Transcript Ch. 19
Ch. 19 1 Ch. 19 • Eye Movements and Sensory Motor Integration. – Extra-ocular muscles – Eye movements – Neural control of saccade movements – Neural control of smooth pursuit movements 2 Control of eye movement • A model for movement control – Extra-ocular muscles adjust eye position – 4 basic eye movements – Cerebellum, basal ganglia, vestibular system used for movement 3 Why eye movement? • Focus objects onto the fovea • Humans move eyes while examining an object • Vision is an active process 4 Neuroanatomy of eye movement • Antagonistic pairs of muscles – Superior & inferior oblique muscles – Superior & inferior rectus muscles – Lateral & medial rectus muscles QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 5 Neuroanatomy of eye movement • Innervation of extra ocular muscles – Three cranial nerves • VI - abducens • IV - trachular nerve • III - oculomotor nerve 6 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 7 Eye movements • • • • Saccades Smooth pursuit movements Vergence movements Vestibulo-ocular movements 8 Saccades • • • • Very rapid (200 msec) Change point of fixation Voluntary or reflex So fast that a saccade can’t be corrected. A second saccade movement starts if the first misses the target. 9 19-4 10 Eye movements 2. Smooth pursuit movements • Voluntary movement • Keep a moving stimulus on the fovea 11 19-5 12 Neural Control of Saccadic Eye Movements • E.g. the abducens nerve (cranial nerve VI) • Lower motor neuron to lateral rectus muscle moves the eye • From oculomotor nuclei • Controls how much the eye moves and the direction of the movement 13 Neural control of saccades • Burst of action potentials prior to movement • Steady baseline of firing holds the eye in its new position 14 19-6 15 What controls the direction of eye movement during eye saccades • The reticular formation has “gaze centers”. When activated, eyes move in a specific direction. • Horizontal gaze center (PPRF) • Vertical gaze center • Activated in concert for oblique movement 16 Sensory Motor Integration in the Superior Colliculus • Roof of the midbrain • Laminated (visual layer, motor layer) • Important for orienting movement of head and eyes. • Receive ganglion cell axons 17 Superior Colliculus • Visual layer has “visual cells” • Motor layer has “motor cells” • Motor cells fire action potentials that command the saccades 18 • Activating different visual cells will activate different saccades • A single brief electrical stimulus delivered to the superficial layer generates a prolonged burst of action potentials. 19 • Visual and motor layers have functional connections required to initiate a command for a visually guided saccade. 20 21 Box B (A) 22 Motor layer neuron Box B (b+c) 23 Auditory System • • • • Sound External ear Middle ear Inner ear 24