Adaptation of the motion-sensitive neuron H1 is generated locally and governed by contrast frequency

Recordings from the motion-sensitive giant neuron H1 of the lobula plate of the fly Lucilia cuprina , indicate that small field, possibly retinotopic, units presynaptic to H1, adapt in response to motion in much the same way as photoreceptors do to their signal, light intensity. As a result their response is related to what we have called velocity contrast. This adaptation is shown to take place during or after the computation of motion, and is strongly dependent on contrast frequency (temporal frequency of a moving grating). Photometric contrast contributes much less to the rate of adaptation. This is best seen by the fact that at low contrast frequencies, strong photometric contrasts can saturate H1, and yet do not adapt H1 quickly. We propose that the dependence of the adaptation upon contrast frequency is a strategy for linking the radical changes of sensitivity and temporal resolution with adaptation to that signal parameter available to H1 that is most indicative of velocity change. The effects of changes in signal parameters not related to motion, such as sudden changes in photometric contrast, which might otherwise be construed as velocity changes, are reduced. Impulse response measure­ments confirm the increase in temporal resolution of velocity changes, during adaptation, as already known in adaptation to frequent impulses. The variable rate of adaptation means that the best measure of respon­siveness is the peak instantaneous spike discharge rate. Taking this into account, the preferred contrast frequency of the H1 neuron in the unadapted state is 8-10 Hz, in close agreement with the optimum contrast frequency for initiating a landing response in tethered flies.