Transcranial Electrical Stimulation offers new prospects to investigate neuroplasticity in visual perceptual learning

The term neuroplasticity refers to the ability of the nervous system to change its structure and function as part of the processes that underlie adaptations to environmental changes. These changes correlate with cognitive plasticity at behavioural level. The capacity of a system to acquire or improve skills and to adapt to new environments through a learning process has been defined 'cognitive plasticity' (Baltes and Willis 1982). Cognitive plasticity has been observed after brain lesions and in response to adaptation and perceptual learning (PL) (Fahle, 2002, Thiele, 2004). Furthermore, recent development of transcranial electrical stimulation (tES) techniques to induce and evaluate cortical plasticity, constitutes a significant important step in understanding the relationship between cognitive plasticity and neuroplasticity. The goal of this work is to investigate whether and how tES can modulate cognitive plasticity in healthy adult brain. Neural plasticity induced by tES protocols has the potential to offer important insights to understand the mechanisms that underlie phenomena of plasticity, and will help to focus and constrain neurocognitive theories of the behavior-brain relationship. We applied tES protocol during the execution of a PL task. PL is a form of implicit memory characterised by an improvement in sensory discrimination after repeated exposure to particular types of stimuli and is considered a manifestation of neural plasticity (Carmel and Carrasco, 2008, Gilbert et al., 2001, Li et al., 2004). In particular we focused on visual PL (VPL). Animal and human studies have demonstrated the specificity of the primary visual cortex (V1) for recognising basic stimulus characteristics, such as orientation and direction. This specificity implies the direct involvement of V1 cells in learning and discriminating stimulus characteristics (Carmel and Carrasco, 2008, Li et al., 2004). Focusing on PL and the visual system, neurophysiological evidences have demonstrated that V1 is highly plastic (Wandell and Smirnakis, 2009). In the first study, we aim to test the interaction between PL and different tES techniques on V1 by applying tES while healthy participants execute an orientation discrimination task (ODT). In particular, we evaluate the effects of two tES, expected to induce facilitator effects at behavioural level, transcranial random noise stimulation (tRNS) and anodal transcranial direct current stimulation (a-tDCS). Although previous studies have shown that tRNS and a-tDCS had similar effects on the motor system (Nitsche et al., 2003a; Terney et al., 2008; Moliadze et al., 2011) our hypothesis was that these two different stimulation protocols would have different effects on visual system. Our results show a greater improvement in performance in the ODT when subjects were stimulated with tRNS than with a-tDCS. This result highlights the potential of the new tRNS technique and advances our knowledge on neuroplasticity induction approaches. The next step was to understand if the tRNS is effective regardless of the timing of application in relation to the state of cerebral activation during the task. We asked ourselves what would happen when the same stimulation protocols are applied before task execution. Consequently the aim of the second experiment was to understand if there was a critical timing for the application tES to obtain the induction of neuroplasticity in the V1 cortex. In this second experiment we applied tES (i.e., tRNS, a-tDCS and cathodal-tDCS) before (offline) or during (online) the execution of an ODT. The results confirm that a critical and "ideal" timing of application exists, and it depends on the stimulation type. tRNS facilitation is present only if applied during the task execution, whereas it's better to apply anodal tDCS before the task in order to induce facilitation in VPL. Overall, these results provide important indications for the designing of rehabilitation protocols, highlighting which of the two excitatory techniques is better to choose in relation to its timing of application. The opportunity to directly influence brain plasticity offered by these data opens new possibilities in cognitive neuroscience and neurorehabilitation.