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Lawrence A. Symons, Bettina Olk, Meena Jassal, Victor Chung, Alan Kingston; The brightness of a looker's iris is not important in determining direction of gaze. Journal of Vision 2004;4(8):911. doi: 10.1167/4.8.911.
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A number of recent reports suggest that reversing the polarity of the iris and sclera affects the ability to determine the direction of gaze in a digitized photo (e.g. Ricciardelli et al., 2000). Ando (2002) has suggested that direction of gaze might be determined by assessing the relative brightness of the sclera on each side of the iris. These findings suggest that the absolute brightness of the iris is less crucial in determining direction of gaze, as long as it is darker than the sclera. The present study assessed the impact of iris brightness on the ability to determine direction of gaze in a task similar to that of Ricciardelli et al. Observers were required to indicate whether a digitized “looker” was looking to the left, right or directly at them. For any given trial, the iris of the looker's eyes ranged in 10 steps from very dark (0.98 cd/m2) to very bright (58.5 cd/m2). The brightness of the sclera remained constant at 19 cd/m2. The observers' accuracy was consistently high for a wide range of iris brightnesses, as long as the iris was darker than the sclera. Importantly, when the iris was brighter than the sclera accuracy was consistently disrupted, regardless of the absolute brightness of the iris. The results suggest that contrast polarity between iris and sclera is important in determining direction of gaze, while absolute brightness of the iris is ignored. Presumably, such an all-or-none process carries several evolutionary advantages with it, such as facilitating efficient communication in the real world where different individuals have different colored eyes.
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