BIOL373L Lecture Notes - Lecture 1: Vergence, Retina, Tennis Ball
5 May 2018
School
Department
Course
Professor
Running Head: SIGHT AND OCULOMOTER CONTROL
Analysis of Eye Movement Involving Sight and Oculomotor Control: Subject Steven Wang
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SIGHT AND OCULOMOTER CONTROL
2
Introduction
The objective of this experiment was to observe, measure, and examine the electrical activity that
occurs in the medial, lateral, and oblique rectus muscles during common eye movements. These
particular movements of the eye involve six muscles which can be further divided into the
muscles along the horizontal, vertical and torsional axes (Coats and Olitsky, 2007). In this
regard, some clarification of eye movement associated with each muscle is required. The
horizontal rectus muscles comprise the superior rectus muscle which elevates and intorts the eye
and inferior rectus muscle which depresses and extorts the eye (Coats and Olitsky, 2007). The
muscles on the vertical axis are the medial and lateral rectus muscles which adducts and abducts
the eye, respectively (Coats and Olitsky, 2007). Around the torsional axis, the superior oblique
muscle intorts and slightly depresses the eye and the inferior oblique muscle extorts and slightly
elevates the eye (Coats and Olitsky, 2007). Simply put, clockwise verses counterclockwise. The
four eye movements being explored in this lab include saccades, vestibular ocular reflex, pursuit,
and vergence. Saccadic eye movement is defined as the rapid change of gaze of both eyes from a
fixed point or target to another and is independent of both the direction of movement and the
initial position of the eye (Shakhnovich, 1977). Common activities that utilize saccadic eye
movement include running, typing, reading, and playing the piano and therefore must possess the
highest degree of accuracy (Shakhnovich, 1977). This complex process involves signaling
between the medial longitudinal fasciculus, the oculomotor and the abducens nuclei (Leppert and
Peipert, 2004). During the vestibular ocular reflex (VOR), the target is stabilized in relatively the
same position on the retina notwithstanding any head movements (Leppert and Peipert, 2004). In
this lab, the subject will rotate their head from side to side whilst keeping focus on a single target
in a visual space thus utilizing the lateral semicircular canals (Vercher and Gauthier, 1990).
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SIGHT AND OCULOMOTER CONTROL
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However, it is important to note that the VOR can work in multiple directions and uses both the
superior and posterior semicircular canals as well. (Armstrong, 2014) Furthermore, the voluntary
movement of the head from side to side is compensated for by the rotation of the eye the same
distance, but in the opposite direction (Leppert and Peipert, 2004). This happens due to the
activation of motor neurons that innervate and inhibit the extraocular muscles on one side of the
head and excite the extraocular muscles on the other side, simultaneously (Armstrong, 2018).
Pursuit eye movements are moderate oculomotor movements that compensate for the retinal slip
of a moving target (in this lab, a tennis ball) using the continuous rotation of the eye. This helps
the target remain fixed on the fovea to prevent motion distortion (Wolf, 2015). The motion itself
is driven by the motion of the target across the retina (Wolf, 2015). Pursuit eye movement
experiences some degree of latency, which is defined as the time between the onset of a target
and the execution of eye movement (Wolf, 2015). Pursuit latency is influenced by a change in
the direction of motion of the target (Wolf, 2015). Lastly, vergence can be classified as
convergence and divergence, two disjunctive eye movements operating in opposing directions.
(Wong, 2007). Convergence occurs when the subject focuses on a distant target first and then
rotates their eyes inward to adjust their focus to a nearer target. The opposite occurs for
divergence and the eyes will rotate outward (D’Yewalle and Van Rensbergen, 1994).). The data
obtained in this lab will be measured and analyzed using an Electrooculogram (EOG), a graph
showing the measure of the electrical potential between electrodes strategically placed onto
points surrounding the eye and is used to study eye movements (Cohen and Salapatek, 1990).
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