Another argument against functionalism I've never thought about before (idea from watching a Steven Lehar lecture):
Everything in complex systems can go wrong. As in, there are mistakes. Software crashes. That's why we do redundancy and stuff.
Obviously, things in the human brain can also go wrong, and they do. We've catalogued hundreds of different ways in the form of mental disorders.
So if human vision were the product of bits, you'd expect to see, well, bit errors! Some people ought to have, e.g., rectangular regions on their visual field where the color is inverted, or that have a static color, or anything in that ball park. There should be some hint that it's made out of pixels when things go wrong. But afaik there are no reports of this. There *are* ways the visual field stops working, but they manifest differently. They can be things like, not being able to bind the field into a coherent hole, not being able to recognize objects, or seeing geometrical patterns anywhere on psychedelics.