Schizophrenia

No schizophrenia case in a person blind from birth has been reported in over 70 years. A 2018 study of ~470,000 Western Australians found none among 66 children with congenital cortical blindness. The effect is specific to early visual-cortex damage, not eye-caused or later blindness.

You can't tickle yourself because your brain predicts outcomes of your own actions and ignores matching sensations. However, schizophrenia patients — whose prediction systems are impaired — can indeed tickle themselves.