Variant Target Effects on Motor Learning With M1 Anodal tDCS

Learning a motor skill usually involves practicing the same task repetitively with the same end target or goal. Many overhand throwing studies have documented accelerated learning when the task is practiced with the addition of anodal transcranial direct current stimulation (a-tDCS) to the primary motor cortex (M1). However, these studies use the same target to throw at each time. The purpose of this study was to replicate previous work where a-tDCS applied to M1 improved performance of a dart-throwing task quicker than with SHAM stimulation, but to have subjects throw to different targets on the dartboard with each throw.

Sixty-four healthy subjects practiced a dart-throwing task with their non-dominant arm. On the first visit, the subjects performed a pre- and post-test, as well as a 20-min practice in between, where they were randomized to receive 2 mA a-tDCS over the contralateral M1 to the throwing hand (n = 33) or very brief stimulation (SHAM, n = 31). The subjects repeated testing 1 and 24 h later to test for retention.

Both groups reduced their endpoint error and the variability in endpoint error (throwing consistency) over time. There were no significant differences between a-tDCS and SHAM conditions in either of these measures.

Unlike many previous studies, stimulation of M1 with a-tDCS did not accelerate learning of a dart-throwing task when the targets were constantly changed. It is hypothesized that this could have been due to the stimulation intensity being too high and possibly having a counter-regulatory effect on cortical activity, or it could have been due to some other learning mechanism that requires more time and practice to develop when the endpoint or target of the task changes. Alternatively, this task may have benefited from stimulation of the cerebellum, where it is known that motor adaptation can efficiently update and improve movements.