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Mechanisms of brightness perception


A physically identical shade of gray on a black background appears lighter than on a white background. This tells us that apparent brightness is not simply a function of how many photons are reflected from a surface, but depends on the surrounding context. This dissertation investigates the mechanisms that underlie this dependence on context. Chapter 1 presents a computational model of apparent brightness, built out of neurally plausible components. This model uses spatial filtering with oriented difference of Gaussians at several different scales. The output of these spatial filters is locally re-weighted to normalize the amount of energy within different scales and orientations. This model can account for a wide range of human brightness illusions, using only simple mechanisms. It suggests that brightness perception might be due to relatively early visual areas, and may not require more high-level calculations (such as inferring the 3d structure of the scene), that have been suggested by previous researchers. If brightness perception is due to early visual areas, then we would expect it to be quite fast. Chapter 2 presents evidence that this is correct. Perceived brightness was measured in human participants who viewed briefly presented stimuli which were then masked to limit the amount of perceptual processing. Subjects were able to report brightness percepts for very brief presentations (as little as 58ms). If brightness is computed in early visual areas, how is it represented? Chapter 3 asks if brightness is represented in a point-for -point neural map that is filled-in from the response of small, contrast sensitive edge detector cells. Subjects adapted to illusory flicker caused by a dynamic brightness induction stimulus, with a modulating surround and a constant center. Flicker sensitivity was reduced when the test region was the same size as the constant center, but not for smaller, inset regions. This suggests that brightness induction does adapt cells along the contrast edge, but that there is no filled-in population of brightness selective cells to adapt. This is compatible with the model presented in chapter 1, which does not require a filling-in mechanism

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