Failure of brightness and color constancy under prolonged Ganzfeld stimulation

We investigated sustained hue and brightness perception in a uniformly-illuminated sphere (Ganzfeld) covering the entire visual field. Under these conditions the perceptions of both brightness and hue do not remain constant, but fade with time due to local adaptation (Troxler effect). Our aim was to quantify the magnitude and time course of hue and brightness fading in physical units. In a first experiment, subjects used magnitude estimation to rate perceived hue and brightness of the sphere when illuminated by constant amounts of red (674 nm), yellow- green (547 nm), or blue (431 nm) light. Within 2 - 7 minutes the perception of hue was found to become desaturated and replaced by a sensation of gray before brightness fading leveled off as well. This final plateau was reached after 5.5 - 7.5 min, depending on wavelength. It was higher for short-wavelength light and lower for long-wavelength light. However, in each case, it was above the intrinsic dark light or Eigengrau. In a second experiment, Ganzfeld luminance was logarithmically reduced with time to obtain correlated brightness estimates in the absence of fading. When the results from experiments 1 and 2 were combined in a Crawford-like transformation, the total perceived brightness loss was found to be equivalent to a luminance decrease of about 1.5 log units and 1.3 log units for Ganzfeld luminances of 0.1 cd/m2 and 1.0 cd/m2, respectively, and for all wavelengths tested. In comparison, the time at which the hue disappeared corresponded to a decrease of luminance ranging from 0.4 to 1.2 log units depending on wavelength.