Topography of brain electrical activity dissociates the retrieval of spatial versus verbal information from episodic long-term memory in humans

Topography and amplitude of slow event-related potentials (ERPs) of the electroencephalogram (EEG) were studied during acquisition and recall of spatial and verbal associations. Subjects learned associations between line drawings and two types of mediators. The latter were either positions in a grid or concrete nouns. In a cued recall test subjects had to decide whether two drawings were linked to each other or not via an associated position or noun. The topography of slow ERPs 1-4 s after stimulus presentation obtained from 18 scalp electrodes dissociated the memory processes: The maximum potential was found over the parietal cortex with spatial and over the left frontal cortex with verbal information. The same topographic pattern emerged during both anticipation learning and cued recall. Moreover, the amplitude at the topographic maximum increased when more associations had to be retrieved. These results are compatible with the idea that memory representations are reactivated in localized cortical cell assemblies specialized for particular codes.