THE RELATIONSHIP BETWEEN INFANTILE STRABISMUS AND LATENT NYSTAGMUS

The so-called infantile strabismus syndrome consists, among other sign s, of (1) strabismus, (2) a defect of pursuit and optokinetic tracking with particular involvement of temporally directed responses on monoc ular viewing, (3) latent nystagmus and (4) adduction preference of the fixating eye. The following causal relationship between these three p henomena is suggested. (1) Binocularity in the visual cortex is impair ed, either as a primary defect or as a consequence of misalignment of the eyes. (2) The reduced binocularity prevents maturation of signal t ransmission from the visual cortex to the brainstem such that slip con trol is evident in poor pursuit and optokinetic responses, particularl y to monocular, temporally directed stimuli. (3) The asymmetry: of the pursuit and optokinetic systems is also evident in latent nystagmus w hich reflects a tonic preponderance, directed nasally with reference t o the fixating eye. The directional preponderance drives the slow phas es of latent nystagmus if the visual input is unbalanced in favour of one eye, Because of the maldeveloped slip control latent nystagmus is not inhibited by visual contours. When both eyes are open the better-f unctioning nasally directed pursuit and optokinetic control systems of the two eyes complement each other and largely prevent drifting of th e eyes. The defect responsible for the abnormal motor control cannot b e located between the retina and the visual cortex because perception of motion is only slightly impaired and a nasal-temporal asymmetry of the motion VEP, typically encountered in infantile strabismus, does no t correlate quantitatively with the asymmetry of the motor control. Ra ther, the defect is located between the cortex and the brainstem. (4) Adduction preference of the fixating eye with a compensatory headturn is due to a gaze-evoked component added to the latent component of the nystagmus. The gaze-evoked component is a purposeful reaction that al lows dampening of the nystagmus in adduction at the expense of an incr ease in abduction.