Motivation increased vigour as measured by the velocity residuals (Fig. 3; F[1, 5.09] = 5.28, p = .022). Crucially, vigour was also increased by pramipexole, (Time × Group interaction, (F[1, 511] = 5.87, p = .015. There was also a practice effect (main effect of Time, F(1, 511) = 9.06, p = .003). None of the other fixed effects were significant (all ps > 0.05).

To further examine the drug effect (group × time), LME models were fitted separately for the pramipexole and control group. For the pramipexole group, there was a significant effect of time, F(1, 102) = 21.6, p = .001, indicating increased vigour from Time 1 (M = -7.18, SE = 2.52) to Time 2 (M = 7.01, SE = 2.51). For the control group, the effect of time is not significant, F(1, 544) = 0.134, p = .714, indicating no change in speed (Time 1, M = 0.561, SE = 2.58, Time 2: M = 0.972, SE = 2.58). Baseline velocity at Time 1 was similar between the pramipexole group (M = 388°, SD = 70) and the control group (M = 381°, SD = 60), t(37) = 0.36, p = .71).

Velocity residuals. Error bars represents +/- 1 SEM

While velocity measures energisation, Reaction Time may measure speed of decision processes (Ghez and Krakauer 2006; Haith et al. 2016). We next asked whether pramipexole also speeds up decisions. There were no drug effects. Reaction time was shorter where outcomes were uncertain (M = 203, SE = 4.84) compared to the noncontingent conditions where outcomes were certain (M = 209, SE = 4.84), (main effect of contingency, F(1, 210) = 6.58, p = .011), and when motivation was high (M = 202, SD = 4.84) compared to when motivation was low condition (M = 208, SD = 4.84) (main effect of motivation, F(1, 210) = 9.47, p = .011). There was also a practice effect, with shorter reaction time at Time 2 (M = 202, SE = 4.84) compared to Time 1 (Time 1: M = 209, SE = 4.84) (main effect of time: F(1, 210) = 15.8, p < .001), with an interaction between motivation and contingency (F(1, 303) = 11.1, p < .001; see supplemental Materials for post-hoc comparisons). However, none of the other fixed effects were significant (all ps > 0.05).

Larger amplitudes of movement often accompany increases in speed. Although pramipexole increased velocity, it had no effect on the amplitude of movement (all p > .05 for drug effects). There was a significant interaction between motivation and contingency, F(1. 311) = 5.94, p = .015. To further examine this, LME models was fitted separately for the contingent conditions and non-contingent conditions, however none of the fixed effects within these separate models were significant (all ps > 0.05).

To test whether the effect of pramipexole on vigour could was accompanied by a speed-accuracy trade-off, we examined end-point variability (S1). There was no drug effect on motor variability (Time x Group, F(1, 35) = 1.86, p = .182) suggesting pramipexole increased speed without trading accuracy. There was greater endpoint variability when outcome was certain (M = 1.06, SD = 0.067) compared to when reward outcome was uncertain (M = 0.868, SD = 0.052) (main effect of contingency (1, 35) = 25.9, p < .001, np2 = 0.425), and when motivation was low (M = 1.06, SD = 0.065) compared to when motivation was high (M = 0.865, SD = 0.051) (main effect of motivation (1, 35) = 44.8, p < .001, np2 = 0.561). There was also a significant interaction between contingency and motivation F(1, 35) = 25.0, p < .001, np2 = 0.417 (see supplemental materials for post-hoc comparisons).

Previous studies (Satterthwaite et al. 2007; Chiew and Braver 2014) showed that motivation and uncertainty both cause pupil dilation, so we wanted to test if pramipexole affects this. Accordingly, pupil dilation was greater when outcomes were uncertain (M = 320, SE = 20.0) compared to when outcomes were certain (M = 297, SE = 19.9) (main effect of contingency, F(1, 187) = 6.11, p = .014), and when motivation was high (M = 219, SD = 19.9) compared to when motivation was low (M = 298, SD = 19.9) (main effect of motivation, F(1, 187) = 5.32, p = .022). Pramipexole did not have any effects on pupil (Time × Group interaction, (F(1, 194) = 1.58, p = .211), unlike on vigour. Contingent motivation caused greater pupil dilation than non-contingent motivation, in alignment with previous studies (contingency x motivation, F(1, 298) = 4.49, p = .035, see Supplemental Materials for post-hoc comparisons).