On 2014 Jan 08, Jean-Jacques Orban de Xivry commented:

In this paper, Hunter and colleagues claim that anodal tDCS on M1 improves motor adaptation to force-field perturbation (Shadmehr R, 1994).

In force-field adaptation, one critical aspect of the task is to control movement speed as the magnitude of the perturbation depends on the velocity of the hand. Indeed, the magnitude of the force pushing the hand away of its trajectory is equal to the velocity of the hand times the magnitude of the field. Unfortunately, I don't think that hand velocity was properly controlled in the paper by Hunter and colleagues. Here are two results that suggest that changes in hand velocity might be a confounding factor:

1) In the present study, anodal tDCS, but not sham tDCS, appears to modulate the velocity of the hand although this effect did not reach significance (p.2995, movement time differences: p=0.07; last panel of Fig.4). (Note that the values in Table 1 do not seem to correspond to the paragraph on movement times on p.2995 or to the last panel of Fig.4). Therefore, one wonders if the larger adaptation with anodal tDCS reported by the authors is not due to a larger perturbation being experienced during anodal tDCS.

2) In force-field tasks, it is well documented that the after-effect observed when the perturbation is suddenly removed at the end of the perturbation period is equal to or slightly lower than the deviation observed after the initial introduction of the observation. Here, for the anodal group, the after-effect appears to be larger than the initial deviation due to the perturbation (Fig.3B and first panel of Fig.5). This is only possible if the magnitude of the perturbation is larger during N5 than during F1. That is, it is only possible if hand velocity is larger during N5 than during F1. Again, this effect appears to be specific to the anodal tDCS groups.

Taken together, these two facts suggest that anodal tDCS but not sham tDCS might influence movement speed in the study by Hunter and colleagues. The larger hand velocity of the anodal tDCS group results in a larger perturbation. A larger perturbation required the subjects to adapt more to it.

To solve the problem of changes in velocity apparently due to anodal tDCS, the authors should 1) provide graphs on peak hand velocity over the course of trials 2) perform an analysis that directly takes into account changes in movement speed. I suggest that the authors use an ANCOVA analysis with Delta Summed Error or Delta Signed Error as dependent variables, group as discrete predictor and movement speed as continuous predictor.

In conclusion, I think that it is safe to conclude that until the authors provide the appropriate analyses, this paper does not provide strong evidence that anodal tDCS enhances adaptation to force-field perturbation. Rather, it suggest than anodal tDCS might modulate movement speed specifically. At least two papers did not demonstrate an improvement in motor adaptation with anodal tDCS (Orban de Xivry JJ, 2011;Galea JM, 2011) (Disclosure: I co-authored both of these papers).

On 2014 Jan 08, Jean-Jacques Orban de Xivry commented:

In this paper, Hunter and colleagues claim that anodal tDCS on M1 improves motor adaptation to force-field perturbation (Shadmehr R, 1994).

In force-field adaptation, one critical aspect of the task is to control movement speed as the magnitude of the perturbation depends on the velocity of the hand. Indeed, the magnitude of the force pushing the hand away of its trajectory is equal to the velocity of the hand times the magnitude of the field. Unfortunately, I don't think that hand velocity was properly controlled in the paper by Hunter and colleagues. Here are two results that suggest that changes in hand velocity might be a confounding factor:

1) In the present study, anodal tDCS, but not sham tDCS, appears to modulate the velocity of the hand although this effect did not reach significance (p.2995, movement time differences: p=0.07; last panel of Fig.4). (Note that the values in Table 1 do not seem to correspond to the paragraph on movement times on p.2995 or to the last panel of Fig.4). Therefore, one wonders if the larger adaptation with anodal tDCS reported by the authors is not due to a larger perturbation being experienced during anodal tDCS.

2) In force-field tasks, it is well documented that the after-effect observed when the perturbation is suddenly removed at the end of the perturbation period is equal to or slightly lower than the deviation observed after the initial introduction of the observation. Here, for the anodal group, the after-effect appears to be larger than the initial deviation due to the perturbation (Fig.3B and first panel of Fig.5). This is only possible if the magnitude of the perturbation is larger during N5 than during F1. That is, it is only possible if hand velocity is larger during N5 than during F1. Again, this effect appears to be specific to the anodal tDCS groups.

Taken together, these two facts suggest that anodal tDCS but not sham tDCS might influence movement speed in the study by Hunter and colleagues. The larger hand velocity of the anodal tDCS group results in a larger perturbation. A larger perturbation required the subjects to adapt more to it.

To solve the problem of changes in velocity apparently due to anodal tDCS, the authors should 1) provide graphs on peak hand velocity over the course of trials 2) perform an analysis that directly takes into account changes in movement speed. I suggest that the authors use an ANCOVA analysis with Delta Summed Error or Delta Signed Error as dependent variables, group as discrete predictor and movement speed as continuous predictor.

In conclusion, I think that it is safe to conclude that until the authors provide the appropriate analyses, this paper does not provide strong evidence that anodal tDCS enhances adaptation to force-field perturbation. Rather, it suggest than anodal tDCS might modulate movement speed specifically. At least two papers did not demonstrate an improvement in motor adaptation with anodal tDCS (Orban de Xivry JJ, 2011;Galea JM, 2011) (Disclosure: I co-authored both of these papers).