Retinal pigment and the Haidinger effect

The cause of the Haidinger effect (that is, the dependence of sensation in the central retina on the polarization of the incident light) has been discussed at length in the literature, but no unanimity has yet been reached. The main possibilities remaining for discussion (Stanworth & Naylor, 1950) are that the effect may be due either to variations in absorption by macular pigment, or to absorption by orientated blue receptors. In the former case the amount of light transmitted by the pigment, and therefore reaching the retina, would vary according to the degree and direction of orientation of the pigment: in the latter case the amount of light absorbed would be dependent on orientation of the receptors themselves. The spectral distribution of the effect would be that of the pigment responsible, and therefore measurements of the effect could be used to decide between the two hypotheses; once that has been done, the magnitude of the effect constitutes a method of estimating the spectral absorption of the pigment. Such measurements have been made by de Vries, Spoor & Jielof (1953). Their method, however, is open to the objection that they use a relatively large test field (30 in one direction) which must cover an area of the retina in which there is a marked variation in amount of orientation of pigment or receptors. Nevertheless, their results seem to establish that the major part of the effect is due to macular pigment. The results reported here were obtained by a different experimental method which utilizes a much smaller field and enables not only the spectral distribution but also the areal distribution of the effect to be measured. There are also some differences in the mathematical bases of the two methods.