Richard Marshall interviews Susanna Siegel in 3:AM Magazine:
3:AM: You talk about cases where prior mental states interfere with perception. Can you talk about this idea and why this might lead to what you call an epistemic downgrade?
SS: Suppose you are afraid that I am angry at you, and your fear makes me look angry to you when you see me. Do you get any reason from your experience to believe that I’m angry at you? There’s something fishy and even perverse about the idea that your fears can get confirmed by fear-induced experience. I focus on the general notion of rationality. I am interested in the epistemic status of the type of “top-down” influences on perception from fears and desires. If you could confirm your fears through such fear-influenced experiences, rational confirmation of fears would be too cheap.
Here’s another example. In the early days of microscopes, the true theory of mammalian reproduction was still unknown. Some early users of microscopes were preformationists: they believed that mammals including people grew like plants, from seeds that were placed in a nutritive environment. Suppose their preformationism made them experience an embryo in a spermcell when they looked under the microscope. (It is probably apocryphal, but this was reported to have happened and is discussed in histories of embryology, such as Pinto- Correira’s excellent book The Ovary of Eve, Chicago, 1998). If favoring preformationism influenced your perceptual experience, that experience could not turn around and provide support for preformationism.
More here.