Focussing their efforts on the melanopsin light sensor, which is responsible for sensing day and night but barely involved in seeing images -- at least in mammals -- Yau's team looked for melanopsin-containing cells in other vertebrates, and found some in the retinal horizontal cells in goldfish and catfish.

Horizontal cells, said Yau, allow cross-talk between neighbouring photoreceptor cells, allowing these cells to compare the light they sense, a process necessary for the brain to see images.

'The brain processes what it sees in context to the surroundings,' said Yau. 'This allows our brain to see borders and contours. Horizontal cells are the reason why we can recognise and see a face, for example.'

These findings were published in Nature.