FOVEATION DYNAMICS IN CONGENITAL NYSTAGMUS .2. SMOOTH PURSUIT

It has been shown that, during 5 seconds of fixation, an individual wi th congenital nystagmus (CN) can repeatedly (beat-to-beat) foveate (SD = 12.87 minarc) and maintain low retinal slip velocities (SD = 118.36 minarc/sec). Smooth pursuit data from several CN subjects showed that eye velocities during these foveation intervals approximated target v elocity. Despite some claims that CN is caused by absent or "reversed" smooth pursuit, those with CN hardly ever experience oscillopsia or e xhibit any accompanying symptoms of such deficits in pursuit; they are able to master sports requiring tracking of rapidly moving small obje cts (e.g. racquetball or handball). We developed and describe several new methods to accurately assess the function of smooth pursuit in an individual with typical idiopathic CN. We investigated the dynamics of CN foveation periods during smooth pursuit to test the hypothesis tha t eye velocities would match target velocities during these periods. U nity or near-unity instantaneous (beat-to-beat) pursuit gains of both experimenter-moved and subject-moved targets at peak velocities rangin g from only a few deg/sec up to 210-degrees/sec were measured. The dyn amic neutral zone was found to shift oppositely to target direction by amounts proportional to the increase in target speed. Our methods pro ved that eye velocity is made to match target velocity during the fove ation intervals and support the conclusion that smooth pursuit in indi viduals with CN is functioning normally in the presence of the CN osci llation. In addition, we hypothesize that the same fixation mechanism that prevents oscillopsia during fixation of stationary targets, also does so during pursuit.