john, you got my curiosity going, so i tried some searches to see if i could come up with anything more on the subject. most of my search results were related to far more advanced studies on the visual system, so advanced that they seemed to assume the reader knew the basics of visual adaptation. the one exception i found was on wikipedia. it has a nice thumbnail description of the mechanism i was talking about: "in the absence of fixational eye movements, visual perception may fade out or disappear due to neural adaptation [1]. (see adaptation (eye).)" that's from: http://en.wikipedia.org/wiki/neural_adaptation there is a nice short article that gives an overview to acquaint you with the topic. hope that will at least offer you a starting point for further reading on the subject. cheers. paul