Transcript Temporal Aspects of Visual Extinction

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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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
Posterior
cavity
Choroid
Vitreous
humor
Fovea
Sclera
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
Aqueous humor
 Ciliary Muscle
Cornea
– Regulates changes by lens (near and far vision)
Pupil
 Iris
– Controls pupil size
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-Vision/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; Meyer’s
Loop) runs under temporal lobe to occipital lobe
Lateral
Geniculate Nucleus
(Thalamus)
V1 Primary Visual Cortex (BA17) 21
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 Horner’s 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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Horner’s 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
B
C
D
E
F
A
Monocular blindness
B
Bitemporal hemianopsia
C
Nasal hemianopsia
D
Homonymous hemianopia
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 Meyer’s Loop
 Lower left Quadrantanopsia: Loss of vision in lower left
quadrant of each eye due to lesion in medial fibers of visual
tract
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