vision

anatomy of eye

functions of eye

• focusing

  • makes sure light is hitting retina

  • passively done by cornea

  • actively done by changing shape of lens

• regulating light

  • pupillary constriction

    • changing the shape of the iris

• recording pattern of light

  • photoreceptors in retina

  • photoreception

photoreceptors

• responsive to light

• retina is made up of specialized cells

  • cones

    • high resolution

    • color

    • active in bright light

    • predominately in fovea (central visual field)

  • rods

    • low resolution

    • black/white

    • night vision

    • active in dim light

    • predominately outside of macula

      • peripheral vision (rods dominant)

the retina

*aim is to produce best image possible

• 6 layers

• peripheral —> higher density of rods

• macula (very tiny portion of eye)

  • central portion of visual field

  • fovea

    • highest density of cones

    • point of greatest visual acuity

  • easily seen on imaging

    • pit

    • blood vessels oriented away —> reduces light absorbed by blood vessels

    • more direct

• optic disc

  • where ganglion cells converge to exit eye

  • start of CN II (optic nerve connected directly to eye)

  • “blind spot” —> where optic nerve exits, no photoreceptors

phototransduction

• converting light signals to electrical signals

• light doesn’t create AP

  • changes in membrane potential cause release of transmitter onto postsynaptic neurons

  • AP isn’t necessary because short distance

• photoreceptors contain specialized proteins that are activated by light

  • rhodopsin

primary visual pathway

• conscious visual perception

• 90% of visual info enters the brain this way

• light hits retina —> optic nerve —> crosses in optic chiasm —> optic tract —> lateral geniculate nucleus (of thalamus) —> white matter pathway (optic radiations) —> occipital lobe —> primary visual cortex (V1)

visual field

• how much someone can see while looking straight ahead

  • 150 degrees (horizontal)

  • 120 degrees (vertical)

• visual fields overlap

  • blind spot (when both visual fields work together —> don’t notice blind spot)

  • stereoscopic

    • 3D vision

    • basis for depth perception (requires both eyes work together)

• temporal crescents

  • monocular

nasal and temporal visual fields diagram

lateral geniculate nucleus (LGN)

• 6 layers

• retinotopically organized (maintains same organization as retina)

  • contralateral eye (1, 4, 6)

  • ipsilateral eye (2, 3, 5)

  • magnocellular

    • input from ganglion cells sensitive to movement and contrast

  • parvocellular

    • input from ganglion cells sensitive to color and shape

  • koniocellular

    • not well known, color sensitivity

superior/inferior optic radiations

• superior optic radiations

  • contain info of LOWER visual fields

• inferior optic radiations

  • contain info of UPPER visual fields

• how we get the “upside-down'“

what does the primary visual pathway being retinotopically organized mean?

we can predict vision loss based on lesion

visual deficits in primary visual pathway

central (macular) sparing

*central visual field still intact

—> meaning not a straight line down the middle loss of vision

“fill in”

*also referred to as perceptual completion

• vision loss does not mean everything is just “black”

• blind spot —> makes it so that we participate in “fill in” all the time

occipital lobe

• primary visual cortex

  • V1

  • striate cortex

• visual information is distributed to specialized areas for further processing

  • motion, color, depth

• sent to temporal and parietal lobes for visual processing

higher order visual processing

• dorsal stream (parietal lobe)

  • where and how to

  • analyzes motion and spatial relationships (where you are in space)

• ventral stream (temporal lobe)

  • what/object processing

  • analyzes form, color, size, texture

  • assigns meaning

midbrain visual projection to superior colliculus

• function

  • spatial map for orienting of the head

• pathway

  • reflexive

  • CN II

  • optic tract

  • superior colliculi

    • auditory info is integrated in superior colliculi

  • attentional

    • CN II

    • optic tract

    • superior colliculi

    • dorsal/ventral stream AND thalamus (pulvinar nuclei)

midbrain visual projection to Edinger-Westphal nucleus

• function

  • regulate amount of light that reaches retina

• mechanism

  • all light passes through pupil

  • size of pupil is regulated by smooth muscles in iris

    • pupillary sphincter

    • pupillary dilater

• pathway

  • light

  • CN II (optic nerve)

  • pretectal area of midbrain

  • BILATERAL PROJECTIONS

    • edinger-westphal nucleus —> stimulate ipsilateral CN III

  • parasympathetic neurons to CN III (oculomotor nerve) —> determines if too much light

  • synapse on pupillary sphincter muscles

    • elicits motor response

pupillary light reflex

*shining light into L eye —> R eye also constricting (consensual response)

• light

• CN II (optic nerve)

• pretectal area of midbrain

• BILATERAL PROJECTIONS

  • edinger-westphal nucleus —> stimulate ipsilateral CN III

• parasympathetic neurons to CN III (oculomotor nerve) —> determines if too much light

• synapse on pupillary sphincter muscles

  • elicits motor response

midbrain projection: accommodation reflex

*near focusing

• function

  • prepare for near viewing

    • change shape of lens

    • additional coordination

      • pupillary constriction

      • convergence of eyes

• complex pathway

  • V1 must be intact

  • visual association areas

  • supraoculomotor nuclei in midbrain

  • edinger-westphal nucleus

    • iris sphincter

    • ciliary muscles of lens

  • CN III motor nuclei

    • convergence

retinohypothalamic pathway

• function

  • regulate sleep/wake cycles

• pathway

  • specialized cells in retina

  • optic nerve —> directly to hypothalamus

    • suprachiasmatic nucleus (damage to chiasm would affect this pathway)

  • spinal cord

    • sympathetic ganglia

  • pineal gland

    • release melatonin

oculomotor system

CN II, III, IV, and VI

• cranial nerves drive the extra ocular muscles

  • CN VI (abducens) —> pons

    • abduction

  • CN IV —> midbrain

    • down and in

  • CN III —> midbrain

    • everything else

CN III deficits

cranial nerve 3 lesion cont.

CN IV deficits

*does down and in —> likely to notice eye resting in elevation

*head tilt occurs away from side of lesion

CN VI deficits

• left eye resting in adduction (CN VI does abduction)

what drives the CN nuclei

• coordination of head and eye movements to get the image of the fovea is incredibly complex

• coordinate movements between CN III, IV and VI

• nerves cannot focus in isolation

• inputs from vestibular system (where head is in space)

• centers

  • horizontal gaze

  • vertical gaze (CN III)

  • convergence (adduction)

• system extremely vulnerable to injury

coordination of eye movements (distribution)

FEF (frontal eye fields) —> plans and initiates lateral gaze

• function

  • plan and initiate lateral eye movements

• pathway

  • initiated through FEF

  • direct or through the basal nuclei

  • superior colliculus

    • contains spatial maps due to retinotopic organization

  • contralateral horizontal/lateral gazes center

  • paramedian pontine reticular formation

  • CN VI —> abduction

  • white matter pathway MLF connects to other eye

  • CN III —> adduction

medial longitudinal fasciculus

• white matter pathway that connects CN III, IV, AND VI

• coordinates lateral gaze

• crosses immediately

• when looking LEFT

  • LEFT CN VI activates for abduction

  • RIGHT CN III activates for adduction

  • inhibition of right VI and left III to allow for proper motion

lesions impacting horizontal gaze

CN VII

• as CN VII exits brainstem it wraps around CN VI —> frequently injured together

• nuclei for CN VII AND CN VI damage

  • lateral gaze palsy with ipsilateral facial paralysis