Motor timing under microgravity

Five participants were tested on their ability to produce accurate and regular inter-response intervals in the 350 to 530 ms time range. Three of them were members of the French-Russian CASSIOPEE 96 spaceflight mission, and the other two were control subjects tested on the ground. During spaceflight, the target inter-response intervals were increasingly undershot and the timing became more variable (less regular). The increase in the timing variability was mostly attributable to the internal timekeeping processes rather than those involved in motor execution. The results are discussed with reference to the physiological mechanisms possibly underlying the timing of fast serial movements.