Physiological segregation of geniculo-cortical afferents in the visual cortex of dark-reared cats

Transneuronal autoradiography in cats dark-reared from birth shows that geniculate terminals from left and right eyes are spread throughout layer IV in area 17 of the visual cortex. In area 18 however, segregation into alternating left and right eye ocular dominance patches appears to be normal. We attempted to correlate this difference between the two areas with the ocular dominance distribution of physiological activity of the geniculate afferents. To do this, microelectrode recordings of multi-unit activity, believed to represent the activity of a number of geniculate terminal axons in the vicinity of the electrode tip, were made in layer IV of areas 17 and 18 in dark-reared cats. The ocular dominance distribution of this activity was predominantly monocular in area 18, as the anatomical results predicted, but surprisingly there were also many regions in area 17 where activity could only be elicited by stimulation of one eye. This has to be reconciled with the anatomical results showing that inputs from both eyes are present throughout layer IV in area 17 of dark-reared cats. Reasons for the discrepancy are discussed.