When a weak tactile stimulus is applied to the siphon of Aplysia californica, the animal withdraws the siphon between the parapodia. This defensive withdrawal reflex can be facilitated (sensitized) if the animal is previously given 4 days of training, consisting of four brief noxious stimuli each day. The sensitization of this reflex can last for up to 3 weeks after training and is mediated by the abdominal ganglion which also mediates long-term habituation. This preparation may provide a system for analyzing the neural mechanism of long-term behavioral modifications of complexity which is intermediate between habituation and associative learning.
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