Abstract
PREVIOUS experimental studies1,2 have established the existence of centrifugal fibres in the avian visual pathway and have shown that these fibres arise in the prominent isthmo-optic nucleus in the midbrain, that for a considerable part of their course they run independently of the optic nerve fibres, cross completely at the optic chiasma and terminate principally around the amacrine cells on the inner aspect of the bipolar cell layer as described in normal material3,4. The only known source of afferent fibres to the nucleus of origin of these fibres is the optic tectum5, and there is evidence for some degree of organization within the projection of the tectum on the isthmo-optic nucleus2. This observation, when taken together with the electrophysiological evidence for the representation of the retina on the tectum6, suggested, first, that there may be an ordered representation of the retina on the isthmo-optic nucleus, and secondly, that the nucleus may project in a similarly organized manner on the retina. This would mean that each part of the retina is reciprocally connected (through the tectum) with that part of the isthmo-optic nucleus from which it receives its centrifugal afferents.
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