Color opponency in horizontal cells of the vertebrate retina

Chromaticity (C-type) horizontal cells have been studied extensively for more than 40 years since the first recording of such units in the fish retina. C-type horizontal cells are seen in almost every retina of cold-blooded species that contains at least two different spectral types of cone. These cells are characterized by photoresponses of polarity that depends upon the wavelength of the stimulating light. There are two basic varieties of chromaticity horizontal cells, biphasic or triphasic cells. Biphasic cells are characterized by one wavelength in which response polarity reverses and triphasic cells have two wavelengths where response polarity reverses. The neuronal network underlying the genesis of color opponency in C-type horizontal cells has been the subject of debate for many years. It is generally accepted now that cones feed-forward excitatory inputs to horizontal cells which in turn exert inhibitory effects on the cones by negative feedback pathways. C-type horizontal cells belonging to the same class are interconnected via gap junctions to form a tight syncytium. However, the spatial properties of these cells depend upon the polarity of the photoresponse because the membrane resistances of the syncytium change with different inputs. Thus, color opponency in C-type horizontal cells depends on the spatial properties of the stimulating light in addition to its dependence upon wavelength, intensity and ambient illumination. The functional role of C-type horizontal cells is to influence the spatial-chromatic organization of the receptive fields of proximal neurons. Thus, the responsiveness of bipolar cells and ganglion cells to surround illumination depend to a great extent upon the horizontal cells. However, the exact mode whereby horizontal cells can affect the organization of the proximal neurons has yet to be elucidated.