We conducted two event-related potential (ERP) experiments that examined the mechanisms of auditory temporal assimilation. Stimulus patterns consisted of two neighboring time intervals marked by three successive tone bursts (20 ms, 1000 Hz). Six stimulus patterns were used in which the first time interval (T1) varied from 100 to 280 ms, while the second time interval (T2) was fixed at 200 ms. Two other stimulus patterns consisted of different T1/T2 combinations were employed. Participants judged whether T1 and T2 had the same duration or not by pressing a button. ERPs were recorded from 11 electrodes over the scalp. Behavioral data showed symmetrical assimilation; the participants judged the two neighboring time intervals as equal when the difference between the time intervals (T1-T2) was -40 to +40 ms. Electrophysiological data showed that two ERP components (P300 and CNV) emerged related to the temporal judgment. The P300 appeared in the parietal area at 400 ms after the 2nd tone burst, and its amplitude decreased as a function of T1. The CNV component appeared in the frontal area during T2 presentation, and its amplitude increased as a function of T1. In Experiment 2, 11 stimulus patterns were presented. In seven stimulus patterns, T1 varied from 80 to 320 ms, and T2 was fixed at 200 ms. ERPs were recorded from 19 electrodes over the scalp. In this experiment, behavioral data showed asymmetrical assimilation; participants judged the two neighboring time intervals as equal when T1-T2 was -80 to +40 ms. Consistent with the results of Experiment 1, electrophysiological data showed the P300 and the CNV during T2. In addition, a slow negative component (SNCt) appeared in the right prefrontal area after the 3rd tone burst, and continued up to about 400 ms after the stimuli. The magnitude of this component was smaller when temporal assimilation occurred. These three ERP signatures seem to correlate with the process of temporal assimilation; (a) the P300 augmentation, which could be related to the participants' attention to the 1st interval and reflect the monitoring of the passage of time, (b) the CNV in the frontal area, which might have accompanied the process of memorizing the lengths of the time intervals, and (c) the SNCt in the right prefrontal area, which showed a reduction when temporal assimilation occurred. Our results showed spatiotemporal characteristics of the cortical processing of short time intervals and may assist the neurophysiological understanding of illusions in time and time perception in general.
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