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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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Apparent motion

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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 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 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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Pupil, Iris, Scelera

Pupil IrisScelera

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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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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

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

27Art by Jay Lonewolf Morales

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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