Visual acuity

the ability to detect and recognize small objects

What does visual acuity depend on?

The refractory (focusing) power of the eye's lens system and the cytoarchitecture of the retina

Visual acuity is measured under:

high illumination, smallest size a dark object in light background can be correctly identified

Color vision

the ability to detect differences in the wavelength of light

Trichromatic visual system includes which colors:

Red, green, and blue

Vision in the visual field center (the macula):

*four properties

1. Best under HIGH illumination

2. Greatest visual acuity and color sensitivity

3. Ten times better than in periphery

4. Represents the operation of the photopic subsystem

Vision in the peripheral visual field:

*four properties

1. More sensitive to dim light

2. Operates best under LOW illumination

3. Has little color sensitivity and poor spatial acuity

4. Represents the operation of the scotopic subsystem

Binocular Fusion

The process of producing a single image from the two disparate monocular images

Depth perception

The binocular disparity between two images

Strabismus

one eye deviates from the normal position or is paralyzed

Amblyopia

Central Blindness. The image from the deviant eye is no longer represented at cortical levels of the nervous system.

Uncorrected, long-term amblyope is

Functionally blind in one eye and has poor depth perception

The temporal hemifield of the left eye is projected onto the

nasal half of the left eye's retina

The nasal hemifield of the right eye is projected onto the

temporal half of the right eye's retina

The left hemifields of both eyes are projected on to the

corresponding right halves of the two retinas

If a viewed object is brought closer to the eye,

the greater the refractive power required to focus the light rays on the retina

The cornea has a

fixed refractive power

Change in the refractive properties of the eye is called

the accommodation process

Presbyopia

Normal distance vision but lens accommodation is reduced with age. Lens loses elasticity and becomes solid mass.

Hyperopia

Refractive power of the eye's lens system is too weak or eyeball too short. "far-sighted"

Myopia

Refractive power of the eye's lens system is too strong or eyeball too long. "near-sighted"

Astigmatism

Cornea surface does not resemble to surface of a sphere. Distant and near objects cannot be focused on the retina.

The retina is the innermost coat of the eye and consists of

the retinal pigment epithelium and neural retina

Neural retina contains five types of neurons:

1. Visual Receptors (Cones & Rods)
2. Horizontal
3. Bipolar
4. Amacrine
5. Retinal Ganglion

What are the layers of the retina?

1. Retinal Pigment
2. Receptor
3. Outer Nuclear
4. Outer Plexiform
5. Inner Nuclear
6. Inner Plexiform
7. Retinal Ganglion
8. Optic Nerve Layer

Retinal Pigment Epithelium

Provides critical metabolism and supportive functions to photoreceptors

Receptor Layer

Contains light sensitive outer segments of the photoreceptors

Outer Nuclear Layer

Contains the photoreceptors' cell bodies

Outer Plexiform Layer

Where the Photoreceptor, Horizontal, and Bipolar cells synapse

Inner Nuclear Layer

Contains the Horizontal, Bipolar, and Amacrine cell bodies

Inner Plexiform Layer

Where the Bipolar, Amacrine, and Retinal Ganglion cell bodies synapse

Retinal Ganglion Cell Layer

Contains the Retinal Ganglion Cell bodies

Optic Nerve Layer

Contains the Ganglion Cell axons traveling to the optic disc

Rods are responsible for the:

What are 4 characteristics?

Scotopic Visual Process.

1. Contain photopigment rhodopsin, breaks down bandwidth of colors, achromatic.
2. Longer outer segments, more outer segment disks, more photopigment
3. More sensitive to light and function at scotopic (LOW) levels of illumination
4. Dominates peripheral retina, color insensitive, poor acuity

Cones are responsible for the:

What are 6 characteristics?

Photopic Visual Process.

1. Contain photopigment that breaks down limited bandwidth of light, chromatic
2. Color Sensitive
3. Less sensitive to light and require high levels of illumination
4. Concentrated in the Fovea
5. Central visual field is projected on them
6. Responsible for photopic, light-adapted vision, high visual acuity, and color vision

Can cause permanent blindness.

Results from liver damage.

Is critical in the synthesis of photopigment.

Produces degeneration of photoreceptors with visual symptoms first presenting as "night blindness".

Vitamin A deficiency

An inherited disorder in which there is a gradual and progressive failure to maintain receptor cells.

The rods do not contain sufficient rhodopsin and do not function as the low illumination receptors.

"Night blindness" and loss of peripheral vision.

Retinitis Pigmentosa

Leading cause of blindness in the elderly.

In the dry form, intraocular proliferation of cells in this area.

In the wet form, capillaries of the choroid coat invade the macular area and destroy receptor cells and neurons.

Visual loss is in the central visual field.

Macular Degeneration

When the neural retina is torn away from the retinal pigment epithelium, there is a loss of vision in the area of detachment.

Retinal Detachment

Microaneurysms and punctate hemorrhages in the retina.

Tiny swollen blood vessels or bleeding in the underlying choroid coat damage the receptor cells and retinal neurons.

Result in blindness in the regions affected.

Diabetic Retinopathy

How does the light-sensitive receptors of the eye convert the image projected onto the retina? (A lot of steps starting from receptors and ending in retinal ganglion cells)

Receptors synapse with bipolar and horizontal cells. Establishes basis for brightness and color contrasts.

Bipolar cells synapse with retinal ganglion cells and amacrine cells. Enhances contrast effects that support form vision and establish the basis for movement detection.

Info from eye is carried by axons of the retinal ganglion cells to the midbrain and diencephalon.

Axons of the 3rd order retinal ganglion cells form the

optic nerve fiber layer of the retina

3rd order afferents exit the eye and form the

optic nerve

Fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina decussate in the _____ to the _____ optic tract.

1. Optic Chiasm

2. Opposite

The temporal hemiretina of the left eye and the nasal hemiretina of the right eyes both have projected on them the

Right halves of their respective visual fields.

The axons in the optic tract terminate in 4 nuclei within the brain.

1. Lateral Geniculate Nucleus of the thalamus
2. Superior Colliculus of the midbrain
3. Pretectum of the midbrain
4. Suprachiasmatic nucleus of the hypothalamus

1. For visual perception
2. For control of eye movements
3. For control of the pupillary light reflex
4. For control of diurnal rhythms and hormonal changes

Beyond the optic chiasm, the corresponding visual hemifields of the two eyes are represented in the contralateral side of the visual pathway.

The left hemifield of both eyes are represented in the _____

right optic tract.

Visual field defects

areas of loss of vision in the visual field

Example #1

• Abnormality detected in his left fundus.
• Visual loss is limited to left eye.
• Peripheral a scotoma in the temporal hemifield, vision loss that doesn't follow the boundaries of the quadrants
• Associated with retinal abnormalities in the nasal hemiretina of the left eye

Conclusion?

• Retinal damage in the left eye
• Damage located in the nasal half of the retina
• Damage related to diabetic retinopathy

Example #2

• Severe vision loss in his left eye, no abnormalities in the eye.
• Right eye's visual field is normal but large visual loss encompassing nearly all of the left eye's visual field

Conclusion?

• Visual defect is retrobulbar, beyond retina
• Limited to optic nerve damage since only one eye is affected

Example #3

• Bitemporal hemianopia, loss of vision in temporal hemifields of both eyes.
• Visual loss is not related to changes in the retina of either eye
• Involves vision in both eyes
• Encompasses only the temporal

Conclusion?

• Damage is retrobulbar
• Involves the optic chiasm
• Fibers of the optic nerve that originate from the nasal half of the retina decussate in the optic chiasm to the opposite tract. These carry info from about objects in the temporal hemifields of both eyes.

Example #4

• No abnormalities, visual loss in the right hemifields of both eyes
• Right homonymous hemianopia, with macular sparing
• Not related to retina damage
• Involves field losses for both eyes
• Involves right hemifields
• Homonymous = congruent
• Spares central visual field

Conclusion?

• Retrobulbar, Retrochiasmatic
• Involves left calcarine cortex
• May involve hemorrhage from branch of left posterior cerebral artery
• Look at lesion 4 in figure 15.14

Example #5

• Suffered stroke & went in for eval
• Is not blind
• Does not have deficits in detecting location or movement of objects
• Does not exhibit symptoms of neglect
• Exhibits deficits in higher visual processing involving color and object recognition

Conclusion?

• Related to damage in the inferior temporal gyrus
• Involving branches of posterior cerebral artery that supplies inferior temporal gyri

Trauma to left temporal lobe

Contralesional superior quadrantanopia with macular sparing

The sublenticular optic radiation fibers carry information about the contralateral superior quadrant of the visual fields and loop through the temporal lobe.