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  • Eye MovementsAnatomy, Physiology and Clinical Implications

  • Introduction

  • Eye movementsOutline for the sessionTo understand and describeThe extraocular muscles and neural circuitry involved in moving the eyes.The different types of eye movements: their purpose, neural structure, and how they differ.

  • Eye movementsThink of this function not as vision, but as an effector system required to move the eyes, therefore a legitimate area of motor control research.

    This relatively simple motor system can be compared to other muscular systems, and the stimulus can be defined precisely.

    Eye movements involve rotation of the eyes in the socket.

  • Why ???

  • Why do the eyes move?We need our eyes to increase the visual range that can be covered.

  • Why do the eyes move?Eye movements bring the image onto the fovea.Receptors for vision located on back of eyeball, on the retina.

    Visualaxis

  • Why do the eyes move?Concentration of receptors providing high resolution (clear image) = fovea.

    More cortical area devoted to foveal region, so need to have image focused here.

  • How

  • The III, IV & VI

    *

  • MLF (medial longitudinal fasciculus)Internuclear connectionNonvestibular pathways

    (among CN nuclei)VI-contralateral IIIIII-VII, VII-V, V-XII, XII-VIIVestibular pathways:EyeEarNeckLimb extensors

    p389

    *

  • Conjugate Eye MovementsYoking mechanismVia MLF

    E.g. CN VI contralat. CN III

    *

  • Extraocular Muscles: three complimentary pairs

  • Muscle propertiesMore complex than somatomotor muscle fibers5 distinct fiber types (vs 2 - fast & slow)Unclear whyMore proprioceptors (?)but proprioception is (too) slowMuch higher innervation ratio (nerve endings/fiber) Built for speed, not for comfort8 ms twitch time (2-3 times faster than fast somatomotor fibers)

    Proprios: no joint capsule for joint receptors, and you really want precision control, but too slow to be useful.innervation ratio: for better control?

  • Muscle innervation (oculomotor nerves)At rest, firing rate of an individual nerve is linear with eye positionDifferent nerves have different slopes and offsetsSum to a non-linear increasing function that matches passive muscle properties

    When the eyes move, you need still more force - activity is proportional to position and velocity (stay tuned)

  • Innervation

  • Types.

  • Five types of eye movementsEach eye movement:1) serves a unique function and2) has properties particularly suited to that function

    Five types:

    Gaze shiftingSaccadesVergenceSmooth pursuitGaze holding4) Vestibular ocular reflex5) Optokinetic reflex (OKR)

  • Saccades

  • SaccadesRapid rotation of the eyes that bring images onto the fovea.Saccades are made spontaneously in response to a sudden appearing object, or to scan a scene or to read.Thus, saccades can be either voluntary or reflexive.

  • SaccadesSaccades allow us to scan the visual field on parts of the scene that convey the most significant information.We make about 3 saccades a second, and > 150,000 saccadic eye movements a day.

  • SaccadesThe trigger for a saccade is position error, the difference where your looking and where you want to look.So when the target isnt centred on the fovea, a saccade brings the eyes onto the target.200 msTarget positionEye positionTime Right Left

  • SaccadesSaccade amplitude ranges from miniature eye movements (0.1o) to movements ~45o amplitude from the straight ahead position.

    Saccade amplitude (deg)Peak velocity (deg/s)Saccade are fast (peak velocity 500o/sec), but peak velocity varies with saccade amplitude.

  • Neural control of saccadesThe discharge frequency of extraocular motor neurons is directly proportional to the position and velocity of the eye.

    Saccade onsetHorizontal eye position Abducens motor neuronAction potential

  • Neural control of saccadesThe saccade signal of motor neurons has the form of a pulse-step.

    Eye position SpikesEye velocityPulseStepSpikes/secHeight of the step determines the amplitude of the saccadeHeight of the pulse determines the speed of the saccade.

  • Neural control of saccadesThe saccade signal of motor neurons has the form of a pulse-step.

    Eye position SpikesEye velocityPulseStepSpikes/secThe pulse is the phasic signal that commands the eyes to move.The step is the tonic signal that commands the eyes to hold in an eccentric position.

  • Neural control of saccadesThe saccade signal of motor neurons has the form of a pulse-step.

    Eye position SpikesEye velocityPulseStepSpikes/secThe duration of the pulse determines the duration of the saccade.

  • Saccadic Eye Movements(saccades)Subtypes often referred to:

    1. Volitional (purposive) -predictive, anticipatory-memory-guided-antisaccades

    2. Reflexive

    3. Express saccades

    4. Spontaneous

    5. Quick phase of nystagmus

  • Velocity, Duration and the Main SequenceVisually Guided SaccadesDeviations from main sequence:

    -saccades in complete darkness

    -saccades to auditory stimuli

    -saccades to remembered targets

    -saccades made in the opposite direction (antisaccades)

  • [abducens, trochlear, om nucleus][cerebellum, brainstem][pprf, mrf][dorsal raphe]

  • Neural control of saccades1) The horizontal gaze centre is in the paramedian pontine reticular formation (PPRF) next to the abducens nucleus.

    The direction of saccades is dictated by premotor neurons in two gaze centres in the reticular formation.

  • Neural control of saccades2) The vertical gaze center is in the rostral interstitial nucleus of the medial longitudinal fasciculus (rostral iMLF) in the mesencephalic reticular formation near the oculo-motor nucleus.

  • Major Pathways for Saccadic Eye Movements

  • Superficial Layers

    Intermediate andDeep Layers

    Retina Major Connections of the Superior ColliculusSC

    *

  • Visual and Motor Related Properties of Cells in the Superior ColliculusSuperficial Layers:Intermediate:

    Deep LayersSCVisual Receptive Fields,Some enhanced Visual Responses, butno Presaccadic (motor) bursts; visualcellsVisual Receptive Fields and PresaccadicBursts before saccades to movement field;visuomotor cells, visually-triggered motor cellsNo visual RFs, just movement fields,Presaccadic burst gets earlier as you go deeper

    *

  • Sparks and Mays, 1980Tuning of SC burst neuron to direction and amplitude of saccades

    *

  • Enhancement of Superior Colliculus Visual Responses and the Need to Dissociate Behavioral ComponentsPassive fixationSaccade toRF targetSaccade toControl target

  • Movement field of Superior Colliculus neuron

  • Map of Stimulation Evoked Saccades amplitudeelevationRostral

    Caudal

  • Major Pathways for Saccadic Eye Movements

  • Lateral Intraparietal Area (LIP): visual, saccade-related and mnemonic responses

  • Incidence of light-sensitive, saccade-coincident and memory activity in LIP

  • Pursuit

  • Smooth pursuitSaccades involve fixating on a point then jumping to the next object of interest.Smooth pursuit involved keeping a visible moving target on the fovea.Although voluntary, smooth pursuit requires a stimulus to track; they cannot be executed in the absence of some environmental stimulus. The trigger for a smooth pursuit movement is a velocity difference between the eyes and the target.

  • Smooth pursuit

    The pursuit system needs to compute the speed of the moving stimulus to produce the proper eye velocity. Fast moving stimuli (30o/s) cannot be tracked with precision, and they usually elicit a saccade.

  • Smooth pursuit

    If a target starts to move1) a pursuit movement is generated after a short delay or latency (~100 ms)2) a saccade is often used to catch up to the target3) finally if the pursuit is perfect, your eye tracks the moving objectTarget movementEye movement100 msCatch-upsaccadeTimeAmplitude123

  • Smooth pursuit

    How well do pursuit movements match the movement of the object being tracked?Slow targets are matched perfectly; less than 0.33 mm retinal slip/sec.Target moving at higher speeds large retinal slips.Retinal slip is the distance between the image of the target on the retina and the fovea.

  • Smooth pursuit vs. Saccade

    Smooth pursuit isnt ballistic, like saccades, and instead moves smoothly.Agonists and antagonists are activated simultaneously in saccades, only muscle agonists are used.So smooth pursuit movements are produced by creating small differences in the tensions of the opposing ocular muscles.

  • Neural control of smooth pursuit

    The sequence of structures that are used to generate pursuit eye movement:Striate CortexMT & MSTPontine nucleiCerebellumBrainstem

  • Neural control of smooth pursuit

    The brainstem structures that are used to generate pursuit eye movement:Abducens nucleusOculo-motor nucleusMedial longitudinal fasciculusVestibulo-cerebellumTrochlear nucleusPontine nucleusVestibular nucleus and PPRF

  • The FEF

  • Microstimulation of the Frontal Eye Field

  • Continuum of Visual and Motor Responses in the FEF

    *

  • Stimulation-Evoked Smooth Pursuit Movements

  • Gaze-holding eye movements

    Gaze holding eye movements include the vestibular ocular reflex and the optokinetic reflex. Their purpose is to keep the image of the whole scene still on the entire retina when the head move

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