If you look at an uncovered lightbulb, and then look at a plain wall, you will see an afterimage. Is this effect caused by eye or brain?
eye. overstimulating eye receptors, which keep sending signals to the brain after initial stimulus is removed.
That’s part of it, but there’s also a physical fatigue mechanism involved. It’s still in the eye, of course. The retina uses photosensitive pigments to absorb light and relay signals to the nerves. It takes some time for the pigment to change state back to the unexcited state once they’ve absorbed light, so for a little while after looking at something for a long time (or a bright light), those rods and cones will not fire. This also explains why afterimages are opposite in color from the stimulus.
These afterimages are of retinal origin. But you could perform a simple experiment to prove that they are of ocular rather than brain origin, if you are skeptical. Look at a bright lightbulb with one eye occluded. The afterimage “appears” only in the non-occluded eye. If the afterimage were in the brain, oy would also seem tp appear in the occluded eye.