Eye Movements and Sensory Motor Integration

Sensorimotor Integration and Eye Movements

Sensorimotor Integration

Sensorimotor integration is the transformation of sensory inputs into motor outputs. It involves the brain's ability to take in sensory information, interpret it, and then generate appropriate motor commands to produce a coordinated movement.

Learning Outcomes

  • Understand sensorimotor integration and the functional role of eye movements in vision.
  • Understand the extraocular muscles and brainstem circuitry involved in eye movements.
  • Differentiate between various types of eye movements, including:
    • Gaze stabilizing eye movements: vestibulo-ocular and optokinetic reflexes.
    • Gaze shifting eye movements: saccades, smooth pursuit, and vergence.
  • Understand the neural control of saccadic eye movements.

Why Study Eye Movements?

  • Behavioral Importance: Eye movements are crucial for visual perception and interaction with the environment; an example is given using the painting 'Le Tricheur' by Georges de la Tour, where eye movements dictate focus and attention.
  • Model System: Studying eye movements provides a simplified model for understanding sensorimotor integration due to several advantages:
    • Accurate Measurement: Eye movements can be measured with high precision.
    • Limited Muscles: Each eye is controlled by only six muscles, each with a specific role.
    • Fewer Degrees of Freedom: The neural circuits controlling eye movements have to compensate for only three degrees of freedom.

What Eye Movements Accomplish

  • Foveal vs. Parafoveal Vision: The fovea is the central part of the retina with high visual acuity due to a high concentration of cones. Eye movements shift the high-resolution fovea to objects of interest.
  • Stabilization of Retinal Images: Vision fades when retinal images are stabilized due to retinal adaptation; constant shifting of the eye prevents this.

Extraocular Muscles and Brainstem Circuitry

Extraocular Muscles

There are three antagonistic pairs of muscles responsible for controlling all eye movements:

  • Lateral Rectus & Medial Rectus: Responsible for horizontal eye movements (abduction and adduction).
  • Superior Rectus & Inferior Rectus: Primarily responsible for vertical eye movements, also contribute to torsion.
  • Superior Oblique & Inferior Oblique: Primarily responsible for torsional eye movements, also contribute to vertical movements.
Terminology
  • Abduction: Movement of the eye away from the midline.
  • Adduction: Movement of the eye towards the midline.
Brainstem Innervation

The extraocular muscles are innervated by lower motor neurons in the brainstem, whose axons form three cranial nerves:

  • Abducens Nerve (Cranial Nerve VI): Innervates the lateral rectus muscle (ipsilateral).
  • Trochlear Nerve (Cranial Nerve IV): Innervates the superior oblique muscle (contralateral).
  • Oculomotor Nerve (Cranial Nerve III): Innervates all other extraocular muscles (ipsilateral).

Types of Eye Movements

Eye movements can be categorized into two functional categories:

  • Gaze stabilizing eye movements
  • Gaze shifting eye movements
Gaze Stabilizing Eye Movements

Gaze stabilizing eye movements include:

  • Vestibulo-ocular reflex
  • Optokinetic eye movements
Vestibulo-Ocular Reflex (VOR)

The VOR is a reflex response that automatically compensates for head movement by moving the eyes the same distance but in the opposite direction. It is driven by sensory information from the semicircular canals in the vestibular system. The VOR is limited to relatively fast head movements and is insensitive to slow movements (< 1 Hz). It does not depend on visual input, but compensatory movements will cease after about 30 seconds of slow rotation without visual input.

Optokinetic Eye Movements

The optokinetic system is sensitive to global or full-field visual motion produced by slow rotational movements (< 1 Hz). It plays an essential role in stabilizing the visual image on the retina by producing compensatory eye movements in the direction of global visual motion.

Complementary Sensitivities

The VOR compensates for rapid head movements (around 1 Hz or above), while the optokinetic system compensates for global visual motion at lower frequencies (around 0.1 Hz).

  • VOR Gain: The ratio of eye movement to head movement. A VOR gain of 1 means that the amplitude of eye movement equals the amplitude of the head movement. For example, a 3030 degree turn of the head to the right results in a 3030 degree rotation of the eyes to the left.
  • OKR Gain: The ratio of eye movement to retinal image motion. An OKR gain of 1 means that the amplitude of eye movement equals the amplitude of global visual motion.
Gaze Shifting Eye Movements

Gaze shifting eye movements include:

  • Smooth pursuit movements
  • Vergence eye movements
  • Saccadic eye movements
Smooth Pursuit Movements

Smooth pursuit movements are used to track moving objects. They are under voluntary control and are visually stimulus-driven, making it difficult to execute them without a moving visual target. Most observers revert to saccades in the absence of a moving target.

Vergence Eye Movements

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). They are reflexive.

Saccadic Eye Movements

Saccades are rapid, ballistic, highly stereotyped movements that change the direction of gaze/fixation. They can be voluntary or involuntary (e.g., microsaccades). Saccades can reflect knowledge of a task domain and are task-specific. Saccades are ballistic movements that cannot be modified by new information once initiated.

Metrics of Saccades
  • Latency: The time between the presentation of a target and the initiation of a saccade (approximately 200200 ms).

Neural Control of Saccadic Eye Movements

  • Amplitude: The amplitude of the saccade is coded by the duration of activity in lower motor neurons in the brainstem.
  • Direction: The direction of the saccade is determined by which eye muscles are activated.
  • Oculomotor Commands: Oculomotor commands are generated in the horizontal and vertical gaze centers in the reticular formation of the brainstem.
Gaze Centers
  • Horizontal Gaze Center: The paramedian pontine reticular formation (PPRF) is responsible for generating horizontal eye movements.
  • Vertical Gaze Center: The rostral interstitial nucleus is responsible for generating vertical eye movements.
  • Oblique Saccades: Joint activation of both gaze centers produces off-axis or oblique saccade trajectories.
Superior Colliculus

The superior colliculus is a laminated structure located in the midbrain that plays an important role in planning and initiating saccades to visual targets. It contains aligned visual and motor maps, such that when an object appears at a particular location in the visual field, neurons are activated in the corresponding part of the visual map. This triggers activity in subjacent neurons in the motor layer that activate upper motor neurons that move the eye just the right amount to center the fovea on the region of visual space that provided the stimulation. Saccades are encoded in movement, not retinotopic coordinates, in the superior colliculus.

Frontal Eye Fields (FEF)

The frontal eye fields are located in the rostral portion of the premotor cortex (Brodmann's area 8) and also play a role in planning and initiating saccades to visual targets.

  • Direct Route: FEF projects directly to the PPRF.
  • Indirect Route: FEF projects indirectly to the PPRF via the superior colliculus (SC).
  • Lesions: Lesions to the FEF result in contralateral saccade deficits that resolve nearly completely over time; patients cannot make saccades without external guidance from a visual target and cannot make anti-saccades. Lesions to the SC alter saccades but do not eliminate them. Lesions to both the SC and FEF result in a permanent deficit in saccade generation.
FEF Function
  • SC lesions selectively impair the ability to make reflexive, short-latency saccades (express saccades).
  • FEF lesions impair the ability to make saccades not guided by an external visual target (e.g., anti-saccades and memory-guided saccades).
  • FEF supports scanning of the visual field to locate objects of interest among distractors; FEF neurons exhibit the highest firing rates when the target is in the response field and lower firing rates when a distractor is in the response field.