Cerebellum and processing of negative facial emotions: cerebellar transcranial DC stimulation specifically enhances the emotional recognition of facial anger and sadness

Abstract

Some evidence suggests that the cerebellum participates in the complex network processing emotional facial expression. To evaluate the role of the cerebellum in recognising facial expressions we delivered transcranial direct current stimulation (tDCS) over the cerebellum and prefrontal cortex. A facial emotion recognition task was administered to 21 healthy subjects before and after cerebellar tDCS; we also tested subjects with a visual attention task and a visual analogue scale (VAS) for mood. Anodal and cathodal cerebellar tDCS both significantly enhanced sensory processing in response to negative facial expressions (anodal tDCS, p=.0021; cathodal tDCS, p=.018), but left positive emotion and neutral facial expressions unchanged (p>.05). tDCS over the right prefrontal cortex left facial expressions of both negative and positive emotion unchanged. These findings suggest that the cerebellum is specifically involved in processing facial expressions of negative emotion.

Figure 1

Experimental protocol. A. Examples of facial stimuli for each of the four facial emotion expressions. B. Schematic diagram of the experimental design during a single session.

Figure 2. Baseline analysis

reaction times (RTs) and answer errors for each emotion. (A) Histograms represent grand average (n=21) of reaction time (RT) for correct answers for each emotion (negative, positive and neutral) and cerebellar transcranial direct current stimulation (anodal cerebellar tDCS, cathodal cerebellar tDCS, sham cerebellar tDCS). y-axis: mean RT values (ms). (B) Grand average (n=21) of answer errors for each emotion (negative, positive and neutral) and stimulation (anodal cerebellar tDCS, cathodal cerebellar tDCS and sham cerebellar tDCS). y-axis: mean number of errors. Error bars represent the 95% confidence interval of the estimated mean (1.96*standard error). * p<0.05, two-way analysis of variance (ANOVA). Note that, at baseline participants identified sadness with lower speed and worse accuracy than all other emotions. Note that no differences were found between sham cerebellar tDCS, anodal cerebellar tDCS and cathodal cerebellar tDCS in RTs and answer errors.

Figure 3. Box plots and trial-to-trial representation showing the effect of cerebellar transcranial direct current stimulation (tDCS) on emotional recognition (negative, positive emotions and neutral)

On each box the central mark is the median, the edges of the box are the 25th and 75th percentiles, the whiskers extend to the most extreme data points not considered outliers, and each outlier is plotted individually (cross). y-axis: RTs (ms). Trial to trial grand average (n=21) of reaction times (RTs) across task stimulus presentation before cerebellar tDCS stimulation (24 answers) and after cerebellar tDCS stimulation (24 answers). x-axis: answers, note that the X axis graphically represents the time elapsing between the end of the task execution before stimulation and the beginning of the task execution after tDCS (15′ tDCS and 35′ rest); y-axis: RTs (arbitrary units, AU). A Note that anodal and cathodal cerebellar tDCS both reduce baseline RTs for negative emotions (two-way ANOVA; *p<0.05). B, C cerebellar tDCS induced no change in RTs from baseline either for positive emotion or neutral emotion. The trial-to-trial representation highlights the finding that anodal and cathodal curves differ from sham curves for negative emotions, but are similar to sham curves for positive and neutral emotions.