A review of the neurophysiology of the turtle retina: Horizontal and bipolar cells

Abstract

Horizontal and bipolar cells are second order neurons of the vertebrate retina. Horizontal cells contribute to the generation of centre/surround receptive field organisation in retinal neurons, primarily via sign-inverting feedback to cones. In turtle, there are two physiological types of horizontal cell, luminosity and chromaticity types. Feedback interactions between horizontal cells and cones underlie the emergence of colour opponent responses in the retina. The functional duality of colour opponent and noncolour opponent channels in the visual system, which in turtle and fish has its roots in interactions in the outer plexiform layer, persists to the cortex. Photoreceptors send output to two functional classes of bipolar cells. ON bipolar cells are depolarised by light and OFF bipolar cells are hyperpolarised by light. The segregation of ON and OFF channels of information, which starts at the level of bipolar cells, has profound significance for visual processing.