P199 tES effects on a visual orientation discrimination task: Noise induction in a non-linear system

Recent studies have shown that the stimulation-dependent model is unable to account for the complex experimental evidence observed when applying transcranial electrical stimulation (tES) outside the motor cortex. An alternative model refers to the concept of stochastic resonance. It is based on the assumption that the injected electric field in the brain is unspecific; therefore, it can be defined as noise induction in a non-linear system (Miniussi et al., 2013). According to the model, the interaction between the injected noise and the ongoing activity of the system (in terms of signal to noise ratio, SNr) is a key factor in determining behavioral effects. The SNr in neural networks may change depending on task demands; consequently, also the response of the system to brain stimulation is expected to vary. However, up to now only few studies have considered elements which may affect the state of the system, such as performance level and interindividual variability. The present study aims to investigate whether the stochastic resonance framework can account for behavioral outcomes when an excitatory type of tES is applied (Paulus 2011; Woods et al. 2016). 36 young healthy participants were asked to perform a visual orientation discrimination task while receiving anodal transcranial direct current stimulation or high frequency transcranial random noise stimulation (hf-tRNS) on the primary visual cortex. In Experiment 1 subjects were presented with a large range of angular differences between the reference and the target stimulus. In Experiment 2 task difficulty was manipulated according to individual performance previously recorded, in order to control for intersubjects variability. Different levels of performance were obtained in a between subjects design. Half of the subjects received sham stimulation. In Experiment 1 an increased performance was observed in the experimental group only for hf-tRNS, while for Experiment 2 no main effect of stimulation was present for any of the conditions. The present study provides evidence not sustaining the stimulation-dependent model, suggesting a more complex relationship between stimulation, brain and behavioral outcomes. Taken together, these findings stress the need of taking into account the state of the system when applying tES