The causal role of prefrontal hemispheric asymmetry in valence processing of words - Insights from a combined cTBS-MEG study.

Hemispheric asymmetries play an important role in multiple cerebral functions. Asymmetries in prefrontal cortex (PFC) function have been suggested to regulate emotional processing in that right-hemispheric dominance biases towards negative affect, whereas left PFC dominance favors positive affect. This study used transcranial magnetic stimulation to test the causal role of prefrontal asymmetries in the processing of emotional stimuli. To experimentally induce hemispheric asymmetries, 21 healthy volunteers underwent two separate sessions of inhibitory continuous theta burst stimulation (cTBS) to the left versus right dorsolateral prefrontal cortex. Each stimulation was followed by magnetoencephalographic (MEG) recordings of event-related fields elicited by visually presented emotional words in a silent reading task and a subsequent behavioral emotion categorization task. The asymmetry manipulation influenced valence processing of words in early, mid-latency and late time intervals in right occipitotemporal and parietal brain regions. Left-sided cTBS (inducing right-hemispheric dominance) consistently resulted in enhanced brain responses to negative words, while right-sided cTBS (inducing left-hemispheric dominance) enhanced responses to positive words. On a behavioral level, right-hemispheric dominance resulted in more categorization matches of negative compared to positive words, while left-hemispheric dominance resulted in reverse effects. These results provide direct evidence that bottom-up valence processing is influenced by prefrontal hemispheric asymmetry.