ADAPTATION TO VISUAL-MOTION IN DIRECTIONAL NEURONS OF THE NUCLEUS OF THE OPTIC TRACT

Extracellular recordings of action potentials were made from direction al neurons in the nucleus of the optic tract (NOT) of the wallaby, Mac ropus eugenii, while stimulating with moving sinewave gratings. When a grating was moved at a constant velocity in the preferred direction t hrough a neuron's receptive field, the firing rate increased rapidly a nd then declined exponentially until reaching a steady-state level. Th e decline in response is called motion adaptation. The rate of adaptat ion increased as the temporal frequency of the drifting grating increa sed, up to the frequency that elicited the maximum firing rate. Beyond this frequency, the adaptation rate decreased. When the adapting grat ing's spatial frequency was varied, such that response magnitudes were significantly different, the maximum adaptation rate occurred at simi lar temporal frequencies. Hence the temporal frequency of the stimulus is a major parameter controlling the rate of adaptation. In most neur ons, the temporal frequency response functions measured after adaptati on were shifted to the right when compared with those obtained in the unadapted state. Further insight into the adaptation process was obtai ned by measuring the responses of the cells to grating displacements w ithin one frame (10.23 ms). Such impulsive stimulus movements of less than a one-quarter cycle elicited a response that rose rapidly to a ma ximum and then declined exponentially to the spontaneous firing rate i n several seconds. The level of adaptation was demonstrated by observi ng how the time constants of the exponentials varied as a function of the temporal frequency of a previously presented moving grating. When plotted as functions of adapting frequency, time constants formed a U- shaped curve. The shortest time constants occurred at similar temporal frequencies, regardless of changes in spatial frequency, even when th e change in spatial frequency resulted in large differences in respons e magnitude during the adaptation period. The strongest adaptation occ urred when the adapting stimulus moved in the neuron's preferred direc tion. Stimuli that moved in the antipreferred direction or flickered h ad an adapting influence on the responses to subsequent impulsive move ments, but the effect was far smaller than that elicited by preferred direction adaptation. Adaptation in one region of the receptive field did not affect the responses elicited by subsequent stimulation in non overlapping regions of the field. Adaptation is a significant property of NOT neurons and probably acts to expand their temporal resolving p ower.