I walked and tried to clear my mind as I walked, and sometimes I'd see everything in clearer focus, and I'd think "yes, this meditation is having an effect!", and then I'd realize that I was walking under a street lamp, and everything was literally brighter.

A similar thing happens when I meditate with my eyes closed.. Some thought will take shape that seems like an alerting of my mental state, but turns out to literally take the shape of some light pattern on my eyelids, or some tension in my muscles (to the extent that those have a "shape").