Exploring memory functions by means of brain electrical topography: a review.

A series of experiments is reviewed which explored whether the functional brain state of long-term memory retrieval is correlated with specific changes in slow, DC-like event-related brain potentials. The main results are: (1) Retrieving associations from long-term memory is accompanied by a slow negative shift of 5-10 microV which prevails about as long as the retrieval process lasts, i.e., in our experiments, for a period of several seconds: (2) When different types of representations have to be reactivated in memory the slow negative wave shows a clearly distinct topography. The maximum was found in a verbal condition over the left frontal, in a spatial condition over the parietal, and in a color condition over the right occipital to temporal cortex. All these conditions were completely equivalent with respect to the established associative structure, the learning procedure, and the performance criterion. (3) The amplitude of the topographic maximum increases with the number of representations which have to be reactivated. This effect is not due to a non-specific increase of effort but specifically related to the number of activated episodic memory contents which had been experimentally established. In contrast, the reactivation of a priori given semantic association did not become manifest in a specific slow wave effect. These findings are compatible with the idea that memory retrieval implies a reactivation of those cortical cell assemblies in the cortex in which the constituting features of a mnestic entity had originally been processed during perception and learning. The results are also discussed with respect to the possible advantages of EEG and MEG recordings for a cognitive psychophysiology in comparison to other brain imaging techniques as PET or fMRI.