Controlling the Effects of Brightness on the Measurement of Pupil Size as a Means of Evaluating Mental Activity

It is well-known that pupil size responds to both brightness and mental activity. However, the correlation between these two phenomena is not clear. This study is about the changes in pupil size in response to mental activity while the effects of brightness are controlled. As a first step, pupillary changes were measured at various levels of brightness, with verbal instructions that were designed to stimulate the mental activity being given and not given. No interaction between pupillary change due to the effects of brightness and pupillary change due to mental activity was found. As a second step, white, gray, and black patterns were presented to the subjects, and pupil sizes were measured at varying levels of brightness. From these measurements, it was possible to develop an an experimental formula that expresses the relationship between pupil size and brightness. Next, the results of the first experiment were compensated using the extracted function, and analysis of variance (ANOVA) showed that brightness did not have an effect on pupil size. Therefore, a method for removing the effects of brightness upon pupillary changes was developed. Finally, the extracted function was applied to the evaluation of pupil size as a function of mental activity for the patterns presented at several levels of brightness. Corrected pupil sizes correlated with pupil sizes when patterns of pictures were presented at the same levels of brightness.