A curiosity of light adaptation

Abstract

By standard psychophysical procedures¹ 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^1–9, including the saturation that we have demonstrated when violet targets are presented on steady blue or violet adapting fields¹⁰. ‘Saturation’ refers to the empirical finding that as the intensity (I) of the short-wavelength field increases, the threshold intensity (ΔI) for detecting a violet target rises much more rapidly than is described by Weber's law (ΔI/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^11,12.