I think this can be a tough line to draw, as we've concluded in the other Meta posts on both Meta.Biology and Meta.CogSci.
In terms of these specific example questions, my sense is that optical illusions are considered to be the result of a "perceptual" process. While perception could be studied in a (perhaps intractable, thank you commenter) biological framework, it has been studied at length in a top-down, abstracted form by cognitive scientists.
I think that the existing disciplinary experience for these perceptual questions lies on that side of the ocean, so to speak. This by no means would preclude an answer on the other site using the framework of systems physiology, as neurobiology as a whole is still on topic on CogSci, but leaving it here would prevent the introduction of a purely cognitive answer.