نبذة مختصرة : As their eye movements are limited, many birds respond to continuous movement of their visual world by a reflexive oscillation of their heads, analogous to optokinetic nystagmus eye movement in primates. To elicit this optocollic response (OCR), we have placed homing pigeons in the centre of a circular arena of high-resolution monitors and then presented horizontally drifting vertical gratings while recording their head movements. If the combination of the spatial frequency and contrast of the grating is beyond their visual capabilities, the reflexive movements cease, allowing us to establish what the animals can and cannot see. The results successfully replicated a previously published spatial contrast sensitivity curve obtained by more invasive electrophysiological methods. In addition to defining the limits of spatial vision, this psychophysical approach can permit an investigation of the effect of stimulus velocity on the OCR. The presentation of a higher velocity drifting pattern only elicited OCR after a prolonged warm up period, which became longer with higher velocities. In humans, such a delay in the development of optokinetic nystagmus is normally only observed in young infants. Moreover, when the drifting pattern is turned off and the bird left in darkness, there is a prolonged after-OCR. This noninvasive approach is clearly applicable to exploring different aspects of visual performance of any animal in which gaze stabilization is achieved by head rather than eye movements.
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