Modelling the effects of a negative feedback circuit from horizontal cells to cones on the impulse responses of cones and horizontal cells in the catfish retina.

A model of the cone-L-HC circuit for the catfish retina is presented with the following features: the outer segment consists of a compression factor and 7 low-pass filters in tandem; the cone pedicle consists of an internal negative feedback circuit in series with a low-pass filter; and the L-HC consists of a low-pass filter and forms a negative feedback circuit with the cone pedicle. By proper adjustment of the various time constants of the low-pass filters and the gain factors, the impulse responses for cones and L-HCs of the catfish retina (and turtle) can be duplicated. The negative feedback gain increases with increasing levels of mean illuminance which causes the monophasic impulse responses to become faster, biphasic and decrease in amplitude, i.e. in gain. This is an expression of the Weber-Fechner law.