Chromatic (red/green) and luminance contrast sensitivity in Monozygotic and Dizygotic twin infants

Abstract

“Purpose: To determine the extent to which contrast sensitivity (CS) development is governed by genetic mechanisms vs. environment, we compared CS between pairs of twin infants and pairs of unrelated infants. If genetics have a strong influence on CS, correlations for Monozygotic (Mz) twin pairs should be greater than those for Dizygotic (Dz) twin pairs, and both Mz and Dz twins should show greater correlations than unrelated infant pairs. By contrast, if genetics have little influence, correlations should be the same for Dz and Mz twins. In this latter scenario, if both Mz and Dz twins show greater correlations than unrelated infant pairs, this suggests a role of shared environment. The current study measured Luminance (light/dark) and Chromatic (red/green) CS to assess sensitivity of the Magnocellular (M) and Parvocellular (P) pathways, respectively. Methods: Nine and 16 pairs of Mz and Dz twin pairs for a total of 50 twin infants were tested (mean age = 5 ±1.5 mos). Zygosity was assessed using a questionnaire and cheek swab kits, when necessary. Luminance and Chromatic CS were obtained using forced-choice preferential looking (˜200 trials per infant; 0.27 cpd; 4.2 Hz; mean luminance = 20 cd/m2). Results: Correlations for both Mz and Dz twins, and for both Luminance and Chromatic CS were significant (all p values 〈 0.002). Monte Carlo simulations on randomly paired unrelated twin infants revealed non-significant correlations (mean r = 0.12 and 0.07 for Luminance and Chromatic), indicating that Mz and Dz twins show greater correlations than unrelated infant pairs, and providing evidence for effects of shared environment. For Luminance CS, the Mz and Dz correlations were nearly identical (rMz = 0.88, rDz = 0.81), suggesting negligible genetic influence. By contrast, for Chromatic CS, the Mz correlation was greater than the Dz correlation (rMz = 0.87; rDz = 0.64), suggestive of a genetic contribution. Multiple regression analyses confirmed that these significant twin effects are not accounted for by small variations in age across infants. Discussion: Results suggest shared environment influences on both Luminance and Chromatic CS, as evidenced by Mz and Dz twins showing greater correlations than unrelated infant pairs. Although more Mz and Dz twins need to be tested, preliminary data suggest less genetic influence on Luminance CS (mediated by the M pathway) than on Chromatic CS (mediated by the P pathway). Future analysis, with more subjects, will employ maximum-likelihood based structural equation modeling (Neale, et al, 2002)”