Background and objectives: Instances of polyrhythm (cf. Vuust et al., 2006) and microtiming (cf. Danielsen, Jensenius, & Haugen, 2015) appear commonly in groove-based music, where they may challenge the listener’s internal framework of timing regularities, often referred to as meter. Two influential theories, The Dynamic Attending Theory (DAT; Jones & Boltz, 1989) and Predictive Coding Theory (PC; Vuust & Witek, 2014, cf. Friston 2005) hypothesize that attentional effort is recruited when metrical frameworks are challenged. The present study addresses this hypothesis by asking: During listening to musical groove-excerpts, are instances of polyrhythm and microtiming asynchrony related to an increase in ‘mental effort’ (as indexed by pupillometry; Kahneman, 1973), as well as a decrease in quality of sensorimotor synchronisation (as indexed by reduced finger tapping accuracy)? In addition, the common assumption that microtiming promotes groove experience is investigated. Of special interest is looking at effects of musical expertise on the processing of polyrhythmic and microtiming events. Method: Two experiments were designed by the author with help from the supervisors. Data collection was done by the author. In Experiment 1, professional jazz musicians (N = 16) and non-musicians (N = 16), matched demographically, were exposed to a groove-based musical excerpt with a 4 against 3 polyrhythmic event that was contrasted with a similar but non-polyrhythmic excerpt (stimuli borrowed from Vuust et al., 2006). In Experiment 2, participants were exposed to original recordings of double bass and drum kit- grooves of varying structural complexity, manipulated into five distinct microtiming bass/drums-asynchrony conditions (-80 ms < X < 80 ms). All musical stimuli were presented in a passive condition (‘listening only’) and an active tapping condition (‘synchronising with the beat’). We recorded pupil diameter sizes and participants gave 1) their subjective ratings after listening to each clip in the passive condition, and 2) tapping responses during the active condition by pressing a key on the keyboard of a PC. Main results: In Experiment 1, as expected, exposure to polyrhythm was related to larger pupil sizes (more effort) and lower tapping accuracy compared to the control condition. In Experiment 2, magnitudes of bidirectional bass/drums-microtiming asynchronies were positively related to pupil dilation and negatively related to tapping accuracy. In both experiments, tapping the beat while listening yielded higher psychophysiological effects than when listening only. Thus, the main effects of polyrhythm and microtiming on pupil response and tapping accuracy supported both DAT and PC accounts. Neither instances of polyrhythm or microtiming generated significant differences in pupil response between musicians and non-musicians. However, professional jazz musicians consistently showed superior tapping accuracy compared to non-musicians, which reflects their enhanced expertise in rhythm perception and performance. On subjective ratings of groove, participants generally preferred the on-the-grid grooves more than the grooves with microtiming. Musicians showed a greater response than non-musicians on these ratings, demonstrating their enhanced sensitivity to microtiming features in musical groove contexts.