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).

there are probably alot of variable that would effect it, such as the amount of friction between your hand and the surface, how steep/long the slope is to the lower part, and along with that how big your hand is in relation to the slope, how rigid you hold your hand/forearm, etc.

Thanks for your input, nin1318. Yep, there are many variables. Let's suppose the entire surface is covered with smooth glossy paper, and let's assume the slope is not sharp. Any rough rule of thumb (I am not looking for exact numbers) as to the depth difference that would trigger a perception of uneveness?

Thanks.

i have no idea...i was just trying to give an outside perspective (i study molecules) sorry i couldn't add anything else...