Speed of Nerve Conduction

Research Neuroscience
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This is from another list but I thought it relevant here. I’ve also posted it to CSGNet.

As a young research student, Hermann Helmholtz was told by his professor that it would be impossible to measure the speed of nerve conduction. It would be too fast. But, like all good students, he ignored this advice. In 1852 he was able to measure the speed of nerve conduction and showed that it was rather slow. In sensory neurons it takes about 20 msec for the nerve impulse to travel 1 meter. Helmholtz also measured “perception time” by asking people to press a button as soon as they felt a touch on various parts of the body. These reactions times turned out to be even longer, being more than 100 msec. These observations show that our perception of objects in the outside world is not immediate. Helmholtz realized that various processes must be occurring in the brain before a representation of an object in the outside world appears in the mind. He proposed that perception of the world was not direct, but depended on “unconscious inferences.” In other words before we can perceive an object the brain has to infer what the object might be on the basis of the information reaching the senses.

Fred Nickols

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Inference is a misleading term. It has literal meaning in logic, and only metaphorical meaning in a description of what the nervous system does. The synapsing of perceptual signals (rates of firing) together in what we call a perceptual input function yields a single perceptual signal (rate of firing) of a higher order or level. As I rest my hands on the computer to type, the regularly changing pressures upon sensors in the skin of my forearms just above the wrists are ‘inferred’ to be a perception of the edge of the laptop.

I inveigh against this because the computational metaphor has misled psychologists and others. Digital computers are programmed with instructions which are essentially equivalent to expressions in mathematical logic. Analog computers do not operate that way.