Classical and instrumental conditioning. (Top) One form of classical conditioning involves conditioning animals to fear a stimulus (7). In the example shown, animals are conditioned to expect an electric shock (unconditioned stimulus, US) when they hear an auditory tone (conditioned stimulus, CS). The neurons involved in learning this connection are situated in the lateral amygdala (LA), which instigates a conditioned freezing response when the tone is heard. The cellular mechanism underlying this learned behavior is a change in the activity of NMDA receptors at specific synapses (long-term potentiation) that then increase AMPA-type glutamate receptor currents in lateral amygdala neurons. (Bottom) Instrumental conditioning in the sea slug Aplysia (4). During instrumental conditioning, a stimulus (S) results in a response (R) that leads to an expected outcome (O), usually a reward. (1) The sea slug exhibits spontaneous biting behavior. (2) Stimulation of the esophageal nerve responsible for ingestion leads to dopamine release onto the B51 sensory neuron. (3) If dopamine is applied to B51 contingent on esophageal nerve activity and biting behavior, there is a change in the resting potential and excitability of B51. (4) This change increases the probability that a biting response will occur.

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Volume 296, Number 5573, Issue of 31 May 2002, p. 1624.
Copyright © 2002 by The American Association for the Advancement of Science. All rights reserved.