Figure 1.

The mixing problem of EEG and trial averaging. Neuronal activity is picked up at scalp electrodes (blue circles), with recordings at each electrode originating from a specific mixture of brain sources (as indicated by the arrows' thickness). To the right, simulated recordings from an electrode are shown. The left of the two columns depicts source activities that are time- or phase-locked to an event. The right column depicts onset times that show some temporal variability across trials. Because at the same point in time (see black bar) the activity can be in different phases of the neuronal response (e.g., either at a peak or a trough), averaging only restores a distorted version of the source activities as recorded at an electrode.