A scotopic ‘blue shift’ obtained by electrical measurements of flicker resonance

In eyes containing rods and cones the well-known Purkinje shift of the luminosity curve or of the corresponding electrically determined sensitivity distribution of individual retinal elements consists in a shift towards the red end of the spectrum with light adaptation. This signifies that cones become responsible for vision. The visual purple (v.p.) absorption curve has its maximum around 5000 A and in terms of the micro-electrode technique is represented by the scotopic dominator. In light adaptation, cones come to the fore. In terms of the same technique this means the appearance of a photopic dominator with maximum around 5600A. Not all elements in the cat's eye are capable of delivering a photopic dominator response upon light adaptation (Granit, 1947). The red shift, which presupposes a considerable amount of light adaptation in the cat's eye (the animal we have used), stands in contrast to the fact that the first photochemical effect of light on the sensitive v.p. pigment actually is a shift of its spectrum towards the blue end. This was discovered by Lythgoe (1937), who called the photoproduct 'transient orange'. He found the substance to be exceedingly thermolabile. Lythgoe & Quilliam (1938a, b) then proceeded to study its absorption spectrum at low temperatures. Their results have been confirmed (see, for example, Colfins & Morton, 1950b; Wald, 1951). It is not necessary to discuss further events and the formation of indicator yellow and colourless products. It is unknown what these blue shifts may mean in vivo. The processes may be fast and too transient to measure, the products formed may act as yellow filters (Dartnall, 1948) or they may serve as photochemically active agents. There are, indeed, in the cat's eye, highly blue-sensitive elements (Granit, 1947; Donner & Granit, 1948) but this may mean that the micro-electrode by chance has struck an element that happens to contain a large number of 'blue cones'.