

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