Synesthesia is pretty rare, occurring in at least 4 percent of people in the world, according to the Cleveland Clinic . Ahead, find out how synesthesia works, the different forms, and how those who find it distressing can get relief, according to doctors. In a 2024 interview , Pharrell Williams said his synesthesia causes him to see “ghost images” after he hears a sound. It’s when someone “associates different pitches and tones to certain colors,” according to 2022 research in the International Journal of Phonosurgery & Laryngology. Mirror-touch synesthesia can cause people to feel phantom sensations from watching someone else be touched, according to older research in NeuroImage .