19. Hippocampal negative event-related potential recorded in humans is not time-locked to the motor response execution

Abstract

A hippocampal-prominent event-related potential (ERP) with a peak latency at around 450 ms is consistently observed as a correlate of hippocampal activity during various cognitive tasks. Some intracranial EEG studies demonstrated that the amplitude of this hippocampal potential was greater in response to stimuli requiring an overt motor response, in comparison with stimuli for which no motor response is required. These findings could indicate that hippocampal evoked activity is related to movement execution as well as stimulus evaluation and associated memory processes. The aim of the present study was to investigate the temporal relationship between the hippocampal negative potential latency (hippocampal slow negativity, hipp SNe) and motor responses. We analyzed ERPs recorded with 22 depth electrodes implanted into the hippocampi of 11 epileptic patients. Subjects were instructed to press a button after the presentation of a tone. All investigated hippocampi generated a prominent negative event-related potential peaking at approximately 420 ms. In 16 from 22 cases we found that the ERP latency did not correlate with the reaction time; in different subjects, this potential could either precede or follow the motor response. Our results indicate that hippocampal slow negativity is not time-locked to the motor response. We suggest that hippocampal evoked activity, recorded in a simple sensorimotor task, is related to the full-value evaluation of stimulus significance within the context of situation.