Hippocampectomized monkeys can remember one place but not two.

In an earlier study by Parkinson et al. (J. Neurosci. 8, 4159-4167, 1988), hippocampectomized monkeys were found to be impaired on a task in which they were required to remember the spatial positions of trial-unique objects overlying two of the wells in a three-well test tray. There were two types of trial in the task. One type (object-place) required memory for the conjunction of object quality and object location, whereas the other (place only) required memory only for the location of the objects, i.e. independent of object quality. The hippocampectomized monkeys performed at near chance levels on both types of trials. The present study sought to determine whether the poor performance of the hippocampectomized monkeys on the place-only trials, which closely resembled spatial delayed response (an ability that is unaffected by hippocampectomy when similarly short delays are used), could have been due to interference from the simultaneous training they had received on the object-place trials. To this end, we examined the effect of hippocampal removals on performance of the "place-only" trial type when that was the only training given. The hippocampectomized monkeys in the present study were found to be just as severely impaired as those in the earlier study, thus ruling out the possible explanation outlined above. Since performance on this modified version of spatial delayed response, unlike performance on the classical version with the same delay, is critically dependent on the hippocampus, it appears that monkeys with hippocampectomy can remember one place after a short delay but not two.