Cortical and callosal connections concerned with the vertical meridian of visual fields in the cat.

IN HIGHER MAMMALS the left half of the visual field is represented in the right visual cortex, and the right half in the left cortex. The fact that our visual fields appear uniform, with no obvious interruption along the vertical midline, would seem to call for connections linking the two hemispheres. The present paper deals with the physiological properties of some of these connections. In the cat the vertical midline of the visual field has its primary representation in and close to the poorly defined boundary between areas 17 and 18 (7, 10). Each half-visual field is represented in both 17 and 18 in mirror fashion, so that as one proceeds away from the 17-18 boundary in either direction, medially along 17 or laterally along 18, the corresponding region of visual fields moves outward from the vertical meridian into the contralateral visual field. Recent anatomical evidence indicates that the part of area 17 adjacent to the 17-18 boundary sends callosal projections to the other hemisphere, ending in 18 near the 17-18 boundary (2,7,8) and possibly also in the adjacent part of 17 (8). Thus the parts of 17 and 18 having to do with the vertical midline of the visual fields are apparently connected on the two sides by the corpus callosum. In contrast, the more medial parts of 17, representing the peripheral parts of the visual fields, seem not to be reciprocally interconnected, and indeed one would hardly expect that cells concerned with a particular part of the left visual field should be connected specifically to cells concerned with the corresponding region in the right field. These anatomical findings have lately been confirmed in the elegant physiological experiments of Choudhury, Whitteridge, and Wilson (1). They showed that with one opti c trac t cut responses to light could be evoked in the ipsilateral hemisphere only near the 17-18 border, and only when the stimulus was applied near the vertical meridian. The responses were abolished either by cooling the corresponding region in the contralateral hemisphere or by cooling or cutting the posterior corpus callosum. Several lines of evidence thus indicate that fibers in the corpus callosum originating in the most lateral part of 17 and projecting to 18 on the other side have special functions involving the vertical midline of the visual field.