The organization of binocular cortex in the primary visual area of the rabbit.

Abstract

AbstractA conscious rabbit which crouches in the ‘freeze’ position has an unequivocal 24° wide binocular field formed by the overlap of the two 12° sectors of uniocular optical field which extend nasal of its midline. Although this investigation reveals that the horizontal representation of the uniocular visual field in cortical visual area I extends more nasal than in earlier reports, it is found to terminate at the midline when the eyes are set in the standard freeze position. The 12° sectors of uniocular field nasal of the midline were not represented in spite of being served by retina. The binocular field of the rabbit in the freeze posture thus appears to have no binocular representation in visual area I.Nevertheless, the presence of a binocular region was confirmed in rabbit visual area I but the projections to it, via the contralateral and ipsilateral eyes, deal with the nonoverlapping sectors of monocular field when the eyes are in the freeze position. The maps of the visual field obtained in one hemisphere via the contralateral and ipsilateral projections were subject to a horizontal divergent disparity of some 18°. The presence of binocular single units in these regions was also confirmed but their two receptive fields were necessarily located in different visual hemispheres and again subject to an 18° mean horizontal divergent disparity. They could not be simultaneously stimulated by any localized feature of an object and are thus precluded from involvement in binocular single vision during the freeze position.The systematic, rather than reportedly random, topography of the ipsilateral projection to visual area I ensured that it could be well fused with the similarly organized contralateral projection by means of an 18° vergence of the eyes from the freeze position. The horizontal receptive field disparity of binocular single units is then brought to a zero mean value which enables their receptive fields through each eye to be simultaneously stimulated by the same part of an image. Tested units then evinced response summation; a minimum requirement for binocular vision. In this equivalent primary position of the eyes, the entire binocular visual field is completely represented in visual area one of both hemispheres.The rabbit thus appears to employ a two‐state cortical system for forward vision. In the freeze position the visual field attains a cyclopean extent of 360° but the receptive fields of cortical binocular units are widely divergent. The classic binocular optical field may then project to some other region of brain which subserves binocular vision. Upon examination of a frontal object the animal must verge its eyes in order to obtain binocular single vision in visual area I; it then temporarily surrenders part of its rear visual field. An explanation in terms of ocular image quality is suggested.