Want to work in medical research, involving diseases - but not mental illness
Employment opportunities etc not a priority, only interest and personal ability
If you want to do medical research (through Honours, I'm assuming), they're not too fussed with what major you have. Just because you major in biochemistry doesn't stop you from doing an Honours project/research in the neuroscience department. It'd certainly help if you've done relevant subjects, but they're also interested in your passion, enthusiasm etc.
I was still tossing up between majors midway this year.
I'd definitely recommend doing biochemistry and lots of practical subjects if you're interested in research. Look at the handbook at majors/third year subjects and make sure you'll have done the right/enough 2nd year prereqs.
You should try out pharmacology which is diseases + therapeutics. You don't have to memorise a bunch of drug names or anything, it's more focused on how different types of drugs work in the body. They have a 2nd year subject in semester 2.