Sampling behavior in the radial maze and operant chamber: role of the hippocampus and prefrontal area.

Abstract

Rats were observed in the radial maze and in an operant setting where "errors" were, within limits, impossible to commit. Under these circumstances, the animals were free to seek information from the environment by varying their behavior, if they chose to do so, without penalty of nonreward. Rats with hippocampal, but not prefrontal or parietal damage, sampled fewer successive arm choices in the maze and exhibited a greatly reduced sensory analysis of the maze and its context. In operant chambers equipped with multiple levers, only hippocampal lesion narrowed the variety of interresponse intervals, which resulted in rhythmic responding. In contrast, the distribution of responses across the three response targets was not influenced by central nervous system injury. This pattern of information deprivation imposed by the hippocampal lesion parallels the profile of performance of such animals on more conventional tasks that depend on such information for optimal performance. Importantly, the sampling deficits were not direct outcomes of the hippocampal damage. Close scrutiny revealed that the rigidity developed over time, albeit quite rapidly. Thus the narrowed behavioral variation following hippocampectomy depends on some interactive component of task experience. We argue that reward is the other critical ingredient.