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 a
nd J. J. DiCarlo (1992) characterizes many of the differences between
simple conditioning and occasion setting. In the framework of this mod
el, it is proposed that the hippocampus modulates (a) the competition
among simple and complex stimuli to establish associations with the un
conditioned 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 stim
ulus configuration, the model correctly describes the effects of these
lesions on paradigms in which stimuli act as occasion setters.