Foveation dynamics in congenital nystagmus IV: vergence

Purpose To evaluate foveation dynamics and characteristics of vergence eye movements during fixation of static targets at different distances and while tracking a target moving in depth in a subject with congenital nystagmus (CN). Method Eye movements of a well-studied subject with CN were recorded using the magnetic search coil technique and analyzed using the OMtools software, including the eXpanded Nystagmus Acuity Function (NAFX). Results Both the phase planes and NAFX values during fixation of targets at various near distances were equivalent to those during fixation of a far target. When applied to vergence data, the NAFX values (“binocular” NAFX) were higher than for the individual eye data. Vergence tracking of targets moving in depth was demonstrated and was accurate for targets moving at speeds up to ~ 35°/sec. Conclusions Target foveation qualities during fixation of targets at various near distances were equivalent to that during fixation of a far target. Stereo discrimination was limited by the foveation quality of the eye with the higher NAFX waveform. Foveation period slopes during vergence tracking demonstrated vergence movements despite the ongoing CN oscillation. Similar to what we found with fixation, pursuit, and the vestibulo-ocular systems, these findings establish that vergence in both static and dynamic viewing conditions functions normally in the presence of the CN oscillation.