 Patients are chosen to participate in this study because they will undergo a particular type of brain surgery to treat their hemifacial spasm. This surgery is called microvascular decompression MVD and involves the facial nerve. The facial nerve is being compressed by one or more blood vessels and this contact produces the facial twitches. The investigators do not know why a blood vessel that touches the facial nerve produces facial twitches. The investigators are interested in investigating this during your surgery. A total of 25 participants are expected to participate in this study. 