Temporal Coding - Finding Meaning in Patterns

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. There are many hypotheses about an encoding method, two of which are rate coding and temporal coding. Many systems of the body utilize a more complex and information rich coding system than could be encoded in a rate code alone. The two are often thought to work in conjunction, as in the gustatory system.

Neurons exhibit high-frequency fluctuations of firing-rates which 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 neurons would have utilized this code over other less robust options. Temporal coding supplies an alternate explanation for the “noise," suggesting that it actually encodes information and 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 ms.

Until recently, scientists had put the most emphasis on rate encoding as an explanation for post-synaptic potential patterns. However, functions of the brain are more temporally precise than the use of only rate encoding seems to allow. In other words, essential information could 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.