Parallel colour-opponent pathways to primary visual cortex

The trichromatic primate retina parses the colour content of a visual scene into ' red/ green' and ` blue/ yellow' representations(1 - 2). Cortical circuits must combine the information encoded in these colour- opponent signals to reconstruct the full range of perceived colours(3). Red/ green and blue/ yellow inputs are relayed by the lateral geniculate nucleus ( LGN) of thalamus to primary visual cortex ( V1), so understanding how cortical circuits transform these signals requires understanding how LGN inputs to V1 are organized. Here we report direct recordings from LGN afferent axons in muscimol- inactivated V1. We found that blue/ yellow afferents terminated exclusively in superficial cortical layers 3B and 4A, whereas red/ green afferents were encountered only in deeper cortex, in lower layer 4C. We also describe a distinct cortical target for ' blue- OFF' cells, whose afferents terminated in layer 4A and seemed patchy in organization. The more common ` blue- ON' afferents were found in 4A as well as lower layer 2/ 3. Chromatic information is thus conveyed to V1 by parallel, anatomically segregated colour- opponent systems, to be combined at a later stage of the colour circuit.