Visual Processing

Question 1

What are the mechanisms that protect eyes from injury?

Answer 1

Eyeball is sheltered by bony socket in which it is positioned; eyelids (act like shutters to protect eye from environmental hazards); eyelashes (trap fine, airborne debris such as dust before it can fall into eye); tears (continuously produced by lacrimal glands and conjunctiva; lubricate, cleanse, bactericidal)

Question 2

Sclera

Answer 2

One of three tissue layers (with cornea) that encloses eye; tough outer layer of connective tissue; forms visible white part of the eye

Question 3

Cornea

Answer 3

One of three tissue layers (with sclera) that encloses eye; anterior, transparent outer layer, allows passage of light rays

Question 4

Choroid body

Answer 4

One of three tissue layers that encloses eye; middle layer underneath sclera which contains blood vessels that nourish retina; contains dark pigment epithelium (melanin) behind the retina; also forms ciliary body, suspensory ligaments and iris

Question 5

Retina

Answer 5

Innermost layer under choroid; consists of: outer pigment cells; rods and cones; primary sensory neurons, and axons of visual nerve fibres

Question 6

Iris

Answer 6

Controls amount of light entering eye; contains two sets of smooth muscle (circular muscle: constrictor; radial muscle: dilator)

Question 7

Pupil

Answer 7

Opening through which light enters the eye

Question 8

Lens

Answer 8

Focuses light

Question 9

Anterior chamber

Answer 9

One of 3 fluid-filled cavities that makes up interior; between cornea and lens; filled with aqueous humor

Question 10

Posterior chamber

Answer 10

One of 3 fluid-filled cavities that makes up interior; between iris and lens; filled with aqueous humor

Question 11

Vitreous chamber

Answer 11

One of 3 fluid-filled cavities that makes up interior; behind the lens, largest chamber; filled with vitreous fluid, a gelatinous mass

Question 12

Tapetum Lucidum

Answer 12

Reflective layer found in many vertebrates (not humans) to replace some of the melanin in the choroid; reflects light back towards retina after its passed the photoreceptors; improves sensitivity of vision under low light, but may cause some blurriness

Question 13

Dilating/constricting the pupil

Answer 13

Changing pupil size controls the amount of light entering the eye (optimization for light and dark conditions; gives the eye a wide “dynamic range”); also controls depth of field (small aperture gives large depth of field, large aperture reduces depth of field; close objects viewed with constricted pupils, so if object moves short distance it stays in focus)

Question 14

What controls pupil size?

Answer 14

ANS (parasympathetic -> ACh; sympathetic -> NE)

Question 15

What has to happen in order to have clear vision?

Answer 15

The focal point must fall on the retina

Question 16

Accommodation

Answer 16

Process of focusing; change in strength of lens; fast: only takes 350ms to change focus from far to near; accomplished by action of ANS on ciliary muscle (PNS -> ACh; ACh causes contraction of ciliary muscle cells, and constriction of the ring -> leads to increasing lens curvature for close objects; decreasing parasympathetic tone relaxes muscle. causes a flatter lens for distant objects)
*natural shape of lens is strong and rounded; when ciliary muscle is relaxed, it pulls the lens to a flatter, weaker shape

Question 17

Myopia

Answer 17

Near-sightedness; occurs when the focal point falls in front of the retina

Question 18

Hyperopia

Answer 18

Far-sightedness; occurs when the focal point falls behind the retina

Question 19

In what order will light pass through the retina?

Answer 19

Fibres of optic nerve -> ganglion cells -> amacrine cell -> bipolar cell -> horizontal cell -> cone -> rod

Question 20

Rods

Answer 20

More of them; grayscale vision; high sensitivity (night vision, low acuity); more convergence onto ganglion cells; mainly located in peripheral retina

Question 21

Cones

Answer 21

Not as many; colour vision (at least 3 varieties); lower sensitivity (high acuity, day vision); less convergence; mainly located in the fovea

Question 22

Macula

Answer 22

Area of a high concentration of cones; has less convergence of photoreceptors to ganglion cells than peripheral areas; directly opposed to pupil so light strikes there; within macula at the fovea light strikes the receptors directly because overlying neurons are pushed aside (very highest density of cones is at the fovea and there is very little convergence of sensory neurons

Question 23

Phototransduction

Answer 23

Changing visual information into APs

Question 24

How does phototransduction occur in darkness?

Answer 24

Rhodopsin is inactive; levels of cGMP are high (cGMP: cyclic nucleotide, intracellular messenger); cyclic nucleotide gated (CNG) Na+/Ca2+ channels and K+ channels are open (cGMP binds to CNG channels and causes them to open); cells are constantly depolarized by entry of Na+ and Ca2+ (tonically release transmitter onto bipolar cells)

Question 25

How does phototransduction occur in light?

Answer 25

Light bleaches rhodopsin and activates it; activated rhodopsin activates transducin (G-protein); transducin activates phosphodiesterase (PD), an enzyme that degrades cGMP; decrease in cGMP causes CNG channels to close, hyperpolarizing the cell (K+ channels still open); causes less transmitter to be released onto bipolar cells

Question 26

Information flow path of the visual pathway

Answer 26

Photoreceptors -> bipolar cells -> ganglion cells (optic nerve) -> optic chiasm -> optic tract -> thalamus -> cortex

Question 27

Amblyopia

Answer 27

Lazy eye; can result from weakness of ocular muscles; if left untreated in young children, proper development of visual cortex will not occur; treatment often includes patching the good eye (strengthens weak muscle; forces the brain to use the information from the weaker eye; strengthens synaptic inputs from the weak eye and allows proper visual cortex development)