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Abstract

Adaptation to a steady background has a profound effect on both color
appearance and discrimination. We determined the temporal
characteristics of chromatic adaptation for appearance and
discrimination along different color directions. Subjects were adapted
to a large uniform background made up of a CRT screen and a 45x64 deg
wall, illuminated by computer controlled lamps. After an instant
change in background color along a red-green or blue-yellow color
axes, we measured thresholds for the detection of increments along the
same axes at fixed times between 25 ms and 121 s. Analogously, color
appearance was determined using achromatic matching. Three components
of adaptation could be identified by their temporal characteristics. A
slow exponential time course of adaptation with a half-life of about
20 seconds was common to appearance and discrimination. A faster
component with a half-life of 40-70 ms - probably due to photoreceptor
adaptation - was also common to both. Exclusive for color appearance,
there was a third, extremely rapid mechanism with a half-life faster
than 10ms. This instantaneous process explained more than 50 of total
adaptation for color appearance and could be shown to act in a
multiplicative manner. We conclude that this instantaneous adaptation
mechanism for color appearance is situated at a later processing
stage, after mechanisms common to appearance and discrimination, and
is based on multiplicative spatial interactions rather than on local,
temporal adaptational processes. Color appearance, and thus color
constancy, seems to be determined in large part by cortical
computations.