My mom has a condition - I can't remember what the actual name is - where she occasionally notices floaters in her eyes. It turned out she's actually had it for years and it's in her medical records, but she only just started noticing it. Generally harmless, but here's the cool part: when the doctor looks into her eyes, it's like looking at a whole galaxy worth of stars. (He told her that and she was like...how nice for you, but prioritize please: am I going blind or not? (She's not.))
Talk about your literal interpretation of "stars in their eyes." One of the major focuses in our lab is retinitis pigmentosa, though for that particular project we mostly work with zebrafish instead of mice. You probably don't think of your eyes as part of your brain most of the time but depending on your definition they really are the most external part of your brain, or at least the retina is...and also the only part you can look at without cutting into anything.