BINOCULAR AND CALLOSAL AREAS IN THE RABBIT VISUAL CORTEX

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In order to reexamine the cortical distribution of the callosal connection, as well as its topographical relation to the binocular area in the rabbit visual cortex, the distribution of cortical responses evoked by electrical stimulation of the optic nerve and by direct cortical stimulation of the opposite visual area were studied in thirty immobilized animals.
1. The early response to ipsilateral optic nerve stimulation was similar in latency and waveform to that of the contralateral stimulation, and the area giving this response was determined as the binocular area. The binocular area which was a band of 6-7mm wide extended from the anteromedial to the posterolateral portion across the visual cortex.
2. Besides the early response, the binocular area showed the later response to ipsilateral optic nerve stimulation. The later response was found to be mediated via the corpus callosum, because it was abolished by cooling the opposited visual cortex. The interval between the beginning of the early and late responses was comparable with the latency of the transcallosal response elicited by direct cortical stimulation. The transcallosal response was also abolished permanently by cutting the posterior half of the corpus callosum. The area of transcallosal response and the area effective in eliciting this response were found to be in good agreement with each other and with the binocular area.
3. The callosal connection between the two binocular areas was not necessarily homotopic. For diffusely distributed points of stimulation in the binocular area of one cortex the points showing the maximal callosal responses in the other cortex were arranged in a narrow band in the middle of the binocular area. This band, about 1mm wide and running obliquely about 45 degrees to the midline, seemed to correspond to the projection line of the vertical meridian (decussation line) determined by Thompson et al.
These results suggest that the whole binocular area of both hemispheres in the rabbit are interconnected via the corpus callosum, and that the callosal fibers from the binocular area of one cortex terminate most densely in the middle of the opposite binocular area.