PHOTORECEPTOR SPECTRAL SENSITIVITIES - COMMON SHAPE IN THE LONG-WAVELENGTH REGION

Previous measurements of mammalian photoreceptor spectral sensitivity have been analysed, with particular attention to the long-wavelength r egion. The measurements selected for study come from rod and cone syst ems, and from human, monkey, bovine and squirrel sources, For the spec tra from photoreceptor electrophysiology and from psychophysical sensi tivity, the frequency scaling applied by Mansfield (1985, The visual s ystem, pp, 89-106, New York: Alan Liss) provides a common shape over a range of at least 7 log(10) units of sensitivity, from low frequencie s (long wavelengths) to frequencies beyond the peak. The same curve is applicable to the absorbance spectrum of bovine rhodopsin, although t he absorbance can only be measured down to about 2 log(10) units below the peak, At the longest wavelengths the results exhibit a common lim iting slope of 70 log(e) units (or 30.4 log(10) units) per unit of nor malized frequency, A simple equation is presented as a generic descrip tion for the alpha-band of mammalian photoreceptor spectral sensitivit y curves, and it seem likely that the equation may be equally applicab le to retinal(1)-based pigments in other species, Despite the lack of a theoretical basis, the equation has the correct asymptotic behaviour at long wavelengths, and it provides an accurate description of the p eak, It also accounts accurately for the experimentally observed ''yel lowing'' of long-wavelength lights that occurs beyond 700 nm.