Sensation and Perception
Alma College
It has long been thought that human beings, due to lateral inhibition and surrounding-light ratios, have a difficult time accurately judging absolute brightnesses of lights presented to them (Wallach 1948). This experiment attempts to show that people can learn to judge more accurately, and attempts also to corroborate previous findings regarding this issue.
METHODS
Three subjects were, individually, seated in a dimly lit room approximately 4.5 feet from a light source, which was covered with white paper (so as to not fatigue retinal receptors), and which was also attached to an experimenter-controlled dimmer. Absolute brightnesses of the stimuli were determined using a "Luna-Pro" lightmeter, and a scale, running from 1-11 was developed for the subjects to use. 1, the dimmest setting, relating to 32 foot-candles, 6, the median setting, relating to 195fc, and 11, the brightest setting, relating to 1000fc, were shown to the subjects prior to the experiment to give a foundation for judgment. During the experiment, 6 was held constant (method of constant stimuli), and shown as a refresher between stimuli. The experiment consisted of three cycles, each containing stimuli between 1 and 11 (except 6) being shown in random order to each of the subjects. After each was presented, the subjects judged according to the scale. After each judgment, the subjects were corrected as to the actual brightness of the stimuli. For example, if a subject said, "...that is a 2," the experimenter might correct by saying, "No... it's actually a 3." After the three cycles were run the subjects were debriefed as to the nature of the experiment.
RESULTS
Subjects showed almost no individual difference. All judged brightnesses well across cycles, but a slight increase in accuracy can be seen in Figure 1 from cycle 1 to cycle 3, as C3 contains more correct judgments.
Figure 1 : Comparison of Subjects' Judgments to Actual Brightnesses Along a 1-11 Scale, with Foot-Candle Conversions.
DISCUSSION
It came as somewhat of a surprise that people could, first, judge so accurately from the beginning, and second, be so similar in their judgments across the cycles. Though Figure 1 shows what seems to be a lack of support for the hypothesis that people cannot judge light brightnesses accurately, it also shows several other interesting things. The shown improvement (however slight), could be the result either of learning or memory. Memory, however, is a doubtful conclusion, as the amount of information that would be needed to be kept in memory is enormous, and seems to defy traditional 7 +-2unit short-term memory storage ideas. Perhaps, then, learning does play a small role.
Another interesting side of the results is the support lent to idea that when people do judge incorrectly, they judge on the low side (Davis & Brooks 1977). This is seemingly caused by light ratios and lateral inhibition, though further experiments seem necessary to validate the findings shown here.
Due to a lack of subjects and time constraints, this experiment cannot answer to questions that it raises, but it can posit ideas for further research. In the future, it would make sense to run the experiment again using a different scale form, such as only showing the low end of the scale previous to the experiment, or only the high end. These changes would allow more subjective interpretation of the stimuli, and could result in some different data coming through.
REFERENCES
Davis, Jeff T. & Charles I. Brooks. "Judged Brightness as a Function of Dark, Intermediate, and Light Adaptation," Peceptual and Motor Skills, Vol. 44, 1977, pp. 987-990.
Wallach, Hans. "Brightness Constancy and the Nature of Achromatic Colors," Journal of Experimental Psychology, Vol. 38, 1948, 310-324.