TEMPORAL PRECISION OF THE ENCODING OF MOTION INFORMATION BY VISUAL INTERNEURONS

Background: There is much controversy about the timescale on which neu rons process and transmit information. On the one hand, a vast amount of information can be processed by the nervous system if the precise t iming of individual spikes on a millisecond timescale is important. On the other hand, neuronal responses to identical stimuli often vary co nsiderably and stochastic response fluctuations can exceed the mean re sponse amplitude. Here, we examined the timescale on which neural resp onses could be locked to visual motion stimuli. Results: Spikes of mot ion-sensitive neurons in the visual system of the blowfly are time-loc ked to visual motion with a precision in the range of several tens of milliseconds. Nevertheless, different motion-sensitive neurons with la rgely overlapping receptive fields generate a large proportion of spik es almost synchronously. This precision is brought about by stochastic rather than by: motion-induced membrane-potential fluctuations elicit ed by the common peripheral input. The stochastic membrane-potential f luctuations contain more power at frequencies above 30-40 Hz than the motion-induced potential changes. A model of spike generation indicate s that such fast membrane-potential changes are a major determinant of the precise timing of spikes. Conclusions: The timing of spikes in ne urons of the motion pathway of the blowfly is controlled on a millisec ond timescale by fast membrane-potential fluctuations. Despite this pr ecision, spikes do not lock to motion stimuli on this timescale becaus e visual motion does not induce sufficiently rapid changes in the memb rane potential.