Target probability affects the likelihood that a P300 will be generated in response to a target stimulus, but not its amplitude.

The ensemble-averaged P300 amplitude is affected by global and local target probability and interstimulus interval. We hypothesize that the local target probability-induced amplitude changes arise from a failure to consistently produce a P300 rather than producing lower amplitude responses. Single-trial analysis was conducted on auditory P300 data from 13 healthy subjects to determine the absence or presence of a P300 component for each target response separately. Selective, normalized averages were computed for each local target probability. Target stimuli preceded by one or two nontargets led to significantly more trials lacking a P300 than the other local target probability categories. The amplitudes of the responsive trials did not differ between the local target probability categories. This suggests that local target probability affects the consistency with which a subject responds to target stimuli, but not the P300 amplitude.