Neuronal responses to motion and apparent motion in the optic tectum of chickens.

The ability to detect moving objects is crucial for survival and is a basic capability in the animal kingdom. In the optic tectum, a major center of the visual pathway of birds, neurons have been found that seem to be perfectly suited to encode motion. These cells, located in the stratum griseum centrale, receive a direct retinal input at their distal dendrites. If the sequential activation of the dendrites is the driving force behind the motion sensitivity of these cells, it is expected that they respond similarly to motion and apparent motion, where the stimulus traverses the exact same trajectory, but in discontinuous steps. To test this hypothesis, we recorded extracellularly from the optic tectum and stimulated the eyes with motion and apparent motion stimuli. Our experiments revealed two different types of responses. The first response type ('Type A') is characterized by an almost equally strong response to apparent motion as to normal motion when the stimulus moves in the preferred direction. The second response type ('Type B') is characterized by a sharp response to rarely occurring visual stimuli, independent of their temporal relation to other stimuli. In addition, these units responded well to stimuli independent of whether they were shown in the preferred, anti-preferred, or pseudorandom order. Units showing these two response patterns could be the units previously characterized as the SGC-III ('Type A') and SGC-I ('Type B') subtypes of the stratum griseum centrale and encode motion and novelty, respectively.