Foveation dynamics in congenital nystagmus II: Smooth pursuit

Abstract

It has been shown that, during 5 seconds of fixation, an individual with 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 velocity. Despite some claims that CN is caused by absent or "reversed" smooth pursuit, those with CN hardly ever experience oscillopsia or exhibit any accompanying symptoms of such deficits in pursuit; they are able to master sports requiring tracking of rapidly moving small objects (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 that eye velocities would match target velocities during these periods. Unity or near-unity instantaneous (beat-to-beat) pursuit gains of both experimenter-moved and subject-moved targets at peak velocities ranging from only a few deg/sec up to 210 degrees/sec were measured. The dynamic neutral zone was found to shift oppositely to target direction by amounts proportional to the increase in target speed. Our methods proved that eye velocity is made to match target velocity during the foveation intervals and support the conclusion that smooth pursuit in individuals with CN is functioning normally in the presence of the CN oscillation. In addition, we hypothesize that the same fixation mechanism that prevents oscillopsia during fixation of stationary targets, also does so during pursuit.

Foveation dynamics in congenital nystagmus III: Vestibulo-ocular reflex

“…Table 2 shows the positions of the dynamic neutral zones (DNZ) [4] for each value of average eye speed in both directions. As previously documented for the DNZ shifts during smooth pursuit [11], the VOR-induced shifts were in the direction opposite to the average eye movement. Within the neutral zones and for pendular waveforms to the right of the neutral zones, the foveating saccades were rightward; to the left of the neutral zones, they were leftward.…”

“…The SD shown in Fig. 7 are higher than those of fixation, as were those measured during pursuit [11]. The means during the light condition were slightly better than during pursuit.…”

“…To facilitate direct comparison to the 5-second records of fixation previously reported [10], the RERfp'S and SD's for all VOR intervals in each 5-second record (combining VOR in both directions) were averaged for each head velocity. A second method that calculated RERfp and its SD for each direction was found to be equivalent to the above method when evaluating smooth pursuit [11] and was not employed in this analysis of the VOR after the Gfp analysis also showed no directional differences in the VOR.…”

“…The input-output relations of the horizontal and vertical VOR during active head movements in the light and the dark were also determined by an additional method (not employed in our previous) analysis of smooth pursuit) [11], the transformation of the position signals into the frequency domain. Position signals were transformed with a Fast Fourier Transformation (FFT) program, after bias and trend had been digitally removed.…”

“…In addition to the above methods, also used in the evaluation of smooth pursuit [11], we computed the average VOR gain using the FFT of head and eye movement (see Methods). If the frequencies of head movement are less than the CN frequency the method will yield accurate values of VOR gain.…”