AnatomyNeuroAnatomy of the Visual System

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Anatomy/Neuro-Anatomy of the Visual System

• Lecture B

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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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The Visual Field

• External world that can be seen by the two eyes
  without movement of the head
• Look straight ahead
• The area that you can see without moving your
  eyes is your visual field
• Divided at midline into right and left and
  superior and inferior halves
• 160-180 degrees horizontally
• 120 degrees vertically

The visual field is created by the retina
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• Each eye has its own visual field and when both
  eyes are open
• the two fields overlap (creates a binocular
  field)
• Images striking center of the visual field are
  seen by both eyes
• known as the binocular zone
• Images striking the far periphery of the field
  are seen by that eye only
• known as the monocular zone or lateral crescent

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Visual Field is Divided into Two Areas

• These two areas reflect the different specialized
  functions of the two types of receptor cells that
  make up the retina

Central visual field is made up of cone cells
Peripheral visual field is made up of rod cells
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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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Retinal Pigment Epithelium (RPE)

• Separated from the choroid layer by a thin layer
  of elastic tissue known as Bruchs membrane
• Cells of the pigment layer receive their
  nourishment by diffusion of the choroid vessels
  through Bruchs membrane
• RPE cells provide nourishment to the rod and cone
  receptor cells of the sensory retina and remove
  waste products created when the photoreceptors
  cells are bleached by light

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Photoreceptor Cells

• Comprise the neural retina
• Two types
• Rods
• Cones
• Both transform light into neural impulses via
  photochemical activation
• Light bleaches the photochemical (aka visual
  pigment) and causes it to break down activating
  the neural impulse
• Photochemical is later reconstituted
• This process is supported by the RPE
• Requires a great deal of energy-high oxygen
  demand thus reason for need for rich vascular
  supply to retina

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Rod Receptor Cells

• Concentrated in periphery of the retina
• Specialized to activate in low illumination and
  to moving stimuli
• Can detect light coming from any direction
• Job is to detect light and general shapes-no
  details
• Provide background information needed to create
  context for a visual scene
• Many more rods than cones

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Cone Receptor Cells
Cone Cell

• Concentrated in the fovea and macula
• Three types red, green and blue
• Each type responds to that specific color of the
  visible electromagnetic spectrum
• Color seen depends on the combination of cones
  stimulated
• Specialized to capture visual details and color
• Provides object identification
• Require direct stimulation and bright light
• Therefore person must be looking directly at an
  object in good lighting to be able to see it
  clearly and identify it

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Macula/ Fovea
Lies opposite the pupil in the central
retinal/visual field approximately 1.5 mm
in diameter Composed entirely of cones Fovea
lies in the center of the macula 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
• Inferior
• Because lens of eye inverts images projected 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

• 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 Summary

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

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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 Summary

• 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
• Requires visual input from macular field
• 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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Posterior Parietal Circuitry

• Visual spatial center
• Requires visual input from peripheral retina
• 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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Prefrontal Circuitry

• Combines visual input from parietal and temporal
  lobes with other information (from other senses
  and from memory) for cognitive application
• Cognitive application includes assessing
  situations, formulating plans, making a decision,
  executing it and then learning from feedback
• Directs visual search for objects
• Wheres Waldo?
• Maintains working visual memory
• Assists in directing and maintaining attention on
  task

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Brainstem Processing of Visual Input

• 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
• Modulate attention
• Key Structures
• Superior colliculus which initiates reflexive
  protective response to visual input
• Reticular activating system which activates
  arousal and attention
• Motor nuclei for cranial nerves 3,4,6 which
  control the extraocular muscles that move the
  eyes

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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 eye movements
• Engagement/direction of voluntary attention

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Brainstem Role cont

• 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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Development of the Visual System
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Visual system develops from three types of tissue

• Neuroectodermal from brain
• Becomes retina, iris and optic nerve
• Surface ectoderm of head
• Forms lens
• Mesoderm
• Forms vascular supply and sclera

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Embryonic Eye Development 3-4 weeks gestation
The eye begins as a groove in the neural fold on
the cranial end of the embryo
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At 4 weeks, the bulb and stalk are fully formed.
The lateral surface of the bulb begins to flatten
(1) and the ectoderm thickens to become the lens
placode (2)
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The placode turns in on itself to form a deep
indentation (the lens pit). The ends of the pit
come to together to form the lens vesicle, which
then is pinched off to become the lens.
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At the same time, the optic vesicle begins to
fold in on itself to form a double walled,
bowl shaped structure called the optic cup.
Optic cup
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The two walls fuse together, the outside wall
becomes the RPE, the inside wall becomes the
sensory retina. The axons of the ganglion cells
converge into the optic stalk to become the
optic nerve.
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Cornea
Retina
Lens
Optic nerve
RPE
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• Rim of optic cup eventually becomes the ciliary
  body and muscle, iris, dilator and sphincter
  muscles
• Mesenchyme cells develop into the choroid and
  sclera-both are extensions of vascular and
  fibrous structures within brain
• Sclera-continuation of dura mater
• Choroid-continuation of pia arachnoid
• Form a sheath around the optic n.

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• The relationship between these structures
  explains why an increase in cerebral spinal fluid
  after brain injury can be diagnosed by observing
  the optic disc for papilledema

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Maturation of Face and Eyes

• As the embryo develops, the eyes migrate from the
  sides to the front as the face matures
• Face is formed by 14 weeks
• During development, structures may fail to fully
  form or to close completely
• Creates many of the congenital eye conditions
  observed in children

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EYES
EYE
EYE
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Maturation of Visual System Pre-natal Post-nat
al

• Rods and Cones
• 25 wks-both begin to develop
• Optic Tract
• 28-38 wks-begins to myelinate
• Superior Colliculus
• Basic structure develops 16-28 wks

• Rods and Cones
• 4 mos-complete with rods finishing first
• Optic Tract
• Rapid myelination first 2 mos continued for 2
  years
• Superior Colliculus
• Myelination completed at 3 mos

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Maturation of Visual System Pre-natal Post-na
tal

• LGN
• Matures after birth
• GC Tracts
• Myelination begins at birth

• LGN
• Process takes 9 mos
• Stereoscopic vision at 3-4 mos
• GC Tracts
• Completed in 4-5 mos

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Maturation of Visual SystemPre-natal Post-natal

• Visual cortex
• 25-28 wks-starts dendritic growth, increasing
  synaptic density, cortical layers develop

• Visual cortex
• Doubles in density first 2 years, adult synaptic
  density and functional maturity by age 11

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Eye Movement

• Child is able to fixate and make basic eye
  movements by 2-3 months
• Takes 2 years to obtain good control
• Up to 9 years to obtain complex control

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Visual Acuity

• Newborn
• 20/200, sees best in 2-75 cm range
• 3 months
• 20/60
• 6 months
• 20/20
• 2 years
• Acute near vision-fine motor skills develop

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Eye Dominance

• Develops sometime in childhood
• Directs fixation
• As resistant to change as hand dominance

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Normal Age Related Changes in Vision
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Reduced Visual Acuity

• Static acuity
• Decreases to 20/30-20/40
• Prevalence 40 by age 70
• Dynamic acuity also decreases reflecting reduced
  gaze stability
• Person experiences more visual blur during head
  movement (as could occur when driving)
• Decrease may be due to reduced OM control

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Loss of Accommodation

• A.k.a. presbyopia
• Result of compacting of protein fibers in center
  of lens
• Lens thickens and loses flexibility
• Occurs gradually beginning in 40s
• Creates need for bifocal

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Floaters

• Strands of protein which float in vitreous
• Float more easily in old eye because vitreous is
  more fluid
• More noticeable in bright light
• Generally benign unless accompanied by bright
  flashes of light or significant increase in number

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Dry Eyes

• Lacrimal glands do not make enough or make poor
  quality tears
• More prevalent in women
• Can be exacerbated by medication
• Causes itchiness, burning, decreased acuity
• Treated with artificial tears or surgery

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Increased Need for Light

• Pupil diameter decreases
• Condition is called senile miosis
• Lens thickens and becomes increasingly yellow
• Combined-these two conditions reduce the amount
  of light coming into eye
• 80 yr old person needs 10x as much light as an
  average 23 year old

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Susceptibility to Glare

• Lens and cornea become less smooth
• Lens vitreous develop protein strands
• Combine to cause light scatter
• Increased discomfort and disability
• Lose acuity under glare condition
• Also takes longer to recover from glare

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Reduced Dark/Light Adaptation

• Takes longer to reform and store visual pigments
  in receptor cells (rods and cones)
• Never reach same level of dark adaptation as
  younger person
• More difficult to go from bright to dark than
  dark to bright

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Reduced Contrast Sensitivity

• Caused by changes in color and density of lens
  and decreased pupil aperture
• 75 year old needs 2x as much contrast as younger
  person
• 90 year old needs 6x as much contrast

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Reduced Color Perception

• Caused by yellowing of lens
• Decrease in sensitivity of retinal cells at
  violet end of spectrum
• May cause white objects to appear yellow

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Reduced Visual Field

• Changes in facial structure
• Nose grows??
• Orbit loses fat and eye sinks in

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Reduced Visual Attention

• Decline in ability to
• Attend to objects in complex, dynamic arrays
• Simultaneously monitor central and peripheral
  visual fields
• Affects driving performance
• Diameter of visual field decreases
• 90 yr olds-40 have an attentional field of less
  than 20 degrees