Is the target-to-target interval a critical determinant of P3 amplitude?

The P3 amplitude is augmented by decrements in target-probability, incremen ts in the number of nontargets preceding the target, and extensions of the interstimulus interval (ISI). Each of these changes prolongs the target-to- target interval (TTI) and, consequently, results attributed to these factor s might, at least partially, be accounted for by the TTI. Recent research a lso indicates that the P3s elicited by targets in one-, two-, and three-sti muli tasks (in which the TTI remains invariant) are similar. However, the T TI has not been examined systematically in previous research. The present s tudy had subjects listen to a randomized ISI (0.5, 1, 2, or 4 s) version of the auditory oddball task in which targets occurred after one, two, three, four, or five nontargets. Event-related potentials were analyzed based on ISI, sequential structure, and TTI. The study examined sequence and ISI eff ects independent of TTI effects and demonstrated that extensions of ISI aff ected N1 but not P3 amplitude, extensions of TTI enhanced P3 amplitude inde pendent of sequential structure, and sequential structure failed to influen ce P3 amplitude when TTI was controlled.