Functional Differentiation Along the Anterior-Posterior Axis of the Hippocampus in Monkeys

Colombo, Michael, Tom Fernandez, Katsuki Nakamura, and Charles G. Gross. Functional differentiation along the anterior-posterior axis of the hippocampus in monkeys. J. Neurophysiol. 80: 1002–1005, 1998. We tested whether the primate hippocampus was functionally heterogenous along its anterior-posterior axis. Two monkeys were trained on both a spatial and nonspatial memory task and the incidence of spatial and nonspatial delay activity in the anterior, middle, and posterior hippocampus was noted. Spatial delay activity (activity in the delay period after the sample stimulus on the spatial memory task) was more common in the posterior than the anterior hippocampus, whereas nonspatial delay activity (activity in the delay period after the sample stimulus on the nonspatial memory task) was evenly distributed throughout the hippocampus. Furthermore, delay neurons in the anterior hippocampus exhibited scalloping delay activity, whereas those in the middle and posterior hippocampus did not. These findings suggest that the hippocampus is functionally heterogeneous and that the posterior regions may be more important for processing spatial information, whereas the anterior regions may be more important for directing or coding movements to points in space.

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Dissociations in the Processing of “What” and “Where” Information in Working Memory: An Event-Related Potential Analysis

Journal of Cognitive Neuroscience ◽

10.1162/jocn.1996.8.5.453 ◽

1996 ◽

Vol 8 (5) ◽

pp. 453-473 ◽

Cited By ~ 71

Author(s):

A. Mecklinger ◽

N. Müller

Keyword(s):

Working Memory ◽

Spatial Memory ◽

Slow Wave ◽

Spatial Information ◽

Test Procedure ◽

Memory Task ◽

Event Related Potential ◽

Study Phase ◽

Object Memory ◽

Spatial Memory Task

Based on recent research that suggests that the processing of spatial and object information in the primate brain involves functionally and anatomically different systems, we examined whether the encoding and retention of object and spatial information in working memory are associated with different ERP components. In a study-test procedure subjects were asked to either remember simple geometric objects presented in a 4 by 4 spatial matrix irrespective of their position (object memory task) or to remember spatial positions of the objects irrespective of their forms (spatial memory task). The EEG was recorded from 13 electrodes during the study phase and the test phase. Recognition performance (reaction time and accuracy) was not different for the two memory tasks. PCA analyses suggest that the same four ERP components are evoked in the study phase by both tasks, which could be identified as N100, P200, P300, and slow wave. ERPs started to differ as a function of memory task 225 msec after stimulus onset at the posterior recording sites: An occipital maximal P200 component, lateralized to the right posterior temporal recording site, was observed for the object memory but not for the spatial memory task. Between-tasks differences were also obtained for P300 scalp distribution. Moreover, ERPs evoked by objects that were remembered later were more positive than ERPs to objects that were not remembered, starting at 400 msec postsimulus. The PCA analysis suggest that P300 and a slow wave following P300 at the frontal recordings contribute to these differences. A similar differential effect was not found between positions remembered or not remembered later. Post hoc analyses revealed that the absence of such effects in the spatial memory task could be due to less elaborated mnemonic strategies used in the spatial task compared to the object memory task. In the face of two additional behavioral experiments showing that subjects exclusively encode object features in the object memory task and spatial stimulus features in the spatial memory task, the present data provide evidence that encoding and rehearsal of object and spatial information in working memory are subserved by functionally and anatomically different subsystems.

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