No effects of anodal transcranial direct current stimulation on contrast sensitivity function

Background: Transcranial direct current stimulation (tDCS) is a well-established non-invasive brain stimulation technique that has been widely applied to modulate cortical excitability in human brain. The results of previous tDCS studies on modulating contrast sensitivity, one of the most fundamental visual functions, were mixed. Objective: We aim to systematically investigate the effects of anodal tDCS on contrast sensitivity functions (CSF), evaluate the responsiveness explanation of tDCS effects, and discuss results along with measurement precision. Methods: We designed a single-blinded, sham-controlled within-subject study. Twenty-seven healthy adult subjects received three sets of 15 min tDCS (two 2-mA anodal and one sham) that delivered at Oz, with CSF measured before and after each tDCS stimulation. Experimental sessions were separated by at least twenty-four hours. CSF was assessed with a Bayesian procedure that accurately estimated CSF within minutes. The anodal tDCS-induced effect was gauged with the change in CSF after stimulation; responsiveness was indexed by correlation between CSF changes in different stimulation; precision was calculated from resampling. Results: Our results indicated that neither the first nor the second session anodal tDCS altered the CSF significantly. Responsiveness was inconsistent between the two anodal sessions, indicating the usual responder/non-responder explanation of tDCS effects was unconvincing. Precision was less than 2 dB and constant throughout the whole experiment. Conclusions: The anodal tDCS, at least with two sessions, has no effect on modulating CSF. The absence of anodal tDCS effect on CSF was not due to subject's responsiveness to tDCS or measurement precision. More studies were needed to determine the optimal vision modulation configuration.