Ocularmotor systems

Ocularmotor systems

1. function of eye movements

   • to allow interesting parts of scene to fall on HD retina
   • to converge the eyes
   • to stablize visual image

2. ocular muscles

   • intra-ocular
     • pupil diameter
   • extra-ocular
     • move eye ball in socket
     • nnervated by specific cranial nerves
       • 3rd CN(ocular motor)
         • medial rectus
         • superior oblique
         • superior rectus
         • interfior rectus
       • 4th CN (Trochlear)
         • superior oblique
       • 6th CN (Abducens)
         • lateral rectus

3. classification

   • deductions
     • adduction or abduction - either moving towards the nose or to the ear
   • versions
     • both eyes moving in the same direction.
   • Vergences
     • both eyes moving in opposite directions

4. Gaze stablization: OptoKinetic Reflex

   • old system
   • reflex to maintain gaze position
     • Minimizes “visual slip”
   • Slow and incomplete
     • driven by error signal (negative feedback)
   • Eyes can only rotate so far
     • OKN nystagmus pattern
       • • Bring eyes back to the middle and start again
       • Nystagmus
         • alternation of slow drift of the eyes and rapid saccades
           • Involuntary eye movements Break down of normal control, cannot fixate eyes
         • normal - when world drifts past your eyes (optokinetic reflex)
           • abnormal - when caused by lesions that abolish positional control of eyes or by abnormal vestibular inputs

5. Vestibular Ocular Reflex

   • Head velocity detected by semi-circular canals (vestibular system)
   • • Reflex arc - detects head movement and drives eye muscles to compensate.
     • means the eyes stay locked on to fixation point
   • • 3 neuron relay to ocular motor neurons (OMNs) to drive extraocular muscles, all neurons are within nuclei in the brainstem
   • very fast
   • • Accurate – 90% of head velocity counteracted
   • Needs tonic signal to maintain eye position at end of head motion
   • Indirect drive from Prepositus Hyperglossi (PPH)
     • keeps the input going so the eyes stay where you put them
   • Requires tuning (“gain”, VOR adaptation) by cerebellum
   • • VOR normally supplemented by visual feedback & by neck proprioceptors
   • DIRECT PATHWAY
     • move your head, vestibular neurones fire and drive the motor neurones fire and then cause the muscles to contract
   • maintain gaze position despite head movement

6. smooth persuit

   • Slow simultaneous (active) movements of both eyes in the same direction
   • Evolutionarily new system
     • to allow tracking of moving objects against a stationary background
   • Requires suppression of the optokinetic reflex (OKR)
     • want to move eyes from the tracking? • Usually driven by a visual target - hard to do voluntarily
       • slow visual “feedback” mechanism:
         • • Driven by visual motion signals from areas MT & MST (middle temporal & middle superior temporal cortex)
       • pursue predicted motion, unseen hand motion, interpolated motion etc

7. Vergence

   • • Medium speed, simultaneous movements of both eyes in the opposite directions
   • Accommodates to viewing distance
   • • Often accompanied or even replaced by asymmetric saccades

8. Gaze shifting: saccades

   • • Fast, simultaneous movements of both eyes in the same direction.
   • • Constant velocity
     • • Need to be fast because vision is badly degraded during movement
   • 2-3 saccades per second
   • • Many different areas contribute to their control
     • Feed-back control” uses signals from sensors (in muscles) to control movement
     • Feed-forward control
       • also called “ballistic” or “open-loop” control
       • the visual system detects error and caused a corrective saccade

9. Superior colliculus

   • map of viusal space according to retina
   • has many layers
   • Inputs (superficial layers)
     • from the cerebral cortex, inferior colliculus, retina, basal ganglia, and spinal cord.
   • •Receives visual, as well as auditory input
   • Outputs (deeper layers)
     • sensorimotor areas of the brain:
       • paramedian pontine reticular formation (PPRF), spinal cord, and elsewhere.
   • • Can generate eye movements without cortical involvement (fast reflective system)
   • Omnipause neurons - Activated when you hold your eyes stationary
     • • Sudden visual (auditory) input causes release of central activity
       • • Turns off “omnipause” neurons, releasing the oculomotor system in the brainstem (PPRF) from inhibition - PPRF drives eye movement
         • releases eyes from fixation
           • • At same time, “hill” of activity drives PPRF to burst and starts saccade
             • saccade occurs, retinal location of the stimulus shifts to the fovea
               • fovea it turns on the omnipause neurones

10. How to make a saccade…

    • Phasic signal in ocular motorneurons (OMNs) driven by burst of action potentials from the left Paramedian Pontine Reticular Formation (PPRF) (BRAIN STEM)
      • tonic signal needed to maintain lateral position, generated by Prepositus Hyperglossi nucleus (PPH)
        • Superior colliculus is activated by visual target
          • • Saccade size & duration are controlled by shifting activation across retinotopic map in opposite “contraversive” Superior Colliculus (SC)
            • • CORTEX: Selection among multiple targets/locations by parietal cortex (lateral intraparietal, LIP)
              • • Voluntary saccades & saccades to remembered targets are controlled by frontal eye field (FEF)
                • • Sequential eye movements controlled by supplementary eye fields (SEF)

11. Visual input > LGN > SC >drive eyemovements > cortex > deeper processing > SC

    • can lead to smooth persuit ? or - voluntary function usingFrontal eye fields