Blough's effect measured in humans: A rise in the cone-mediated detection threshold during dark adaptation.

Abstract

Blough$\sp1$ found that pigeons' detection thresholds for long wavelength lights fall initially in the dark, but then rise at a later time. This result is interesting because it is not consistent with the traditional understanding of the physiological mechanisms responsible for the shape of psychophysical dark adaptation functions. In the present study, 12 experiments confirm that the Blough effect may be measured in humans, and that it is not likely the result of any methodological artifact. Dark adaptation functions for 5 trichromats, 3 protanopes and 2 deuteranopes were measured at 15 degrees in the temporal retina using a Maxwellian-view, computer controlled dark adaptometer. Following a 2 minute exposure to a monochromatic (500nm) bleaching field (3.62 log Tp), the cone-mediated threshold for a 1 degree diamater test flash (701nm) falls initially to a minimum at 3 minutes in the dark, but then rises by approximately 0.15 log units. Other experiments indicate that the effect is not caused by interocular interactions, nor does it involve a decrease in either the spatial or temporal integrating capability of the detection mechanism. A much larger rise in threshold (0.6 log units) was found for small 14$\sp\prime$ diameter test fields suggesting that for larger test fields, increases in spatial integration during dark adaptation may partially balance the loss of sensitivity involved in Blough effect. Dark adaptation functions measured after exposure to different chromatic bleaching fields show a similar Blough effect following scotopically equated bleaches but not following photopically equated bleaches, while dark adaptation functions measured on a series of (504nm) rod-stimulating backgrounds show a rise in threshold if the background was 1.20 log Ts (or less), but not if the background was 1.82 log Ts (or greater). Based on these results, it is concluded that the Blough effect represents a real underlying physiological feature of human dark adaptation, which may be caused by a tonic, inhibitory influence of rods on cones. ftn$\sp1$Blough, D. S. (1958). Rise in the pigeon's threshold with a red test stimulus during dark adaptation. J. Opt. Soc. Am., 48: 274.