Adaptation of transient responses of a movement-sensitive neuron in the visual system of the blowfly Calliphora erythrocephala

We studied temporal response properties of the H1 neuron by extracellular recording. This neuron is a wide-field movement-sensitive element in the visual system of the blowfly (Calliphora erythrocephala). If the neuron is stimulated with a stepwise pattern displacement in its preferred direction, it responds with a burst of action potentials. By repeating the stimulus step one obtains the average of the step response: a 20ms latency time followed by a sharp increase in average firing rate and a slower decay to the resting activity.We report that the characteristic decay time of the step response depends on the stimulus history. If the stimulus moved prior to the step, the higher the pattern velocity, the faster was the decay of the step response to the resting level. In quantitative terms, for velocities in the range 0.4–100°/s, the decay time-constant varies from 300–10ms and is smaller for higher velocities. The time-constant is only weakly affected by other stimulus parameters such as modulation depth or spatial wavelength, and is set independently in different areas of the visual field where it is tuned to the local velocity.We discuss a possible advantage of this form of adaptation for the processing of visual signals: The performance of the nolinear operations that extract information from the visual input can be optimized by prefiltering signals in the individual visual columns with a time-constant that decreases with stimulus velocity. It will be shown that both the test step response and the response to continuous movement can be described reasonably well by a correlation model with input filters that adapt their time-constants.