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Transcript of 1 Chapter 8 Visual System Chris Rorden University of South Carolina Norman J. Arnold School of...
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Chapter 8 Visual System
Chris RordenUniversity of South CarolinaNorman J. Arnold School of Public HealthDepartment of Communication Sciences and DisordersUniversity of South Carolina
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Visual Perception Events
Refraction of light rays by lens and corneaConversion of electromagnetic energy of light
to nerve impulseTransmission of action potential to CNSPerception 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 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 long5/6 in orbital cavityAnterior 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 cavity
Posterior cavity
Anterior chamberPosterior chamber
Vitreous humor
Fovea
Macula lutea
Optic disk
Optic nerve
Retina
Choroid
Sclera
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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
Pupil
Lens
Vitreous humor
Ciliary body
Iris
Cornea
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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://web.mit.edu/bcs/schillerlab/research/A-Vision/A3-1.html
http://www.webexhibits.org/colorart/ganglion.html
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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
LateralGeniculate 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 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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Other Common Disorders
Presbyopia - decrease in vision with ageCataract - Increase in protein in lensGlaucoma - Increased intraocular pressure Infections - Inflammation of the eyeRetinitis 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 copyBut 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
AC
D
B
EF
F
E
D
C
B
AL R Monocular blindness
Bitemporal hemianopsia
Nasal hemianopsia
Homonymous hemianopia
Homonymous left Superior quadrantopsia
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