Laboratory #3: Oculomotor Muscle Control

Objective

To measure and investigate the activity of the medial and lateral rectus muscles during various eye movements, including saccades, pursuit, vestibular ocular reflex (VOR), and vergence. This will help in understanding how the oculomotor system functions in real-time conditions, offering insight into normal and abnormal eye movement patterns.

Background

The human eye possesses six extraocular muscles that collaborate to control its movement:

• Medial Rectus: Primarily responsible for adducting the eye, allowing it to turn towards the nose, essential for coordinated gaze shifts.

• Lateral Rectus: Responsible for abducting the eye, enabling it to turn away from the nose, which plays a role in peripheral vision.

• Superior Rectus: Elevates and intorts the eye, critical for upward gaze and adjusting the eye's axis when looking up.

• Inferior Rectus: Depresses and extorts the eye, facilitating downward gaze and contributing to eye stabilization during head movements.

• Superior Oblique: Intorts and slightly depresses the eye, aiding in rotation and stabilization of the ocular image.

• Inferior Oblique: Extorts and slightly elevates the eye, enhancing the ability to follow objects above the eye level.

Innervation occurs via motor neurons, generating two types of muscular activity:

• Tonic Activity: Maintains static eye position during fixation.

• Phasic Activity: Involves modulation of muscle tone for quick eye movements, controlling the velocity of eye motion across the visual field.

Commands for eye velocity travel along a direct neural pathway, while commands for eye position are derived from an integration of signals from various sources, primarily managed by a complex network functioning as a neural integrator. This integration is crucial for maintaining stable vision during head movements.

Types of Eye Movements

• Saccades: Rapid and ballistic movements designed to reposition the fovea to focus on objects of interest.

  • They can reach peak velocities upwards of 800 degrees/second, allowing for quick shifts in gaze.

  • Saccadic movements are highly accurate due to predictive internal mechanisms that estimate the eye's destination.

• Pursuit Movements: Smooth eye movements that enable tracking of a moving target, such as a bouncing ball.

  • Notably, there is an initial latency before smooth tracking initiates, with saccades being employed to initially align the gaze on the target.

• Vestibular Ocular Reflex (VOR): A stabilizing reflex ensuring that images remain focused on the retina during head rotations.

  • This quick phasic response is facilitated via a simple reflex arc that involves three neurons working to maintain visual stability.

  • The signals for head movement are sourced from the semicircular canals, which detect angular motion.

• Vergence Movements: Facilitate the alignment of both eyes on a target when adjusting focus between near and far objects.

  • Eye movements are opposite; convergence occurs for near objects, and divergence occurs for far objects, preventing double vision and aiding depth perception.

• Optokinetic Reflex: Engaged when there is substantial motion in the visual field, typically helping to create a stable perception during prolonged visual stimulus presentation.

  • This reflex operates slower than VOR but complements it for sustained movements, coordinating eye movements with visual stimuli.

• Exercise 1: Saccades

  • Record electrooculogram (EOG) while the subject reads a selected paragraph.

  • Instructions: Ensure subjects avoid head or body movements, marking recordings at the initiation and conclusion of reading. Analyze and interpret eye movement patterns alongside the EOG results to understand quick ocular shifts.

• Exercise 2: VOR

  • Record EOG while the subject rotates their head side to side, fixating on a stationary target.

  • Ensuring no bodily movements, this will allow for precise analysis of how EOG patterns vary with different head rotation speeds.

• Exercise 3: Pursuit

  • Record EOG as the subject follows a moving target, such as a pendulum.

  • Ensure that only the eyes are moving, facilitating a comparison of EOG patterns between fast and slow moving targets to identify tracking efficiency.

• Exercise 4: Vergence

  • Record EOG while focusing on two targets at different distances.

  • The subject will alternate focus between distant and near targets over several cycles; this provides data to analyze the effects of convergence and divergence reflected in the EOG patterns.

Questions for Analysis

1. Compare EOG patterns for slow vs. fast saccades, pursuits, and varying head movements during VOR.

2. Describe the differences in eye movements (smooth vs. saccadic) across the exercises conducted.

3. Discuss how the EOG changes with various visual tasks, focusing strategies, and velocities, linking to eye muscle activity.

Conclusion

This laboratory exercise provides a comprehensive understanding of the oculomotor system and its intricate neurological processes governing diverse types of eye movements. By emphasizing muscular activity, the importance of precise and effective eye control during visual tasks is highlighted, showcasing the coordination necessary for optimal visual performance.