Dynamics of the disparity vergence fusion sustain component

Abstract

The stereotypical vergence response to a step stimulus consists of two dynamic components: a high velocity fusion initiating component followed by a slower component that may mediate sustained fusion. The initial component has been well-studied and is thought to be controlled by an open-loop mechanism. Less is known about the slow, or fusion sustaining component except that it must be feedback controlled to achieve the positional precision of sustained fusion. Given the delays in disparity vergence control, a feedback control system is likely to exhibit oscillatory behavior. Vergence responses to 4 deg step changes in target position were recorded in eight subjects. The slow component of each response was isolated manually using interactive graphics and the frequency spectrum determined. The frequency spectra of all isolated slow vergence movements showed a large low frequency peak between 1.0 and 2.0 Hz and one or more higher frequency components. The higher frequency components were found to be harmonics of the low frequency oscillation. A feedback model of the slow component was developed consisting of a time delay, an integral/derivative controller and an oculomotor plant based on Robinson’s model. Model simulations showed that a direction dependent asymmetry in the derivative element was primarily responsible for the higher frequency harmonic components. Simulations also showed that the base frequencies are primarily dependent on the time delay in the feedback control system. The fact that oscillatory behavior was found in all subjects provides strong support that the slow, fusion sustaining component is mediated by a feedback system.

Objective assessment of eye alignment and disparity-driven vergence in Parkinson’s disease

“…Similar to PD subjects, the controls were tested at both 55 cm and 30 cm. Convergence was measured using LED targets at 20, 55, 150, and 244 cm distances along the sagittal plane where the subjects were asked to shift their gaze from the distantly located target to near (Searle and Rowe, 2016;Kang et al, 2018;Semmlow et al, 2019Semmlow et al, , 2021.…”

“…There are two key sensory drives for inducing vergence while looking at targets at different depths, which are (1) visual blur under monocular viewing that induces accommodative vergence; and (2) retinal disparity under binocular viewing, i.e., differences in the spatial position of the image on two retinas that induces disparity-driven vergence. There are two key motor components of disparity-driven vergence, which are (1) the fusion initiation component, i.e., the open loop or pulse of neural activity facilitating the initial movement responsible for vergence, and (2) the fusion maintenance component driven by visual or internal feedback to align the eyes accurately to maintain a fusion of the binocular image of the target (Semmlow et al, 2019(Semmlow et al, , 2021. A handful of studies have quantified vergence in PD and have reported increased latency with variable effects on convergence velocity and gain (Hanuška et al, 2015;Gupta et al, 2021a,b).…”