The Neural Mechanism for Latent (Fusion Maldevelopment) Nystagmus

Abstract

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.

Comparison of perceptual eye positions among patients with different degrees of anisometropia

“…Besides FMNS, there were other findings, including interocular suppression, strabismus, and small vertical oscillations of the eyes, with the most necessary and sufficient factor being binocular decorrelation. [30] The mechanisms of all these findings may be the lack of connectivity and suppression of temporalward neurons of major visual areas V1, V2 (prestriate cortex), medial temporal (MT), and medial superior temporal (MST) of the cerebral cortex. [30,31] In the present study, we found a high deviation of vertical PEP in severe anisometropes without amblyopia.…”

“…[30] The mechanisms of all these findings may be the lack of connectivity and suppression of temporalward neurons of major visual areas V1, V2 (prestriate cortex), medial temporal (MT), and medial superior temporal (MST) of the cerebral cortex. [30,31] In the present study, we found a high deviation of vertical PEP in severe anisometropes without amblyopia. The finding is consistent with the small vertical oscillations of primate models reported by Tychsen et al [30] In our opinion, the results indicate that in nonamblyopes, the deviation of vertical PEP is associated with the severity of anisometropia.…”

Fixation control and eye alignment in children treated for dense congenital or developmental cataracts

“…28,29 Nevertheless, the predominant model of INS posits that the degraded visual experience as a result of cataract during an early critical period induces an adaptive ocular motor response that leads to INS. 28,29 Because INS develops in the presence of multiple sensory system diseases, it has been hypothesized that INS may be the result of failure of a portion of the smooth pursuit system to calibrate its own internal gain during a critical period of development.…”

“…28,29 Nevertheless, the predominant model of INS posits that the degraded visual experience as a result of cataract during an early critical period induces an adaptive ocular motor response that leads to INS. 28,29 Because INS develops in the presence of multiple sensory system diseases, it has been hypothesized that INS may be the result of failure of a portion of the smooth pursuit system to calibrate its own internal gain during a critical period of development. 28,29 The results presented here, that is, that dense cataracts present during the first 5 years of life were associated with either FMNS or INS, suggests that each motor outcome is a result of its own critical combination of time, duration, and type of associated visual system perturbation.…”

“…28,29 Because INS develops in the presence of multiple sensory system diseases, it has been hypothesized that INS may be the result of failure of a portion of the smooth pursuit system to calibrate its own internal gain during a critical period of development. 28,29 The results presented here, that is, that dense cataracts present during the first 5 years of life were associated with either FMNS or INS, suggests that each motor outcome is a result of its own critical combination of time, duration, and type of associated visual system perturbation. Moreover, persistent asymmetry in monocular afferent visual system maturation after the onset of the ocular motor anomaly may result in interocular asymmetry in motor development.…”