In Golgi preparations of goldfish retina we have observed three types of horizontal cell which receive exclusively from cones and one which receives exclusively from rods. The cone horizontal cells were designated H1, H2 and H3, in order of increasing dendritic spread, increasing separation from the outer synaptic layer, decreasing size of perikaryon, and decreasing density of cone contacts. Slender appendages with knobby terminal enlargements project horizontally from the perikarya and larger dendrites of both rod and cone horizontal cells.
We determined patterns of cone inputs to Golgi-impregnated horizontal cells by analyzing serial 1 μm sections with the light microscope. The probable inputs, in terms of visual pigments in the cones which contact them, are: H1, red + green + blue; H2, green + blue; H3, blue. Analysis of previously published work suggests (1) that H1 cells generate monophasic or L-type responses, H2 cells generate biphasic or C1-type responses, and H3 cells generate triphasic or C2-type responses; (2) that H1 cells receive direct functional input at least from red-sensitive cones, H2 cells from green-sensitive cones, and H3 cells from blue-sensitive cones; and (3) that H1 cells constitute pathways from cones to H2 cells, and H2 cells constitute pathways from cones and H1 cells to H3 cells. The precise location and route of the transfers, from H1 to H2 and from H2 to H3, are not yet known.