ANALYSIS OF DIRECTION-TUNING CURVES OF NEURONS IN THE TURTLES ACCESSORY OPTIC-SYSTEM

Visual-movement sensitivity of neurons in the turtle's accessory optic system was investigated. Neuronal responses to stimulus direction and speed were analyzed to determine whether they reflect processing by a one-dimensional encoder of visual motion or whether they indicate dir ectional integration of presynaptic direction-sensitive responses whos e maximal-response directions are distributed. Both of these mechanism s make predictions about the functional relationship between stimulus direction and response. The responses of single units in the basal opt ic nucleus to visual stimulation in different directions were describe d by both cosine and wrapped normal fitting functions. The wrapped nor mal function (a Gaussian curve mapped onto a circle) performed at leas t as well as the cosine function and described directional tuning curv es of varying widths. Unlike cosines, the addition of two wrapped norm als could describe multi-lobed directional data. Next, it was demonstr ated that these neurons did not encode visual motion projected onto a single, spatial axis. Responses to the projected speed along the maxim al-response direction were systematically lower than responses to the actual speed along that direction. Thus, for speeds above 1 degrees/s, neuronal response varies with respect to direction but not speed. Sum mation of presynaptic direction-sensitive responses with distributed m aximal-response directions (referred to as directional integration) is discussed as a means of accounting for these results.