A dynamic model of a two-synapse feedback loop in the vertebrate retina

Electronic simulation of cones, horizontal cells and bipolar cells of generalized vertebrate cone retina

Electronic analogue of my theoretical model of generalized vertebrate cone retina [Siminoff: J. Theor. Biol. 86, 763 (1980)] is presented. Cone mosaic is simulated by 25 X 21 grid of phototransistors that have colored filters mounted in front of then to produce red-, green-, and blue-sensitive cones arranged in a "trichromatic" retina. Each retinal element is simulated by Summator-Integrator and unit gain voltage invertes are used to give correct polarities to output voltages. Dynamic properties of retinal elements are developed solely by temporal interplay of antagonistic input voltages with differing time courses, and spatial organization of receptive fields is developed by unit hexagons that precisely define cone input voltages to subsequent elements. Electronic model contains both color- and non-color-coded channels. Negative feedback from L-horizontal cells to cones, electrical coupling of like-cones, and electrical coupling of like-horizontal cells are simulated by feedforward circuits. Stray light is present due to light scattering properties of colored filters used to simulate color selectivety of cones. Stationary and moving spots of white and colored lights of varied sizes and intensities are used to study characteristics of electronic analogue. Results demonstrate practicality of electronic simulation to function analogues to real cone retinas to process visual stimuli and give information to higher centers as to size, shape, color and motion of objects in visual world.

An analogue simulation of the luminosity-channel in the vertebrate cone retina. 2. Frequency analysis

A frequency analysis was performed on an analogue model of the vertebrate cone retina. Bode plots of magnitude and phase angle shifts were obtained from the various stages of the Model. While the cones were non-linear, the rest of the Model was linear and results could be explained by cascading of low-pass filters with linear summation of antagonistic inputs of differing time constants. The cut-off frequency of the Model was determined by the first stage, which was the cone pedicle, while the slope of the db magnitude versus frequency curve increased for each "synaptic delay" in the pathway. An initial increase in the magnitude of the output voltage at any given stage occurred up to the cut-off frequency and was due to the antagonistic input voltages of differing time courses. Physiological data were discussed in terms of the Model.