The vergence-mediated gain increase of the vestibulo-ocular reflex during near-viewing
What is it about?
This review explores current knowledge about the processes that maintain clear vision of near objects during fast, unpredictable head movements. The vestibulo-ocular reflex (VOR) is a fast-responding three-neuron reflex arc that evolved to drive compensatory eye-rotations during rapid head movements. During near-viewing (vergence), the response/gain of the VOR increases to compensate for the relatively larger translation of the eyes with respect to the target - known as the vergence-mediated gain increase (VMGI). In the last decades, the clinical examination of vestibular dysfunction has yielded new knowledge concerning the differential diagnosis of central and peripheral vestibular pathologies by using transient, high-velocity/high-acceleration vestibular stimuli. In addition, the recent clinical availability of video goggles that can measure binocular eye movements during fast head movements have made it easier to study the VMGI. In fact, our two groups and others have published several papers showing transition of this topic from the experimental into the clinical field. Anticipating further developments concerning the VMGI during the VOR as a clinical tool we thought it timely to summarize recent findings. In this review, we present the available literature on VMGI testing, physiology, and pathology. We explain how the vestibular system drives compensatory eye movements at every target distance, and at every physiologically relevant head acceleration and direction. Briefly, the analysis of the stimulus starts peripherally using a whole spectrum of information created during the sensory process, which is then used to build a central a three-dimensional model. This model continually updates to provide the optimal compensatory output, even under the most demanding situations.
Why is it important?
Organizing and summarizing this knowledge may stimulate research into the VMGI during the VOR. Currently there are technical barriers that hinder measurements in the clinic, especially when measuring the VMGI/VOR during head-translation (as opposed to head rotation). Currently, there are problems with data interpretation due to the non-uniformity of stimuli and protocols. We anticipate that the next generation of clinically available video goggle systems will allow easier and more reliable binocular tracking during high acceleration/high velocity head movements, thus opening a whole new area of research for discovery.