Scalp electroencephalography (EEG) can be used to record event-related brain potentials (ERPs). ERPs appear as transient changes in the ongoing electrical brain activity, time-locked to the onset of the event. ERPs are hypothesized to result from sudden and synchronized increases of postsynaptic activity, occurring in similarly oriented neuronal populations.
128-channel EEG system.
To extract evoked potentials from the ongoing, non event-related, electrical brain activity, the event is usually repeated such as to allow the averaging of successive peristimulus EEG recordings. The principle underlying time-domain averaging techniques is that averaging successive EEG epochs should cancel out the contribution of signals which are not ‘time-locked’ or ‘stationary’ to the onset of the event while it should preserve evoked activity which is assumed to occur with a constant time-delay. The fraction of the signal which is cancelled-out by the averaging procedure is often referred to as ‘additive noise’.
Event-related potentials typically consist of a series of voltage polarity changes, observed as peaks and troughs in the average waveform. These potentials can be classified according to their relative timing to stimulus onset, their polarity, and their magnitude. In most cases, each individualized ERP deflection corresponds to neural activity arising from several temporally overlapping sources. As ERPs provide a high temporal resolution, they can be used to characterize the chronometry of the different neural processes involved in perception. Indeed, depending on their modality, sensory stimuli elicit a series of sensory or exogenous ERP peaks which reflect the initial processing occurring in modality-specific cortical areas. Following these peaks, later components may be recorded, which are thought to reflect more integrative and endogenous aspects of perception.