Transcranial direct current stimulation combined with cognitive training on sustained attention performance for healthy elderly

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Sustained attention is integral to advanced cognitive processes; however, it declines with age. Both cognitive training and non-invasive transcranial direct current stimulation (tDCS) altering cortical excitability in the human brain have yielded inconsistent results in attenuating cognitive aging in the elderly. Furthermore, prior studies with synchronized application of anodal tDCS and cognitive training to induce plasticity effects for optimization of intervention efficacy nevertheless obtained heterogeneous results, which might at least partially be due to stimulation parameter differences between studies. To optimize tDCS effects with simultaneous cognitive training on vigilance decrement in older adults, a systematic evaluation of different tDCS protocols for sustained attention improvement in older adults, with respect to target regions and stimulation intensity, is required.
To this aim, tDCS modulations regarding laterality and intensity with synchronized multidimensional cognitive training tasks were applied in Study 1. Fifty-four healthy elderly were randomly assigned to three groups of different tDCS montages: left, right, and bilateral prefrontal tDCS, with three intensities: 2mA, 3mA, and sham, as a withinsubject factor. The AX-CPT with rare targets was chosen as the outcome measure to investigate possible synergistic effects of tDCS combined with cognitive training on sustained attention, and cognitive training task performance during tDCS was utilized as online measurement of anodal tDCS effects hinting for possible synergy. The results suggested that a single session of 30-min bilateral 3mA tDCS protocol in combination with cognitive training might reduce age-dependent vigilance decrement, as it shortened response time in the AX-CPT compared to baseline. No tDCS effects on simultaneous cognitive training task performance were found. To clarify the individual contribution of cognitive training on vigilance decline in elderly, we compared effects of cognitive training task in isolation with video-watching session as a control condition in forty healthy older adults in Study 2, shown results were of no differences.
In conclusion, no tDCS effects on synchronized cognitive training tasks were found. For reducing age-dependent vigilance decline in combination with cognitive training, no other active tDCS protocols except bilateral tDCS with 3mA, shortened response time compared to its baseline, but not to its sham condition, in Study 1. Further analysis in Study 2 indicated that the cognitive training task alone had no positive effects on vigilance decrement in older adults. This result pattern provided a hint of a preliminary positive effect of 3 mA prefrontal bilateral stimulation combined with cognitive training on sustained attention in the elderly.