Barrie Frost: Neural selectivity for different classes of complex visual motion

Many tectal neurons are selective for local relative motion such as that produced by moving objects or animals, and this selectivity is produced by a double opponent process, centre-surround directionally specific mechanism. This motion system is subsequently differentiated so that higher order cells can perform figure/ground discrimination, or are sensitive to moving occluding edges, or are specifically responsive to 'time to collision' with moving objects. We are currently investigating other higher order cells in the tectofugal pathway, to determine if they selectively respond to "Biological Motion" patterns produced by conspecific behaviour, particularly those involved in the dynamics of courtship displays. In contrast, the Accessory Optic System (AOS) contains neurons responsive to large areas of optic flow, that are implicated in the optokinetic control of eyes, head and posture, and maybe are even involved in navigation.

Recent experiments on both the Tectofugal animate motion pathway and the AOS self-motion pathway will be described. Nucleus rotundus neurons that respond very selectively to looming objects will be shown to fall neatly into three classes, one group computes rho or rate of image expansion, another computes tau or relative rate of expansion, and a third group computes eta, an early warning device for approaching objects. In the AOS our recent research has shown that translation specific neurons prefer simulated panoramic translation flow-fields and are maximally responsive to translation along three orthogonal axes (vestibular frame of reference) oriented 45 degrees on either side of the midline, and vertical. The different functions these different classes of motion neurons are performing will be discussed.