Replication and further studies of neural mechanisms of spatial mnemonic processing in humans

Changes in neuronal firing rates during periods of time when subjects are r equired to remember information (retention delays) have been reported in no n-human primates. In humans, tests for such functional changes using hemody namic markers of neural activity have typically relied on cognitive subtrac tion. However, the temporal resolution of fMRI allows a more direct test th an that afforded by cognitive subtraction of the idea that certain brain re gions may increase their neural activity during retention delays in humans. Using a method that exploits this temporal resolution, increased functiona l activity attributable to a retention delay for spatial information in reg ions proximate to/within the right frontal eye field and the right superior parietal lobule were detected tin four out of four and three out of four s ubjects, respectively; this is an internal replication of the results of [E . Zarahn, G.K. Aguirre, M. D'Esposito, Temporal isolation of the neural cor relates of spatial mnemonic processing with fMRI, Cognit. Brain Res., 7 (19 99) 255-268.]). Second, a model in which ventral and not dorsal prefrontal cortex in humans is involved in simply maintaining spatial information was tested. The results disputed this model as increases in fMRI signal attribu table to the retention delay were detected more frequently in dorsal than v entral prefrontal cortex. Third, a model which posited that the intensity o f neural activity is causally related to the accuracy of spatial mnemonic r epresentation was tested by comparing retention delay signal between correc t and incorrect trials. The results did nor support this model in any of th e regions tested. (C) 2000 Elsevier Science B.V. All rights reserved.