BIOLOGY 2C03 Lecture Notes - Extrastriate Cortex, Cerebellum, Vergence
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Outline of Lecture 43 (03-07 B; Walker)
Ocular Motor System
I. Saccadic system
- Function: to redirect gaze from one point to another (change fovea target)
- Saccades are fast (peak velocity 500°/s, typical latency 200ms) and accurate
- Target position info is first processed by the PPC and frontal cortex (especially the parietal and
frontal eye fields)
- Both the PEF and FEF project to the superior colliculus (which has an ocular motor map) which
then projects to the brainstem saccade generator
- The brainstem saccade generator consists of (omnipause,) burst and integrator neurons (be able
to describe how these function together to give saccadic pulse)
- The locations of the burst and integrator neurons differs for horizontal (pons) and vertical
(midbrain) saccades
- The cerebellum can modify saccades (from below, can predict lesion effects)
- Fastigial nucleus facilitates contraversive and inhibits ipsiversive saccades
- Vermis inhibits fastigial nucleus
II. Pursuit system
- Function: track moving objects (keeps target on fovea)
- Pursuit consists of 100ms latency, initiation (accel. of eye), and sustained tracking (gain=1)
- Pursuit can inhibit the VOR reflex to allow head tracking as well
- Pursuit involves the frontal, striate, and extrastriate cortex, which project to pons, cerebellum,
and ocular motor nuclei in succession; visual feedback is involved
- Lesions have ipsiversive effects; reduced pursuit gain is nonspecific
III. Vergence system
- Function: adjust alignment of both eyes to see depth
- Vergence is disconjugate whereas saccades are conjugate
- Convergence vs. divergence
- Anatomy is unclear; it involves mesencephalic reticular formation and tonic/burst/mixed neurons
- Strabismus: static eye misalignment, various etiologies
IV. VOR reflex system (see Lec 41 – Vestibular System)
V. Ocular motor adaptation (plasticity) takes place in the cerebellum
- Saccadic adaptation: see part I above
- VOR adaptation: e.g. glasses, takes place in flocculus
- See figure on p.10 for details of pathway
Summary of major points
- Understand the 4 types of ocular motor systems
- Be able to predict the effects of cerebellar lesions, especially on saccades