The effect of stimulus direction on phasic and tonic vergence

Abstract

Purpose : Vergence eye movements are controlled by two processes, phasic (PV) and tonic (TV), with different time constants. We have previously reported a method in which the main sequence (MS) of the TV element can be isolated. The stimulus to the tonic system is hypothesized to be the faster, pulse-step output of the phasic system. To date, this relationship has not been quantified directly using similar metrics. The purpose of this study was to define this relationship to convergent and divergent step-stimuli separately, using a main sequence (MS) analysis.

Methods : Four subjects (25-32 years) viewed a fixation cross dichoptically at 40 cm from each eye using a haploscope. Images were set at an equal accommodative and vergence demand initially. Vergence responses were then induced via disparity steps of 2,4,8,12 deg convergence or -2,-4,-6 deg divergence and recorded at 250hz using video eye tracking (EyeLink 2). PV responses were defined by the MS plot of peak velocity vs. amplitude of the saccade-free movements in 4 trials where the stimulus was stepped randomly at 3-5s intervals. TV responses were characterized by the MS plot of maximum velocity vs. amplitude of phoria change during 3 minutes of sustained binocular viewing of one step amplitude (18 separate trials). Regression analysis were applied to the MS plots in each system and compared. A RM-ANOVA was used to compare the phasic convergence and divergence responses to the non-congruent vergence step-stimuli.

Results : MS regressions of PV and TV responses to convergent stimuli demonstrated significant, positive linear MS effects in all subjects (R² > 0.35, p<0.01). No such relationships were found in divergence for any subject (R² < 0.3, p > 0.05). RM-ANOVA of the average peak velocity data (+/- 2,4 deg stimuli) stimuli yielded a significant main effect of stimulus direction, F(1,3)=29.3, p = 0.012 and amplitude, F(1,3)=49.2, p=0.006, indicating that convergence responses are significantly faster than divergence. Interaction effects were also significant, F(1,3)=19.1, p = 0.02.

Conclusions : This study uniquely quantifies MS differences between convergence and divergence responses in both PV and TV systems. This is consistent with the view that convergence and divergence arise from seperate neural mechanisms. Similar MS characteristics between the phasic and tonic convergence systems support model assumptions that TV changes are driven by the pulse-step output of the PV system.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.