Moving background patterns reveal double-opponency of directionally specific pigeon tectal neurons

The experiments reported in this paper were carried out to determine the effect moving background patterns have on the response characteristics of directionally specific neurons in the pigeon optic tectum. First, care was taken to select the optimal single stimulus for each cell, then large textured patterns were added to the test stimulus and moved either "in-phase" or "anti-phase". Altogether 214 cells were studied in 77 white Carneaux pigeons and it was found that all cells below a depth of 400 microns were inhibited by backgrounds moved "in-phase" with the optimal test stimulus, while few cells above this level were affected in any way by backgrounds. All directions of background motion containing an "in-phase" vector resulted in rather profound inhibition of the directional response while directions with an "anti-phase" vector produced less inhibition and sometimes were even facilitated by direct "anti-phase". The velocity tuning curves obtained with an optimal single test stimulus and by "anti-phase" movement of backgrounds were essentially similar. "In-phase" inhibition can also be produced by a second spot stimulus located some distance from the test stimulus. This latter effect was used to map the outer margins of the inhibitory receptive fields of deep tectal neurons displaying these effects and it was found they were extremely large, often in excess of 100 degrees in diameter. When masks were used to prevent the moving background from stimulating the excitatory receptive field "anti-phase" movement always produced facilitation. This suggests a double opponent-process directionally specific receptive field organization. These neurons seem well suited to respond to local (object) motion and to ignore translation of the visual image arising from body, head and eye movements.