On the “Purkinje rod-cone shift”

by Gerald Huth on May 8, 2006

Reading “The Purkinje rod-cone shift as a function of luminance and retinal eccentricity” by Anstis (Vision Research, 42, (2002), 2485-2492).

The effect defined from the paper’s abstract: “In the Purkinje shift, the dark adapted eye becomes more sensitive to blue than red as the retinal rods take over from the cones”

And further on from Results and Discussion: “In conclusion, the data plotted in Figs.3 and 5 show that the blue sensitivity increases as a joint function of increasing eccentricity and of decreasing luminance (emphasis mine). Relative blue sensitivity increases linearly with eccentricity, which projects the stimulus on to a larger population of rods, and this effect is multiplied by dark adaptation, which increases the activity of this enlarged population of rods relative to cones. This increasing activity in larger numbers of rods leads to a progressive increase in blue sensitivity”.

But first, there is no mention in the paper of ‘blue-sensitive cones’…? Rather, the author points to an ‘increasing activity’ (whatever that may mean?) of rods as the cause of increased blue sensitivity. ?????

The science of vision gets itself into a greater and greater tangle of confusion!

I will explain. The retina beyond 20 degrees eccentricity is composed chiefly of rods and it is, according to my explanation, rod-rod appositions that are sensitive to short blue wavelengths (with the precise dimension between adjacent rods actually defining the short wavelength limit of visual response). Cone response is not involved.

I have proposed that the function of the total population of blue-sensitive peripheral rods acts (in parallel) as a wide angle ‘light meter’ controlling pupillary constriction and thus light entrance into the eye. The ‘increased activity’ cited by the author follows in this explanation from, under the condition of dark adaption (i.e., when the pupil is dilated), a larger area of blue-sensitive rod-rod appositions is illuminated.

In case I have left anything out…blue wavelengths are refracted by the body of the eye (or in obsolete terminology: ‘chromatically abberrated”) beyond the fovea to eccentricities of ~ 20 degrees. It is obvious that, to the extent that any ability is left to discern color in peripheral vision, the short wavlength limit (blue) will dominate.

I remind: the sensation that we term ‘color’ is determined by ratioing intensities of wavelengths refracted to either side of the ‘pure green’ or ‘Land’ point at 7-8 degrees of retina eccentricity. It is obvious that this sensation will diminish at the larger retina angles of peripheral vision exactly as has been found.

THE PURKINJE SHIFT AND THE RESULTS REPORTED IN THIS PAPER ARE COMPLETELY EXPLAINED BY THE GEOMETRIC MODEL FOR LIGHT INTERACTION WITH THE RETINA.

I will be writing shortly more of my thoughts about the work of Edwin Land.

GCH

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