Habituation and sensitization have been studied in a variety of contexts and in a number of organisms, from simple protozoans (such as Stentor) to human subjects. Such studies have focused on the adaptive significance of these simple learning processes, their neurological control, and the range of behavioral responses that result from interaction between these two forms of learning.
One particular organism in which the neurological basis of habituation and sensitization has been extensively studied is the marine slug Aplysia. Eric Kandel and his associates at Columbia University showed that when the mantle of this organism is prodded, the slug quickly withdraws its gills into a central cavity. After repeated prodding, it learns to ignore the stimulus; that is, it becomes habituated. Conversely, when the slug is stimulated with an electric shock, its sensitivity to prodding increases greatly, and it withdraws its gills in response to even the slightest tactile stimulation (that is, it becomes sensitized).
Because Aplysia possesses only a few, large neurons, it is an excellent organism in which to study the physiological basis of learning. Capitalizing on this unique system, Kandel and his colleagues have been able to establish the neurological changes that accompany simple forms of learning. In the case of habituation, they have shown that repeated stimulation interferes with calcium ion channels in the nerve which, under normal circumstances, causes synaptic vesicles to release neurotransmitters, which in turn relay a nervous impulse between two neurons. Thus, habituation results in a block ing of the chemical signals between nerves and thereby prevents gill withdrawal.
When Aplysia is stimulated (or sensitized) by an electric shock, an inter-neuron (a closed nerve circuit contained within one part of the nervous system) stimulates the sensory neuron by opening calcium ion channels, increasing neurotransmitter production, and promoting gill withdrawal. Thus, the proximate neurological changes that take place during sensitiza-tion and habituation are nearly opposite, but they are achieved by very different neurological circuits.
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