Non-invasive brain stimulation: an innovative tool to study language abilities

Non-invasive brain stimulation (NIBS) techniques have been to date extensively used for the investigation of the cerebral functions. Thanks to their ability to interact with the central nervous system in a non-invasive and non-painful way they have been applied to investigate not only basic motor and visual processes, but also superior cognitive functions (e.g., language, memory, attention). In the language domain, for example, repetitive transcranial magnetic stimulation (rTMS) has proved to be efficacious in shortening the vocal reaction times of young subjects during an action naming task (Cappa et al., 2002). This work has confirmed the role of the left dorsolateral prefrontal cortex (DLPFC) in action naming. The involvement of the left DLPFC in naming task has been highlighted by both neuroimaging and rTMS studies (Cappa et al., 2002, Cotelli et al., 2006, Cotelli et al., 2008, Perani et al., 1999, Shapiro et al., 2006). The aim of my research is to understand which NIBS and stimulation protocols are the most effective to obtain a facilitation in a picture naming task in healthy subjects. The final goal is to apply these protocols in aphasic patients, to support and enhance the effects of the classical logopedic rehabilitation. First of all, we created two different versions of the naming task, to adopt accordingly with the age of the experimental subjects/patients (young vs. aged). Subsequently we applied transcranial direct current stimulation (tDCS) on the left DLPFC in young healthy subjects before the execution of a picture naming task. The results highlighted a facilitation (i.e., shortening of the vocal reaction times) only after the stimulation with anodal polarity (Fertonani et al., 2010). The same task was subsequently tested in young healthy subjects with a tDCS bilateral montage (i.e., anode on the left DLPFC and cathode on the right DLPFC), but this study have not had significant results. Then we applied tDCS in healthy aged subjects, varying the timing of anodal stimulation (i.e., before vs. during the task execution). The results showed a superiority of online stimulation. Summarizing, the most effective approach was the unilateral tDCS montage, in consequence we applied unilateral tDCS in one aphasic patient, in combination with a logopedic rehabilitation. In addition, considering the previously demonstrated efficacy of DLFPC rTMS (Cappa et al., 2002; Cotelli et al., 2006; Cotelli et al., 2008), other three aphasic patients were treated with rTMS plus logopedic rehabilitation (Cotelli et al., 2011b). Results are encouraging and provide additional evidence for the beneficial effects of brain stimulation in combination with targeted logopedic training in aphasic patients suffering from anomia. In particular, a longlasting effect of combined tDCS and behavioural therapy was observed on the therapy list of stimuli still at 48 weeks after the beginning of the combined stimulation-speech therapy intervention. This result is consistent with previous reports on enhanced cognitive performance following NIBS (i.e., rTMS or tDCS) to specific cortical areas in patients with a variety of neurological diseases (Miniussi et al., 2008, Miniussi and Vallar, 2011a). Moreover, this result is also consistent with previous evidence regarding the increased efficacy of daily combined rTMS or tDCS plus cognitive rehabilitation (Baker et al., 2010, Fiori et al., 2011, Floel et al., 2011, Fridriksson et al., 2011, Kakuda et al., 2010b, Kang et al., 2011, Martin et al., 2009b, Naeser et al., 2010b, Weiduschat et al., 2011). The possibility of non-invasively interacting with the functioning of the brain and its plasticity mechanisms, leading to cognitive and behavioural modifications, opens new and exciting scenarios in the cognitive neurorehabilitation field.