Codes in the Neurons: How is Information Represented in Brain Tissue?
Since the nervous system is above all an information receiving, processing, generating, and transmitting system, the question how it encodes and represents information is a first order one. The evidence shows there is not a single code but multiple forms of representation, none digital but various forms of analog, including pulse coded analog representation. The title question becomes multiplexed because of parallel codes and compounded due to sequential encodings, decodings and reencodings after integrating other inputs or in-built proclivities. There are those who reject the terminology of codes, representation, and recognition but I find it relevant and heuristic, the only shorthand way to ask many meaningful questions. Obviously terms such as encoding do not imply the conscious intentionality of a human operator but only the change in some aspect of neural activity that is felt by the next neuron and carries the information about time and intensity of whatever aspect of the stimulus, input or state change the encoder senses. The concept applies not only to sensory but to all neurons.
Subthreshold graded activity in cells, dendrites, axons and terminals provides a large class of codes, both electrical and chemical. They may be reencoded as spike trains or cause transmitted influence upon the next neurons without spikes (Chapter 3). I like to recognize impulse trains in single neurons as providing a whole class of candidate codes, for many of which there is now some evidence. These are single channel codes. Activity in parallel arrays of units may be considered as ensemble representations; they too may exist as spatiotemporal patterns of spikes or of non-spike, slower, graded, compound fields. It is less certain whether this last class act as signals, exerting influence, or whether they represent merely effects, like the noise of a car. I am betting that some of the larger fields can exert a significant degree of influence.
Implicit in this view of neural coding is the prediction that different messages can be carried in parallel in the same channel, by different codes read by different receivers, just as we use spoken and body languages or linguistic and tone of voice communication.
One further class of codes is the labels on the lines. A very large number of labels must be known to the brain, via genetics and experience. This applies to primary modalities (visual, auditory, etc.) and modali?ties presumed to depend on combinations of them (tickle, texture, some kinds of pain), submodalities (color, acoustic frequency, sweet, sour, yaw, roll, and pitch) and local sign (part of the body, place on the sensory surface).
In addition many qualities are known to have labelled neurons, for example: direction of movement in visual, tactile and auditory spheres; small, dark, mov?ing profile within a specified 3 degrees of visual space; faces; food; the water bottle missing. These bespeak hierarchies of converging, less specific labelled lines.
KeywordsSpike Train Neural Code Electric Fish Ensemble Representation Neuron Alization
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