Motion Detection in the Presence and Absence of Background Motion in an Anolis Lizard

Anolis lizards respond to a moving object viewed in the periphery of their visual field by turning their eye to fixate the object with their central fovea. This paper describes the relative effectiveness of different patterns of motion of a small black lure in eliciting these eye movements and the way motion of a backdrop of vegetation affects the response. The stimulus was positioned 45 degrees from the animal's line of gaze and oscillated in the vertical axis at different frequencies between 0.5 and 10 Hz. At each frequency, the amplitude of the oscillation was increased until the lizard flicked its eye towards the stimulus. The minimum amplitude needed for response (0.22 degrees of visual angle) was independent of frequency and waveform. The probability of any response occurring was, however, lower at higher frequencies (7 and 10 Hz) and a 1.5 Hz square wave evoked the greatest proportion of responses. Sinusoidal oscillation of a background of vegetation at 1.6 Hz during or before motion of the stimulus lure reduced the probability of an eye flick but did not raise the minimum amplitude needed for a response. The suppressive effect was greatest when the lure was oscillated at frequencies close to that of the background. It is concluded that Anolis, which rely upon motion to detect objects in the periphery of the visual field, filter out irrelevant motion such as that of windblown vegetation by responding preferentially to particular patterns of motion and short term habituation to commonly present patterns of motion.