Progress in solving the circuit for seeing black, white, and colour

We have successfully treated colour blindness in an adult primate using gene therapy. Our results raise the major question of what properties of the neural circuit make the addition of a new dimension of colour vision possible. To answer this, we probed the circuit for colour vision in the rodent using a gene therapy approach in which new long-wavelength sensitivity was targeted to either S-cone or M-cone pathways. Gene therapy using cone-class specific transcriptional regulatory elements enabled us to express human L-opsin in a mosaic of either M- or S-cones. Functional consequences of the expanded spectral sensitivity were explored after therapy, by measuring the neuronal response throughout the visual system with fMRI and behavioural tests of colour vision. Dramatic expansion of colour vision to include a red-green dimension of colour vision in these rodents was observed when L-opsin was targeted to S-cones but not when targeted to Mcones. This result indicates that the novel red-green colour vision observed in animals treated with gene therapy is served by a portion of the pre-existing circuit for blue-yellow colour vision involving the