Declarative Memory: A Reply

In Nadel’s comment on our study (Zola-Morgan et al., 1994), he reiterates the view that the hippocampus has a selective role in spatial memory, i.e., the hippocampus stores information about nonegocentric space (viewpoint-independent space; O’Keefe and Nadel, 1978; Nadel, 199 1). In the context of our report, he points out that four monkeys with lesions limited to the hippocampal region (the H lesion, which includes the cell fields of the hippocampus proper, the dentate gyrus, and the subiculum) were impaired on the delayed nonmatching to sample task only at a delay of 10 min’, and not at shorter delays of 15 s and 60 s. The monkeys were removed from the testing apparatus during the 10-min delay and returned to their home cages. Because task performance under this condition could conceivably require re-identifying the spatial context in which the sample was originally presented, Nadel suggests that this version of the nonmatching task, with long delays, could be construed as a spatial memory task and that our results could therefore be interpreted as supporting the spatial mapping theory of hippocampal function. To our mind, Nadel’s commentary simply identifies that our study (which was designed to determine the severity of impairment following H lesions and other lesions, not to distinguish between theories of hippocampal function) may be consistent with more than one view of hippocampal function. In the sections that follow, we make two specific points about our findings, one about removing animals from the testing apparatus and one about the nature of the lesions. We then make a more general point about the spatial memory hypothesis of hippocampal function.