Thresholds for detecting slowly changing Ganzfeld luminances

Detection thresholds for luminance increments or decrements are normally re lated to rapid light changes. The goal of this study was to determine detec tion thresholds far slowly changing achromatic Ganzfeld luminances before a nd after adaptation to a constant Ganzfeld illumination, subsequently calle d Ganzfeld adaptation. During Ganzfeld adaptation, perceived brightness dec reased slowly and leveled off(on average after 5-7 min), despite constant i llumination of the retina. The state of adaptation was characterized by usi ng magnitude estimation. Comparing detection thresholds for changing light intensities before and after Ganzfeld adaptation showed that the sensitivit y for luminance changes is independent of the perceived brightness. A furth er issue addressed was the time dependence of the luminance change. is ther e a limit below which a change of luminance is no longer perceivable? Even for the slowest gradient tested (0.01 log/min), subjects were able to detec t the change of luminance, although they were not able to perceive a contin uous brightness change. Similar thresholds (ca. 0.24 log unit) for shallow and steep luminance gradients suggest an absolute luminance detection mecha nism. Possible underlying mechanisms and neurophysiological substrates are discussed. (C) 2000 Optical Society of America [S0740-3232(00)00608-6].