Just Noticeable Difference Thresholds of Asynchrony and Non-isochrony in a Multi-Instrumental Groove-based Context

It has been claimed that musicians can convey different timing ‘feels’ in groove-based performance by manipulating onset asynchrony between instruments (playing ahead/on/behind the beat) and the durational ratio of metrical subdivisions (non-isochrony/‘swing’). The extent to which listeners can actually perceive such microrhythmic nuances, however, has rarely been tested in ecological, multi-instrumental contexts. While pupil dilation has been linked to asynchrony, it has not been studied for swing, even though it has been shown to reliably index attentional allocation and sensory conflict, offering a useful physiological complement to behavioral measures of microrhythm. We measured just-noticeable difference (JND) thresholds of asynchrony (Exp. 1) and swing (Exp. 2) in a naturalistic funk pattern (IOI ≈143 ms) featuring guitar, bass, and drums (kick, snare, hi-hat). Sixty-four participants (32 musicians, 32 non-musicians) completed a 1IFC staircase task with global onset displacements (asynchrony: ±1–100 ms; swing: +1–71.5 ms). Pupillary responses were recorded throughout. Across both experiments, thresholds varied systematically by instrument. For asynchrony, Kick, Snare, and Hi-hat yielded the lowest JNDs (≈19–24ms), while Bass and Guitar required the largest displacements (≈31–35ms). For swing, thresholds were lowest when Drums or All Instruments swung together (≈24–26ms), and highest for Bass and Guitar (≈32–34ms). An asymmetric effect of temporal direction was found for detection of asynchrony, with late displacements (≈28ms) harder to detect than early ones (≈23ms). In addition, musical training conferred a general advantage, with musicians achieving lower thresholds compared to non-musicians (asynchrony: ≈20ms vs. 31ms ; swing: ≈23ms vs. 34ms). We also found a linear relationship between pupil size and both asynchrony and non-isochrony, further indicating that the pupil indexes saliency in the processing of microrhythm more generally.