Characteristics of braking saccades in congenital nystagmus.

Several of the characteristic waveforms of congenital nystagmus (CN) contain braking saccades. We test the hypothesis that braking (including foveating) saccades, while not always satisfying the standard relationships for saccades, are normal; any differences are due to the presence of high-velocity, slow-phase eye movements. Better measurements of saccadic properties, including position- and velocity-based measures and skewness, can eliminate some of this apparent distortion. We also evoked an analogous effect in normal subjects by use of a ramp-step-ramp stimulus. Finally, we used a model to further demonstrate this distortion in the saccades of normals, deviating from their intended magnitude as a function of the magnitude of the opposing velocity. The saccadic analysis methods developed herein are applicable to all saccades made during ongoing eye movements, whether normal or pathological. The above findings support the hypothesis that the braking saccades integral to many CN waveforms have normal characteristics and are the result of a normal saccadic system's responses to a slow-eye-movement oscillation.