|
==Finding meaning in patterns==
Simply put, a neural code can be defined as the minimum number of symbols necessary to express all biologically significant information.<ref name="Theunissen F 1995">Theunissen F, Miller JP. ''Temporal Encoding in Nervous Systems: A Rigorous Definition''. Journal of Computational Neuroscience, 2, 149—162; 1995.</ref> There are many hypotheses about an encoding method, two of which are [[neural coding#Rate coding|rate coding]] and temporal coding. Many systems of the body utilize a more complex and information rich coding system than could be considered reasonableencoded forin a rate code alone.<ref name="van Hemmen 2006">J. Leo van Hemmen, TJ Sejnowski. 23 Problems in Systems Neuroscience. Oxford Univ. Press, 2006. p.143-158.</ref> The two are often thought to work in conjunction, as in the gustatory system.<ref name="Carleton A 2010">Carleton, Alan, Riccardo Accolla, and Sidney A. Simon. (2010). [http://dx.doi.org/10.1016/j.tins.2010.04.002 "Coding in the mammalian gustatory system"]. ''Trends in Neurosciences'', 33(7):326–334.</ref> </b>
Neurons exhibit high-frequency fluctuations of firing-rates which either could be noise or could carry information. Rate coding models suggest that these irregularities are noise, while temporal coding models suggest that they encode information. If the nervous system only used rate codes to convey information, a more consistent, regular firing rate would have been evolutionarily advantageous and likely would have been selected for over time.<ref name="van Hemmen 2006"/> Temporal coding supplies an alternate explanation for the “noise”“noise," andsuggesting allowsthat forit aactually moreencodes information-rich codeand affects neural processing. To model this idea, binary symbols can be used to mark the spikes: 1 for a spike, 0 for no spike. Temporal coding allows the sequence 000111000111 to mean something different than 001100110011, even though the mean firing rate is the same for both sequences, at 6 spikes/10 msec.<ref name="Theunissen F 1995"/>
Until recently, scientists had put the most emphasis on rate encoding alone. However, functions of the brain are more temporally precise than the use of only rate encoding would seem to allow. In other words, essential information wouldcould be lost due to the inability of the rate code to capture all the available information of the spike train. In addition, responses are different enough between similar (but not identical) stimuli to suggest that the distinct patterns of spikes contain a higher volume of information than is possible to include in a rate code.<ref name="Zador, Stevens">{{cite web|last=Zador, Stevens|first=Charles, Anthony|title=The enigma of the brain|url=https://docs.google.com/a/stolaf.edu/viewer?a=v&pid=gmail&attid=0.1&thid=1369b5e1cdf273f9&mt=application/pdf&url=https://mail.google.com/mail/u/0/?ui%3D2%26ik%3D0a436eb2a7%26view%3Datt%26th%3D1369b5e1cdf273f9%26attid%3D0.1%26disp%3Dsafe%26realattid%3Df_h0ty13ea0%26zw&sig=AHIEtbQB4vngr9nDZaMTLUOcrk5DzePKqA|work=© Current Biology 1995, Vol 5 No 12|accessdate=4/08/12}}</ref>
==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.<ref name="Theunissen F 1995"/> 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,<ref>{{cite book|last=Reike, Warland, de Ruter van Steveninck, Bialek|first=Fred, David Rob, William|title=Spikes: Exploring the Neural Code|year=1997|publisher=Massachusetts Institute of Technology}}</ref> but once patterns have been establishedidentified, there is still the problem of decoding the messagesinformation thatfrom liespikes withininto 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. In this case, theThe 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.
==Sensory systems==
|
