week 3: oculomotor

week 3: oculomotor

1. eye movement = most common, 160,00 movements a day. constantly focusing on different objects

2. important sensory and behavioural consequences. diplopia = double vision, nystagmus - drift when reading

   • function = to move across a visual scene and image to fall into retina. stabilise the image onto fovea despite motion of eye or scene

3. Lens = focuses light on retina, Retina = connected to optic nerve, optic nerve = blind spot where info leaves the eye, rods = low density in fovea, cones = high density in fovea

   • night = only peripheral, black and white, poor resolution, low acuity

4. reading requires eye movement. gaze paths = read the 1st few letters and move on. high frequency words skipped entirely. saccadic eye movement

   • eye movement is voluntary. behaviourally relevant scanning of the scene. goal directed movements to access info

5. ocular muscles = MEDIAL- nose, LATERAL - ear, SUPERIOR - above, INFERIOR - below

   • when you contract one side, you stretch the other side. will want to relax back to its central fixation point. MUST KEEP CONTRACTING THE MUSCLE SO YOUR GAZE IS FOCUSED.
     • EXTRAOCULAR = move the eyeball within the socket. INTRAOCULAR = control pupil diameter
       • oblique muscles = tortian to stabilise the eyes (twisting)
         • paired extraocular muscle action = work in pairs. left and right inferior rectus muscle will pull the eye down
     • VERSIONS = both eyes moving in the same direction VERGENCES = both eyes moving in opposite directions DUCTIONS = involving either eye

6. GAZE STABILISING = keeping the visual scene static on the retina, (OKR + VOR.). TRACKING AND FOLLOWING OBJECTS (saccades, smooth pursuit, vergence movement) GAZE FIXATION

   • Optokinetic Reflex (OKR) - gaze stabilsation. maintains gaze position, assumes the world is stationary and YOU are moving. eyes drawn with an image moving across a scene. DRIVEN BY ERROR/ NEGATIVE FEEDBACK = SLOW
     • VESTIBULAR OCULAR REFLEX (VOR), faster way to respond to head movements. vestibular = inner ear, semicircular canals, brain areas involved in balance and orientation
       • make direct use of head movement information, correct where we are looking without waiting for negative feedback. USE OWN KNOWLEDGE OF HEAD POSITION
       • Signals into semicircular canal caused by head moving in one direction or another. > vestibular nucleus > motor neurons > lateral and medial muscles which rotate the eye
       • accounts for 90-95% of head correction movements, fast
         • afferent pathway = VISUAL. efferent psthway = OCULOMOTOR, can be controlled cognitively from the cortex
       • uses error feedback from the visual system, cerebellum involved, neck proprioceptors
   • NYSTAGMUS = eyes can only rotate so far and therefore, have to re-set. slow movement, then flick back. (lesions indicate issues in the brain stem, the eye will constantly be moving and world constantly blurred
   • gaze shifting = smooth pursuit. slow and simultaneous movements of both eyes in same direction. follows the target. Requires suppression of OKR to track a single object. limited to low velocities. slow visual feedback. driven by motion signals from MT and MST
     • vergences = medium speed, simultaneous movements of both eyes in opposite directions, accomodates to viewing distance
     • saccades = jumping from one point to another. 600deg./sec. cannot see eye movement. we de-emphasise the info which occurs during the movement to not notice the saccade
       • getting from A-B as quickly as possible. feedforward = no control during the movement. when you miss = a period when you realise and causes a catch up saccade shortly after
         • superior colliculus = layered. superior layer = info about the visual scene from cerebral cortex, inferior colliculus, retina, basal ganglia and spinal cord
           • through layers will transform the sensory command - motor will cause the saccade to the target
             • light coming in > superficial layer > transformed to a motor command > periphery will cause a hill of activity > hill > sweep to middle to pull eye in one direction > when in middle we use omnipause neurons to HOLD IT!
           • retinotopic = updates as we move around for fovea in middle
           • shift the stimulus from the outside of the map into the middle of high acuity area
         • relay = LGN which sends information to the visual system and Superior Colliculus (SC). flash of light in peripheral gets sent to SC and drives eye movement. FAST CIRCUIT AND INVOLUNTARY

7. when you get to end of head movement the eyes Prepositus Hyperglossi (PPH) provides a memory of the movement

   • activity comes in < direct to PPH and motor neuron. PPH = reverberating neural circuit to keep memory and produce activation which keeps the eye looking in that direction as it is continuously sending the signal to the motor neurons.
     • MAINTAIN EYE MOVEMENTS AT THE END OF HEAD MOVEMENTS = THE INDIRECT PATHWAY