Thalamic neurons in the pigeon compute distance-to-collision of an approaching surface.

The thalamofugal and tectofugal pathways in birds are two parallel visual pathways to the telencephalon and might be comparable to the geniculocortical and colliculo-pulvinar-cortical pathways in mammals, respectively. It is known that some tectal neurons in the tectofugal pathway can signal the time-to-collision of an approaching object. Here we show by single cell recording in the pigeon that a population of visual neurons in the nucleus opticus principalis thalami (nOPT) in the thalamofugal pathway is able to detect the distance-to-collision of a large surface approaching towards the animal. These neurons began response firing when the surface reached a threshold distance to the viewing eye and thereafter their firing rates increased exponentially until collision occurred at distance = zero. The response onset distance is nearly constant for a wide range of stimulus velocities and is equal to the product of velocity of approaching stimulus and response onset time of a nOPT cell. Furthermore, onset distance of looming responses in nOPT cells is close to that causing cardioacceleration in the pigeon viewing the approaching surface. It appears that nOPT and tectal neurons are dichotomized functionally to a large extent for detecting imminent dangers, and the pigeon's ability to compute the distance-to-collision of an approaching surface may benefit avoiding large obstacles during flight.