The cerebellar oculomotor vermis is involved in reflexive but not tonic vergence adaptation

Abstract

Purpose : Two different neural mechanisms underlie vergence adaptation; a ‘fast’ mechanism that modifies reflexive fusional vergence dynamics and a ‘slow’ mechanism that gradually alters tonic innervation during prolonged fixation (heterophoria adaptation). The oculomotor vermis (OMV) of the posterior cerebellum (PC) is known to play a critical role in saccadic adaptation, however its role in vergence adaptation is unclear. We used inhibitory repetitive transcranial magnetic stimulation (rTMS) to investigate the OMV’s involvement in vergence adaptation.

Methods : Eye movements (n = 14) were recorded binocularly at 250Hz with infrared video oculography while disparity stimuli were presented dichoptically at 40cm. Tonic vergence changes were measured objectively via the heterohphoria before and after reflexive convergence responses were adaptively lengthened using a double-step disparity stimulus (2°+ 2°, 175ms delay). Active and sham continous theta-burst TMS was applied to the PC for 40 seconds on separate visits at 80% of the individual’s active motor threshold using a 75mm butterfly coil. Stimulation sites were localized using BrainSight® neuro-navigation and anatomical landmarks.

Results : Active TMS impaired the rapid adaptation of reflexive fusional vergence gain (active: 11.9% ± 7% vs sham: 32.6% ± 4%, p = 0.002) and peak velocity (active: 7.0% ± 6 vs sham: 28.1% ± 4%, p = 0.02) to the double-step stimuli. Vergence response latencies were unchanged in either condition after adaptation (active: 5.6% ± 10, p = 0.09 vs sham: 4.1% ± 10, p = 0.17). The magnitude of heterophoria change after adaptation was significantly more esophoric in each condition (active: 0.79° ± 0.3°, p = 0.01 vs sham: 0.84° ± 0.3°, p = 0.03), as expected. This change in heterophoria was not different between stimulation conditions (p = 0.86). Baseline vergence response dynamics and heterophoria were the same at each visit (p > 0.36).

Conclusions : Inhibitory rTMS applied to the PC impairs adaptation of reflexive but not tonic vergence mechanisms. This indicates a specific role for the OMV in the adaptive control of vergence eye movements.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.