Quantal Determination and Statistical Evaluation of Absolute Foveal Luminosity Thresholds and of Threshold Variability*

Abstract

Foveal luminosity thresholds to 31 stimulus wavelengths of light were obtained from five observers and expressed in radiometric units. The 31 wavelengths ranged from 400 mµ to 700 mµ at intervals of 10 mµ. The flash duration and test patch size were 50 msec and one degree of arc. Each observer sat for ten experimental sessions. One complete luminosity curve based upon five determinations at each stimulus wavelength was determined at each session. Variability values, in terms of coefficients of variation, were computed for each observer, session, and wavelength.The corneal threshold value, at 550 mµ averaged for all five observers, was approximately 14 600 quanta. The average retinal threshold value, i.e., the corneal value corrected for absorption by ocular media, was approximately 8000 quanta. The latter figure yielded a cone threshold of two quanta for three cones.An analysis of covariance of the standard deviations revealed that wavelength exerted a very significant effect (P<0.001) on these SD’s for three observers; a lesser effect (P<0.02) for one observer; and exerted no effect for the fifth observer. Pearson correlation coefficients, expressing this wavelength-variability relationship, were computed. The results paralleled exactly those of the covariance analysis: the same three observers exhibited a significant negative relationship (P<0.01); the same individual yielded a barely negative relationship (P<0.10); and the fifth displayed no correlation between wavelength and variability.