Response modulations by static texture surround in area V1 of the macaque monkey do not depend on feedback connections from V2

We analyzed the extracellular responses of 70 V1 neurons (recorded in 3 ane sthetized macaque monkeys) to a single oriented line segment (or bar) place d within the cell classical receptive field (RF), or center of the RF. Thes e responses could be modulated when rings of bars were placed entirely outs ide, but around the RF (the "near" surround region), as described in previo us studies. Suppression was the main effect. The response was enhanced for 12 neurons when orthogonal bars in the surround were presented instead of b ars having the same orientation as the center bar. This orientation contras t property is possibly involved in the mediation of perceptual pop-out. The enhancement was delayed compared with the onset of the response by about 4 0 ms. We also observed a suppression originating specifically from the flan ks of the surround. This "side-inhibition," significant for nine neurons, w as delayed by about 20 ms. We tested whether these center/surround interact ions in V1 depend on feedback connections from area V2. V2 was inactivated by GABA injections. We used devices made of six micropipettes to inactivate the convergent zone from V2 to V1. We could reliably inactivate a 2- to 4- mm-wide region of V2. Inactivation of V2 had no effect on the center/surrou nd interactions of V1 neurons, even those that were delayed. Therefore the center/surround interactions of V1 neurons that might be involved in pop-ou t do not appear to depend on feedback connections from V2, at least in the anesthetized monkey. We conclude that these properties are probably shaped by long-range connections within V1 or depend on other feedback connections . The main effect of V2 inactivation was a decrease of the response to the single bar for about 10% of V1 neurons. The decrease was delayed by,20 ms a fter the response onset. Even the earliest neurons to respond could be affe cted by the feedback from V2. Together with the results on feedback connect ions from MT (previous paper), these findings show that feedback connection s potentiate the responses to stimulation of the RF center and are recruite d very early for the treatment of visual information.