Categorical rhythm perception as a result of unilateral assimilation in time-shrinking

In previous studies, we established an illusion of time perception that we called time-shrinking: an empty time interval, immediately preceded by a slightly shorter time interval, is underestimated. In the first experiment of the present study, we examined the perceived duration not only of the second interval (t2), but also of the first interval (t1). The empty time, intervals t1 and t2, making a total duration of 90, 180, 360, or 720 ms, were presented such that the time ratio between them changed systematically. The points of subjective equality of t1 and t2 were established by the method of adjustment. In the patterns typically susceptible to time-shrinking, that is, in which t2 was underestimated, t1 was perceived almost veridically. In the second experiment, listeners had to bisect an empty duration of 180 ms, marked by sound bursts. The bisecting sound marker was positioned closer to the initial marker than to the final one. Thus, t1 had to be shorter than t2 in order for a regular pattern to be perceived. In the third experiment, just-noticeable forward and backward displacements of the middle sound marker were measured by a transformed up-down method. The prediction that the interval of uncertainty was closer to the initial than to the final sound marker was confirmed. The three experiments demonstrated the existence of unilateral temporal assimilation, and it is argued that this perceptual mechanism causes a category of 1:1 rhythms, despite a considerable change in temporal ratio between two contiguous time intervals.