THE BLUE-ON OPPONENT PATHWAY IN PRIMATE RETINA ORIGINATES FROM A DISTINCT BISTRATIFIED GANGLION-CELL TYPE

COLOUR vision in humans and Old World monkeys begins with the differen tial activation of three types of cone photoreceptor which are maximal ly sensitive to short (S), medium (M) and long (L) wavelengths. Signal s from the three cone types are relayed to the retinal ganglion cells via cone-specific bipolar cell types1-4. Colour-coding ganglion cells fall into two major physiological classes: the red-green opponent cell s, which receive antagonistic input from M- and L-sensitive cones, and the blue-yellow opponent cells, which receive input from S-sensitive cones, opposed by combined M- and L-cone input. The neural mechanisms producing colour opponency are not understood. It has been assumed tha t both kinds of opponent signals are transmitted to the lateral genicu late nucleus by one type of ganglion cell, the midget cell5,6. We now report that a distinct non-midget ganglion cell type, the small bistra tified cell, corresponds to the physiological type that receives excit atory input from S cones, the 'blue-on' cell. Our results thus demonst rate an anatomically distinct pathway that conveys S-cone signals to t he brain. The morphology of the blue-on cell also suggest a novel hypo thesis for the retinal circuitry underlying the blue-yellow opponent r esponse.