Light-sensing cells in the eye that have long been associated with colour vision could also be involved in non-colour vision.

The retina is lined with colour-sensing cone cells, and with rod cells, which are responsible for night vision. Ramkumar Sabesan at the University of California, Berkeley, and his colleagues used light to stimulate individual cone cells in two male volunteers and asked them to report the colour they saw. The team found that one group of cones was involved in sensing red and green, whereas another, larger group allowed the participants to see white.

The findings may reflect an evolutionary trade-off that favoured high-resolution non-colour vision over the ability to see fine-grained colour.

Sci. Adv. 2, e1600797 (2016)