The present study examined the relationship between the amplitude of N1 component of event-related potentials (ERPs) elicited by task-irrelevant auditory probes and the observer's level of interest in co-occurring visual stimuli. Participants watched short animated video clips (about 400 s) played either forward (interesting) or backward (boring) accompanied by task-irrelevant sequence of auditory probes. The tone frequency of probes was fixed in a monotonous sequence condition but randomly varied in a variable sequence condition. The mean stimulus onset asynchrony of probes was 600 ms in both sequence conditions. Results showed that the N1 amplitude for probes in the variable sequence condition became smaller when participants watched interesting animated videos compared with their watching boring ones; a parallel effect was not observed in the monotonous sequence condition. Furthermore, analysis of sub-blocks (i.e., 360 s of the analysis time window for each animated video was divided into 20 s x 18 sub-blocks) showed a significant correlation between the forward-minus-backward differences in scored interest levels with the N1 amplitude in the variable sequence condition. This finding points to the possibility that the observer's interest can be estimated by neurophysiological data just for 20s. The present study should remarkably extend the usability of the task-irrelevant probe technique. (C) 2014 Elsevier B.V. All rights reserved.