Thursday, May 26, 2005
THE NEUROSCIENCE AND PHILOSOPHY WORKSHOP 2
Grossberg then pointed out that learning in the two systems seems to be complementary, in the following sense. Sensory and cognitive expectations are matched via an excitatory matching process; cf., the example of looking for a blue pencil within a second for a large monetary reward. Motor expectations are matched via an inhibitory matching process. Such a motor expectation, in the hand-arm system, for example, codes where you want to move. It is matched against where your hand is now. When your hand is where you want it to be, you stop moving. Hence motor matching is inhibitory. The corresponding learning in the sensory and cognitive domain is "match" learning; in the motor domain it is "mismatch" learning. Match learning solves the stability-plasticity dilemma; it allows us to keep learning rapidly throughout life without also suffering rapid and inappropriate forgetting. Mismatch learning does suffer catastrophic forgetting, but in the motor domain, this is good, since we only want to remember the most recent motor control parameters, that are appropriate for controlling our bodies as they are now, not as they were, say, when we were children. Grossberg remarked that, if you study lots of brain architectures from a fundamental point of view, you see over and over again that they are organized into complementary processing streams, which individually realize a hierarchical resolution of uncertainty. He predicted that these complementary streams arise through a process of global symmetry-breaking during development. Complementarity and uncertainty principles are also found throughout the physical world, and he interpreted their occurrence in the brain as being a manifestation of the brain's role in measuring and modeling the physical world in an adaptive fashion. THE NEUROSCIENCE AND PHILOSOPHY WORKSHOP 3 Eichenbaum's first major philosophical claim is that there is no coding purely for the function of memory. Rather, memory is simply the modification of the normal process of representation or coding that occurs as part of perception or interaction. There is nothing that functions like a tape recorder, no storehouse of memory. There is no strict separation, Eichenbaum claimed, between perception and memory. What we call memory is best explained as a modification of perceptual coding properties.
. . .There are two ways to form a memory, by repetition and arousal. The hippocampus, according to the questioner, might play a role in repetition pathways, whereas memories mediated by the amygdala might account for the arousal circumstance.
. . .Eichenbaum, began by emphasizing that there is a great deal more plasticity in the hippocampus, it is the best and the quickest of the memorizers. The cortex, by comparison is a slow memorizer, remodeling itself very gradually. This two component model turns out to be an engineering solution to the problem of catastrophic interference. Neural network models are notoriously susceptible to catastrophic interference. McClelland overcame this problem by juxtaposing a more rigid network over and above the highly plastic and fast-acting learning device.
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