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Integrative cortical dysfunction and pervasive motion perception deficit in fragile X syndrome

From the Departments of Educational Psychology and Neurology and Neurosurgery (Dr. Cornish), Psychology (Drs. Boutet and Chaudhuri, C.S. Kogan), and Pediatrics and Human Genetics (Dr. Der Kaloustian), McGill University; École d’Optométrie (A. Bertone and Dr. Faubert), Université de Montréal; and Departments of Neurology and Neurosurgery and Human Genetics (Dr. Andermann), McGill University and the Montreal Neurological Hospital and Institute, Canada.

Address correspondence and reprint requests to Cary Kogan, Department of Psychology, 1205 Dr. Penfield Avenue, Stewart Biologic Sciences Building, Room W8/1, McGill University, Montréal, Québec, H3A 1B1, Canada; e-mail: cary{at}hebb.psych.mcgill.ca

Background: Fragile X syndrome (FXS) is associated with neurologic deficits recently attributed to the magnocellular pathway of the lateral geniculate nucleus.

Objective: To test the hypotheses that FXS individuals 1) have a pervasive visual motion perception impairment affecting neocortical circuits in the parietal lobe and 2) have deficits in integrative neocortical mechanisms necessary for perception of complex stimuli.

Methods: Psychophysical tests of visual motion and form perception defined by either first-order (luminance) or second-order (texture) attributes were used to probe early and later occipito-temporal and occipito-parietal functioning.

Results: When compared to developmental- and age-matched controls, FXS individuals displayed severe impairments in first- and second-order motion perception. This deficit was accompanied by near normal perception for first-order form stimuli but not second-order form stimuli.

Conclusions: Impaired visual motion processing for first- and second-order stimuli suggests that both early- and later-level neurologic function of the parietal lobe are affected in Fragile X syndrome (FXS). Furthermore, this deficit likely stems from abnormal input from the magnocellular compartment of the lateral geniculate nucleus. Impaired visual form and motion processing for complex visual stimuli with normal processing for simple (i.e., first-order) form stimuli suggests that FXS individuals have normal early form processing accompanied by a generalized impairment in neurologic mechanisms necessary for integrating all early visual input.

Received March 16, 2004. Accepted in final form June 23, 2004.

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