This opened a new avenue to explore the
requirements for establishing the neural circuits for a new dimension
of colour sensation. Classic visual deprivation experiments1
have led to the expectation that neural connections established
during development would not appropriately process an input
that was not present from birth. Therefore, it was believed that
the treatment of congenital vision disorders would be ineffective
unless administered to the very young. However, here we show
that the addition of a third opsin in adult red–green colourdeficient
primates was sufficient to produce trichromatic colour
vision behaviour. Thus, trichromacy can arise from a single addition
of a third cone class and it does not require an early developmental
process. This provides a positive outlook for the potential
of gene therapy to cure adult vision disorders.