Physiological Engineering Model Of The Retinal Horizontal Cell Layer

Abstract

Color and spatial opponent signals are essential properties of the visual system. These properties are formulated at a very early stage in the so-called retinal outerplexiform layer. To elucidate the underlying neural mechanisms, we constructed a model of the horizontal cell layer. The model consists of red- and green-sensitive cones, L- and R/G-type horizontal cells. The light induced transmitter release from photoreceptors is mimicked by a third order linear system. The horizontal cell is expressed by Hodgkin-Huxley type equations in terms of relevant ionic currents. To make the horizontal cell layer, each horizontal cell is connected by gap junctional linear conductance. The main input to L- and R/G-type horizontal cells comes from red- and green-sensitive cones, respectively. The L-type horizontal cell has a negative feedback connection to photoreceptors. Computer simulation of a stimulus displacemnet produced dynamic charactersitics very similar to the experimental results in the L-type horizontal cell layer. However, the dynamic response of the R/G-type horizontal cell differed from the experimental data without a negative feedback from R/G-type horizontal cell to green-sensitive cone. This suggests that R/G-type horizontal cell may also have feedback synapse to the green-sensitive cone, although conclusive physiological evidence has not yet been found. We also simulated bipolar cell responses and confirmed that bipolar cells respond to the local contrast change by mediation of the surrounding effect from horizontal cells.