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.

The significance of microsaccades for vision and oculomotor control.

Over the past decade several research groups have taken a renewed interest in the special role of a type of small eye movement, called 'microsaccades', in various visual processes, such as the activation of neurons in the central nervous system, or the prevention of image fading. As the study of microsaccades and their relation to visual processes goes back at least half a century, it seems appropriate to review the more recent reports in light of the history of research on maintained oculomotor fixation, in general, and on microsaccades in particular. Our review shows that there is no compelling evidence to support the view that microsaccades (or, fixation saccades more generally) serve a necessary role in improving oculomotor control or in keeping the visual world visible. The role of the retinal transients produced by small saccades during fixation needs to be evaluated in the context of both the brisk image motions present during active visual tasks performed by freely moving people, as well as the role of selective attention in modulating the strength of signals throughout the visual field.