Balance affects prism adaptation: evidence from the latent aftereffect.

In prism adaptation experiments, the effect on throwing to a target is reduced (primary aftereffect is smaller) when the throwing condition with prisms removed (first test phase) is different from the throwing condition with prisms (the training phase). The missing adaptation, however, can be revealed through further testing (second test phase) in which the throwing condition during training is fully reinstated. We studied throwing underhand to a target flush with the floor. During training, participants wore left-shifting prism glasses while standing on the floor (Group 1) or on a balance board (Groups 2 and 3). Tests 1 and 2 following training involved the same underhand throwing. For Group 2, Test 1 was on the balance board and Test 2 on the ground; for Group 3, the order was reversed; and for Group 1, both tests were on the ground. The Group 3 Test 1 aftereffect was smaller, and the Test 2 aftereffect was larger than the respective tests for Groups 1 and 2, with the aftereffect sum the same for all three groups. A parallel was noted between prism adaptation and implicit memory: whether given training (study) conditions lead to better or poorer persistence of adaptation (memory performance) at test depends on the fit between the conditions at test relative to the conditions at training (study). In the general memory case, those conditions will involve nonobvious contributors to memory performance, analogous to the support for upright standing in the adaptation of the visual system to prismatic distortion investigated in the present research.