Ross, J. & Burr, D. (2008).

The knowing visual self,Trends Cogn Sci, 10 (12), 363-364.

Like all information-processing systems, biological visual systems are limited by internal and external noise; but this noise never actually impinges on our conscious perception. An article recently published in the Journal of Vision suggests that, at least for orientation judgments, the visual system has access to its own noisiness and sets thresholds accordingly. This could well be a general principle in perception, with important and wide ranging consequences.