Time dependent effects of transcranial direct current stimulation and caffeine on vigilance performance during extended wakefulness

Abstract

Background: Previously, we found that transcranial direct current stimulation (tDCS) preserved vigilance performance approximately twice as well and three times as long as caffeine during a period of extended wakefulness. Vigilance performance often declines linearly over the period of watch, but in our previous study the performance trends over the period of watch were not analyzed. Hence, it was not known whether the intervention applied reduced the vigilance decrement, or simply shifted the performance to a higher mean value while maintaining a similar slope. Objective: Our objective was to evaluate the time-dependent effects (within each period of watch) of anodal transcranial direct current stimulation (tDCS) applied to the pre-frontal cortex at 2 mA for 30 min. We then compared these results to those of caffeine as well as the effects of both interventions on arousal. Methods: The period of watch was segregated into equal time segments and target identification accuracy was averaged across subjects in each group. These values were used in an analysis of covariance (separately for each session) as the dependent variable. Factors were group and subject nested in group. Results: The results indicated there is not a significant difference in slope (i.e. vigilance decrement) between the treatment conditions (tDCS, caffeine, and sham) within each period of watch. However, as reported previously, there was a significant difference in mean change from baseline between the treatment conditions. Conclusion: Our data suggests that tDCS does not prevent the vigilance decrement within the period of watch. Rather, it shifts performance to higher mean values by a scalar multiple while maintaining a similar slope.