Control of Eye movement

1. Describe the fovea

2. What are the three axis of eye movement

Fovea:

• area of high visual resolution

• area subtending about one degree of visual angle.

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The eye moves around three axis of rotation

• X axis: vertical movements

• Y axis: horizontal movements

• Z axis: torsion

[REVIEW] actions of EOMs

How do we clinically test EOMs

Describe the eye movement mechanics

• Resistance?

• Requires?

Eye movement mech:

• Resistance:

  • Orbit’s soft tissues + muscles → resistance to movement → sluggish dynamics

• Eye movement requires:

  • Force to overcome viscous drag

  • Force to maintain eccentric position

  • THUS: requires pulse and step activity of motor neurons

Describe brainstem’s role in eye movement

• Nuclei’s

• Gaze Centers

Describe the mech of horizontal eye movements:

• Pulse signals

• Step signals

• Omnipause cells?

Brainstem:

• EOM’s Nuclei:

  • Oculomotor nucleus

  • Trochlear nucleus

  • Abducens nucleus

  • Nuclei connected to each other via MLF

• Gaze Centers:

  • PPRF: horizontal eye movement

  • riMLF: vertical eye movements

    • ***P-H; R-V; Potter Had Rhino Virus***

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Horizontal eye movement mech:

• Pulse Signals:

  • via excitatory + inhibitory burst neurons in Gaze Centers

• Step signals (tonic position)

  • Nucleus prepositus hypoglossi: horizontal saccades

  • Interstitial nucleus of Cajal: vertical saccades

    • ***P-H; C-V: Potter Had Crucio Virus***

• Omnipause cells

  • provide tonic inhibition of burst cells during fixation;

  • pause as saccade trigger

Draw out the circuits for horizontal eye movement

Describe Gaze stabilization

• Why needed?

• Two systems?

• VOR

  • Afferent

  • Function

• Optokinetic System:

  • function/Mech

  • Input

  • Pathway

Gaze Stabilization:

• Why needed?

  • Head Movement impacts image stabilization.

• Two reflex response systems

  • Vestibulo-ocular reflexes (VOR)

  • Optokinetic system

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VOR:

• A:

  • vestibular apparatus (ampullary cristae)

    • coordinated by vestibular nuclei

• Function:

  • Produce Equal/Opposite eye movements to angular head movement.

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optokinetic system

• Function/Mech: compensates for sustained or slow head movements.

  • Produces slow eye movement matching (direction + velocity) retinal slip w/ rapid repositioning phase (opposite direction).

    • Optokinetic Nystagmus

• Input:

  • Visual Input is used → infer direction+ speed of head motion

    • particularly whole field movement of visual scene (retinal slip)

• Pathway:

  • Activates wide-field retinal ganglion cells → nucleus of optic tract + accessory optic nuclei.

  • Project to vestibular nuclei + vestibulocerebellum (indirectly)

Draw out the circuitry of VOR

Draw out circuitry of optokinetic System

Describe how eye motor neurons uses both pulse/step components

Describe the Superior Colliculus’ Role in eye movement:

• Contains

• Function

• A/E

Describe the function of the cerebellar vermis in eye movement

Superior colliculus

• Contains:

  • retinotopic map of Contra visual space used in directing eye movement

    • (deeper layers are visuomotor)

• Function:

  • Translate sensory information → motor error signal (desired change in position)

  • Reflex orienting movements

• A/E:

  • A: cortical eye fields + SNpr

  • E: brainstem gaze centers + frontal cortex via thalamus (md)

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cerebellar vermis

• Function:

  • calibrating saccades

  • long term adaptation in eye movement control

    • EX: adjust for muscular weakness,

    • EX: adjust for difference in elastic restorative forces between positions — same amplitude

Draw out the circuiltry of the superior colliculus

Describe how the frontal eye fields and parietal eye fields contribute to eye movement

• A/E

• Function

Frontal Eye Fields (BA 8)

• A/E:

  • A:

    • visual association cortex + thalamus (md)

      • (regards target location)

  • E:

    • brainstem gaze centers + superior colliculus

• Function:

  • Volitional or memory guided saccades

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Parietal Eye Fields (posterior IPS)

• Indirect influence

• A/E:

  • Reciprocally connected to FEF

  • E: superior colliculus

• Function:

  • visual selection (attention)

    • provides "salience map"

      • ***NOTE: Salience map = highlights the most attention‑grabbing or behaviorally relevant parts of the visual field***

  • Reflexive saccades

Draw out the circuitry of the FEF and PEF

Explain how the brain tracks a moving object

• How?

• Mech?

• Draw it Out

Shifting Gaze:

• How?:

  • Object moving → Saccade brings fovea into alignment → smooth pursuit of eye

• Mech:

  • Extrastriate visual cortex (MT and MST) + FEF + posterior parietal cortex (info about target motion)→ Dorsolateral pontine nucleus (DLPN) (encodes direction/velocity of pursuit) → vestibulocerebellum → brainstem oculomotor system via vestibular nuclei.

Describe Vergence

• What is it?

• Describe the neural circuitry:

  • Where are motor error commands created?

  • Location of Premotor Neurons? Function?

Vergence:

• What is it?

  • Eye movement required to maintain Fovea on target as distance changes

  • NOTE: Gaze shifts often involve both version and vergence movements

    • Version = both eye same movement; vergence = opposite

• Neural Circuitry:

  • Motor error commands

    • by neurons of visual cortex w/ binocular visual fields in response to retinal disparity.

      • Binocular = area where both retina share

      • Retinal disparity = difference in images in L/R Retina

  • Premotor Neurons:

    • found in supraoculomotor area in midbrain

      • drives vergence/accomodation

Describe antisaccade task

• Procedure

• Neural Circuitry

• Diagnosis

• Consequence of FEF stroke

Antisaccade Task:

• Procedure:

  • Pt shown a dot that moves rapidly → instructured to resist the reflexive saccade + shift gaze in opposite direction

• Neural Circuitry invovled:

  • Dorsolateral prefrontal cortex (DLPFC): saccade inhibition

  • FEF: triggering (anti)saccade motion

• Diagnosis:

  • Increasing errors = frontal lobe dysfunction (b/c stroke/dementia)

    • b/c Difficult to ignore or override parietal cortex saliency signal

• FEF Stroke:

  • Transient gaze deviation to side of lesion

  • difficulty directing gaze to contralateral side.

[REVIEW] Horizontal eye movement control

Describe what happens when these areas area damaged:

• Flocculus

• Oculomotor Vermis and fastigial nuclei

• Nodulus and ventral uvula

• Vestibulocerebellum

• Flocculus:

  • ipsilateral smooth pursuit impairment

  • inability to hold eccentric eye positions

• oculomotor vermis and fastigial nuclei:

  • saccade dysmetria

• nodulus & ventral uvula:

  • produce periodic alternating nystagmus (PAN)

  • spontaneous horizontal nystagmus (ipsilesional)

• Vestibulocerebellum:

  • VOR disturbances