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 plays a critical role in memory for spatial location. In spite of this compelling evidence, a number of important neuropsychological and neuroanatomical issues remain unresolved. In the present study, the functional anatomy of object-location memory was investigated using positron emission tomography (PET) with magnetic resonance imaging (MRI). Regional cerebral blood flow (rCBF) was measured while normal volunteers encoded, and then retrieved, the locations of eight familiar objects presented on a computer screen. In two analogous conditions, designed to fractionate object-location memory into its component processes, the subjects were simply required to encode, and then to retrieve, eight distinct locations represented by identical white 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 Retrieving Object-Location condition. In contrast, when the Encoding Location 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 anterior fusiform gyrus. In addition, the two encoding conditions activated left-hemisphere regions preferentially, whereas the two retrieval conditions 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 finding that encoding and retrieval appear to be lateralized to the left and right hemispheres respectively, is discussed with reference to current models of episodic memory, and alternative hypotheses are considered.