LONG-RANGE COMPETITION AMONG THE NEURONS PROJECTING CENTRIFUGALLY TO THE QUAIL RETINA

Birds have a well-developed centrifugal pathway from the isthmo-optic nucleus to the retina. In the present study, receptive-field propertie s of the isthmo-optic (IO) neurons and suppressive effect of remote st imuli far beyond the ''classical'' receptive fields were examined elec trophysiologically in the Japanese quail. Receptive fields emitting mo re than half the maximal ON-OFF response to a small spot of light meas ured 4.3 +/- 1.9 deg (n = 37) in diameter. A stationary spot of light was presented at a remote point (35-76 deg) away from the receptive-fi eld center, besides a stimulus for the receptive-field center, with va rious onset time difference. The peripheral spot, when turned on 50-10 0 ms before the center spot, maximally suppressed the ON and OFF respo nses to the center spot. In most of the IO neurons examined (13 out of 15 units), even small remote stimuli (2-5 deg in diameter) significan tly suppressed the ON-OFF responses to the center stimulus. The suppre ssive effects of remote stimuli were seen to be extended to, at least, two thirds of the entire visual field of a single eye. Such wide supp ressive fields indicate that the IO neurons may compete with each othe r for activity in a very long-range scale. Simulation using a simple s tatic model based on three basic principles of processing of the centr ifugal visual system-topographical input, long-range competition, and local gain enhancement-suggests that the system could function as an a utonomous mechanism for attentional object selection.