Matched filtering by a photoreceptor membrane.

This study demonstrates how phototransduction cascades and membranes tune photoreceptor response dynamics to image quality, and eliminate noise introduced in cell signalling. Intracellular recordings from intact retina confirm that the light-adapted photoreceptors of the crane fly Tipula paludosa (Diptera; Tipulidae) have a slow response, appropriate for their visual ecology. To provide a slow response, the phototransduction cascade's impulse response fails to narrow with light-adaptation, despite reductions in the timescales of latency and quantum bumps. The photoreceptor membrane acts as a passive RC-filter, because light induced depolarization inactivates voltage-gated potassium currents. The frequency response of the membrane equals the cascade's and, as a result, the membrane is a matched filter that suppresses photon shot noise. This type of broad-band filter, matched to the predictable dynamics of preceding processes to remove noise, could be widely employed in vision and in many other chains of cellular communication.