Visual-temporal processing in reading-disabled and normal twins

Percent contrast thresholds for the detection of 1 or 8 cycle/degree sinusoidalgratings flickering in counter-phase at 1 or 10Hz were evaluated in 356 8–19 year old twinsand their school-age siblings. 107 of the twinshad a school history of reading disability.Subjects adjusted contrast levels to thresholdfrom above and below in a total of 32 trialsover about 12 minutes. Internal reliability foreach of the 4 stimulus conditions was 0.8, andeach stimulus condition was similarlycorrelated at 0.2–0.3 with several measuresof reading and phonological processing,suggesting modest relations with readingdeficits for both parvocellular (sustained) andmagnocellular (transient) visual processes. Thevariance in the word-reading measure that wasrelated to visual contrast thresholds waslargely shared with full-scale IQ, although IQand reading each also accounted for small(1%) but statistically significant amounts ofadditional independent variance in contrastthresholds. Word reading and nonword readingalso accounted for largely overlapping variancein contrast thresholds, but with a small (1%)amount of independent variance for nonwordreading. A behavioral-genetic analysis of themonozygotic and dizygotic twin correlationsindicated no significant genetic influence onindividual differences in contrast-thresholdlevels.