The tentorium cerebelli is the membrane that separates the brain from the lower parts of your nervous system. Therefore, a supratentorial problem is NOT a way in which doctors can call you crazy without you knowing it. Rather, it is a way for them to define the cause of the neurological problem. Left supratentorial damage causes sensory and motor dysfunction on the right side of your face and body, and vice versa.
Patient: I cannot feel the left side of my face or the left side of my body, doc!
Doctor: Sounds like supratentorial damage.
Patient: Are you saying it's all in my head?
Doctor: Kinda. Damage to your right brain has caused you to lose all feeling on the left side of your face and body.