DYNAMICS OF ORIENTATION TUNING IN MACAQUE PRIMARY VISUAL-CORTEX

Orientation tuning of neurons is one of the chief emergent characteris tics of the primary visual cortex, V1 (refs 1, 2). Neurons of the late ral geniculate nucleus, which comprise the thalamic input to V1, are n ot orientation-tuned, but the majority of VI neurons are quite selecti ve. How orientation tuning arises within V1 is still controversial(1,3 -17). To study this problem, we measured how the orientation tuning of neurons evolves with time(18-20) using a new method: reverse correlat ion in the orientation domain. Orientation tuning develops after a del ay of 30-45 milliseconds and persists-for 40-85 ms. Neurons in layers 4C alpha or 4C beta, which receive direct input from the thalamus, sho w a single orientation preference which remains unchanged throughout t he response period. In contrast, the preferred orientations of output layer neurons (in layers 2, 3, 4B, 5 or 6) usually change with time, a nd in many cases the orientation tuning may have more than one peak Th is difference in dynamics is accompanied by a change in the sharpness of orientation tuning; cells in the input layers are more broadly tune d than cells in the output layers. Many of these observed properties o f output layer neurons cannot be explained by simple feedforward model s(1,3-6), whereas they arise naturally in feedback networks(7-17). Our results indicate that V1 is more than a bank of static oriented filte rs; the dynamics of output layer cells appear to be shaped by intracor tical feedback.