Distributed cortical systems in visual short-term memory revealed by event-related functional magnetic resonance imaging.

The spatio-temporal distribution of brain activity as revealed by non-invasive functional imaging helps to elucidate the neuronal encoding and processing strategies required by complex cognitive tasks. We investigated visual short-term memory for objects, places and conjunctions in humans using event-related time-resolved functional magnetic resonance imaging that permitted segregation of encoding, retention and retrieval phases. All conditions were accompanied by the activation of a widespread network of parietal and prefrontal areas during the retention phase, but this retention-related activity showed additional modulations depending on task instructions. These modulations confirmed a posterior-anterior and right-left dissociation for spatial versus non-spatial memory and revealed that conjunction memory does not rely on a linear addition of the component processes.