Thresholds for detecting slowly changing Ganzfeld luminances

Detection thresholds for luminance increments or decrements are normally related to rapid light changes. The goal of this study was to determine detection thresholds for slowly changing achromatic Ganzfeld luminances before and after adaptation to a constant Ganzfeld illumination, subsequently called Ganzfeld adaptation. During Ganzfeld adaptation, perceived brightness decreased slowly and leveled off(on average after 5-7 min), despite constant illumination of the retina. The state of adaptation was characterized by using magnitude estimation. Comparing detection thresholds for changing light intensities before and after Ganzfeld adaptation showed that the sensitivity for luminance changes is independent of the perceived brightness. A further issue addressed was the time dependence of the luminance change. Is there 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 detect the change of luminance, although they were not able to perceive a continuous brightness change. Similar thresholds (ca. 0.24 log unit) for shallow and steep luminance gradients suggest an absolute luminance detection mechanism. Possible underlying mechanisms and neurophysiological substrates are discussed.