In class when we were discussing the neurolgical aspects of vision we concluded the human sensory devices for vision were designed to notice "edges"; sharp changes in light intensity. This explains why many optical illusions "fool" the human eye. My question is the following: what would happen if one took a had a long strip which was black on one end and VERY gradually lightened until it was white on the other end? At what point would the eye register the change in light intensity? From what we have learned so far it would seem that one would not register the continueing change, but instead would register a series of small rectangles each lighter than the next. For example, the eye would register black until the change in intensity of was significant enough for it to register a lighter shade. Therefore one actually "sees" an area of black where it actually is getting ligher but the change is so subtle that one cannot tell the difference. From the series of rectangles the brain would then be able to contruct a picture of a strip that is gradually shifting from black to white.
Nice question. Try looking at a wall sometime, with ceiling lights on. There's more light reaching your eye from the top of the wall than the bottom (since its closer to the source of light), but you don't notice that (normally), and instead see a homogenous wall (NOT a series of step changes in brightness, since no edge detectors are activated). That help? But yes, the question of how rapidly a brightness change must occur to be detected still remains (as well as the question of how come you CAN notice the difference in light intensity on the wall if you "pay attention to it"). The answer is that not all ganglion cells are SHARP edge detectors. There are also ganglion cells which respond better to slower changes in light intensity (and even, in some organisms at least, some which actually DO, more or less, report local light intensity). PG