Latent nystagmus (LN) is the by-product of fusion maldevelopment in infancy. Because fusion maldevelopment-in the form of strabismus and amblyopia-is common, LN is a prevalent form of pathologic nystagmus encountered in clinical practice. It originates as an afferent visual pathway disorder. To unravel the mechanism for LN, we studied patients and nonhuman primates with maldeveloped fusion. These experiments have revealed that loss of binocular connections within striate cortex (area V1) in the first months of life is the necessary and sufficient cause of LN. The severity of LN increases systematically with longer durations of binocular decorrelation and greater losses of V1 connections. Decorrelation durations that exceed the equivalent of 2-3 months in human development result in an LN prevalence of 100%. No manipulation of brain stem motor pathways is required. The binocular maldevelopment originating in area V1 is passed on to downstream extrastriate regions of cerebral cortex that drive conjugate gaze, notably MSTd. Conjugate gaze is stable when MSTd neurons of the right and left cerebral hemispheres have balanced binocular activity. Fusion maldevelopment in infancy causes unbalanced monocular activity. If input from one eye dominates and the other is suppressed, MSTd in one hemisphere becomes more active. Acting through downstream projections to the ipsilateral nucleus of the optic tract, the eyes are driven conjugately to that side. The unbalanced MSTd drive is evident as the nasalward gaze-holding bias of LN when viewing with either eye.
From the Departments of Ophthalmology and Visual Sciences (LT, AW, PF) and Anatomy and Neurobiology (LT, AB), Washington University School of Medicine, St Louis, Missouri; Department of Ophthalmology and Vision Sciences (MR, AW), University of Toronto, Ontario, Canada; and the Yerkes Regional Primate Research Center (DB), Atlanta, Georgia.
Supported by a Grant EY10214 (L.T.) from the National Institutes of Health, A Walt and Lilly Disney Award for Amblyopia Research from Research to Prevent Blindness (L.T.), Summer Student Research Scholarship from the University of Toronto Faculty of Medicine (M.R.), Grant MOP 67104 (A.W.) and a New Investigator Award (A.W.) from the Canadian Institutes of Health Research.
Address correspondence to Lawrence Tychsen, MD, Room 2 South 89, St Louis Children's Hospital, Washington University School of Medicine, One Children's Place, St Louis, MO 63110; E-mail: firstname.lastname@example.org.