Pretectal neurons responding to slow wide-field retinal motion: could they compensate for slow drift during fixation?

The visual response properties are described of a group of retinal slip neurons in the wallaby pretectum, referred to as slow cells. Their responses to motion are direction-selective: tempero-nasal and naso-temporal motion over the contralateral eye increase and decrease, respectively, the firing rate relative to the spontaneous level. Slow cells are maximally sensitive to image velocities from 0.08 to 10 degrees/s. The present study focuses on slow cells that are maximally sensitive to image velocities below 1 degree/s. An interesting characteristic of 82% of slow cells is that once motion stops, the firing rate exhibits a same-sign after-response. This is characterized by a slow exponential return from the firing rate during motion to the spontaneous rate. The time constants of the after-responses are independent of the temporal frequency velocity, duration and direction of the motion stimulus. It is proposed that the neurons may assist the stabilization of eye position during fixation.