Information Rate and Capacity in a Crayfish Photoreceptor Nerve Channel

The theoretical basis of our experimental approach to information rate and capacity in a crayfish photoreceptor nerve channel is that used by Shannon [16]. Dispersion curves of conduction times were studied for pulses following short, medium, and long intervals. Although the means were equal (2.6 ms), the standard deviation in the case of pulses following short intervals was larger (0.29 ms) than for medium or long intervals (0.18 and 0.21 ms, respectively). A statistical description of nerve-fiber noise was found and used to compute the information rates (¿ = 280, 140, and 80 bit/s) for various light intensities. By maximizing ¿ over all possible input interval distributions, the channel capacity was calculated to be 360 bit/s. Extending the channel to include the entire system defined from light stimulus to probabilistic behavioral output, the overall mutual information rate was estimated to be 0.04 bit/s, the discrepancy between this overall behavioral rate and the capacity of the nerve channel is noted.