The influence of rod photoreceptors on color perception
Date
2015
Authors
Baker, Lucinda Susan, author
Volbrecht, Vicki J., advisor
Nerger, Janice L., advisor
Tobet, Stuart A., committee member
Dik, Bryan J., committee member
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Abstract
Since the 19th century, the human visual system has been described as two separate and non-interacting visual systems, the photopic system, mediated by cone photoreceptors, and the scotopic system, mediated by rod photoreceptors. The photopic system operates at high light levels, and provides us with color perception, while the scotopic system operates in low light levels, and allows us achromatic vision. It has come to be accepted that there is some overlap, or simultaneous activity, of these two visual systems at moderate, or mesopic, light levels. Anecdotal and empirical evidence has suggested that when rod and cone photoreceptors are simultaneously active, color perception is altered in two general ways: there is an increase in the perception of blue, and there is desaturation, or overall decrease in the perception of chromatic content of colored stimuli. Various research groups have investigated the effect of rod photoreceptor input on color perception using a variety of research methods. The studies reported here extend previous work from this laboratory, and were conducted to characterize the development of rod influences on perceived hue and saturation during the course of dark adaptation, to reveal how the relationship between achromatic and chromatic perception is altered over time. The first study, which involved collecting descriptions of observers’ hue and saturation perceptions, provided data that were used to predict the results of the second set of studies, in which observers identified the particular wavelengths of light that appeared to be of pure, or unique, hues under various viewing conditions. In addition, observers also identified wavelengths of light that appeared to be equal mixtures of two neighboring hues, e.g., blue/green and green/yellow, under the same viewing conditions. These wavelengths are called binary hues. Results from the first hue scaling study were used to derive wavelength predictions for the second set of studies, with the expectation that the results from the two different experimental methods would produce the same pattern of changes in color perception correlated with rod photoreceptor activity. This was not what was found, however. The results of all studies described herein provide only partial support for the hypotheses that increased rod input correlates with increased perception of blue and a decrease in perceived saturation of colored stimuli. What these results do show is that there was a great deal of variability in the responses provided by the four observers who participated in the hue scaling study, and noticeable differences in the hue loci identified by the three observers who participated in the second set of studies. The predictions derived from the hue scaling study, for both unique and binary hues, did not match the loci measured with a staircase procedure for the two observers who participated in all studies. The nature of the experimental procedures followed for these and other studies were considered, and some suggestions were offered to explain why the present results are not consistent with many already in the literature. The human visual system is very complex, and the methods employed in the present studies may not be sufficient to tease apart the effects of rod photoreceptor input from those of other anatomical and physiological differences at multiple levels of the visual system.
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Subject
photoreceptors
color perception
psychophysics