C11- Ocular movements

Ocular Movements

• Eye movements are easier to study than movements of other body parts due to the relative simplicity of muscle actions on the eyeball.
• There are 6 extraocular muscles, each with a specific role in adjusting eye position.
• Eye movements are stereotyped, with partially distinct central circuits governing each.

Retinal Stabilization

• Spatial resolution of the retinal transduction system is limited by retinal photoreceptor mosaic properties.
• Retina is not homogeneous, with visual acuity varying across its parts.
• Fovea is the high-acuity region of the retina, containing cones with small receptive fields.
• Fovea has a 1:1 receptor-to-ganglion cell correspondence in some areas.
• The fovea's high acuity allows for precise object details focusing on it.
• Fovea size limits how much of a large object can be focused.
• Eye movement degrees of freedom need adjusting to maintain visual acuity with body movements.
• Retinal stabilization reflexes (involuntary) compensate for these limitations by adjusting eye movement in response to body movement to keep images steady.
• Different types of stabilization movements exist based on aim; - Gaze stabilization compensates for self-motion to avoid blurry vision. - Gaze shifting accurately positions the fovea for targeted stimuli. - Vergence is part of binocular vision, adjusting eye angles for clear focus of one object.

Gaze Stabilization (all animals)

• These movements counteract self-motion effects on visual acuity, keeping images stable during head movement.
• Adjusted eye movements compensate for motion.

Vestibulo-ocular Reflex (VOR)

• This involuntary reflex uses vestibular information to counteract head movements.
• The eyes move in the opposite direction to the head to maintain visual stability.
• Works rapidly during brief, transient head movements.
• Less effective for slow or sustained head rotations.
• VOR is active from the lowest to the highest velocity of head movement.
• VOR is used to examine or assess the integrity of the brainstem.

Optokinetic Reflex (OKR)

• This reflex uses visual cues (the visual field) to counteract visual movement by moving the eyes in the opposite direction.
• Especially sensitive to slow visual field movements.
• Response builds up slowly.
• Works well when head movements are slow.
• OKR is complementary to the VOR.

Gaze Shifting

• This specific type of movement (only in animals with retinal specializations) accurately points the foveal regions of the retina at a target in order to maintain visual stability.
• Two types of movement exist: saccades and pursuit movements - Saccades: swift, jerky movements rapidly changing eye direction. - Pursuit movements: smooth, tracking eye movement necessary to follow a moving stimulus to maintain it focused on the fovea.

Vergence

• Unique to animals with binocular vision.
• Adjusting eye angles for the focus of an object at varying distances.
• Crucial for maintaining a unified view of the scene.

Eye Movements overview

• All eye movement mechanisms rely on a set of common motor neurons and muscles organized in antagonistic pairs.
• Three cranial nerves (III, IV, and VI) control the eye muscles.

Nystagmus

• A reflexive eye movement that involves quick ("fast") and slow movements.
• Occurs when trying to rotate the eye past its maximum ability to counteract motion.
• A visual cue is used to reset and adjust the eyes location from other movements

Optokinetic Reflex overview

• Extracting visual information to determine relative visual field motion and responding with a compensating eye movement.
• Compensating for slow visual movements using inputs from the retina
• Damage to the AOS/pretectum can disrupt this visual feedback response.

Saccades

• Rapid, precise eye movements.
• Typically high velocity (as fast as 800°/sec)
• Used to shift focus from one location to another.
• Evoked by a variety of stimuli (visual, somatosensory, and auditory).

Smooth Pursuit

• Continuous tracking movements following a moving image.
• Eye and image movement are roughly the same speed.
• A dedicated neural circuit processes information for these subtle eye movements.

Clinical Aspects (III cranial nerve lesion)

• Complete ptosis (drooping eyelid)
• Divergent strabismus ("down and out") eye position
• Dilated pupil, unresponsive to light or accommodation.