Signal transmission in the catfish retina. II. Transmission to type-N cell.

Responses from channel catfish type-N (sustained amacrine) cells were evoked either by step changes in illuminance, i.e. brightening or dimming from a mean illuminance, or by a white-noise modulated light stimulus. Current injected into the horizontal-cell soma or axon produced responses in type-N cells that were very similar to those produced by light stimuli. Light- and current-evoked responses had linear and second- and third-order nonlinear components; the former contributed 40-50%, whereas the latter contributed 20-30% to the total response. The remainder of the response could have been due to higher-order nonlinearities or to intrinsic as well as extrinsic noise. Nonlinear components in the light- and current-evoked responses were sharp transient peaks, which were prominent in white-noise-evoked responses, and oscillatory wavelets. The high-frequency components in the cell's response, which result from nonlinearity, were absent in the responses from bipolar and horizontal cells. The nonlinear responses were predicted by the second- and third-order kernels. The type-N cell response was complex because the response had both linear and nonlinear components, and because of the complexities of second- and, probably, third-order kernels. The cell's complex response reflects the complex nature of the cell's function as well as its synaptic organization.