Unmasking the linear behaviour of slow motor adaptation to prolonged convergence

Abstract

Adaptation to changing environmental demands is central to maintaining optimal motor system function. Current theories suggest that adaptation in both the skeletal--motor and oculomotor systems involves a combination of fast (reflexive) and slow (recalibration) mechanisms. Here we used the oculomotor vergence system as a model to investigate the mechanisms underlying slow motor adaptation. Unlike reaching with the upper limbs, vergence is less susceptible to changes in cognitive strategy that can affect the behaviour of motor adaptation. We tested the hypothesis that mechanisms of slow motor adaptation reflect early neural processing by assessing the linearity of adaptive responses over a large range of stimuli. Using varied disparity stimuli in conflict with accommodation, the slow adaptation of tonic vergence was found to exhibit a linear response whereby the rate and amplitude of the adaptive effects increased proportionally with stimulus amplitude. These results suggest that this slow adaptive mechanism represents an early neural process, implying it is a fundamental physiological process that is potentially dominated by subcortical and cerebellar substrates.

Reflexive Fusional Vergence and Its Plasticity Are Impaired in Convergence Insufficiency

“…We have reported directional asymmetries between reflexive fusional convergence and divergence responses in healthy, binocularly normal controls, which also extended into their adaptive capacities. 50,51 In addition to being slower at baseline than convergence, reflexive divergence responses demonstrated limited recruitment of larger, faster responses after completion of an adaptive lengthening double-step paradigm. 52 This result suggests that a saturation limit in the preprogramed pulse-generating divergence neural mechanism was reached.…”

Asymmetries between convergence and divergence reveal tonic vergence is dependent upon phasic vergence function

“…Main sequence plots for PV were regressed based on the disparity stimulus direction ( Figure 5). In the SV trials, heterophoria was defined as the average vergence angle of the last 3 s of the RE stimulus off periods (Erkelens et al, 2016;Han et al, 2010). Each heterophoria measure was plotted as a function of time and an exponential curve was fit to this plot for a single trial.…”

“…(PV ¼ phasic vergence; PA ¼ phasic accommodation; SV ¼ slowtonic vergence; SA ¼ slow-tonic accommodation; VA ¼ vergence accommodation; AV ¼accommodative vergence; EOMs ¼ extraocular muscles; CB ¼ ciliary body. (B) Graphical illustration of the replacement of PV by SV as fixation time increased adapted from (Schor, 1979b), reprinted with permission from Erkelens, I. M., Thompson, B., Bobier, W. R., & Munoz, D. (2016). Unmasking the linear behaviour of slow motor adaptation to prolonged convergence.…”

“…A detailed description of the SV trial procedures is published in previous work (Erkelens, Thompson, & Bobier, 2016), which was based on work in other laboratories (Han, Guo, Granger-Donetti, Vicci, & Alvarez, 2010;Satgunam, Gowrisankaran, & Fogt, 2009). In each trial the subject first viewed a fixation cross ( Figure 2B) binocularly for 2 s at an initial vergence demand of 2.5 meter angles (8.448), which was cue-congruent to the accommodative and proximal demand at 40 cm.…”

“…Slow trials: SV was measured as the rate and amplitude of heterophoria change (sampled every 15 s) during 3 min of disparity viewing. Heterophoria was measured by extinguishing the right stimulus to simulate occlusion, above right reproduced with permission fromErkelens, I. M., Thompson, B., Bobier, W. R., & Munoz, D. (2016). Unmasking the linear behaviour of slow motor adaptation to prolonged convergence.…”