Certain non-isochronous sound trains are perceived as more isochronous when they start on beat.

Abstract

Perceiving the duration of neighboring time intervals is vital for rhythm perception. We discovered a phenomenon in which the perceived equality/inequality of neighboring time intervals in a sound sequence is changed by its metrical interpretation. The target sound sequence consisted of eight short sound bursts marking seven neighboring time intervals, which were repetitions of two durations (T1 and T2) presented in alternation (T1-T2-T1-T2 …). There were three tempos, which corresponded to T1 + T2 being 210, 420, and 630 ms. The physical difference between T1 and T2 (T1 - T2) was varied systematically for each tempo in the ranges of -100 to 100 ms (when T1 + T2 was 210 or 420 ms) or -150 to 150 ms (when T1 + T2 was 630 ms). Participants reported the level of perceived equality/inequality of these neighboring time intervals. For each target sequence, four isochronous lower-pitched preceding sounds were added at different phases so that the beginning of either T1 (Beat-on-T1 condition) or T2 (Beat-on-T2 condition) coincided with the beat induced by these preceding sounds. When T2 was longer than T1 by up to 60 ms, the neighboring time intervals of the same target sequence were perceived as more "equal" in the Beat-on-T1 condition compared with the Beat-on-T2 condition. Such a difference in the perceived equality/inequality appeared significantly only at the intermediate tempo of T1 + T2 = 420 ms. The difference in equality/inequality perception at limited temporal conditions could be accounted for by the occurrence of an illusion in time perception called time-shrinking.