Neuroplasticity induction in a perceptual learning task. Which is the best timing to apply tES?

Perceptual learning is considered a manifestation of neural plasticity in the human brain. We investigated brain plasticity mechanisms in a visual perceptual learning task using non-invasive transcranial electrical stimulation (tES, i.e., direct current stimulation tDCS and random noise stimulation tRNS). tES induces changes in the brain activity, leading to alterations in the performance of behavioral tasks. Our hypothesis is that different types of tES would have varying actions on the nervous system, which would result in different efficacies of neural plasticity modulation. Moreover, we hypothesize that in the visual system the effects of tES are dependent on the state of cerebral activation. Therefore the same type of stimulation, applied in different moments, may have different effects. We applied tRNS and tDCS before (offline) or during (online) a visual perceptual learning task. 106 subjects participated in the experiment, divided in seven groups: anodal-tDCS offline and online, cathodal-tDCS offline and online, tRNS offline and online and sham stimulation. Our results confirm that exists an ideal timing of application, depending on the type of stimulation. We observed an improvement of the performance when a-tDCS is applied before the task, whereas with tRNS we have a great improvement in the performance only during the task. Surprisingly an analogous improvement is present with offline c-tDCS whereas online c-tDCS is similar to sham. These results are important for the designing of rehabilitation protocols, highlighting which stimulation is better to choose in relation to its timing of application.