Limits of the Optically Active Cortex of the Rabbit. ∗

Abstract

The approximate coincidence of the optically excitable area with the area striata as defined histologically may be inferred from the work of Kornmuller 1 on the Berger rhythm, and of Bartley and Bishop 2 on responses to stimulation of the optic nerve. We have here examined the limits of optic cortex histologically and physiologically in the same animals. The physiological technique has been described (Bartley and Bishop 2 )., Ten animals were used. In 5 the indifferent electrode was fastened to bone adjoining the exposed cortex; in 5, it was plunged through the cortex to the subjacent white matter. In the animals of both series the local recording electrode was moved across the margins of the optic area, recording consecutively from series of points. Identifying stabs served to locate recorded points in 20 micron frontal sections cut serially from the cortex studied. The serial sections from each brain were used to reconstruct the histo-logical boundaries of the area striata at 10 diameters (Fig. 1A). Upon these reconstructions were entered the physiological data.

The lateral border of the area striata, determined by the histolog-ical criteria of Rose, 3 coincided closely with the functionally defined limit of the optically active area. In our histological preparations a transition zone of 1 to 2 mm. width occurred between the area striata and the neighboring area occipetalis which we could not assign exclusively to either field., The stabs which identified recorded points of minimal response on the lateral margin fell within the his-tological transition zone.

The medial border of the histologically defined area was significantly overlapped by the optically active area in each of the 10 animals used in the experiments. Records taken from serial points on frontal planes between the anatomical center of the area striata and the midline (Fig. IB) demonstrated that the medial margin of the optically active area fell within the transition zone, about 2 mm. wide, between the areas parastriata and retrosplenialis (Figs. 1A and 1C). The area parastriata in the rabbit, which underlies a shallow parasagittal fissure, is, therefore, a part of the optic projection area; and from this and other evidence we conclude that it represents no more than a fissural adaptation of the histological pattern of the area striata.

The anterior histological border coincides approximately with the limit of the optically active cortex in this direction.

The posterior margin determined histologically or physiologically falls at the posterior pole of the hemisphere. Due to the rapid curving of the cortex at this point and the consequent disarrangement of the layer pattern in frontal sections, the correlation of histological and elecitrophysiological data was less reliable than on the other borders.

With the complication noted at the medial border, the coincidence between the previously defined architectonic area striata and the field of optically induced cortical response is satisfactory. This appears to justify utilizing the electro-physiological method in defining the boundaries of other projection areas from which induced responses can be recorded following stimulation of the appropriate sensory nerves.