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Faraz Farzin, David Whitney, Flora Tassone, Susan M. Rivera; The Role of the FMR1 Gene in Infant Contrast Sensitivity. Journal of Vision 2010;10(7):21. doi: 10.1167/10.7.21.
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© ARVO (1962-2015); The Authors (2016-present)
Fragile X syndrome (FXS) is characterized by the expansion of a CGG trinucleotide repeat within the FMR1 gene located on the X chromosome. Depending on the size of the CGG expansion, the mutation can be categorized as full (> 200 CGG repeats), which is associated with gene silencing and little to no FMR1 protein (FMRP) production, or premutation (55 - 200 CGG repeats), which is associated with high levels of FMR1 mRNA and decreased FMRP levels particularly in the upper permutation range. Several previous studies have reported impaired processing of temporally dynamic stimuli in infants, adolescents and adults with the FXS full mutation (Kogan et al, 2004, Farzin et al, 2008), and most recently in adult premutation carriers (Kéri and Benedek, 2008). Here, we assessed contrast sensitivity for second-order static and dynamic stimuli in male and female infants with the full mutation and the premutation, and compared their performance to mental age-matched typically developing infants. Our goal was to examine the molecular correlates of this visual deficit in infants with the disorder.
A forced-choice preferential looking paradigm was presented on an eye tracker to measure contrast sensitivity for second-order (texture-defined) static and dynamic stimuli. Michelson contrast levels were varied at four points between 10 and 42%. Infants' looking time to the side of the display that contained the stimulus, relative to their overall looking time, was measured to calculate a Visual Preference (VP) score. Logistic psychometric functions were fitted to the mean VP score at each contrast level to determine the minimum contrast value at which a 0.75 VP score was demonstrated. Our results revealed a significant difference in contrast sensitivity for second-order dynamic stimuli between TD infants and infants with the full mutation. Importantly, a significant negative correlation was found between contrast sensitivity for second-order dynamic stimuli and CGG repeat length, but not for sensitivity for second-order static stimuli. These findings confirm the role of the FMR1 gene in infant visual development, and specifically its involvement in sensitivity for parietally-mediated, attention-based dynamic stimuli. Critically, no molecular link was present for sensitivity for static stimuli. This is the first study to correlate performance on a psychophysical visual detection task with molecular measures in individuals with fragile X spectrum disorders. These results support the hypothesis that an abnormal molecular phenotype of the FMR1 gene is associated with the specific visual deficit of processing dynamic stimuli.
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