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

Abstract

Horizontal vergence eye movements are controlled by two processes, phasic and slow-tonic. Slow-tonic responses are hypothesized to be stimulated by the faster, pulse-step neural output of the phasic system. This suggests that the general behavior of each system should be similar; however, this relationship has yet to be investigated directly. We characterize the relationship between phasic and tonic vergence by quantifying directional asymmetries in the response properties of each mechanism to the same disparity amplitudes. Four subjects viewed symmetric steps in disparity dichoptically at 40 cm while eye movements were recorded with infrared oculography. First- and second-order phasic and slow-tonic convergence response properties increased linearly with disparity demand (p < 0.01), whereas divergence responses did not (p > 0.05). Phasic divergence responses were slower than convergence (p = 0.012) and were associated with a higher frequency of saccades (p < 0.001). The average rate of slow-tonic change was correlated to the average peak velocity of phasic vergence at the same vergence demand in both directions, r = 0.78, p < 0.0001. Clear directional asymmetries were observed in phasic and tonic vergence responses. The response properties of the slow-tonic mechanism varied directly with the peak velocity of the complementary phasic system. These results provide empirical evidence of the relationship between phasic and slow-tonic vergence, suggesting that the latter depends on the motor function of the former, specifically the peak velocity. The recruitment of additional oculomotor mechanisms, such as saccades, improved the phasic response properties of the slower divergence mechanism but did not directly influence the response behavior of the slow-tonic mechanism.

A differential role for the posterior cerebellum in the adaptive control of convergence eye movements

“…Vergence stimuli for experiments #2 & 3 were created in a custom-built haploscope, which has been described in detail in our previous work [96]. In this apparatus the participant was seated (head supported and immobilized with a custom chin rest) 12 cm in front of two infrared passing mirrors placed orthogonally to each eyes line of sight.…”

Reflexive Fusional Vergence and Its Plasticity Are Impaired in Convergence Insufficiency

“…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. Beyond this limit, the width of the velocity profile increased in response to the double-step stimuli; however, the overall efficacy of this alterative process was significantly reduced.…”

“…Beyond this limit, the width of the velocity profile increased in response to the double-step stimuli; however, the overall efficacy of this alterative process was significantly reduced. 52 The following study aims to test two separate, but dependent, hypotheses in order to better characterize the oculomotor deficits that underpin the clinical condition of convergence insufficiency. First, individuals with convergence insufficiency will demonstrate a reduced capacity to adaptively lengthen their convergence responses when compared to binocularly normal controls.…”

“…Beyond this limit, the width of the velocity profile increased in response to the double-step stimuli; however, the overall efficacy of this alterative process was significantly reduced. 52 …”

Functional anatomy of human extraocular muscles during fusional divergence

“…For example, prior convergence effort diminishes divergence amplitude (Fray 2017), and resting vergence may adapt to the habitual wearing of prismatic spectacles (Sreenivasan and Bobier 2014). Phasic divergence is slower than corresponding phasic convergence (Erkelens and Bobier 2017;Hung et al 1994Hung et al , 1997, although both are proportional to their tonic performance (Erkelens and Bobier 2017). Divergence usually has lower speed and greater latency than convergence (Hung et al 1997;Semmlow and Wetzel 1979), although this asymmetry is reversed in a minority of people (Tyler et al 2012).…”