We used anterograde and retrograde transsynaptic pathway tracing techniques to reveal the retinal origin and the cortical
termination of the expanded retino-geniculo-middle suprasylvian (MS) cortex pathway in adult cats which sustained lesions
of areas 17 and 18 on the day of birth (P1) or at 1 month of age (P28). Following anterograde transsynaptic transport of tritiated
amino acids from the eye, four major results were obtained: (1) a strong and specific pathway from retina through dorsal lateral
geniculate nucleus (dLGN) to the posterior half of MS cortex was identified; this pathway is a substantial expansion of an
insignificant pathway present in intact cats; (2) the terminus of the pathway was lower layer III and layer IV; (3) contralateral
projections were stronger than ipsilateral projections; (4) projections in P28 cats were stronger than those in P1 cats. Following
retrograde transsynaptic transport of WGA-HRP from posterior MS cortex, four additional results were obtained: (1) the pathway
was enlarged and visuotopically organized; (2) the pathway arose primarily from α- and γ-retinal ganglion cells; (3) a small
number of β-cells in P1 cats and a modest number in P28 cats also contribute to the pathway; (4) the combined numbers of γ-
and β-cells relative to α-cells was greater in temporal retina than in nasal retina. The combined demonstration of both origin
and terminus of the pathway with transsynaptic tracers argued strongly for high levels of coupling between primary and secondary
pathway limbs in both P1 and P28 cats. This level of coupling, as well as other features of the pathway, have much in common
with the retino-geniculo-17/18 pathway of intact cats. However, the retino-geniculo-MS system in P1 cats transmits primarily
Y and W signals, in P28 cats X, Y, and W signals; whereas the retino-geniculo-17/18 pathway transmits primarily X and Y signals.
These results have implications for understanding the repercussions of early visual cortex lesions in monkeys and humans.
Key words Adaptive plasticity - Behavioral sparing - Extrastriate cortex - Transsynaptic labeling - Monkey - Human
Received: 17 November 1997 / Accepted: 10 February 1998