Brain generators of evoked potentials: the late (endogenous) components.

Locating the synapses which generate endogenous potential components recorded at the scalp would permit some aspects of the neural activity directly underlying cognition to be probed non-invasively. Knowing which synapses are functioning abnormally in certain neurological and psychiatric diseases may suggest rational treatment strategies aimed at local brain regions. Although it is not possible to localize the generating synapses using scalp topography, large potentials can be recorded in the hippocampal formation during the same task conditions which evoke scalp endogenous potentials. Phase-reversal of these hippocampal potentials over short distances, accompanied by changes in simultaneously recorded unit-activity, indicates that they are generated locally. Demonstration that they volume-conduct to the scalp would, however, require observation of lesion effects, and recordings from other candidate structures, as well as a realistic model for propagation.