Temporal Coding - Evidence

Evidence

Because it is unclear when a neuron begins encoding a stimulus, neuroscientists must choose a point of reference to compare different spike trains, and may form different conclusions based on the same spike train pattern. Even so, by observing trends between the stimuli and the response, it is possible to find different patterns which are more likely to be elicited by a certain type of stimulus. Each stimulus can elicit a variety of responses, and there does not seem to be a one-to-one, stimulus-to-response pattern. Despite this, scientists have found that there is a higher likelihood of certain response trends with specific stimuli, but once patterns have been identified, they must be decoded by cells into synaptic neurotransmitter release and resulting postsynaptic potentials.

For very brief stimuli, a neuron's maximum firing rate may not be fast enough to produce more than a single spike. Due to the density of information about the abbreviated stimulus contained in this single spike, it would seem that the timing of the spike itself would have to convey more information than simply the average frequency of action potentials over a given period of time. This model is especially important for sound localization, which occurs within the brain on the order of milliseconds. The brain must obtain a large quantity of information based on a relatively short neural response. Additionally, if low firing rates on the order of ten spikes per second must be distinguished from arbitrarily close rate coding for different stimuli, then a neuron trying to discriminate these two stimuli may need to wait for a second or more to accumulate enough information. This is not consistent with numerous organisms which are able to discriminate between stimuli in the time frame of milliseconds, suggesting that a rate code is not the only model at work.