Smooth Pursuit and Congenital or Acquired Nystagmus

The first quantitative studies of smooth pursuit in a subject who had congenital nystagmus (CN) were carried out in the ROCKY MOUNTAINS in an electrical engineering laboratory at the University of Wyoming in 1965 [1], In retrospect it was serendipitous that the particular CN subject studied had a pendular form of CN rather than a jerk nystagmus. It was fortuitous that a pendular waveform was studiecT first since it enabled the experimenter to correctly interpret the data and not be misled by the obligatory saccades that are a part of all jerk waveforms. No such saccades ware present in the response and it was observed that the eye movements produced by a person with pendular CN consisted of that same pendular oscillation riding on a change in eye position that followed exactly the target motion. This was hypothesised to be the result of a normally operating smooth pursuit system adding its output to the internally generated CN oscillation. The additive response was observed for targets moving with constant ve city (ramps) and for those moving in a sinusoidal (pendular) manner. This hypothesis was embodied in a model of the ocular motor system that contained both a saccadic and smooth pursuit system and that duplicated the pulse, step, ramp and step-ramp (Rashbass) responses of both normals and the patient with pendular CN [1-3]. Included in the model was the variation with gaze angle of the CN amplitude and the nulling of the CN at a gaze angle equal to that of the patient.