We observed that participants typically initiated reaching to a single target more rapidly than to an odd-color target among distractors, reflecting differentiated states of sensorimotor readiness for easy versus hard tasks,
F(1, 8) = 16.7,
p < .005. This is in accord with the prediction that participants can adjust their initiation latencies appropriately for the difficulty level of each trial (
Figure 2).
Yet, this occurred only when trial types were presented in separate blocks, t(8) = 6.3, p < .001, showing a significant interaction effect between trial type and block type, F(2, 16) = 36.5, p < .001. The difference between the two trial types vanished, on average, in the mixed condition, t(8) = 1.6, p < 1. Compared to the blocked condition, single-target trials became significantly slower, t(8) = 6.87, p < .001, and odd-color-target trials became faster, t(8) = 3.09, p < .02, showing a homogenization effect. In all conditions, target selection accuracies were more than 98%, F values < 1.
The alternated condition represented the most critical result, where the two predictions diverge. Our results showed that there was no difference between the two trial types, t(8) = .4, p < 1, demonstrating the homogenization effect like that occurring in the mixed condition. Also, we observed that, compared to those in the blocked condition, single-target trials became significantly slower, t(8) = 6.57, p < .0001, and odd-color-target trials became faster, t(8) = 4.75, p < .001.
Thus, in contrast to the blocked condition, the mixed and alternated conditions demonstrated that, on average, motor execution strategies were not differentiated for each trial type. It is worth noting that participants have perfect explicit knowledge of upcoming trial types in the alternated condition. Yet, participants did not actively adjust their motor execution strategies by relying on their explicit knowledge. Therefore, the results support the passive cumulative learning hypothesis that trial repetitions are critical for adjusting motor readiness.