Evaluation of smooth pursuit in the presence of congenital nystagmus

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Abstract
Studies of the ability of patients with congenital nystagmus (CN) to pursue a moving target have yielded conflicting conclusions. This paper reviews pursuit records dating from the first quantitative eye movement study of smooth pursuit and CN (with its hypothesis of superposition of normal pursuit onto the CN) to more recent records obtained when such patients pursue a moving target. Correct interpretation of the resulting ocular motility data is dependent upon the understanding of CN waveform and direction changes, null shifts and target foveation that has emerged in the 20 years since that first study. The data support the conclusion that the smooth pursuit system in CN patients is intact, using both retinal slip velocity and position information to match eye velocity and position to those of the target. An updated model of the author's original hypothesis is included to illustrate how this is possible. Thus, the mechanisms used are identical to those employed by normals and differences in the resulting eye movements are due to the presence of the ever-changing CN waveform superimposed upon normal efferent pursuit commands.