on-going “background” oscillatory activity. Furthermore, scalp electrodes also record activity from non-brain sources including muscle (eye-movements, blinking, heartbeat) and in some cases from non-physiological electrical sources (e.g., line-noise). Filtering and artifact rejection reduces the influence of some of these unwanted contributions to EEG, however, the spatial mixing of numerous brain-based processes means that the signal of interest, i.e., the signal associated with the cognitive task, is mixed with signals from task-unrelated processes and is therefore difficult to observe and measure on a trial-by-trial basis.
