1 Center for Interdisciplinary Studies in Rhythm, Time, and Motion, University of Oslo, Oslo, Norway.
2 Department of Musicology, University of Oslo, Oslo, Norway.
3 Penhune Laboratory for Motor Learning and Neural Plasticity, Department of Psychology, Concordia University, Montreal, Canada.
4 Department of Psychology, University of Oslo, Oslo, Norway.

Musicians convey timing "feels" in groove-based performance by manipulating onset asynchrony between instruments and the durational ratio of metrical subdivisions (non-isochrony, or "swing"). The present study tested whether listeners perceive such fine-grained timing deviations by measuring the 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, an index of attentional allocation and sensory conflict, were recorded throughout. Results show that: (1) JND thresholds of asynchrony and swing are higher in realistic, multi-instrumental groove-based contexts than in previously reported non-/quasi-musical contexts; (2) instrument type strongly modulates sensitivity with percussive instruments yielding the lowest thresholds and stringed instruments the highest; (3) listeners exhibit a bias whereby late displacements require larger magnitudes for detection, suggesting asymmetric temporal prediction in auditory timing; (4) musical training enhances sensitivity globally but especially benefits the perception of less salient instruments; and (5) pupil responses track the absolute magnitude of microrhythmic expression related to swing as well as asynchrony, providing a physiological index of salience scaling with microrhythmic displacement.