TIME TO COLLISION IS SIGNALED BY NEURONS IN THE NUCLEUS ROTUNDUS OF PIGEONS

THROUGHOUT the animal kingdom, the sight of a rapidly approaching object usually signals danger and elicits an escape response 1-6. Gibson 7 suggested that the symmetrical expansion of an object's image (looming) is the critical variable determining that the object is on a collision course with the observer. Similarly, large expanding flow-fields like those produced by locomotion may precipitate manoeuvres such as turning or landing 8,9. From such observations it has been shown that the optic flow parameter, tau, which specifies time to contact with the approaching object best fits the behavioural data 10,11. We describe a subpopulation of neurons in the nucleus rotundus of the pigeon brain that respond selectively to objects moving on a collision course towards the bird. These neurons give their maximum response at a constant time before contact occurs, even when the size of the stimulus or its velocity is varied widely. We propose that these neurons are signalling the time to collision of approaching objects.