Visual processing of color and shape in people with leprosy

Abstract The aim of this study was to analyze the contrast sensitivity of spatial luminance and the color discrimination thresholds of the protan, deutan, and tritan axes of people with leprosy. This study included 8 subjects with leprosy (M = 4, W = 4, M = 33.38 ± 8.7) and 8 healthy subjects (M = 4, W = 4, M = 30.89 ± 5.8). The contrast sensitivity was evaluated by the Metropsis software version 11.0 with vertical sinusoidal grids of frequencies of 0.2, 0.5, 1, 5, 10, and 16 cycles per degree of visual angle (cpd) and color vision by the desaturated Lanthony D15 tests and the trivector and ellipse protocols of the Cambridge Color Test. The results showed significant differences between the groups in the processing of spatial frequencies of 0.2 (U = 14; p = .018); 5.0 (U = 45.0; p = .001); 10.0 (U = 45.0; p = .001), and 16.0 (U = 45.0; p = .001) cpd. The difference in color recognition through D15d (U = 4.0; p = .002). Ellipse 2 (U = 10.0; p = .012) and ellipse 3 (U = 9.0; p = .009) were discriminated against. Overall, the results indicate that leprosy changes the visual processing of low, medium, and high spatial frequencies, as well as the sensitivity of the short wavelength (tritan line of confusion) and long (protan line of confusion) cones.

Chromatic and luminance sensitivity for skin and skinlike textures.

Despite the importance of the appearance of human skin for theoretical and practical purposes, little is known about visual sensitivity to subtle skin-tone changes, and whether the human visual system is indeed optimized to discern skin-color changes that confer some evolutionary advantage. Here, we report discrimination thresholds in a three-dimensional chromatic-luminance color space for natural skin and skinlike textures, and compare these to thresholds for uniform stimuli of the same mean color. We find no evidence that discrimination performance is superior along evolutionarily relevant color directions. Instead, discriminability is primarily determined by the prevailing illumination, and discrimination ellipses are aligned with the daylight locus. More specifically, the area and orientation of discrimination ellipses are governed by the chromatic distance between the stimulus and the illumination. Since this is true for both uniform and textured stimuli, it is likely to be driven by adaptation to mean stimulus color. Natural skin texture itself does not confer any advantage for discrimination performance. Furthermore, we find that discrimination boundaries for skin, skinlike, and scrambled skin stimuli are consistently larger than those for uniform stimuli, suggesting a possible adaptation to higher order color statistics of skin. This is in line with findings by Hansen, Giesel, and Gegenfurtner (2008) for other natural stimuli (fruit and vegetables). Human observers are also more sensitive to skin-color changes under simulated daylight as opposed to fluorescent light. The reduced sensitivity is driven by a decline in sensitivity along the luminance axis, which is qualitatively consistent with predictions from a Von Kries adaptation model.

Impaired contrast sensitivity among leprosy patients with normal visual acuity.

Contrast sensitivity is a person's ability to identify an object from its background. Patients with normal visual acuity can have reduced contrast sensitivity and may experience trouble in identifying objects at night or moving around in dimly lit places. Contrast sensitivity has never been studied in leprosy patients having normal visual acuity. This study aimed to determine if contrast sensitivity is impaired in leprosy patients who have normal visual acuity and to identify possible associations with demographic, leprosy and ocular characteristics. A hospital based study measuring and comparing contrast sensitivity using the VCT 6500 chart in 127 consecutive leprosy patients without clinically apparent ocular complications and 123 non-leprosy controls was done. Contrast sensitivity was impaired in leprosy patients in all five spatial frequencies (1.5, 3.0, 6.0, 12 and 18 cycles/degree) investigated. Among leprosy patients, contrast sensitivity falling outside the normative range was associated with increasing age (adjusted OR 1.28, 95% CI: 1.14-1.42), being female (adjusted OR 11.05, 95% CI: 2.93-41.69) and having a grade 2 deformity (adjusted OR 6.43, 95% CI:1.68-24.61). Contrast sensitivity is impaired in leprosy patients having normal visual acuity. Elderly, deformed, female patients are particularly burdened with this vision loss.