The function of the midget cell system in primate color vision

Abstract

Polyak described a neural circuit in primate retina which can mediate the output of a single cone. This circuit connects a cone with a ganglion cell by either of two types of midget bipolar cells, the synapotology of which has recently been examined by electronmicroscopy. I believe that I have detected the responses of ganglion cells of this midget cell system in Rhesus monkey retina. These ganglion cells have small receptive field centers; they are excited (either at “on” or at “off”) by only one cone mechanism (either red, green or blue sensitive) in the center of their receptive field; they have small axons based on conduction velocity measurements; they are concentrated near the fovea; they respond continuously to a maintained stimulus of appropriate wavelength. Antidromic stimulation of these cells produces a distinct graded potential in the central retina. This potential can be elicited by stimulation of the parvo, but not the magno-cellular layers of the lateral geniculate nucleus implying that the midget cell system terminates in the parvo-cellular layers of this nucleus. Cells in the parvo-cellular layers tend to resemble midget ganglion cells in having only one of the three cone mechanisms mediating their receptive field center response. Striate cortical cells receive signals from several cone mechanisms in the center of their receptive field making some (those excited at “on” by one or more cone mechanisms and at “off” by another one or more) especially sensitive to color boundaries. Cells sensitive to a particular color boundary moving in one direction seem to be located next to those sensitive to the same boundary moving in the opposite direction suggesting an organization based on color contrast rather than color, per se. A model for color vision will be proposed in which six channels (based on the receptive field center mechanism of cells in the midget cell system) excite in various combinations single color sensitive cortical cells. Excitation of such a cell either at “on” or at “off” always produces the same color sensation.