The effects of manual tracking and allocation policy on the “negative difference” (Nd) event-related brain potential (ERP) were investigated. Five measures of performance were analyzed—root-mean-square-error, reactions time, sensitivity, N1 and P2 amplitude and N1 and P2 latency. Two results are discussed. The first results discussed show that the “negative difference” potential is unaffected by either the manual tracking task or the priority given the Nd task. The ERP data is further corroborated by the behavioral data. Both the sensitivity measure and reaction time were unaffected by the difficulty of the manual tracking task or the priority given the auditory task. However, the auditory task affected the manual tracking task. This asymmetric pattern of interference is explained in terms of the mechanism of preemption (Wickens, 1987; Wickens & Liu, 1988). The second results discussed show that high and low tones differentially effected Nd. Nd was greater at N1 for the low tone and greater at P2 for the high tone. This result is discussed in light of the different reaction times for low and high tones.