why do people see human shadows in their peripheral vision?
* metaphysically speaking, their third eye might be open, thereby giving them the paranormal ability to perceive what others cannot. treatment: close guidance and training from people who have matured and learned to control their third eye abilities.
* spiritually speaking (especially in the christian context), it might be some form of oppression by spiritual forces. treatment: exorcism by catholics; deliverance by born-again christians.
* psychiatrically speaking, the person might be experiencing visual hallucinations which could be symptoms of psychosis, depression, or substance/alcohol intoxication. treatment: anti-psychotic medication.