Retina Neuronal Circuitry

In bright light, cone-photoreceptors are active and colour vision derives from a comparison of signals in cones with different visual pigments. This comparison begins in the retina, where certain retinal ganglion cells have 'colour-opponent' visual responses— excited by light of one colour and suppressed by another colour 1. In dim light, rod-photoreceptors are active, but colour vision is impossible because they all use the same visual pigment. Instead, the rod signals are thought to splice into retinal circuits at various points, in synergy with the cone signals 2. Here we report a new circuit for colour vision that challenges these expectations. A genetically identified type of mouse retinal ganglion cell called JAMB (J-RGC) 3 , was found to have colour-opponent responses, OFF to ultraviolet (UV) light and ON to green light. Although the mouse retina contains a green-sensitive cone, the ON response instead originates in rods. Rods and cones both contribute to the response over several decades of light intensity. Remarkably, the rod signal in this circuit is antagonistic to that from cones. For rodents, this UV-green channel may play a role in social communication, as suggested by spectral measurements from the environment. In the human retina, all of the components for this circuit exist as well, and its function can explain certain experiences of colour in dim lights, such as a 'blue shift' in twilight. The discovery of this genetically defined pathway will enable new targeted studies of colour processing in the brain. Like most mammals, the mouse has one type of rod and two types of cone photoreceptors, with absorption maxima in the ultraviolet (S pigment) and green (M pigment) region of the spectrum. As in other small mammals, the retinal organization of the cones is inhomogeneous: the M and S pigments are largely segregated in the dorsal and ventral retina, respectively 4. At the level of ganglion cells, the spectral sensitivity essentially follows this cone distribution 5,6 , which severely limits any local comparison of signals across cone pigments. Because behavioural experiments show that mice can indeed 'see colour' 7 , it has been suggested that colour vision in mice operates on very different principles from primates 8. Surprisingly, as we demonstrate here, the mouse does have a dedicated ganglion cell type with clearly opponent responses to light of different wavelengths. It uses an unexpected retinal circuit that circumvents the obstacle caused by the spatial segregation of cone pigments. We recorded the visual responses of J-RGCs in the retina of a mouse line that labels these neurons fluorescently 3 (Fig. 1a). When probed with white light, the receptive field has OFF-type sensitivity in the centre and ON-type sensitivity in the surround (Fig. 1b). As reported previously, the surround is stronger on the side of the asymmetric den-dritic arbor 3. Stimulation using coloured lights led to a surprise. Many J-RGCs produce an OFF response to uniform UV light,