Encoder adaptation modulates the visual responses of crayfish interneurons

The responses of sustaining and dimming fibers were characterized by the time varying firing rates elicited by extrinsic current and flashes of light. These data were simulated by an adaptive integrate-and-fire model. A postimpulse shunt conductance simulated spike-frequency adaptation. The correlation between observed and model current-elicited impulse rates was 0.94-0.98. However, except for a difference in input resistance (both measured and simulated), the voltage to impulse encoders of the two cell groups was similar and exhibited comparable degrees of spike-frequency adaptation (40 to 45%). The encoder model derived from current-elicited responses (with fixed parameters) was used to simulate visual responses elicited by light flashes. These simulations included a synaptic current derived from the time course of the postsynaptic potential (PSP). The sustaining fiber visual response consisted of a large excitatory PSP and high-frequency transient burst that adapted (by approximately 80%) to a low-frequency plateau discharge. The simulations indicated that spike-frequency adaptation had no effect on the transient discharge but reduced the plateau firing rate by approximately 60%. Encoder adaptation enhances the sustaining fiber response to the time derivative of the stimulus. In dimming fibers, the light flash elicits an inhibitory PSP that interrupts the "dark discharge" and an off response following the end of the flash. The simulations indicated that spike-frequency adaptation reduces the firing rate of both the dark discharge and the off response. Thus the model suggests that different effects of encoder adaptation on the two cell types arise from the same encoder mechanisms, but different actions are determined by differences in impulse rate and the time course of the discharge.