Abstract
Cathodal transcranial direct current stimulation (c-tDCS) is usually considered an inhibitory stimulation. From a physiological perspective, c-tDCS induces hyperpolarization at the neural level. However, from a behavioral perspective, c-tDCS application does not always result in performance deterioration. In this work, we investigated the role of several important stimulation parameters (i.e., timing, presence of pauses, duration, and intensity) in shaping the behavioral effects of c-tDCS over the primary visual cortex. In Experiment 1, we applied c-tDCS at two different times (before or during an orientation discrimination task). We also studied the effects of pauses during the stimulation. In Experiments 2 and 3, we compared different durations (9 vs. 22 min) and intensities (0.75 vs. 1.5 mA) of stimulation. c-tDCS applied before task execution induced an improvement of performance, highlighting the importance of the activation state of the cortex. However, this result depended on the duration and intensity of stimulation. We suggest that the application of c-tDCS induces depression of cortical activity over a specific stimulated area; but to keep reactivity within given limits, the brain react in order to restore the equilibrium and this might result in increased sensitivity in visual performance. This is a further example of how the nervous system dynamically maintains a condition that permits adequate performance in different environments.
Highlights
Transcranial direct current stimulation is a technique that allows the modulation of cortical excitability
Multiple post-hoc comparisons revealed that the offline paused and offline continuous (0.623 ± 0.114) conditions were significantly different from the online paused (0.304 ± 0.121), online continuous (0.346 ± 0.118), and sham (0.368 ± 0.083) conditions. These results support the initial hypothesis that c-Transcranial direct current stimulation (tDCS), applied before the orientation discrimination task (ODT), modulates behavior, while Cathodal transcranial direct current stimulation (c-tDCS) applied during the task does not modify the final outcome
In this work, we demonstrated that c-tDCS, which is often considered to be inhibitory at the behavioral level, can induce facilitatory effects, enhancing subjects’ performance
Summary
Transcranial direct current stimulation (tDCS) is a technique that allows the modulation of cortical excitability. If there is ongoing activity (i.e., background activity determined by state or taskinduced activity), the change in membrane potential induced by tDCS can promote more effective “excitation” or “inhibition” in a polarity-specific manner (Creutzfeldt et al, 1962; Bindman et al, 1964; Stagg and Nitsche, 2011). These tDCS-induced changes in the neuronal threshold during stimulation result from changes in membrane permeability, including depolarization of the soma by anodal stimulation (a-tDCS) and hyperpolarization by cathodal stimulation (c-tDCS) (Liebetanz et al, 2002; Nitsche et al, 2003b, 2004a,b). Polarization effects outlast the tDCS period (Nitsche and Paulus, 2000, 2001), and these after-effects are due to changes in receptor activity at the synaptic level, in addition to membrane polarity shifts (Nitsche et al, 2003a, 2004a,b)
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