This study (paywalled) http://www.sciencedirect.com/science/article/pii/S1053811910... shows learning increasing with an increase in applied current, at least — 'This relationship between current strength and learning as measured by change in performance with training was well predicted by a linear model of current strength (r = 0.437, p = 0.0015). This suggests that there was a strong relationship between the amount of current administered during training and the amount of learning, within the range of current strengths tested here.'