Optimal Filtering in the Salamander Retina

The dark-adapted visual system can count photons wit h a reliability limited by thermal noise in the rod photoreceptors the processing circuitry bet.ween t.he rod cells and the brain is essentially noiseless and in fact may be close to optimal. Here we design an optimal signal processor which estimates the time-varying light intensit.y at the retina based on the rod signals. \Ve show that. the first stage of optimal signal processing involves passing the rod cell out.put. t.hrough a linear filter with characteristics determined entirely by the rod signal and noise spectra. This filter is very general; in fact it. is the first st.age in any visual signal processing task at. 10\\' photon flux. \Ve iopntify the output of this first-st.age filter wit.h the intracellular voltage response of the bipolar celL the first anatomical st.age in retinal signal processing. From recent. data on tiger salamander phot.oreceptors we extract t.he relevant. spect.ra and make parameter-free, quantit.ative predictions of the bipolar celll'esponse to a dim, diffuse flash. Agreement wit.h experiment is essentially perfect. As far as we know this is the first successful predicti ve t.heory for neural dynamics.