The role of timing in the induction of neuromodulation in perceptual learning by transcranial electric stimulation

Background: Transcranial electric stimulation (tES) protocols are able to induce neuromodulation,
offering important insights to focus and constrain theories of the relationship between brain and
behavior. Previous studies have shown that different types of tES (i.e., direct current stimulation e tDCS,
and random noise stimulation e tRNS) induce different facilitatory behavioral effects. However to date is
not clear which is the optimal timing to apply tES in relation to the induction of robust facilitatory effects.
Objective/hypothesis: The goal of this work was to investigate how different types of tES (tDCS and tRNS)
can modulate behavioral performance in the healthy adult brain in relation to their timing of application.
We applied tES protocols before (offline) or during (online) the execution of a visual perceptual learning
(PL) task. PL is a form of implicit memory that is characterized by an improvement in sensory
discrimination after repeated exposure to a particular type of stimulus and is considered a manifestation
of neural plasticity. Our aim was to understand if the timing of tES is critical for the induction of
differential neuromodulatory effects in the primary visual cortex (V1).
Methods: We applied high-frequency tRNS, anodal tDCS and sham tDCS on V1 before or during the
execution of an orientation discrimination task. The experimental design was between subjects and
performance was measured in terms of d’ values.
Results: The ideal timing of application varied depending on the stimulation type. tRNS facilitated task
performance only when it was applied during task execution, whereas anodal tDCS induced a larger
facilitation if it was applied before task execution.
Conclusion: The main result of this study is the finding that the timing of identical tES protocols yields
opposite effects on performance. These results provide important guidelines for designing neuromodulation
induction protocols and highlight the different optimal timing of the two excitatory
techniques.