The Transfer of Abnormal Visual Field from the Dorsal Lateral Geniculate Visual Cortex in Siamese Cats Representations Nucleus to The

It has previously been shown that Siamese cats have abnormal retino-geniculate pathways. In the lateral geniculate nucleus of the Siamese cat two layers (A and C) represent the nasal retina of the contralateral eye in a normal manner. Two other layers (A1 and C1) have some segments that carry a normal retinotopically organized representation of the visual field, and other segments in which the rePresentation of the visual field is abnormal and reversed 26. In order to determine the pattern of input that the visual cortex receives from these normal and abnormal geniculate segments, the visual field representations in the visual cortex of Siamese cats have been studied. Recordings have been obtained from cortical neurons or neuron groups with micro-electrodes, and the retrograde cell changes that occur in the lateral geniculate nucleus following lesions of the visual cortex have been studied. The geniculo-cortical pathway is modified in one of two quite distinct ways. Each modification produces a complete and orderly sequential representation of the visual field upon the cortex. One involves the suppression of geniculo-cortical input from layers A1 and C1, without any modification of the pathway from the normal layers, and the other involves a rearrangement of the whole geniculo-cortical projection , with a reversal of the input from the abnormal segments of layers A1 and C1 (see ref. 36). In Siamese cats that suppress input from layers A1 and C1 the suppression can be reduced but not abolished by lid suture. That is, after a unilateral lid suture in Siamese cats, input relayed through the normal geniculate layers still dominates over that coming through the abnormal layers, even on the side on which the normal layers were deprived by the suture. However, the suppression of the input from the abnormal layers is less on this side than it is in a normal Siamese cat.