What is the mask caused by Parkinson's disease?

The main pathological changes of Parkinson's disease are atrophy and degeneration of substantia nigra and striatum, which leads to dopamine reduction, acetylcholine hyperactivity, muscle stiffness change, and even facial muscles and eye movements are affected. The patient's face looks like wearing a mask, so it's called a mask face. Because facial muscles can't make their own decisions, it is very difficult to make expressions, such as smiling and rolling their eyes, including making expression changes. For example, when telling jokes, people's laughter and smiling faces should be basically the same under normal circumstances, but Parkinson's patients can only laugh, which is embarrassing.