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Donald I. A. MacLeod, R. Dirk Beer; Vision works by concatenating factors of change. Journal of Vision 2005;5(8):248. doi: https://doi.org/10.1167/5.8.248.
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© ARVO (1962-2015); The Authors (2016-present)
Light adaptation is regionally restricted, rapid and (at least approximately) reciprocal.
We demonstrate the reciprocity by allowing two Gaussian adapting blobs of different luminance, I and J, to fade completely under several seconds of steady fixation. If the two blob intensities are then suddenly changed, by unequal increments, to kI and kJ, the blobs reappear but remain matched. The brightnesses remain the same if the factors of intensity change are the same, as expected if retinal signals specify factors of change rather than absolute intensity. Hence the brightness gains in the two regions are quite unequal, and are reciprocally related to the initial intensities I and J.
But, if the new intensities are introduced early during the fading of the blob pair, the two brightness gains turn out to be nearly equal, even though threshold sensitivities have already approached their steady-state reciprocal values.
The very different behavior in the “ faded” and “unfaded” conditions is understandable if the subjectively perceived “baseline” of local brightness, perceptually present before the sudden change of intensity, combines multiplicatively, not additively, with the reciprocally scaled retinal signal in the revision of our perception.
This enables perception to be nearly veridical despite reciprocal adjustments of retinal sensitivity. The brain can make appropriate use of retinal signals that specify factors of change only if it considers what the factor of change is applied to.
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