The iris usually regulates the intensity of light reaching the retina. The iris, located in the anterior part of the eye, can be highly pigmented owing to the presence of melanin. Iris color is predominantly inherited [1]. Iris melanocytes are innervated by parasympathetic and sympathetic nerve endings [2]. However, there has been little research on the relationship between iris color and autonomic nerve activity.
A cross-sectional study was conducted on iris color and diurnal changes in autonomic nerve activity in 621 brown-eyed adults [3]. Iris colors were photographed with a digital camera and classified based on the analysis results of Picture Color Analyzer RBG (Red–Blue–Green) software. Autonomic nerve activity was estimated using LF and HF, calculated based on Holter electrocardiography. The frequency of subjects with a decrease in the LF/HF ratio from day to night was the highest in the group in which the quantity of melanin seemed higher because of the dark color compared to the other groups. The strengths of this study are the originality of the research theme and objective measurement of iris color. The lack of standardization of eye color with no widely accepted classifications has been because color print illustrations or color photography can fade with time [4]. Digital and computer-based eye color classifications of red/green/blue, which were used in this study, could detect changes in iris color [5].
In another cross-sectional study, the association between iris color and emotional health status was analyzed in 2346 middle school students [6]. Iris colors were determined by two independent graders and emotional health status was assessed using a questionnaire. A darker iris color tended to have a less negative emotional status (i.e., fear, sadness, and anger). A reduction in parasympathetic activity appears to be linked to negative affective states in non-clinical populations [7].
These studies suggest the potential use of iris color analysis to detect changes in autonomic nerve balance. Iris color may be useful not only for detecting major eye diseases [8] but also for checking autonomic nerve activity.
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The author declares that he has no conflict of interest.
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References
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