Besides THAT part of it...
The brain is a funny thing. There can be problems with the "input" side of things, but there can also be problems with the "processing" side of things. Vision is no different than hearing or other "perceptive" systems.
If there's too much brain-fog, the processing systems are bouncing all over the place, and stuff gets missed, or mis-placed, or scrambled.
And... MJ is one of the substances known to be used for self-medicating ADHD...
Hmmm...
Maybe he isn't quite so stupid after all... as in, recognizing his own problems - now he needs a REAL solution (other than MJ)