Is there an effect of weightlessness on mental rotation of three-dimensional objects?

We studied the performance of eight cosmonauts in a mental rotation paradigm with simultaneously presented perspective views of three-dimensional objects. The cosmonauts were tested successively on earth, in microgravity aboard the Russian MIR station and again on earth. Their performance was compared to performance of a control group of five subjects tested on earth on the same dates. We particularly tried to disambiguate the effect of microgravity, procedural bias and practice. Our results show that the microgravity did not alter the mental rotation process. The performance of cosmonauts increased with practice, similarly to the performance of control group's subjects suggesting that the weightlessness did not impair implicit learning as well. Finally, we propose an explanation of previous contradictory results.