Transcranial Direct Current Stimulation Improves Semantic Speech-Gesture Matching in Patients With Schizophrenia Spectrum Disorder

Abstract

Background: Patients with schizophrenia spectrum disorders (SSD) have severe deficits in speech and gesture processing that contribute considerably to the burden of this disorder. Brain imaging shows left inferior frontal gyrus involvement for impaired processing of co-verbal gestures in patients with schizophrenia. Recently, transcranial direct current stimulation (tDCS) of the left frontal lobe has been shown to modulate processing of co-verbal gestures in healthy subjects. Although tDCS has been used to reduce symptoms of patients with SSD, the effects of tDCS on gesture processing deficits remain hitherto unexplored.

Objective: Here we tested the hypothesis that inhibitory cathodal tDCS of the left frontal lobe decreases pathological dysfunction and improves semantic processing of co-verbal gestures in patients with SSD.

Methods: We measured ratings and reaction times in a speech-gesture semantic relatedness assessment task during application of frontal, frontoparietal, parietal, and sham tDCS to 20 patients with SSD and 29 healthy controls.

Results: We found a specific effect of tDCS on speech-gesture relatedness ratings of patients. Frontal compared to parietal and sham stimulation significantly improved the differentiation between related and unrelated gestures. Placement of the second electrode (right frontal vs parietal) did not affect the effect of left frontal stimulation, which reduced the preexisting difference between patients and healthy controls.

Conclusion: Here we show that left frontal tDCS can improve semantic co-verbal gesture processing in patients with SSD. tDCS could be a viable tool to normalize processing in the left frontal lobe and facilitate direct social communicative functioning in patients with SSD.

Keywords: co-verbal gestures; gesture processing; left frontal lobe; left inferior frontal gyrus (IFG); tDCS.

Fig. 1.

Study design and speech–gesture relatedness assessment task. (A, top) Study design. Each subject underwent four stimulation sessions (L = left; R = right; F = frontal; P = parietal; C = cathode; A = anode) on 4 days. The colors indicate electrode polarization (orange = right anodal stimulation; blue = left cathodal stimulation). (B, bottom): Speech–gesture relatedness assessment task, performed during stimulation. Example clips for each of the 4 gesture types presented, from right to left: metaphoric related, iconic related, metaphoric unrelated, and iconic unrelated. Figure adopted from Schülke and Straube.

Fig. 2.

Group dependence of relatedness ratings. Mean relatedness ratings (relatedness rated on a scale from 1 = very low to 7 = very high relatedness). SSD = schizophrenia spectrum disorder. Error bars indicate the standard error of the mean (SEM).

Fig. 3.

Stimulation, group, and relatedness dependence of ratings. (A, top) Mean ratings (relatedness rated on a scale from 1 = very low to 7 = very high relatedness). (B, bottom) tDCS improvement in differentiation between related and unrelated gestures: Difference between real stimulation conditions and sham condition, regarding the difference in ratings between related and unrelated gestures of each condition. L = left; R = right; F = frontal; P = parietal; C = cathode; A = anode; eg, LFC-RFA = left frontal cathodal and right frontal anodal stimulation; LFC-RPA = left frontal cathodal and right parietal anodal stimulation; LPC-RPA = left parietal cathodal stimulation. Electrode positions illustrated by head drawings above (blue = cathode; orange = anode). Error bars indicate the standard error of the mean (SEM).

Fig. 4.

Stimulation, gesture type, and relatedness dependence of mean reaction times across the entire group of patients and healthy controls. Light red: iconic unrelated. Dark red: iconic related. Light green: metaphoric unrelated. Dark green: metaphoric related. Electrode positions illustrated by head drawings above (blue = cathode; orange = anode). Error bars indicate the standard error of the mean (SEM).