It is well-known that pupil size response to both brightness and to mental activity. However, the correlation between these two phenomena is not too clear. This study, therefore is on changes in pupil size in response to mental activity while controlling the effects of brightness. As a first step, pupillary changes were measured at variable brightnesses with and without verbal instructions designed to stimulate mental activity. No interactions between pupillary changes due to the effects of brightness and those of mental activity were 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 this, it was possible to develop an experimental formula expressing the relationship between pupil size and brightness. Next, the results of the first experiment were corrected by function and using analysis of variance (ANOVA) showed that brightness did not have an effect. Therefore, a method for removing the effects of brightness upon pupillary changes was developed. Finally, the function was applied to the evaluation of pupil sizes as a function of interest in patterns presented at several levels of brightness. Corrected pupil sizes correlated to pupil sizes for patterns presented at the same levels of brightness.
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