Intérêt des potentiels évoqués cognitifs dans l'étude du vieillissement chez l'homme

An evoked potential (or “evoked response”) is an electrical potential recorded from the nervous system of a human, following presentation of a stimulus (visual, auditory, somatosensory) as distinct from spontaneous potentials as detected by electroencephalography (EEG). Evoked potential amplitudes tend to be low, ranging from less than a microvolt to several microvolts, compared to tens of microvolts for EEG. To resolve these low-amplitude potentials against the background of ongoing EEG and ambient noise, signal averaging is usually required. The signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. An event-related potential (or cognitive evoked potential) measured brain response directly resulting of a thought or perception. It is any stereotyped electrophysiological response to an internal or external stimulus. The P300 (P3) wave is an event related potential which can be recorded via EEG as a positive deflection in voltage at latency of roughly 300 ms regardless of the stimulus presented: visual, tactile, auditory or olfactory. The signal is typically measured most strongly by the electrodes covering the parietal lobe. The presence, magnitude, topography and time of this signal are often used as metrics of cognitive function in decision-making processes. The N400 is an event-related potential component typically elicited by unexpected linguistic stimuli. It is characterized as a negative voltage deflection (topologically distributed over central-parietal sites on the scalp), peaking approximately 400 ms (300-500 ms) after the presentation of the stimulus. The N400 plays a significant role in language processing. The mismatch negativity is an event-related potential to an odd stimulus in a sequence of stimuli. It has most frequently been studied for audition and for vision, with a rare deviant stimuli occuring after an infrequent change in a repetitive sequence of stimuli (the entire sequence is called an oddball sequence). The evoked potential are very useful in the study of the chronology of neurocognitive process. The interest of evoked potential has been shown in several neurological disorders and aging. Their application in the understanding of brain ageing make up a challenge for physicians.