... neuroscientists at the University of California Irvine showed people with a specific gene variant performed more than 20% worse on driving tests than people without it. This gene variant reduces the availability of brain-derived neurotrophic factor during activity, which keeps memory strong and brain cells functioning at a maximum capacity. In people with the variant, a smaller portion of the brain is stimulated than in those without it. As part of the study, 29 people were subjected to a driving test that required learning elements of the track, including difficult curves. Seven of those tested had the gene variant. The test was repeated four days later. Results showed drivers with the gene variant performed worse on both tests and remembered less the second time through. However, studies have found people with it maintain mental sharpness longer when neurodegenerative diseases, such as Huntington's, Parkinson's and multiple sclerosis are present."