A troubling paradox of invisibility: how does the invisible man see? If light passes through the retina, there are no nerve impulses to send to the brain. The Invisible Man is blind. (Wells solved the problem by making the retina partially visible, "an attenuated pigment...fainter than mist," but this seems arbitrary and awkward.)
A blind, invisible man is an interesting concept, but the parasite provides a solution. Photons don't just travel unimpeded through the body; the parasite passes the photons through. The parasite senses the light, and that sensory data can be accessed by the host. But what does that look like?
Initially, I think the invisible man is blind, and has to learn to make sense of alternative optical sensory input. In some ways, every cell in his body is a photoreceptor, and he has not only a 360° field of vision, but the whole sphere (4π sr). He sees the whole of the room without himself in it.1 He probably can intuit where he is, but there's no visual cue which way he's facing. This is more than just the awkwardness of figuring out how to walk when you can't see your limbs; there's a nauseating disorientation as he tries to move--the room shifts and reels about him unexpectedly.
But that disorienting movement helps focus the mind. Though invisible, the brain isn't changed, and the habits of seeing are strong, even instinctual. The parts of the brain that "see" will still do their job, and once the mind knows where the eyes are, it will prioritize that perspective because that's what it's used to.
Eventually, this will become an interesting advantage. He sees clearly in all directions at once, an obvious advantage in a fight. (I'd been thinking he could stick a finger out and "see" around a corner, but why would he need to do that? So he wouldn't be seen?) But there will be persistent disadvantages. I expect the nausea will be pretty chronic, as the mind fights with both the inner ear and the clarity of the view from the "eyes in the back of his head." There will be times when the physiology asserts control, when the brain pays attention to those dormant optic nerves and the blindness returns. And then there's the problem of sleeping when you can't really close your eyes.
1 More on this in a future post exploring the missing self.↩