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, Monozygotic (Mz) twin siblings should be more similar (and more strongly correlated) than Dizygotic (Dz) twin siblings, 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 and Parvocellular pathways, respectively. Methods: Ten and 26 pairs of Mz and Dz twin pairs were tested (mean age = 4.5±1.5 mos). Zygosity was assessed using a questionnaire and cheek swab kits. Luminance and Chromatic CS were obtained for sinusoidal gratings using forced-choice preferential looking (∼200 trials per infant; 0.27 cycles/degree; 4.2 Hz). Results: Multiple regression was conducted on 100 runs of random Twin-1/Twin-2 orderings. Results indicated that the CS of one twin predicted 35-40% of the variance (p <0.0001) in the CS of the other twin, while controlling for postnatal age, gestational length, and gender. Simulations with randomly-matched twins suggest these findings are not due to chance. Discussion: Results suggest shared environment and/or genes influence both Luminance and Chromatic CS. With more subjects, we will address whether Luminance vs. Chromatic CS is more or less affected by shared genes/environment. Once we have enough data to compare Mz vs. Dz twins, we can ask which factor, genes or environment, is more influential using structural equation modeling (Neale et al, 2002).