Cerebellar transcranial alternating current stimulation in the theta band facilitates extinction of learned fear responses

Fear extinction is a major component of exposure therapy for anxiety disorders. There is initial evidence that the cerebellum contributes to fear extinction learning, i.e., the ability to learn that certain stimuli are no longer associated with an aversive outcome. So far, however, knowledge of the cerebellums role in extinction is scarce. In the present study, 6 Hz cerebellar transcranial alternating current stimulation (ctACS) was used to modulate cerebellar function during extinction learning in young and healthy human participants in an MRI study. A two- day differential fear conditioning paradigm was used with acquisition and extinction training being performed on day 1, and fear extinction recall being tested on day 2. 6 Hz ctACS reduced spontaneous recovery of the initial fear association during recall, stabilizing extinction effects compared to sham ctACS. fMRI data during recall revealed significantly reduced activation in cortical areas involved in initial fear acquisition, such as the anterior cingulate and insula, in the verum ctACS group compared to the sham group. During extinction training, on the other hand, the verum group exhibited more widespread cerebral activation compared to the sham group. Group differences were significant in occipital cortical areas. Although direct stimulation effects cannot be excluded, increased activation in the visual cortex may reflect enhanced encoding and processing of visual information during fear extinction learning. The findings suggest that theta-range oscillatory interactions between the cerebellum and cortical areas support extinction processes and provide causal evidence of the cerebellar role in the human fear extinction network. Significance StatementWhile the cerebellum is well known for its role in associative learning, its contribution to fear extinction learning remains underexplored. This study used 6 Hz cerebellar transcranial alternating current stimulation (ctACS) to modulate cerebellar function during extinction training in healthy participants. The present data demonstrate that ctACS enhances cerebral cortical activation during extinction training which is followed by enhanced recall of the extinction memory and subsequently reduced activation of cerebral cortical areas associated to spontaneous recovery of fear memory. These findings provide causal evidence for the cerebelluar involvement in the human fear extinction network and suggest that enhancing cerebellar theta oscillations may be useful to support exposure therapy.