Unlike Parkinson disease, corticospinal tract lesions cause Paresis (weakness or paralysis), preferentially in distal antigravity muscles Hyperreflexia Extensor plantar responses (Babinski sign) Spasticity that increases muscle tone in proportion to the rate and degree of stretch placed on a muscle until resistance suddenly melts away (clasp-knife phenomenon) The diagnosis of Parkinson disease is supported by the presence of other signs such as infrequent blinking, lack of facial expression, and gait abnormalities.