Vergence fusion sustaining oscillations
Introduction: Previous studies have shown that the slow, or fusion sustaining, component of disparity vergence contains oscillatory behavior. Given the delays in disparity vergence control, a feedback control system would be expected to exhibit oscillations following the initial transient period. This study extends the examination of this behavior to a wider range of frequencies and a larger number of subjects.
Methods: Disparity vergence responses to symmetrical 4.0 deg step changes in target position were recorded in 15 subjects. Approximately three seconds of the late component of each response were isolated using interactive graphics and the frequency spectrum calculated. Peaks in these spectra associated with oscillatory behavior were identified and examined.
Results: All subjects exhibited oscillatory behavior with primary frequencies ranging between 0.45 and 0.6 Hz; much lower than those identified in the earlier study. All responses showed significant higher frequency components. These higher frequency components were related in both frequency and amplitude with the primary frequency indicating that they are harmonics probably generated by nonlinearities in the neural control processes. A correlation was found across subjects between the amplitude of the primary frequency and the maximum velocity of the fusion initialing component probably due the gain of shared neural pathways.
Conclusion: Low frequency oscillatory behavior was found in all subjects adding support that the slow, or fusion sustaining, component is mediated by a feedback control. Data have clinical implications in that dysfunction in feedback control may manifest as additional vergence error which may be reflected in the frequency spectrum.