Directional adaptation of reactive saccades and hand pointing movements is not independent

It is a matter of debate whether reactive saccades and hand pointing movements share common adaptive mechanism. To find out, the authors used a double-step paradigm in which the direction either of eye or of hand movements was adaptively modified in a first block of 300 trials, and the direction of the other motor system was then modified with opposite polarity in a second block of 300 trials. In a third block, single-step stimuli were used to test for after effects. The authors found that subjects adapted in the second block less well than in the first, and that aftereffects were adequate for the first rather than the second block. When the second block was extended to 500 trials, adaptation was still poor but aftereffects were now adequate for the second block. From this the authors concluded that double-step adaptation of the first motor system interferes with the subsequent adaptation of the other motor system (i.e., the adaptive mechanisms for eyes and hand are not independent).