Week 10 Lecture - Eye Movements

  • Sensorimotor Integration: Transforms sensory inputs into motor outputs.

  • Eye Movements: Studied for their behavioral importance and simplicity as a model system for sensorimotor integration.

    • Accurately measured.

    • Controlled by 6 muscles.

    • Neural circuits require minimal load compensation.

  • Eye Movement Functions:

    • Shifts the high-resolution fovea to objects of interest.

    • Prevents vision fading due to retinal adaptation.

  • Extraocular Muscles: Controlled by 3 antagonistic pairs (Lateral rectus, Medial rectus, Superior rectus, Inferior rectus, Superior oblique, Inferior oblique).

  • Brainstem Motor Neuron Innervation:

    • Abducens nerve (cranial nerve VI) > lateral rectus (ipsilateral)

    • Trochlear nerve (cranial nerve IV) > superior oblique (contralateral)

    • Oculomotor nerve (cranial nerve III) > other muscles (ipsilateral)

  • Types of Eye Movements:

    • Gaze Stabilizing: Vestibulo-ocular and optokinetic reflexes.

    • Gaze Shifting: Saccades, smooth pursuit, and vergence.

  • Vestibulo-Ocular Reflex (VOR): Stabilizes gaze during self-motion; compensates for head movement. Limited to fast movements (>1 Hz). Doesn't depend on visual input.

  • Optokinetic System: Stabilizes gaze during world motion; sensitive to global visual motion (<1 Hz).

  • Smooth Pursuit Movements: Tracks moving objects voluntarily; requires a moving visual target.

  • Vergence Eye Movements: Shifts gaze in depth; aligns fovea with targets at different depths (disconjugate).

  • Saccadic Eye Movements: Rapid, ballistic shifts of gaze; can be voluntary or involuntary. The system doesn’t respond to target position changes after initiation.

  • Neural Control of Saccades:

    • Amplitude coded by duration of activity in brainstem motor neurons.

    • Direction determined by activated eye muscles.

    • Horizontal and Vertical Gaze Centers in reticular formation (PPRF and Rostral interstitial nucleus).

  • Superior Colliculus: Visual and motor map alignment in the midbrain; saccades encoded in movement coordinates.

  • Frontal Eye Fields: Saccade control via direct (FEF > PPRF) and indirect (FEF > SC > PPRF) routes. Supports visual field scanning and suppresses unwanted saccades.