J. Physiol. (1983), 337, pp. 331-349 331 With 7 text-figures Printed in Great Britain POSSIBLE FUNCTIONS OF THE INTERHEMISPHERIC CONNEXIONS BETWEEN VISUAL CORTICAL AREAS IN THE CAT BY COLIN BLAKEMORE*, YUNCHENG DIAO, MINGLIANG PU, YUNGKAI WANG AND YUEMEI XIAO From the Institute of Biophysics, Academia Sinica, Beijing, The People's Republic of China (Received 13 October 1981) SUMMARY 1. The functions of interhemispheric axons linking the borders between cortical areas 17 and 18 on the two sides of the brain were investigated by two techniques. 2. A well-matched sample of neurones was recorded in the 17/18 border region before and after an extensive lesion was made in the corresponding part of the other hemisphere. The proportion of binocularly driven cells fell from 96 % to 67 %, confirming the results of Dreher & Cottee (1975). Orientation- and direction-selectivity, as well as the responsiveness of the population of neurones, seemed unaltered. The reduction in binocularity was much less convincing for cells in the body of area 17, even very close to the callosal-recipient zone. 3. Reversible cooling of the 17/18 border had no effect on the few cells recorded outside the callosal zone in the other hemisphere nor on eighteen of the thirty-five cells recorded in the callosal zone. However, in ten cells the receptive field disappeared completely in one eye; in five cells there was a general reduction in responsiveness; two cells lost a portion of the receptive field, on the ipsilateral side, in both eyes. 4. The receptive fields that were apparently transmitted via the corpus callosum lay around the vertical meridian of the visual field and were not restricted to the visual hemifield ipsilateral to the receiving hemisphere: their distribution overlapped that provided by the direct geniculo-cortical input. 5. The principal function of the callosal projection between the 17/18 borders may be to contribute to binocular convergence on cortical cells and perhaps to play a part in stereoscopic vision. INTRODUCTION In all mammalian species so far examined there are reciprocal interconnexions, via axons running through the splenium of the corpus callosum, between mirror-image points in the occipital visual areas of the two hemispheres. However, the distribution of callosal-efferent cells and of their axon terminations is not uniform; it is concen- trated in bands, some of which coincide with the borders between cytoarchitectonic zones (e.g. Whitteridge, 1965; Ebner & Myers, 1965; Garey, Jones & Powell, 1968; * On leave from the University Laboratory of Physiology, Parks Road, Oxford, OXI 3PT. Correspondence and requests for reprints should be sent to this address. ) by guest on July 12, 2011 jp.physoc.org Downloaded from J Physiol (