To take Van Gogh as an example, he was a highly intelligent, open and articulate person. In the letters he wrote to his brother he describes many times his artistic intentions and the forging of his artistic voice; his paintings were not random splashes of paint, they served an artistic purpose. Therefore, even within the mental problems he had, Van Gogh could articulate and carry out an artistic program. He was able to develop a new visual language and through this language express his emotional state in his paintings; which I believe became the meaning of his art.
The efficiency and effectiveness in which he carried out his intentions was probably affected by his health. This may have also impacted the efficiency of the thought process Van Gogh went through to formulate his artistic intentions but (as evidenced by his letters) not the content of that thought.
I believe that within disabilities that allow at least periods of lucid thought (e.g. Parkinson’s) there is a difference between the intact formulation of intention and the lack of ability to carry out the intended act. Indeed, the disparity between formulating a reasonable intention (e.g. getting a glass of water) and being unable to fulfil its demands may be a measure of the impact of the disability. Of course, within a disability affecting the content of reasonable intention this logic does not apply (e.g. a delusional person); the content and act are both missing.
It is wrong to assume that Van Gogh or a person with Parkinson’s lack conscious intention within the physical and mental confines of their disability; either when the perception or actuality of the disability masks that intention.