Vision

I. Background

A. Processes

1. Light energy is transduced into neural activity

2. Neural activity is processed by the brain

 

Note: By way of analogy, you can imagine taking a picture with a camera. The eye is the camera, the retina, which is a specialized part of the brain at the back of the eye, is the film, and the parts of the brain that process visual information is the photoshop.

 

B. Human visual systems permit light reflected off distant objects to be:

1. Localized relative to the individual within his or her environment

2. Identified based on size, shape, color, and past experience

3. Perceived to be moving (or not)

4. Detected in a wide variety of lighting conditions

C. Sequence of events

1. Light entering the eye is focused on the retina

2. Retina converts light energy into neuronal activity

3. Axons of the retinal neurons are bundled to form the optic nerves

4. Visual information is distributed to several brain structures that perform different functions

 

II. Anatomy of the Eye

A. Structural levels

1. Gross anatomy

2. Opthalmoscopic appearance

3. Cross-section anatomy

B. Gross anatomy

1. External features of the eye

††††††††††† a. Pupil--opening that allows light to reach the retina

††††††††††† b. Iris--circular muscle that controls the diameter of the pupil

††††††††††† c. Aqueous humor--fluid behind the cornea

††††††††††† d. Sclera--outermost layer that forms the eyeball

††††††††††† e. Extraocular muscles--attached to the eye and skull and allow movement

††††††††††† f. Conjunctiva--membrane inside the eyelid attached to the sclera

††††††††††† g. Optic nerve--axons of the retina leaving the eye

††††††††††† h. Cornea--transparent surface covering the iris and pupil

 

2. Opthalmoscopic appearance (Retina as seen through the pupil)

 

Aside: in photographs, the red appearance of the eye is actually the retina photographed. Double flash camera causes the pupil to constrict.

 

a. Optic disk (blind spot)--no vision is possible

††††††††††† i. Blood vessels originate here. The vessels shadow the retina

††††††††††† ii. Optic nerve fibers exit here

††††††††††† iii. No photoreceptors

b. Macula--area of the retina responsible for central vision (vs. peripheral)

c. Fovea--center of the retina (where most of the cones are)

3. Cross sectional anatomy

a. Lens--transparent surface that contributes to the formation of images (w/i 9 meters)

b. Ciliary muscles--change the shape of the lens and allow focusing

c. Vitreous humor--more viscous than the aqueous humor

i. Lies between the lens and the retina

ii. Provides spherical shape

††††††††††† d. Retina

i. Inner most layer of cells at the back of the eye

ii. Transduces light energy into neural activity

 

III. Image Formation

A. Processes

1. Refraction by the cornea

2. Accommodation by the lens

3. Pupillary light reflex

B. Refraction by cornea

1. Distant objects

††††††††††† a. Light rays run in parallel

2. Light rays slow

a. Cornea

b. Aqueous humor

3. Light rays bend

a. Perpendicular to the angle (radius of the cornea) between the curve of the cornea and the plane they are traveling on

4. Focal distance

a. Distance between the refractive surface and where the light rays converge

b. Depends on the curvature of the cornea

i. 2.4 cm

ii. Distance between the cornea and the retina

C. Accommodation by the lens

1. Objects within 9 meters

a. Light rays do not travel in parallel

i. Some diverge

2. Lens adds refractive power

a. Provided by changing the shape of the lens

3. Contraction of ciliary muscles

a. Tension on the suspensory ligaments is released

b. Lens becomes rounded

c. Greater the curvature provides greater the refraction

D. Pupillary light reflex

1. Pupil contribute to optical qualities of the eye

a. Adjusts for different light levels

b. Contributes to simultaneous focusing on near and distant objects

2. Accommodation alters light rays that would otherwise run in parallel

a. Light rays are no longer focused on the retina by the cornea

3. Closing the aperture of the pupil

a. Only light rays that are primarily in the center of the cornea and lens are allowed in

b. These are generally not focused

c. Permits seeing things in the foreground and background in focus

E. Additional terms and concepts

1. Visual field

a. Total space that can be viewed by the retina

i. 150 degrees

ii. 90 on temporal side

iii. 60 on the nasal side

2. Image formed on the back of the retina is reversed and inverted

3. Emmetropia (normal vision)

a. Parallel light rays are focused on the retina without accommodation

4. Hyperopia (farsightedness)

a. Eye ball is too short

b. Image is focused at a point behind the retina

c. Lens can accommodate for distant objects but not for near

d. Condition can be corrected with a convex lens (e.g., increase refractive power)

5. Myopia (nearsightedness)

a. Eye ball is too long

b. Light rays converge in front of the retina

c. Lens can accommodate for near objects but not distant

d. Condition can be corrected with a concave lens

 

III. Microscopic Anatomy of the Retina

A. Specialized cells of the retina convert light energy into neural activity

B. Cellular architecture of the retina

 

1. Cell types

††††††††††† a. Photoreceptors--the only light sensitive cells in the retina

i. Transduce light energy into neural signals

††††††††††† b. Bipolar cells--connect photoreceptors to ganglion cells

††††††††††† c. Ganglion cells--fire action potential and send axons to the brain

d. Horizontal cells--receive inputs from photoreceptors and project laterally to bipolar cells

e. Amacrine cells--receive inputs from bipolar cells and project laterally to ganglion cells

2. Layers (3 primary, but there are subdivisions)

††††††††††† a. Ganglion cell layer--cell bodies of the ganglion cells

††††††††††† b. Inner nuclear layer--cell bodies of the bipolar cells

††††††††††† c. Outer nuclear layer--cell bodies of the photoreceptors

3. Characteristics:

††††††††††† a. Photoreceptors are the only cells that respond to light

††††††††††† b. Ganglion cells are the only output cells

††††††††††† c. Light travels through the other cell layers to reach the photoreceptors

d. At the back of the eye is a pigmented epithelium that absorbs any light not absorbed by the photoreceptors

 

Side point: Inside many mammalian eyes, there is an additional layer of cells between the photoreceptors and the epithelial layer that reflects the light back out again. The photoreceptors have two opportunities to be exposed--greatly enhances night vision.

 

C. Photoreceptors--two kinds based on appearance and function

 

1. Rods--long, cylindrical, many disks

††††††††††† a. Photopigment is in the disk

††††††††††† b. Rods have a much higher pigment concentration

††††††††††† c. 1000x more sensitive to light than cones

††††††††††† d. Function in scotopic conditions

i. Nighttime lighting

††††††††††† e. All rods have the same pigment

i. Rhodopsin

2. Cones--shorter, tapering outer segment, relatively few disks

††††††††††† a. Photopic conditions

i. Daytime lighting

ii. Primarily cones

††††††††††† b. 3 different types of cones based on type of photopigment

i. Pigments are differentially sensitive to wavelength of light

3. Retina is therefore a duplex

a. Scotopic retina using only rods

b. Photopic retina using primarily cones

 

4. Distribution of rods and cones

a. Rods and cones are distributed regionally

b. Center of the eye (i.e., the fovea)

i. Only cones

††††††††††† c. Peripheral retina

i. Primarily rods

ii. Few cones

5. Connectivity

††††††††††† a. Central retina

i. 1:1 (approximately) correspondence between photoreceptor and ganglion

††††††††††† b. Peripheral retina

i. Many photoreceptors (rods) converge on a single output ganglion cell

††††††††††† c. Peripheral retina is more sensitive to light

 

IV. Phototransduction

A. Photoreceptors transduce (change) light energy into changes in membrane potential

1. Analogous to transduction of chemical signals into electrical signals that occurs during synaptic transmission at G-protein coupled receptors

2. Events at G-protein coupled receptors

††††††††††† a. Binding of NT activates G-proteins

††††††††††† b. G-protein activation stimulates various effector enzymes

††††††††††† c. Enzymes alter the intracellular concentration of cytoplasmic second messengers

d. 2nd messengers either directly or indirectly alter membrane ion channels which alter membrane potential

3. Events during phototransduction

††††††††††† a. Light stimulation of photopigment activates G-proteins

††††††††††† b. G-proteins activate various effector enzymes

††††††††††† c. Enzymes decrease intracellular concentrations of 2nd messengers (cGMP)

††††††††††† d. Change in 2nd messenger concentration closes a Na+ channel

 

B. Functional considerations

1. In complete darkness there is a steady influx of Na+ which depolarizes the photoreceptor membrane

††††††††††† a. Movement of + charge across the membrane is called the dark current

2. Na+ channels responsible for this current are gated by cGMP (cyclic guanosine monophosphate)

††††††††††† a. cGMP is produced continually in photoreceptors

i. Na+ channels stay open in the dark

3. In the light

a. cGMP is converted to GMP (phosphodiesterase hydrolyxes cGMP)

b. Membrane hyperpolarizes in response to light

††††††††††††††††††††††† i. Na+ channels close

4. Rhodopsin

††††††††††† a. Photopigment

††††††††††† b. Located in stacked disks in the outer segment of the rods

c. Comprised of retinal and opsin

††††††††††††††††††††††† i. Opsin absorbs light

5. Bleaching

††††††††††† a. Photoreceptors no longer respond at particular light intensities

b. Activation of rods by light bleaches the photopigment

i. Changes the wavelengths absorbed by rhodopsin

6. Cones also contain opsins

††††††††††† a. Three different opsins

i. Each maximally activated by different wavelengths of light

ii. Blue--430 nm

iii. Green--530 nm

iv. Red--560 nm

††††††††††† b. All colors are created by mixing the proper ratio of red, green and blue

c. Colors are assigned by the brain based on a comparison of the readout of the three cone types

i. White results from equal activation of all three

C. Dark and light adaptation

 

Note: Most of have had the following experiences. Get up at night and turn on the bathroom light; leave a brightly lit room to go down the basement when there are no lights on. Remember there are two different visual systems--one for daytime that utilizes all cones and one for nighttime that utilizes all rods. There is a time course necessary for the photoreceptors to "come on line".

 

1. Changes associated with adaptation

a. Pupil diameter changes

b. Regeneration (or generation) of unbleached (bleached) rhodopsin

c. Change the functional circuits to allow 1:1 rod to ganglion or reverse that to allow 1:1000

 

V. Connectivity in the Retina

A. Communicating changes in photoreceptor function to the brain

1. Only ganglion cells fire action potentials

††††††††††† a. Axons of ganglion cells form the optic nerve

B. Photoreceptor receptors to ganglion cells

1. Light energy (or its absence) is transduced into a chemical signal.

a. In response to dark, photoreceptors are depolarized and release NT (glutamate).

2. Photoreceptors make synaptic contact with bipolar cells either directly or indirectly via horizontal cells.

3. Bipolar cells, in response to the glutamate released by photoreceptors, are either depolarized or hyperpolarized.

a. Based on their response to glutamate, bipolar cells can be classified as:

i. OFF cells ("off" refers to light being off) depolarize when there is no light. In darkness, the glutamate released by the photoreceptor causes an EPSP in the bipolar cell

ii. ON cells ("on" refers to light being on) hyperpolarize when there is no light (they depolarize when there is light)

b. In darkness, the glutamate released by the photoreceptor causes an IPSP in the bipolar cell

C. Ganglion cells

1. Output neurons of the retina

2. Types

††††††††††† a. M-type ganglionólarge

††††††††††† b. P-type ganglionósmall

 

VI. Neural Circuitry

A. Pathway

1. Retina to LGN (lateral geniculate nucleus of the thalamus)

2. LGN to the primary visual cortex

3. Primary visual cortex to other cortical areas

B. Connection between eyes and brain

1. Optic nerve, optic chiasm, optic tract

2. Functional considerations

††††††††††† a. Information from the right visual field crosses to the left side of the brain

††††††††††††††††††††††† i. Decussation

††††††††††† b. Information from the left crosses to the right

††††††††††† c. Not all information crosses

††††††††††††††††††††††† i. Partial decussation

C. General considerations

1. Left and right visual worlds are processed contralaterally

††††††††††† a. Information about the left visual field is processed by the right side of the brain

††††††††††† b. Information about the left that is seen by the right eye does not cross over

2. Right and left eyes perceive parts of both visual worlds

3. Image is inverted and reversed

 

Exercise

Look straight ahead. Imagine a vertical line dividing the right and left side.

1.†††††††† Objects appearing to the left are in the left visual hemifield.

2.†††††††† Objects appearing to the right are in the right visual hemifield.

 

Close your left eye. Your right eye sees part of the left visual hemifield.

Remember that images as seen on the retinal are reversed. Objects in the temporal part of the left hemifield are focused onto the nasal retina of the left eye. Objects in the nasal part of the right hemifield are focused on the temporal retina of the left eye. The temporal retinal output does not cross over.

 

D. Target of optic tract

1. Primary target is the LGN

2. 10% goes to the superior colliculus in the midbrain

3. Hypothalamus (SCN-circadian rhythm)

4. Pretectum-reflex control of the pupil and lens

E. Lateral geniculate nucleus (LGN)

1. Part of the dorsal thalamus

2. Arranged in six (6) layers (Draw--bended knee; 6 dorsal, 1 ventral)

3. Layers 1 + 2 (most ventral) contain large neurons and are referred to as magnocellular LGN layers.

4. Layers 3 - 6 contain small neurons and are referred to as parvocellular LGN layers.

5. The information from the two separate eyes is kept separate by projecting to different layers of the LGN

a. Remember the nasal retinal sees the temporal part of the hemifields.

i. This information crosses over

b. The temporal retina of the opposite eye sees the retinal part of the opposite hemifield

i. This information does not cross over

F. Connection between retina and LGN

(Given the left hemifield)

1. Nasal retina projects to layers 1, 4 and 6

a. Information about the temporal part is seen by the nasal retina of the left eye

i. This information goes to the right LGN layers 1, 4 and 6

2. Temporal retina projects to layers 2, 3 and 5

††††††††††† a. Information about the nasal part is seen by the temporal retina of the right eye

††††††††††††††††††††††† i. This information goes to the right LGN layers 2, 3 and 5

G. Specificity of projections based on ganglion type

1. M- and P-type ganglion cells

††††††††††† a. M-type cells are large and they project to the magnocellular layers (1 or 2)

b. P-type cells are small and they project to the parvocellular layers (3, 4, 5 or 6)

 

Exercise

Where would a P-type ganglion cells in the right nasal retina project?

(Left LGN layer 4 and 6)

Where would a P-type ganglion cells in the left temporal retina project?

(Left LGN layer 3 and 5)

Where would a M-type ganglion cells in the right nasal retina project?

(Left LGN layer 1)

Where would a M-type ganglion cells in the left temporal retina project?

(Left LGN layer 2)

 

H. Connection between LGN and primary visual cortex

1. Cortical organization

a. Arranged in a number of layers

i. 6 layers

b. Layer IV is subdivided into three separate layers--IVA, B, and C.

2. LGN projects primarily to layer IVC

3. Layer IVC is divided into two tiers

a. Alpha

b. Beta

4. Magnocellular LGN layers project to IVC alpha

5. Parvocellular LGN layers project to IVC beta

 

Exercise

Now where would a P-type ganglion cells in the right nasal retina project?

 

VII. Higher Level Cortical Processing

Not Discussed