Week 10 - Eye Movements

Eye Movements and Sensory Motor Integration

What Eye Movements Accomplish

  • Eye movements are behaviourally important, evident in the motor repertoire of most animals.

  • They are crucial for survival:

    • Searching for food.

    • Scanning for dangers and risks.

  • They play a significant role in social settings:

    • Diverting attention.

    • Engaging or grasping attention.

  • Eye movements serve as a valuable model system for investigating sensorimotor integration:

    • They can be measured accurately.

    • Only 6 muscles control each eye's position, each with specific roles.

    • Neural circuits controlling eye movements don't need to compensate for variable loads, as eyes maintain essentially the same volume over a lifespan.

  • They enable high-resolution perception of the environment, beyond just the foveal view.

  • Visual perception fades when retinal images are stabilized.

Actions and Innervations of Extra-Ocular Muscles

  • Three antagonistic pairs of muscles control all eye movements.

  • Each movement is controlled by a separate pair of muscles.

  • These muscles facilitate movements along three different axes:

    • Horizontal: Adduction (toward the nose) or abduction (away from the nose).

    • Vertical: Elevation (upward) or depression (downward).

    • Torsional: Intorsion (top of the eye toward the nose) or extorsion (top of the eye away from the nose).

  • Horizontal movements:

    • Controlled by the medial and lateral rectus muscles.

    • Medial rectus: adduction.

    • Lateral rectus: abduction.

  • Vertical movements:

    • Controlled through coordinated action of two pairs of muscles: the superior and inferior rectus muscles, and the inferior and superior obliques.

    • Elevation: superior rectus and inferior oblique muscles.

    • Depression: inferior rectus and superior oblique muscles.

Types of Eye Movements and Their Functions

Stabilizing vs. Shifting Gaze

  • Stabilizing eye movements:

    • Vestibulo-ocular and optokinetic movements, which stabilize gaze in response to body or world motion, or both.

  • Shifting gaze eye movements:

    • Saccades, smooth pursuit, and vergence, which destabilize gaze and move it to points of interest in the environment.

Stabilizing Eye Movements

Vestibulo-ocular

  • Stabilizes gaze due to self-motion.

  • Example: Stabilizing gaze on a fixed point while moving the head from side to side.

  • It is a reflex response that automatically compensates for head movement by moving the eyes the same distance (at the same velocity) but in the opposite direction.

Optokinetic

  • Sensitive to global or full-field visual motion produced by slow rotational movements.

  • Plays an essential role in stabilizing the visual image on the retina.

  • Produces compensatory eye movements in the direction of global visual motion.

  • Stabilizes gaze due to world motion.

Shifting Eye Movements

Saccades

  • Shifts gaze to objects of interest (e.g., diverting gaze to faces).

  • Rapid, ballistic, highly stereotyped movements that change the direction of gaze or fixation.

  • Can be voluntary or involuntary (whenever the eyes are open).

  • Can reflect knowledge of a task domain (task-specific).

  • Ballistic movement cannot be modified by new information once initiated.

  • The saccade-generating system doesn't respond to subsequent changes in target position during a saccade.

Smooth Pursuit

  • Shifts gaze to track moving objects (e.g., a bouncing ball).

  • Under voluntary control; the observer can decide whether or not to track a moving stimulus.

  • Difficult to make a smooth pursuit movement without a moving visual target.

Vergence

  • Shifting gaze in depth.

  • Vergence movements align the fovea of each eye with targets located at different depths.

  • Unlike other eye movements where both eyes move in the same direction (conjugate), vergence movements involve both eyes moving in different directions (disconjugate).