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

Area of Science:

• Oculomotor control
• Neuroscience
• Vision science

Background:

• Horizontal vergence eye movements involve distinct phasic and slow-tonic processes.
• Slow-tonic vergence is hypothesized to be driven by the phasic system's neural output.
• The direct relationship between phasic and tonic vergence mechanisms remains under-investigated.

Purpose of the Study:

• To investigate the relationship between phasic and tonic vergence mechanisms.
• To quantify directional asymmetries in vergence responses to disparity.
• To determine if tonic vergence depends on phasic vergence characteristics.

Main Methods:

• Four subjects participated in the study.
• Symmetric disparity steps were presented dichoptically.
• Eye movements were recorded using infrared oculography.
• Phasic and slow-tonic vergence response properties were analyzed.

Main Results:

• Both phasic and slow-tonic convergence increased linearly with disparity demand.
• Divergence responses did not show a linear increase with disparity.
• Phasic divergence was slower than convergence and involved more saccades.
• Slow-tonic vergence rate correlated with phasic vergence peak velocity (r = 0.78, p < 0.0001).
• Directional asymmetries were evident in both phasic and tonic vergence.

Conclusions:

• The slow-tonic vergence mechanism's properties are directly influenced by the phasic system's peak velocity.
• Empirical evidence supports the dependence of slow-tonic vergence on the motor function of the phasic system.
• Saccades enhanced phasic divergence but did not affect slow-tonic responses.