When using mannitol for medical purposes, it is given intravenously. When consumed orally, as a dietary sweetener or laxative, mannitol is poorly absorbed by the gut. Oral mannitol is being tested for benefits in Parkinson disease on ClinicalTrials.gov. It seems unlikely for oral mannitol to reach high enough blood levels to reach the brain. It may have some effect within the bowel lumen, by interaction with the gut biome. I tried it myself in 2013, and experienced troublesome flatus, and no noticeable benefits.