Visual neurons in the pigeon brain encode the acceleration of stimulus motion

Seeing target motion is a vital capability of the visual system in humans and animals. Physically, motion is described by its acceleration, speed, and direction. Motion-sensitive neurons in all the visual areas examined to date are selective for the direction and speed of motion. Here, we show by single-unit recording that one-third of motion-sensitive neurons in the pigeon's pretectal nucleus also encode the acceleration of stimulus motion. These neurons are characterized by plateau-shaped speed-tuning curves in which the firing rate is the same over a wide range of speeds, a feature that allows these neurons to encode unambiguously the rate of change of speed over time. Acceleration-sensitive neurons also show transient responses to the offset of motion in the preferred and/or nonpreferred directions; acceleration-insensitive neurons do not. We observed the same sensitivity to target acceleration for brief ramps of stimulus speed and for sinusoidal modulation of speed. The locations of acceleration-sensitive and -insensitive neurons are segregated in the pretectal nucleus. The visual responses of pretectal neurons indicate that the visual and vestibular systems share not only a spatial but also a temporal reference frame that can detect the acceleration produced by self-motion of an organism.