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 properties of the isthmo-optic (IO) neurons and suppressive effect of remote stimuli far beyond the "classical" receptive fields were examined electrophysiologically in the Japanese quail. Receptive fields emitting more than half the maximal ON-OFF response to a small spot of light measured 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-field center, besides a stimulus for the receptive-field center, with various onset time difference. The peripheral spot, when turned on 50-100 ms before the center spot, maximally suppressed the ON and OFF responses 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) significantly suppressed the ON-OFF responses to the center stimulus. The suppressive 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 suppressive fields indicate that the IO neurons may compete with each other for activity in a very long-range scale. Simulation using a simple static model based on three basic principles of processing of the centrifugal visual system-topographical input, long-range competition, and local gain enhancement-suggests that the system could function as an autonomous mechanism for attentional object selection.