Human medial temporal lobe potentials evoked in memory and language tasks

Abstract

Lesion evidence indicates that the human medial temporal lobe (MTL) makes an important contribution to Recent Memory formation and retrieval very soon after a stimulus item is encountered. For verbal materials, this contribution is lateralized to the language dominant hemisphere. Evoked potentials recorded from the MTL during verbal recognition memory display two late endogenous components. Both show differences between repeated target words and non-repeated distractor words. The first component is usually negative and has an average latency of 460 msec. It is also observed in lexical decision and picture naming tasks. This component is similar in latency, morphology, and task correlates to the scalp-recorded N4 potential. This ‘MTL-N4’ is smaller in amplitude to words recognized as repeats and is largest in amplitude in the left MTL. The second component is usually positive and has an average latency of 620 msec. It is similar in morphology and MTL topography to the P3-like component evoked at 360 msec to rare tones in auditory discrimination tasks. This ‘MTL-P3’ is larger in amplitude to words recognized as repeats. Both components are of very large amplitude and invert polarity over short distances within the MTL. Hence, they appear to be locally generated in the MTL. The extent to which volume conduction of these MTL potentials contribute to scalp-recorded EPs is unclear. The MTL-N4 might be involved with memory formation and retrieval processes, and the MTL-P3 might index completion of the detection-recognition cycle.