Stimulus configuration, occasion setting, and the hippocampus.

N. A. Schmajuk, J. Lamoureux, and P. C. Holland (in press) showed that an extension of a neural network model introduced by N. A. Schmajuk and J. J. DiCarlo (1992) characterizes many of the differences between simple conditioning and occasion setting. In the framework of this model, it is proposed that the hippocampus modulates (a) the competition among simple and complex stimuli to establish associations with the unconditioned stimulus, and (b) the configuration of simple stimuli into complex stimuli. Under the assumptions that (a) nonselective lesions of the hippocampal formation impair both configuration and competition, and (b) selective lesions of the hippocampus proper impair only stimulus configuration, the model correctly describes the effects of these lesions on paradigms in which stimuli act as occasion setters.