Processing of motion stimuli by cells in the optic tectum of chickens

The ability to detect motion is crucial for the survival of animals. In the avian optic tectum, motion-sensitive output neurons in the stratum griseum centrale have large dendritic fields and receive direct retinal input at their distal dendrites (bottlebrush endings). It has been hypothesized that the activation of each ending elicits a burst in the neuron. Thus, an object moving across the receptive field would lead to a fixed number of bursts, independent of movement speed. However, experimental confirmation of this hypothesis is still missing. We measured the response of tectal neurons to moving stimuli in vivo and found that in 'fast-bursting' units (~500 Hz within burst), the number of bursts was independent of stimulus speed. These results indicate that the number of bursts might indeed be related to the sequential activation of the bottlebrush endings by visual stimuli.