A study by Diane McGuinness (1975) testing binocular scotopic threshold indicated the existence of sex differences in minimum threshold. In her experiment, females were consistently more sensitive to light at different stages of dark adaptation. The purpose of this study is to test the hypothesis that women are more sensitive to light than men after dark adaptation in low light settings. After 30 minutes in a room concealed from light, will women see darker shades of gray than men?
Methods
For this experiment, the subjects were 19 (12 male and 7 female) Alma College and Central Michigan University undergraduates. In a completely dark room, low levels of light were introduced to all subjects using a light source with a 100 watt light bulb which would allow the experimenters to control the amount of light reaching all subjects. There were 11 preset light levels; at each level the subjects were asked to identify the number of gray rectangles clearly visible. There were ten successive shades of gray, ranging from white to black, randomly arranged on a black background.
A period of dark adaptation was induced for thirty minutes. The lowest light level was introduced, and the subjects were given thirty seconds to record the number of gray rectangles clearly visible. This procedure was repeated for each of the 11 light levels. As a control, subjects viewed the stimuli in normal light conditions.
Results
The results of this experiment show no statistically significant difference between males and females in minimum thresholds after dark adaptation. The largest difference in average minimum thresholds was at light level 3 in which males were more sensitive than females. At light level three, males reported clearly detecting an average of 9.4 of gray sheets of paper, while females reported seeing an average of 9.0 gray sheets. Other small discrepancies occurred at light levels 4 and 5, with males being more sensitive at these levels as well. Average gray sheets visible reached 10 (maximum) beginning at light level eight.
Figure 1. Average number of rectangles recorded in 11 different light levels for males and females
Eye color seemed to affect the number of rectangles perceived, but the difference was not great enough to be statistically significant. At light level 1, blue-eyed subjects identified an average of 8.00 rectangles, brown-eyed subjects identified 6.57, subjects with hazel eyes identified 6.75, and subjects with green eyes identified 4.75. Once again differences in the average number of rectangles perceived for light level 2 exist for the different eye colors--blue-eyed people once again identify more rectangles than subject with other eye colors (See Figure 2).
Figure 2. Average number of rectangles recorded in 11 different light levels for brown, green, blue, and hazel-eyed subjects
Discussion
Contrary to the hypothesis, a difference in the sensitivity of males and females after dark adaptation did not exist. The data show that males and females have equal abilities perceiving the stimuli. After the 30 minutes of dark adaptation, the male averages were only slightly different from the female averages--these differences were not significant. Though there were only 19 subjects, it is likely that similar results in males and females would appear even in a larger group because the data were so close.
Though an initial hypothesis about eye color was not made, the data seem to suggest that a trend may exist between the number of stimuli perceived and the subject's eye color. This is an area where further research could be done. Why do blue-eyed subjects seem to perceive darker gray rectangles at low light levels? Why do green-eyed subjects perceive only the lighter rectangles? Statistical tests indicate that this happened by chance, but that could just be because there was a small number of subjects. What would happen if 10 or 20 subjects for each eye color were tested?
References
McGuinness, D. (1975). Away from a unisex psychology: individual differences in visul sensory and perceptual processes. Perception, 5, 279-294.
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