Pulse Logic

Electrical signaling in the nervous system was, according to legend, discovered during a demonstration given by Luigi Galvani to a class in the late 18th century. While Master Galvani was dissecting a frog, an electrostatic generator (which discharged electric sparks) was being played with by a bored student. Each time he drew a spark, the frog's leg (held in a forceps by Galvani) twitched. Galvani pursued this curious observation and published his treatise on Animal Electricity in 1791. Galvani felt that animal electricity was the material of excitation of nerves and of the contraction of muscles. Only after his death did his followers demonstrate that animals indeed generated their own electrical currents, rather than merely responding to applied electricity (Freeman, 1972).

The development of the microelectrode and appropriate electronic support equipment after 1940 revealed discrete electrical nerve impulses to be ubiquitous throughout the central and peripheral nervous systems. Electrophysiological techniques afforded an approach to complex problems by recording the activity of individual cells or small groups of cells. For example, the neural events involved in perception of touch were studied by recording signals from a neuron that terminates in the skin and whose function-touch sensation-was unambiguous. These signals consist of brief electrical pulses about 0.1 volt in amplitude, they last for about one millisecond, and they move along nerves at a speed up to 120 meters per second. Electrophysiological pulses within "Sherringtonian centers" had been correlated with appropriate forms of behavior, for example bursts of firings of neurons in medullary reticular formation were related to breathing. Neural activity was conceptually limited to actuation of information stored in specific neural regions. Newer approaches introduced the possibilities of encoding by dynamic neural firing patterns.

In 1943 McCulloch and Pitts proposed that neurons might be approximately described by the following assumptions: 1) the output activity of a neuron is an "all or none" process, 2) a certain number of input synapses must be excited within a brief period to excite the neuron's output, and 3) the only significant delay within the nervous system is synaptic delay. These "McCulloch-Pitts neurons" connected in networks could, in theory, perform any computation. Behaviorally significant information ("psychons") were conceived as action potential patterns in single neurons much like mass discharge in "centers" had been previously considered. A finer structure of neural information processing was perceived.

All or none action potentials which were the central element of pulse logic are relatively new over the course of evolution compared to graded wave-like events which occur in more primitive nerve nets. In the olfactory bulb, granule cells have no axons and do not generate extracellularly detectable action potentials at all. Thus the nerve impulse is not the only basis for transmission either between neurons or within neurons. What is the basic substrate of information transfer and processing within the nervous system? McCulloch (1943) considered:

... Why have I chosen to quantize in nervous impulses? ... If we think (of the brain) in terms of its ultimate particles, one might split this at the level of the atoms or one might split this at the level of the neurons and so on. The question is at what level can one split the behavior so as to define a set of units in terms of which to work? And obviously the nervous impulse at the level of the neuron is a fairly nice unit for working ... . But what I'm looking for is something that will perform a logical task. I'd like a thing that has a grain and I'd like to take that grain as my unit.

The grain which would explain information processing within all eukaryotic systems is the dynamic activity of subunits within cytoskeletal proteins. McCulloch, Pitts and others had guessed that information was coded in the pattern and sequence of nerve impulses somewhat like Morse code in telegraph lines. However, Adrian showed in 1947 that the frequency of firing in a nerve cell is a quantitative measure of the intensity of the stimulus. When skin is pressed, the stronger the pressure applied to the skin, the higher the frequency and the better maintained the firing of the cell. Frequency coding appears limited to information about the intensity of a stimulus and impulses in a given cell appear identical with those in other nerve cells. The significance and meaning are quite specific for each cell; for example skin sensory neurons indicate that a particular part of the skin has been pressed. Although there appears to be no reason why a great deal of information could not be conveyed by any predetermined signal including a code made up of different frequencies, the frequency or pattern of discharges do not appear to stand on their own as qualitative information. Following this recognition, the meaning of a signal became attributed to origins and destinations of the nerve fibers which convey it: "connectionism." The importance of connections is exemplified by sensations of light produced by nonvisual stimulation of the eyeball, or the phantom limb phenomenon in which an amputee may have sensation of a limb long since removed. In each case, mental representation is determined by the location of stimulation.

McCulloch and Pitts elaborated their model by adding a term which included the possibility that a firing decision might depend on inputs of times more remote than the synaptic delay period (dendritic memory) and by considering circular neural networks: closed pathways with logical feedback and reverberation. Thus pulse logic evolved into connectionism and neural networks as media of neural information representation.

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