Consequences of traumatic brain injury for human vergence dynamics.

Christopher W Tyler,Lora T Likova,Anas M Elsaid,Spero C Nicholas,Kristyo N Mineff

doi:10.3389/fneur.2014.00282

Abstract

Purpose: Traumatic brain injury involving loss of consciousness has focal effects in the human brainstem, suggesting that it may have particular consequences for eye movement control. This hypothesis was investigated by measurements of vergence eye movement parameters.Methods: Disparity vergence eye movements were measured for a population of 123 normally sighted individuals, 26 of whom had suffered diffuse traumatic brain injury (dTBI) in the past, while the remainder served as controls. Vergence tracking responses were measured to sinusoidal disparity modulation of a random-dot field. Disparity vergence step responses were characterized in terms of their dynamic parameters separately for the convergence and divergence directions.Results: The control group showed notable differences between convergence and divergence dynamics. The dTBI group showed significantly abnormal vergence behavior on many of the dynamic parameters.Conclusion: The results support the hypothesis that occult injury to the oculomotor control system is a common residual outcome of dTBI.

Highlights

The coordination of the horizontal movements of the two eyes requires effective management of the action of the four horizontal rectus muscles
Disparity vergence eye movements were measured for a population of 123 normally sighted individuals, 26 of whom had suffered diffuse traumatic brain injury in the past, while the remainder served as controls
The mean vergence angle was reduced by 1.8° relative to the 9° vergence angle of the screen, implying that the diffuse traumatic brain injury (dTBI) group exhibiting a further degree of convergence relaxation on average, the tracking amplitude was similar to that for the control group

Summary

Introduction

The coordination of the horizontal movements of the two eyes requires effective management of the action of the four horizontal rectus muscles (two per eye, the lateral and medial recti in each eye). Symmetric (disjunctive) eye movements between targets at different distances (known as vergence eye movements), are typically about a factor of five slower than parallel (conjunctive) eye movements at different locations at the same distance (known as saccades). If there was independent eye movement control of the two eyes, vergence and saccadic movements should be rapid, since there is no obvious evolutionary value in downregulating the speed of fixation at different distances if the capability had been available. The inference from this differential behavior is that vergences and saccades must be considered strong evidence that they are controlled by separate neurophysiological mechanisms [see Ref. We focus on the analysis of the normal dynamics of the weaker system, vergence, and its susceptibility to disruption by diffuse traumatic brain injury (dTBI)

Methods

Results

Discussion

Conclusion