Modulating semantic speech-gesture matching in healthy subjects and patients with schizophrenia spectrum disorder via transcranial direct current stimulation

Background: Severe deficits in speech and gesture processing are an important characteristic of patients with schizophrenia spectrum disorders. Given that co-verbal gestures are a vital part of human communication, it is not surprising that deficits in co-verbal gesture perception and performance contribute significantly to the suffering of these patients. Brain imaging studies have shown that the left frontal cortex plays a major role for processing co-verbal gestures, both in healthy subjects and in patients with schizophrenia spectrum disorders. The left inferior frontal gyrus seems to be particularly important for the perception of metaphoric gestures, that is, gestures accompanying abstract sentence content (e.g., lifting the hand to illustrate the high quality of a discussion), compared to iconic gestures, i.e., gestures accompanying concrete sentence content (e.g., circular hand movement to illustrate a round table). Moreover, the left frontal brain area appears to be activated excessively in patients with schizophrenia spectrum disorders. So far, no study had probed whether transcranial direct current stimulation could influence co-verbal gesture processing in patients with schizophrenia. Objective: In the first part of our study (publication 1), we investigated the functional relevance of the left frontal lobe for processing metaphoric co-verbal gestures in healthy subjects using transcranial direct current stimulation. We hypothesized a polarization dependent effect of left frontal transcranial direct current stimulation on reaction times and ratings in a speech-gesture semantic relatedness assessment task. In the second part of the study (publication 2), we investigated the effect of transcranial direct current stimulation on co-verbal processing of patients with schizophrenia spectrum disorders. We hypothesized that inhibitory cathodal transcranial direct current stimulation of the left frontal lobe would improve patients’ performance in the speech-gesture semantic relatedness assessment task. Methods: We applied anodal, cathodal and sham stimulation to the frontal, parietal and frontoparietal areas of twenty-nine healthy subjects and twenty patients with schizophrenia spectrum disorders. During stimulation, subjects watched video clips of an actor saying concrete or abstract sentences that were accompanied by semantically related or unrelated, iconic or metaphoric gestures. After each video clip, subjects immediately rated to what extent gestures were related to the sentence content (prompt: “Does the sentence content match the gesture?”, the answer was to be given on a scale from one “very badly” to seven “very well”). Results: For the first sample of seventeen healthy subjects (publication 1), we found electrode localization- and polarization-dependent changes in reaction times and ratings for metaphoric co-verbal gestures. Anodal stimulation of the left frontal lobe decreased reaction times and relatedness assessments for this type of gestures. When comparing healthy subjects and patients (publication 2), we found a specific effect of transcranial direct current stimulation on speech-gesture relatedness ratings of patients with schizophrenia spectrum disorders. Left frontal cathodal stimulation significantly improved the differentiation between related and unrelated gestures, thus reducing the pre-existing difference in speech-gesture assessment between patients and healthy controls. Conclusion: First, we demonstrated that left frontal transcranial direct current stimulation influences processing of co-verbal metaphoric gestures in healthy subjects (publication 1). Subsequently, we showed that transcranial direct current stimulation may also improve semantic speech-gesture matching in patients with schizophrenia spectrum disorders (publication 2). In the future, transcranial direct current stimulation could be a viable tool to normalize processing in the left frontal lobe and improve social communication deficits in patients with schizophrenia spectrum disorders.