Summary: Rapid communication
The Purkinje rod-cone shift as a function of luminance and
Stuart Anstis *
Department of Psychology, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0109, USA
Received 14 September 2000; received in revised form 25 June 2002
In the Purkinje shift, the dark adapted eye becomes more sensitive to blue than to red as the retinal rods take over from the cones.
A striking demonstration of the Purkinje shift, suitable for classroom use, is described in which a small change in viewing distance
can reverse the perceived direction of a rotating annulus. We measured this shift with a minimum-motion stimulus (Anstis &
Cavanagh, Color Vision: Physiology & Psychophysics, Academic Press, London, 1983) that converts apparent lightness of blue
versus red into apparent motion. We filled an iso-eccentric annulus with radial red/blue sectors, and arranged that if the blue sectors
looked darker (lighter) than the red sectors, the annulus would appear to rotate to the left (right). At equiluminance the motion
appeared to vanish. Our observers established these motion null points while viewing the pattern at various retinal eccentricities
through various neutral density filters. Results: The luminous efficiency of blue (relative to red) increased linearly with eccentricity at
all adaptation levels, and the more the dark-adaptation, the steeper the slope of the eccentricity function. Thus blue sensitivity was a
linear function of eccentricity and an exponential function of filter factor. Blue sensitivity increased linearly with eccentricity, and
each additional log10 unit of dark adaptation changed the slope threefold.
Ó 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Dark adaptation; Rods; Cones; Purkinje shift; Equiluminance; Photometry; Minimum motion