Neuro Anatomy of the Visual System

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Neuro Anatomy of the Visual System
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Lecture Objectives

• Identifies and describes the functional
  components of the eye and CNS structures involved
  in visual perceptual processing
• Understands the neurological organization of the
  visual perceptual processing

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Eyeball has three layers

• Outer protective layer
• Sclera and cornea
• Middle vascular layer
• Uveal tract
• Consists of iris, ciliary
• body and choroid
• Inner sensory layer
• Retina

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Sclera

• Encloses eyeball except for cornea
• Extension of the dura mater of CNS
• Protects inner contents of eye
• Helps maintain shape of the eye
• Extraocular muscles attach to its surface

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Cornea

• Outermost layer of eye
• Avascular and transparent
• 5 layers
• Epithelium,Bowmans membrane, stroma,Decemets
  membrane, endothelium
• Stroma makes up 90 thickness
• Epithelium is bathed in tears and
• must be kept moist to maintain its
• transparency
• Protects inner contents of eye
• Refracts light

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Aqueous

• Continuously produced drained away
• trabecular meshwork
• canal of Schlemm
• Maintains health of lens and cornea
• Maintains shape pressure within eye

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Iris

• Pigmentation protects retina
• Controls pupil aperture
• Dilator muscle
• Dilates pupil
• Under sympathetic control
• Eyes dilate with fear
• Spincter muscle
• Constricts pupil

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Lens

• Avascular
• 65 water 35 protein
• Refracts light to focus image onto retina
• Shape controlled by ciliary muscles
• Balls up for near focus
• Flattens for distance focus
• Fibers form throughout life
• Gradually become more compacted into center of
  lens
• Lens loses flexibility
• Presbyopia develops

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Ciliary Body

• Ciliary muscle
• Shapes lens
• Controlled by CN III
• Ciliary process
• Secretes aqueous

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Vitreous

• Transparent gelatinous substance
• Maintains transparency and form of eye
• Holds retina in place

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Conjunctiva

• Thin transparent membrane
• covering sclera and
• inner eyelid
• Provides protection
• and moisture
• Many blood vessels
• Few pain fibers
• Conjunctivitis common

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Choroid

• Vascular supply for eye
• Capillaries and veins
• Choroid capillaris
• Separate blood supply for macula foveal area

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Retina
Direction of light
Retina as viewed through an ophthalmoscope

• Three dimensional-lines posterior 2/3rd of eye
• First neural center for integration of visual
  input
• Captures images in the visual field
• Maps out the first representation of the visual
  field

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Visual Field
External world that can be seen by the two
eyes without movement of the head Divided at
midline into right and left and superior and
inferior halves
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• Images striking center of the field will be seen
  by both eyes
• known as the binocular zone
• Images striking the far periphery of the field
  will be seen by
• that eye only
• known as the monocular zone or lateral crescent

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General Features of the Retina
6 layer structure the consistency of glue-only.02
microns thick Inside out structure light
passes through the first 4 layers to contact the
5th layer of photoreceptor cells- rods and
cones-then makes a return trip 6th layer
consists of retinal pigment epithelium cells
which support and nourish the photoreceptors
Direction of light
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Rod Receptor Cells
Rod Cells

• Concentrated in periphery
• Activate in low illumination
• Detect general form, not details
• Provide background information

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Cone Receptor Cells
Cone Cell

• Capture detail and color
• Require direct stimulation
• Bright light
• Concentrated in fovea

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Macula/ Fovea
Lies opposite the pupil in the center of the
retinal/visual field Composed entirely of
cones Fovea lies in center and is most densely
packed with cones Provides detail vision and
color Has a very limited field of view-only about
4 square inches at a Distance of 8 feet Function
of eye movements is to Keep images focused on the
fovea
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Function of Retina

• Breaks down visual input into its spatial
  components
• Each photoreceptor cell is designed to respond
  only to certain type of stimuli
• Some respond only when light comes on
• Some only with light goes off
• Some only to a certain color wave length
• Blue, red, green
• Some only to certain orientation of line
• Vertical, horizontal, diagonal, curved
• Some only to certain contrast
• Some only to motion

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Function of retina continued

• Each photoreceptor has its own receptive field
  (aka retinal point)
• Area of retina that when illuminated will
  stimulate this cell
• Cell responds only if light strikes in its
  receptive field
• Images create a mosaic of cell activity
• Job of CNS is to make sense of this

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Working together, cells map out the visual world
and project it onto the retinal field

• Retinal field is divided into regions
• Temporal-area next to temples
• Nasal-area next to nose
• Superior-dorsal
• Inferior-ventral
• Because lens of eye invert images onto the retina
• Images in superior visual field are projected
  onto inferior retinal field
• Images in inferior visual field are projected
  onto superior retinal field

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Images in the left visual field are projected
onto the nasal retinal field in the left eye and
the temporal retinal field in the right eye
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Images from the right visual field are projected
onto the nasal retinal field in the right eye and
the temporal retinal field in the left eye
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Retinal Processing Pathway
Axons form Optic nerve
Ganglion cells
Bipolar cells

• Each retina contains 100 million photoreceptor
  cells
• Impulses converge onto bipolar cells
• Converge again onto ganglion cells
• Axons of ganglion cells merge to form the optic
  nerve (CN 2) and exit at optic disc

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• Retinal field is initially just a disorganized
  mess of firing cells
• As input passes back through the four layers of
  the retina, impulses are converged first onto
  bipolar cells, then ganglion cells
• Several rod cells converge onto one bipolar
• But only one cone cell converges onto one bipolar
• provides cone cells with greater sensitivity
• Convergence process refines and compresses the
  image data
• Each retinal point is encoded several times
  through filters that are receptive to objects of
  different size, spatial and temporal
  organization, contrast and color

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• By the time visual input reaches the ganglion
  cells in the last layer, only 1 million pieces of
  visual input remain from the 100 million
  originally captured
• Sufficient to provide the CNS with several
  descriptions of objects in slightly different
  representations
• Axons from the ganglion cells converge at the
  optic disc to form the optic nerve

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Optic Nerve

• Exits eye at the optic disc
• Located just medial to the fovea
• No receptor cells are here so the retinal field
  is inactive
• Creates a 5 degree physiologic blind spot
• Light coming from a single point in the binocular
  zone of the visual field never strikes both blind
  spots so when both eyes are open, never aware
  that you have a blind spot
• Only become aware of the blind spot in specific
  monocular situations

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Optic Nerve

• Cranial nerve II
• Exits eye carrying a map of
• the images contained in the
• visual field
• 1 million fibers in each eye
• Core of nerve contains
• macular/foveal input
• Periphery contains
• peripheral field input
• Travels back towards
• occipital lobe

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At the Optic Chiasm

• Fibers carrying information
• from the nasal retinal field
• in each eye cross over
• and enter the optic tract
• on the opposite side
• Temporal field fibers
• dont cross

Nasal fields
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Basis of Binocular Function

• Before chiasm
• Two separate, independent sets of information
  coming from the eyes
• After chiasm
• Input from the two eyes is merged so that now
  visual processing is integrated and each
  hemisphere is concerned with processing visual
  input from the contralateral half of the visual
  field
• Left hemisphere will process right visual field
  information
• Right hemisphere will process left visual field
  information
• The optic nerve changes names and now becomes the
  optic tract

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Lateral Geniculate Nucleus(LGN)

• Majority (but not all) fibers in optic tract
  synapse in LGN
• Contains orderly map of the
• contralateral visual hemifield
• left LGN-right VF
• right LGN-left VF
• Not all parts of retinal field are equally
  represented
• Contains over representation of central field
• Represents 5 of field but gets half of the LGN
  map
• Because more fibers of optic tract carry central
  field input

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LGN has Two Primary Jobs

• Assists CNS to tune into salient
• features
• Filters out more extraneous
• input and refines image
• Puts two eyes together for
• binocular vision
• 6 layered structure
• Layers alternate maps of contralateral and
  ipsilateral visual fields
• Provides foundation for stereoscopic vision
• Each image has slightly different representation
  because of distance between the two eyes

Signals from ipsilateral eye enter layers
2,3,5 signals from contra- lateral eye enter
layers 1,4,6
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Geniculocalcarine Tracts

• Arise from the LGN and
• terminate in the calcarine
• fissure of the occipital pole
• Carry visual field map of the
• contralateral space
• Two loops
• A Parietal-inferior visual field
• B Temporal-superior visual field
• Temporal loop has more curve
• due to growth of temporal lobe
• Parietal loop fibers have
• straighter line of travel

A
B
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Occipital Lobe
Fibers from the geniculo- calcarine tract
terminate in the V1 area of the occipital
pole Serves as a portal or gateway for visual
input traveling to cortex for processing-all
input must go through here Sorts out incoming
input and dispatches it through either temporal
or parietal circuitry
V 1
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Occipital Lobe

• Made up of a complex layering of cells in
  horizontal and vertical columns
• Each cell in each column is responsible for
  extracting a specific quality about the object
  seen
• Like retinal photoreceptors, some cells respond
  only to movement in a certain direction others
  only to a specific color etc.
• Collectively, the cells in OL determine the
• Spatial orientation of an object
• Brightness
• Form
• Movement direction
• Color

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Occipital Lobe

• Screens out irrelevant and incongruent visual
  input
• Dispatches the rest for cortical processing
  through parietal and temporal lobes

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Cortical Processing

• Job is to enable the use of vision for adaptation
• Cognitive application of vision requires that
  visual input be integrated with other information
  (memory, emotion, other sensory input) in
  prefrontal circuitry of brain
• Visual input takes two routes to the prefrontal
  lobe
• Northern route through posterior parietal
  circuitry and Southern route through posterior
  temporal circuitry

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Posterior Temporal Circuitry

• Visual object center
• Responsible for recognition, classification of
  objects
• Concerned with features of objects
• Color, juxtaposition of line, shape, size
• Puts this information together in a composite
  which is sent to the prefrontal circuitry
• Prefrontal circuitry takes information, compares
  it with memory to check the accuracy of the input
  and decides what to do with the information
• Composite includes appropriate emotional tag
• I like this object, I dont like it

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Temporal Circuitry

• To do its job, temporal lobe must get very
  precise visual input
• Utilizes macular input
• And must obtain that information in an organized
  sequential fashion
• Utilizes selective attention

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Posterior Parietal Circuitry

• Visual spatial center
• Incoming sensory input is integrated to create a
  sensory representation of the contralateral side
  of the body and surrounding space
• Known as an internal map
• Map is used to orient the body to objects in
  space on the contralateral side of the body
• Not a detailed map but a holistic, general
  representation of objects and space

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Posterior Parietal Circuitry

• Close your eyes and visualize the space
  surrounding you
• You have a general idea of objects and their
  relationship to you but you lack specific details
• Objects closest to you can be visualized with the
  most detail
• Receive the largest representation on the map
• And therefore most likely to activate attention
• Because can do you the greatest harm
• If your eyes were open, objects moving towards
  you would also receive greater representation
• Critical that they be detected before they get
  too close.

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Posterior Parietal Circuitry

• To have useful internal map, must get the big
  picture
• Quickly assimilate a lot of information
• Information doesnt need to be very precise
• If a big blurry object is moving towards you, its
  not necessary to identify it as a Ford Explorer
  to know that you need to get out of its way
• Circuitry relies primarily on peripheral visual
  field input

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Prefrontal Circuitry

• Combines visual input with other information for
  cognitive application
• Two important visual centers
• Frontal eye fields
• Dorsal lateral prefrontal cortex

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Frontal Eye Fields

• Direct voluntary search of environment for needed
  objects
• Direct search by anticipating where an object
  will be found
• Utilize visual memory
• Where are fire extinguishers located?
• Where are exit signs found?
• The more experience and knowledge you have with
  an environment, the faster and more efficient you
  will execute visual search

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Dorsal Lateral Prefrontal Cortex

• Supplies short term working visual memory
• Maintain an active memory only as long as needed
  to complete the task/objective
• Example remembering where you parked your car at
  the mall
• DLPC keeps areas in parietal and temporal lobes
  active with information needed to relocate the
  car
• Temporal lobe image of the car color, make etc
• Parietal lobe location of the car
• As soon as the car is relocated, this information
  is dumped from memory
• Important in keeping attention on task and
  maintaining organization

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Limbic Circuitry

• Supplies short term and long term memory circuits
  for storage of visual images
• Many connections between prefrontal areas and
  anterior medial temporal lobe
• Facilitates storage by attaching emotional stamp
  to visual memory
• Strongest, most resilient memories are those of
  events with great emotional relevance
• Babys first smile
• Twin trade towers collapse

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Limbic Circuitry

• Anterior medial temporal lobe
• Assigns motivational relevance to events
  occurring in the contralateral space according to
  past experience and current need
• Objects with greater emotional relevance acquire
  more space on the internal parietal map so that
  they are more likely to engage attention and
  visual search
• A jelly donut sitting on a plate to your left
  side when you are really hungry
• Your dog
• Injury to this area causes indifference to events
  on the contralateral side
• Visual search is not executed

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• All of these cortical structures interact and
  work with each other to provide a coordinated
  network for acquiring and applying visual
  information
• Network is held together by long and short fiber
  tracts that enable continuous communication
  between areas
• Each area of the network contributes a certain
  advantage to visual processing
• No area works alone or independent of other areas
• Damage in any one area will affect the whole
  network
• Various deficits will be observed

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Brainstem

• Fibers break off the visual pathway before the
  LGN and travel to the brainstem
• 12 separate centers in BS are involved in visual
  processing
• Modulate visual reflexes
• Pupillary responses, accommodation, blink reflex
  etc
• Integrate vision with other sensory input
  especially vestibular
• Control cranial nerves involved in vision
• Modulating attention
• Key Structures
• Superior colliculus
• Reticular activating system
• Motor nuclei for cranial nerves 3,4,6

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Superior Colliculus
Midbrain structure-one on each side Responsible
for visual capture of any novel or unexpected
visual stimuli Contains a map of the retinal
field If a novel or unexpected stimulus moves
into the field, the SC captures it and directs
persons attention towards it
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Superior Colliculi

• Responds primarily to movement rather than
  luminosity
• Very attentive to peripheral visual field input
• Response is automatic and reflexive mediated
  through sub cortical channels as a protective
  response
• SC respond to ALL novel stimuli
• Cant inhibit the response
• Each SC directs eye movements towards
  contralateral visual space

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Reticular Formation

• Loose collection of cells
• scattered throughout mid
• brain
• Control autonomic functions
• The Four As arousal
• sequence
• asleep
• awake
• alert
• attending

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Reticular Formation

• Visual system strongly influences arousal
  sequence
• Example might be sitting there ready to fall
  asleep during this lecture when suddenly a bloody
  hand moves out from behind the screen..
• Sight of the hand would instantly engage your
  attention
• The more threatening or important the visual
  input, the greater the arousal
• Visual input also elicits autonomic response
  through this system
• 13 year old girl catches a glimpse of her crush
• Eyes dilate, heart starts pounding, shortness of
  breath

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Reticular Formation

• Important to remember that RF responds most
  strongly to somatosensory and vestibular input
• When working with low functioning patients want
  to provide this input prior to attempting to
  engage visual attention
• Combine movement with visual attention tasks
• RF also contains motor centers for contol of
  conjugate eye movements

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Reticular Formation

• Parapontine reticular formation (PPRF)
• Center for generating saccadic eye movement
• Control center for horizontal gaze
• Involved in execution of all kinds of rapid
  automatic eye movements
• Receives input from cortical, cerebellar areas,
  vestibular nuclei and spinal cord
• Integrates this input and sends impulses to CN 3,
  4, 6 to initiate eye movements
• Injury to this area would wipe out horizontal eye
  movements and disrupt other eye movement

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Cranial Nerves Controlling Extraocular Eye Muscles

• Motor Nuclei are located in
• the brainstem
• CN III Oculomotor
• CN IV Trochlear
• CN VI Abducens
• Receive input from vestibular nuclei, cortex,
  cerebellum

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Summary of Brainstem Role in Visual Processing

• Acts independently of cortex for some functions
• Protective eye responses
• Accommodation
• Reflexive attention to environment
• Works in conjunction with cortex and cerebellum
  for other functions
• Synergistic control of smooth pursuits and
  saccades
• Engagement/direction of voluntary attention

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Brainstem role

• Houses structures that control machinery of the
  extraocular and internal musculature of the eyes
• Nucleus of 3,4,6 cranial nerves
• Works with vestibular system for control of gaze
  stability
• Functions primarily as the soldier not the
  general
• Implements decisions made by others
• Damage it and there is no one to implement the
  commands/desires of the cortex and cerebellum

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Cerebellum (CB)

• Integrates vestibular, proprioceptive and visual
  input to add synergy to control of eye movements
• Puts together information regarding eye position,
  head position, eye velocity,head velocity to
  ensure that coordinated product is delivered
• Requires extensive communication with cortex,
  basal ganglia, brainstem

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Cerebellum continued

• Major role as repair shop for eye movements
• All motor commands/plans go through CB
• If cortical/kinetic system initiate a poor plan,
  cerebellum cleans it up before its executed
• Adapts eye movements to ensure they are
  appropriate for the visual stimuli
• When CB is damaged, plasticity of oculomotor
  system is reduced
• Accounts for longstanding nature of oculomotor
  deficits following cerebellar damage

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Summary of Cerebellar Role in Visual Processing

• Functions as the first lieutenant
• Makes sure that all plans go correctly
• Utilizes a massive spy system to do its job
• Connected to every other part of CNS
• Adds synergy to all movement and thought
• Acts on the rest of CNS through the rostral part
  of the brainstem
• Rostral brainstem is one of the most vulnerable
  areas of CNS
• Damage here mimics CB damage

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Cerebellum

• CB damage is rare but when it occurs, it is long
  lasting and debilitating
• Disruption of the effort, force, direction and
  timing of eye movement is observed
• Dysmetria
• Unable to place eye on target or smoothly tract a
  moving target
• Nystagmus
• Uncontrolled quivering of eye which causes blurry
  vision and reduces visual acuity

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Majority of CNS is Involved in Visual Processing

• Areas are tied together by fiber tracts
• Commissural tracts
• Tie the two hemispheres
• together
• Corpus callosum is the
• largest
• Also many others

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Association Tracts
Localized to a hemisphere or a side of the
brainstem Comprised of short and long
fibers Short fibers tie gyri together
billions of them Long fibers tie lobes
together Two important ones for vision
superior longitudinal fasciculus inferior
occipital frontal fasciculus
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Superior Longitudinal Fasciculus
Carries visual spatial information from occipital
lobe through posterior parietal lobe to frontal
eye fields in prefrontal cortex
Inferior Occipital Frontal Fasciculus
Carries visual object information from occipital
lobe through posterior temporal lobe to
prefrontal
Provide a northern and southern route to
prefrontal cortex for visual processing.
Reciprocal pathway carries input both directions.
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• Pathway system is vulnerable to diffuse axonal
  injury in head trauma
• Fasciculi are superficial, easily injured
• Those on a decided curve will experience more
  shearing and damage

Both of these fasciculus tracts are on
significant curves