Temporal coding: Difference between revisions

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Studying neural coding is a complex process. Because it is unclear when a neuron begins encoding a stimulus, neurologists must choose a point of reference to compare different spike trains and may make different conclusions regarding the same encoded message. 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.<ref>Theunissen F, Miller JP. ''Temporal Encoding in Nervous Systems: A Rigorous Definition''. Journal of Computational Neuroscience, 2, 149—162; 1995.</ref> Each stimulus can elicit a variety of responses, and unfortunately there is no one-to-one, stimulus-to-response pattern. However, scientists have found that there is a higher likelihood of certain response trends with specific stimuli. Once a pattern has been established, it is another matter altogether to be able to assign meaning to the spike trains.

In addition, the temporally precise nature of neuronal interactions should be considered when attempting to establish a probable code. [[Spike-timing-dependent plasticity]] is one canonical example in which the synchronicity between two neural codes is vital for synapse strengthening or weakening. In the hippocampus, when two EPSPs[[EPSP]]s arrive simultaneously, the likelihood of their producing an action potential is much higher than the smaller, temporally dispersed [[EPSP]]sEPSPs. An ''in vivo'' study done by intracellular analysis of pyramidal cells in the monkey motor cortex and simultaneous recordings from connected neurons along the thalamocortical and intracortical transmission chain showed that the EPSPs from the cortical cells had a much more drastic impact on surrounding neurons when they were synchronized within two milliseconds of each other. The importance of spike-timing-dependent plasticity in the learning and evolution process is another strong indicator that the neural code is temporal in nature. <ref name="Current Opinion in Neurobiology">{{cite web|last=Singer|first=Wolf|title=Time as coding space?|url=www.biomednet.com/elecref/0959438800900189|publisher=Elsevier Science Ltd|accessdate=4/08/12}}</ref>

If a neuron is capable of firing at a maximum rate of one hundred spikes per second, then a stimulus of less than ten milliseconds would likely elicit only 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 a lot more information than 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, where 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.