Effects of Stimulus Probability and Task-Relevance on Event-Related Potentials

Stimuli were delivered to 12 subjects in a sequence that included regularly occurring standard tone pips, random warning tone pips, and random while noise bursts. Half the noise bursts were preceded by warning tones (high probability), and half were not (low probability). There were four runs, each having one of two noise burst intensities, and each having the warning tone and noise bursts either both task-relevant to a reaction time task or both task irrelevant. Event-related potentials (ERPs) were obtained by signal averaging in F₂, C₂, P₂ and eye leads. The P₃ to the warning tone was largest in P, and was not affected by task relevance. CNV_s preceded all the high probability noise bursts. These CNV_s had an amplitude of 5μV even when the bursts were task irrelevant. Low probability noise bursts elicited larger P3_s in all leads for the task-irrelevant condition, but only in P_z for the task-relevant conditions. In C_z, the amplitudes of NI. P₂, and P₃ to the high probability. task-relevant noise bursts correlated with the amplitude of the preceding CNV.