A SPECIFIC ROLE FOR THE RIGHT PARAHIPPOCAMPAL GYRUS IN THE RETRIEVAL OF OBJECT-LOCATION - A POSITRON EMISSION TOMOGRAPHY STUDY

A plethora of studies, across many species, have now demonstrated that the hippocampal region play a critical role in memory for spatial loc ation. In spite of this compelling evidence, a number of important neu ropsychological and neuroanatomical issues remain unresolved. In the p resent study, the functional anatomy of object-location memory was inv estigated using positron emission tomography (PET) with magnetic reson ance imaging (MIR). Regional cerebral blood flow (rCBF) was measured w hile normal volunteers encoded, and then retrieved, the locations of e ight familiar objects presented on a computer screen. In two analogous conditions, designed to fractionate object-location memory into its c omponent processes, the subjects were simply required to encode, and t hen to retrieve, eight distinct locations represented by identical whi te boxes on the screen. An increase in rCBF was observed in the region of the right parahippocampal gyrus corresponding to entorhinal cortex when the Retrieving Location condition was subtracted from the Retrie ving Object-Location condition. In contrast, when the Encoding Locatio n condition was subtracted from the Encoding Object-Location condition , no significant rCBF changes were observed in the hippocampal region although significant activation was observed, bilaterally, in the ante rior fusiform gyrus. In addition, the two encoding conditions activate d left-hemisphere regions preferentially, whereas the two retrieval co nditions activated right-hemisphere regions. Together, these findings suggest that the human right hippocampal region is critically involved in retrieving information that links object to place. The secondary f inding that encoding and retrieval appear to be lateralized to the lef t and right hemispheres respectively, is discussed with reference to c urrent models of episodic memory, and alternative hypotheses are consi dered.