Implicit and Explicit Adaptation Just Don’t Add Up


Our ability to adapt movements is likely supported by both implicit and explicit adaptation. Many studies and methods (tacitly) rely on the assumption these two add linearly, but this seems an unlikely neural mechanism and has not been tested rigorously. Here we test this by measuring implicit and explicit adaptation independently, using exclude strategy trials and re-aiming responses. We have two control groups that did not do re-aiming trials, and either received instructions on the perturbation or not. Testing predictions from additivity using both simple sums (strictly additive) and weighted sums (loosely additive), does not confirm additivity: there is no relation between implicit and explicit adaptation in our data. We also re-analyze data from nine other studies (total N=831) and only observe loose additivity in 6/43 subgroups (N=128). While this larger data set suggests explicit and implicit adaptation are combined in some way, it does not support linear addition. We conclude that implicit and explicit adaptation should both be directly and separately measured. More importantly, we are far away from understanding how various motor adaptation processes combine to shape behaviour.