Chapter%208%20Visual%20System

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Chapter 8 Visual System

• Chris Rorden
• University of South Carolina
• Norman J. Arnold School of Public Health
• Department of Communication Sciences and
  Disorders
• University of South Carolina

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(No Transcript)
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Apparent motion
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Visual Perception Events

• Refraction of light rays by lens and cornea
• Conversion of electromagnetic energy of light to
  nerve impulse
• Transmission of action potential to CNS
• Perception of visual image in visual cortices

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Terminology

• Optic Nerve
• Visual fibers from retina to optic chiasm
• Optic Tract
• Optic fibers between chiasm to lateral geniculate
  body of thalamus or fibers that bypass thalamus
  to superior colliculus
• Optic Radiation
• Fibers project to visual cortex via
  geniculocalcarine fibers (optic radiation)

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Visual Pathway

• Cortical and subcortical processing

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Visual Field

• Visual Field area you see before you - outside
  world
• Retinal Field Focused representation of visual
  field
• Reversed (right/left, up/down)
• Monocular Visual Field Lateral portion perceived
  in only one eye
• Binocular Visual Field Common area seen by both
  eyes

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Eyeball

• Weighs 7.5 g and 2.4 cm long
• 5/6 in orbital cavity
• Anterior Chamber filled with aqueous humor
• Made by choroid plexus of the ciliary processes
• Drains through canal of Schlemm
• Need to maintain pressure and link to circulatory
  system

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Cavities and Chambers of Eyeball
Anterior chamber Posterior chamber
Anterior cavity
Retina Choroid Sclera
Posterior cavity
Vitreous humor
Fovea
Optic disk
Optic nerve
Macula lutea
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Ocular Layers

• Fibrous Tunic (blue)
• Sclera White of eye
• Cornea Nonvascular and transparent fibrous
  region of eye
• Vascular Tunic (yellow)
• Choroid
• Iris
• Ciliary Body
• Nervous Tunic (red)
• Retina Rods and Cones

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Functions

• Lens
• Focuses images on the Retina
• Ciliary Muscle
• Regulates changes by lens (near and far vision)
• Iris
• Controls pupil size

Aqueous humor
Cornea
Pupil
Iris
Lens
Ciliary body
Vitreous humor
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Pupil, Iris, Scelera

• Pupil
• Iris
• Scelera

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Anatomy of Retina

• Rods
• Night vision
• Cones
• 3 types sensitive to long, medium and short
  wavelength
• Often red, green, blue but actual peak
  sensitivity is yellow, yellowish-green, and blue
• Bipolar Cells
• Ganglion Cells
• Light passes through cell layers and then back to
  the ganglion cells.

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Photo receptors
http//www.webexhibits.org/colorart/ganglion.html
http//web.mit.edu/bcs/schillerlab/research/A-Visi
on/A3-1.html
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Illusions from the retina
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Blindspot

• There are no rods or cones in the optic disk.
• Close your right eye, and look at the 'x' in the
  figure. Move either closer or further away from
  the screen until you notice the that circle with
  the dot inside vanishes altogether.

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Photosensors

• Cones (30 million)
• Discriminate color and sharp vision
• Cone cells in macula lutea
• fovea centralis
• Rods (100 million)
• Discriminate in dim light
• Sensitive to shape and movement
• Lateral peripheral retina
• You can often see things better at night if you
  do not look directly at them!
• We will not cover photochemistry of retina and
  optical mechanism.

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Central Visual Mechanism

• Visual pathway from retina to primary visual
  cortex
• Optic nerve fibers exit optic foramina and move
  to optic chiasm
• Optic tract move to lateral geniculate body
  (Remember it is part of thalamus)
• Travel to occipital lobe to visual cortex

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Visual Pathway

• Each eye sees both left and right visual field.
• Ipsilateral information crosses over at optic
  chiasm.
• Some connections to superior colliculi.
• Reflexive eye movments
• Others go to thalamus (lateral geniculate nuclei)
  and then cortex.

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Retinal Representation

• Nasal and temporal visual fields
• When you are looking at an object, these form the
  medial (nasal) and lateral (temporal) hemifield
  of vision for each eye.
• Reversed to opposite halves of retinal
  representative fields
• Also inverted
• Nasal visual fields project to temporal retinal
  fields and do not cross at optic chiasm
• Temporal visual field project to nasal retinal
  fields and cross at optic chiasm

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Lateral Geniculate Nucleus to Visual Cortex

• Optic Radiation (geniculocalcarine fibers
  Meyers Loop) runs under temporal lobe to
  occipital lobe

Lateral Geniculate Nucleus (Thalamus)
V1 Primary Visual Cortex (BA17)
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Reflexes

• Pupillary Light Reflex
• Involves Edinger-Westphal Nucleus and oculomotor
  CN (III)
• Pupil contracts with light (consensual response)
• Damage to system may be due to Horners syndrome
  (always constricted pupil) or CN III lesion
  damage to afferents to one eye
• Accommodation Reflex The focus reflex
• Modifies lens curvature when object moves closer
  to eyes
• Lens flexibility important
• Lens tends to become less flexible around age 45

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Horners syndrome

• Injury to sympathetic nervous system
• First-order neuron disorder Central lesions that
  involve the hypothalamospinal pathway (e.g.
  transection of the cervical spinal cord).
• Second-order neuron disorder Preganglionic
  lesions (e.g. compression of the sympathetic
  chain by a lung tumor).
• Third-order neuron disorder Postganglionic
  lesions at the level of the internal carotid
  artery (e.g. a tumor in the cavernous sinus).
• ptosis (drooping eyelid), miosis (constricted
  pupil) and dilation lag.

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Clinical Conditions

• Hypermetropia (farsightedness)
• Can see distant objects normally but problem in
  near objects
• Due to short eyeball and inadequate refractory
  power of the lens
• Myopia (nearsightedness)
• Can see near objects but not distant
• Due to abnormally long eyeball and too strong
  refractory power

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Clinical Conditions

• Astigmatism
• Focus disorder of vertical and horizontal rays
• Caused by irregular shape or the cornea, lens, or
  both
• Can typically be corrected with glasses with
  relatively cylindrical rather than dish shaped
  lenses.

Standard Cylindrical
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Clinical conditions

• Color vision disorders (usually males)
• First documented by John Dalton (1798)
• Dichromacy Loss single type of cone, e.g. of
  long (yellow, protanopia), medium (yellow-green,
  deuteranopia) or short (blue, tritanopia)
  wavelength.
• Monochromacy Total color blindness due to
  absence of cones or abnormal cones

Normal
Protanopia
Deuteranopia
Tritanopia
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• Art by Jay Lonewolf Morales

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Other Common Disorders

• Presbyopia - decrease in vision with age
• Cataract - Increase in protein in lens
• Glaucoma - Increased intraocular pressure
• Infections - Inflammation of the eye
• Retinitis Pigmentosa - familial disorder causing
  loss of rod cells. Includes peripheral visual
  loss and night blindness

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Visual defects following stroke

• Damage to early visual centers causes blindness
  (see next slides).
• Damage to temporal/parietal lobes cause
• Neglect failure to respond to contralesional
  stimuli (usually right hemisphere injury)
• Achormatopsia color blindness
• Akinetopsia Motion blindness (very rare)
• Agnosia failure to recognize objects
• Ataxia reaching deficits
• Simultanagnosia only see one thing at a time

Neglect
Agnosia can copy But not recognize
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V1 (BA 17)

• Primary visual cortex (V1) lies in calcarine
  fissure.
• Complete damage leads to Homonymous hemianopia.
• Partial damage leads to scotomas
• Point-to-point mapping with retina.

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Types of Field Defects

• Left optic tract carries info from right visual
  field in each eye
• Right optic tract carries info from left visual
  field in each eye
• Simplified in that some overlapping present

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Types of Field Defects
L
R
A
Monocular blindness
A
B
Bitemporal hemianopsia
B
C
C
Nasal hemianopsia
D
E
D
Homonymous hemianopia
F
E
Homonymous left Superior quadrantopsia
F
Homonymous left Inferior quadrantopsia
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Visual Field Defects

• Homonymous
• Similar regions affected in each eye
• i.e. Right visual fields of both eyes
• Heteronymous
• Different regions affected in each eye
• i.e. Left visual field of one eye and right
  visual field of other eye

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Specific Deficits

• Monocular Blindness Blindness in one eye due to
  optic nerve lesion before optic chiasm
• Bitemporal (Heteronymous) Hemianopsia Loss of
  temporal visual fields of each eye, lesion at
  optic chiasm
• Nasal Hemianopsia Loss of nasal vision in one
  eye due to lesion in lateral edge of optic
  chiasm.
• Homonymous Hemianopsia Loss of left or right
  visual fields for both eyes due to lesion in
  right optic tract
• Upper Left Quadrantanopsia Loss of vision in
  left upper quadrant of each eye due to lesion in
  Meyers Loop
• Lower left Quadrantanopsia Loss of vision in
  lower left quadrant of each eye due to lesion in
  medial fibers of visual tract