Reading and Writing: An Interdisciplinary Journal 15: 127–149, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Visual-temporal processing in reading-disabled and normal twins
RICHARD OLSON & HELEN DATTA
University of Colorado, Boulder, Colorado, USA
Abstract. Percent contrast thresholds for the detection of 1 or 8 cycle/degree sinusoidal
gratings ﬂickering in counter-phase at 1 or 10 Hz were evaluated in 356 8–19 year old twins
and their school-age siblings. 107 of the twins had a school history of reading disability.
Subjects adjusted contrast levels to threshold from above and below in a total of 32 trials
over about 12 minutes. Internal reliability for each of the 4 stimulus conditions was 0.8,
and each stimulus condition was similarly correlated at 0.2–0.3 with several measures of
reading and phonological processing, suggesting modest relations with reading deﬁcits for
both parvocellular (sustained) and magnocellular (transient) visual processes. The variance
in the word-reading measure that was related to visual contrast thresholds was largely shared
with full-scale IQ, although IQ and reading each also accounted for small (1%) but statistically
signiﬁcant amounts of additional independent variance in contrast thresholds. Word reading
and nonword reading also accounted for largely overlapping variance in contrast thresholds,
but with a small (1%) amount of independent variance for nonword reading. A behavioral-
genetic analysis of the monozygotic and dizygotic twin correlations indicated no signiﬁcant
genetic inﬂuence on individual differences in contrast-threshold levels.
Keywords: Contrast-sensitivity, Dyslexia, Genetic, Reading Temporal, Visual
Several studies have reported that groups of dyslexic children and adults have
lower visual-contrast sensitivity when compared to groups of normal children
and adults. This deﬁcit has been particularly notable for low compared to
high spatial frequencies, and for high compared to low temporal frequencies
(Lovegrove, Martin, Bowling, Blackwood, Badcock & Paxton 1982; Martin
& Lovegrove 1984, 1987). These results and those from more recent studies
reporting deﬁcits in certain kinds of motion or apparent-motion perception
in dyslexic groups have been viewed as evidence for a related and possibly
causal deﬁcit for dyslexia in the magnocellular (transient) visual system (cf.
Cornelissen, Richardson, Mason, Fowler & Stein 1995; Farmer & Klein
1995; Eden, VanMeter, Rumsey, Maisog, Woods & Zefﬁro 1996; Slaghuis,
Twell & Kingston 1996). In contrast to the parvocellular visual system that
primarily mediates foveal color vision and the perception of higher spatial
frequency at lower temporal frequency, the magnocellular system is more