Attention is a fundamental cognitive process—without it, we would be helplessly adrift in an overload of sensory input. There is considerable interest in techniques that can be used to enhance attention, including transcranial electrical stimulation (tES).
Is transcranial electrical stimulation an effective tool to enhance attention?
We present an overview of 52 studies that have paired attention tasks with tES, mostly in the form of transcranial direct current stimulation (tDCS). A brief summary of the methods and results of the included studies is presented in the article (in Tables 1–4), clustered by the most frequently used types of tasks: visual search, spatial orienting (e.g., Posner cueing tasks), spatial bias (e.g., line bisection tasks), and sustained attention.
Some promising results have been reported in each of these domains. However, drawing general conclusions about the efficacy of tES is at present hampered by a large diversity in study design and inconsistent findings.
- Visual search: Anodal stimulation to the right parietal and frontal cortex might speed up learning and reaction time in visual search, but robust results were only reported by one group for one particular visual search task.
- Spatial orienting: Parietal tDCS may enhance visuospatial processing (on the side of the visual field contralateral to the stimulation), but many studies also reported null results or even performance decrements.
- Spatial bias: The most exciting and consistent results may be those showing that tDCS can shift spatial biases and thereby ameliorate symptoms in hemispatial neglect patients. However, even this field is not without its contradictory results, and it remains to be seen whether this effect stands up to well-controlled and larger clinical trials.
- Sustained attention: prefrontal tDCS may improve sustained attention by countering the performance decrements normally observed after prolonged time-on-task, but this effect remains to be replicated independently, and tDCS did not produce consistent effects in other sustained attention tasks.
In the discussion, we highlight some pitfalls and opportunities for future research:
- The large variability in stimulation parameters is one important factor that may explain the lack of consistent results—–we rarely came across two studies that used the same protocol.
- Prior neuroimaging studies can help make more informed choices about stimulation sites (MRI) and waveforms (M/EEG).
- Pre-registered, direct replications could help clear up inconsistencies in the literature.
- Considering that tDCS effects vary highly between individuals (due to a range of factors, from differences in head anatomy to baseline brain state), future studies should use higher sample sizes.
- For cognitive enhancement, we must understand the factors that drive responsiveness to tES, lest we risk actually causing detrimental effects in some individuals.
We hope that our overview of current studies and recommendations for future research will help to determine the efficacy of tES for enhancing attention.