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Martin Banks, Robert Held; The Functional Advantage of Slit Pupils. Journal of Vision 2012;12(9):445. doi: 10.1167/12.9.445.
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© ARVO (1962-2015); The Authors (2016-present)
There have been three hypotheses about the utility of slit pupils: 1) Larger adjustments in area with simple musculature; 2) better image quality for contours perpendicular to the pupil’s long axis; 3) preserves chromatic-aberration correction in some lenses when pupil is constricted. These hypotheses do not explain why slits are always vertical or horizontal relative to the upright head, nor why they are vertical in terrestrial predators (e.g., domestic cats) and horizontal in terrestrial grazers (horses). Humans use blur to estimate depth in front of and behind fixation where depth from disparity is imprecise (Held et al., 2011). We simulated retinal images with various pupils to determine depth-of-field blur for different kinds of natural scenes. This leads to a new hypothesis concerning slit pupils. With slit pupils, depth of field is astigmatic: shorter for contours orthogonal to the pupil’s long axis; longer for perpendicular contours. Thus, depth from blur is more precise for orthogonal contours. The ground is a common environmental feature for terrestrial predators and grazers. With the head upright, the ground is foreshortened vertically in the retinal image, increasing the prevalence of horizontal contours. Vertical slits of terrestrial predators align the orientation of the shorter depth of field with horizontal contours allowing these animals to make finer depth discriminations along the ground, an advantage in their niche. Eyes of terrestrial grazers are laterally positioned in the head, so when the head pitches downward to graze, the pupils are roughly vertical relative to the ground. Again this aligns the orientation of the shorter depth of field with horizontal contours along the ground, which is advantageous (at least while grazing). We hypothesize that the orientation of slit pupils is an adaptation that provides some animals advantageous depth discrimination relative to common contours in their environment.
Meeting abstract presented at VSS 2012
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