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

Musicians 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, or ‘swing’). The present study asked whether listeners per-ceive such fine-grained timing deviations, which place demands on perceptual grouping, attention-al allocation, and temporal prediction, in complex, multi-instrumental groove contexts. We meas-ured just-noticeable difference (JND) thresholds of asynchrony and swing in a naturalistic funk pattern featuring guitar, bass, and drums (kick, snare, hi-hat). Sixty-four participants (32 musicians, 32 non-musicians) completed a 1IFC staircase task with uniform onset displacements (asynchrony: ±1–100 ms; swing: +1–71.5 ms). Pupillary responses were recorded throughout. These have been shown to index attentional allocation and sensory conflict, offering a useful physiological comple-ment to behavioral measures of microrhythm. 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, with late displacements (≈28ms) harder to de-tect than early ones (≈23ms). Musical training generally conferred lower thresholds. We also found a linear relationship between pupil size and both asynchrony and non-isochrony, further in-dicating that the pupil indexes saliency in the processing of microrhythm. The findings expand our understanding of perceptual boundaries for microrhythmic expression in multi-layered groove mu-sic and highlight how both acoustic and experiential factors shape sensitivity to timing nuances.