The clash of expectancies: Does the P300 amplitude reflect both passive and active expectations?

We investigated the clash of different types of expectations by studying event-related potentials and behavioural correlates of passive and active expectations. In the three tasks we used, target stimuli could either confirm or disconfirm both expectations, or confirm one while disconfirming the other. Depending on the task, expected events were related to either increased or decreased P300 amplitude. This was contingent on whether the expected item was probable given the context, or whether it was a target match. This suggests that the effect of expectancy on the P300 amplitude can be direct or indirect. In the case of a direct effect, the evidence suggests that most likely only unconscious, automatic estimation of stimulus probability is reflected in the size of P300 amplitude. However, active, conscious expectancies can influence the P300 activity via indirect routes, by influencing stimulus significance, by leading to the closure of perceptual epoch, or by changing the level of difficulty of processing. Such indirect effect of expectancy can have an opposite direction to the effect of automatically formed expectancies-that is, expected stimuli could be related to larger P300 amplitudes.