ASSOCIATIVE ENCODING OF PICTURES ACTIVATES THE MEDIAL TEMPORAL LOBES

It remains unresolved whether the medial temporal lobe activations fou nd in recent neuroimaging studies are mediated by novelty detection al one, by specific kinds of encoding or consolidation operations, or bot h. This study attempted to see whether associative encoding or consoli dation is sufficient to cause such activation by matching for novelty across conditions. Using single-photon emission computer tomography (S PECT) (with Tc(99m)HMPAO), we compared the activation patterns produce d by the associative encoding and the perceptual matching of novel com plex scenes in 10 normal subjects using both statistical parametric ma pping (SPM) and a regions-of-interest (ROI) approach. During the encod ing condition, significant activations were detected in the left hippo campal/parahippocampal region, the left cingulate cortex, and the righ t prefrontal cortex, using both statistical techniques. Additionally, activation was found in the right cingulate cortex, and a trend toward s activation was found in the right hippocampal/parahippocampal region using the ROI approach. In contrast, no medial temporal activations w ere found during the matching condition, which produced bilateral occi pito-parietal and right posterior inferior parietal (supramarginal gyr us) activations. These results not only confirm that the associative e ncoding and/or consolidation of complex scenes is partially mediated b y medial temporal lobe structures, but also demonstrate, for the first time, that associative encoding/consolidation is sufficient to produc e such an activation. The implications of the high degree of consisten cy revealed by the results of the SPM and ROI comparison are discussed . (C) 1998 Wiley-Liss, Inc.