A curiosity of light adaptation.
By standard psychophysical procedures it is possible to arrange that visual discriminations depend only on signals originating in the violet-sensitive receptors of the eye. A person's vision then shows several characteristic properties: differential sensitivity is lower than when the green- and red-sensitive cones contribute to detection, spatial and temporal resolution is poorer, and a number of anomalies of adaptation reveal themselves, including the saturation that we have demonstrated when violet targets are presented on steady blue or violet adapting fields. Saturation" refers to the empirical finding that as the intensity (I) of the short-wavelength field increases, the threshold intensity (delta I) for detecting a violet target rises much more rapidly than is described by Weber's law (delta I/I = constant). These earlier results (replicated here; see Fig. 2, right-hand data), were obtained in what would conventionally be regarded as equilibrium conditions, in that thresholds were measured after 4 min of adaptation to the steady field. Here we examine a very odd aspect of the phenomenon: the threshold reaches its saturated state only after passing through a much lower value.
D000222:Adaptation, Physiological; D006801:Humans; D008027:Light; D010786:Photoreceptor Cells / Q000502:physiology; D012160:Retina / Q000502:physiology*; D013057:Spectrum Analysis; D014796:Visual Perception / Q000502:physiology*