In this system, the motion-sensitive neurons emit spikes very often and very precisely, said Nemenman. Historically, people have observed a lot more random spike intervals. This research is a departure from the traditional understanding in that we see that the precision of spike timing that carries information about the flys rotation is a factor of ten higher than even the most daring previous estimates.
Similar-though-much-simpler experiments on different subjects, including flies, and going back to the seminal work of E. D. Adrian and Yngve Zotterman in 1926, seemed to show that sensory neurons would fire a certain number of impulses during a given period, but that the precise timing of the impulses was largely irrelevant. Nemenman and his team believe the timing of the spikes was not as crucial during those early experiments largely because the artificial stimulation was in some sense unnatural, bordering on the monotonous and predictable.
Biological organisms have an interest in conserving energy, Nemenman said. Fly eyes account for about one-tenth of the flys energy consumption. The fly wants to be very efficient, but it costs energy and molecular resources to emit many precise spikes in the neurons.
If you are presenting simple stimuli where little changes with time, then the most efficient way to encode them may be to generate few randomly positioned spikes, which would be sufficient to convey whatever small changes, if any, happened. Similarly, if the stimulus is unnaturally fast, the neurons may not be able to encode it well.
However, if you put an organism in an environment with fast and naturally changing velocity profiles, the fly starts using all the bandwidth available to it, Nemenman said. The motion-sensitive neuron adjusts its coding strategy and it uses the precise positioning of the spikes t
|Contact: James E. Rickman|
DOE/Los Alamos National Laboratory