Cognitive evoked potentials as modulatory processes in human memory formation and retrieval.

The complex experiential nature of human cognitive memories implies that their formation and retrieval result from neural activity that is widespread and patterned. This integration of widespread activity into coherently patterned gestalts is accompanied by event related brain potentials (ERPs). ERP component N4, reflecting associative activation, declines as words or faces become more familiar. ERP component P3, reflecting cognitive closure, generally grows larger with familiarity. Similar changes in the N4/P3 are seen to familiarity in remote semantic memory. The N4/P3 may be generated directly by the multitude of specific neuronal interactions that constitute associative activation and contextual closure. Alternatively, the N4/P3 may be generated by diffuse modulatory pathways exerting non-specific control over these same interactions. In any case, the N4 and P3 appear to be generated simultaneously in several neocortical and subcortical structures, including the medial temporal lobe (MTL), where lesions are known to produce a profound deficit in learning new cognitive material. The MTL in the dominant hemisphere is necessary for the extratemporal N4/P3 generators to alter their response to words that are familiar in context. The mechanism whereby the MTL controls cortical activation is unknown. However, some hippocampal neurons are activated by particular words, suggesting that the MTL contributes specific information to the cortex during memory retrieval. These properties of the N4/P3 bear a close similarity to modulatory processes which have been found to aid formal associative networks in creating patterns that best represent current input in light of past experience. These modulatory processes consist of an initial generalized activation that encourages partially specified widespread elements to emerge as a network, followed by inhibition that focuses the emerging pattern into a coherent gestalt.