VISUAL CONTROL OF VELOCITY OF APPROACH BY PIGEONS WHEN LANDING

Films of pigeons flying to a perch were analysed to test a theory of how speed of approach and timing of foot extension in preparation for landing are visually controlled. Rapid neural computation of distance to perch and of speed and deceleration would seem to be required. However, according to the theory, none of this is necessary. Simpler control is possible based solely on the value of the tau function of certain optic variables x, where the tau function of x is x divided by its rate of change; i.e. tau(x)=x/x. Tau(x) is a first-order approximation of time to contact with the perch and so could be used for timing foot extension. Controlled braking is possible by simply keeping tau(x), the rate of change of tau(x), constant. The results indicated that pigeons did regulate their braking when approaching the perch by keeping tau(x) constant and initiated foot extension when tau(x) reached a threshold value of approximately 150 ms. They followed this procedure even when they had one eye covered, and so binocular vision was not necessary for regulating braking or timing foot extension. It is shown that an optic variable that the pigeons could be using is the width of the optic projection of the gap between foot and perch. It is further shown that they could be using the same optic variable for controlling the trajectory of their feet to contact the perch.