Simple random-interval generation reveals the irresistibly periodic structure of perceived time.

Whole-integer ratios in musical rhythm are culturally universal. The reliable periodicity of rhythm inspired us to determine whether time perception, which is foundational to and inherently less structured than rhythm, is subject to similar biases. We created a random-interval generation task that exploits the nonrandom tendencies in perception and action in order to uncover the structural biases underlying temporal duration perception. Participants listened to and watched an audiovisual suprasecond temporal cue and were asked to subdivide it as randomly as possible in a prescribed number of responses. The results showed that the subdivision probability distributions were distinctly nonrandom, and closely resembled multimodal distributions with a number of equally spaced, symmetrical peaks equal to the number of subdivisions required. These patterns were thus highly periodic and isochronous, despite explicit instructions to act as randomly as possible. We interpreted this bias as an organizing heuristic that divides perceived time into smaller, equal-duration chunks in order to facilitate representation.