The topographical relations of the visual field with the superior colliculus of the rat was investigated using constant small visual stimuli and recording the electrical response of aggregate unit activity with fine tip metal microelectrodes. A precise essentially linear projection onto the contralateral colliculus was demonstrated after appropriate corrections for brain curvature were performed. The general pattern and arrangement of the retinotopic projection is similar to that found in infra-mammalian vertebrates. An ipsilateral projection appears to be absent. The phasic properties and rhythmic discharge patterns of single units in the colliculus were studied with stationary and moving luminous stimuli.Several recent studies indicate a n orderly projection of the retina onto the optic tectum i n fishes (Jacobson and Gaze, '64; Schwassmann and Kruger, '65a j, amphibians (Gaze, '58), reptiles (Heric and Kruger, '65) and birds (Hamdi and Whitteridge, '54), and an essentially similar spatial pattern has been described in all forms studied. In mammals, the superior colliculus, the presumed homologue of the optic tectum in other vertebrates, does not constitute the principal visual projection, although retinal fibers are known to terminate in this region in an orderly fashion (Bodian, '37; Lashley, '34; Barris et al., '35). Electrophysiological studies of mammals have provided some detailed information concerning the retinotopic pattern of the superior colliculus of cat (Apter, '45), rabbit and goat (Cooper et al., '53; Hamdi and Whitteridge, ' 5 3 ) , but to date no mammal has been mapped with sufficient precision for comparison with the results obtained in sub-mammalian forms.Specification of the detailed organization of the superior colliculus and its specialized properties should be useful in evaluating the persistent view that spatial analysis is solely dependent upon the thalamo-cortical visual pathway, and that the superior colliculus merely subserves total luminous flux discrimination after removal of the principal projection to the cerebral cortex (Lashley, '34; Kluver, '41 j . The present study was initiated in order to J. COMP. NEUn., 127: 435-444. map the retinotopic projection to the superior colliculus of a mammal with a relatively unspecialized visual system as a preliminary to a more extensive analysis of receptive field organization and properties of visual neurons in this region. The rat, by virtue of possessing an almost completely crossed optic pathway and a rod retina, should provide a suitable basis for evaluation of the retinal specialization and phyletic status of other forms. A preliminary account of the present findings has been presented (Siminoff et al., '65).
MATERIALS AND METHODSSuccessful experiments were performed in 29 pigmented rats of the LongEvans strain. Several attempts to map the retinal projection to the superior colliculus in albino rats proved unsatisfactory because of difficulty in delimiting receptive fields adequately. Animals were anesthetized with urethane, ...
