I am wondering if anyone can help me with the following scenario:
I have a continuous board surface that I perceive as flat after swiping my hand across the surface. If I now decide to progressively but infinitesimally lower a section of the surface and then, blindfolded, swipe my hand across the surface each time. I am wondering how much I would need to lower the section before I my brain begins to perceive the surface as "not flat". Let's assume that the higher section of the board transitions smoothly and continuously to the lower section of the board (i.e. there are no sharp, sudden drops).