Differential effects of the stimulus sequence on CNV and P300.

During the presentation of stimulus sequences in oddball paradigms, participants tend to implicitly evaluate the conditional probability of target occurrence. It is not sure, however, if subjective estimation of conditional probabilities modulates target expectancy and target processing in the same manner. In the present experiment, the amplitudes of CNV and P300 were studied separately to compare preparatory and decision mechanisms and their sensitivity to variations in target probability. Amplitudes of both components were measured at different positions in the stimulus sequence, which modulates target probability as a function of distance from the preceding target. Results showed shorter RTs with increased probability of target occurrence (that is, with longer distance to the previous target). CNV amplitude was low when target probability was zero and maximal when the occurrence of a target was possible, regardless of its probability. Conversely, a gradual increase with augmenting probability was observed for P300 amplitude. Thus, preparatory activity as reflected by the CNV showed an all or nothing response to variations in probability, whereas action closure mechanisms, indexed by P300 components, were found to be more sensitive to subtle differences in stimulus expectancy.