2: The Beginning of the Perceptual Process

Question 1

The distance between the peaks of the electromagnetic waves is called what?

Answer 1

Wavelengths.

Question 2

Short, medium, and long wavelengths appear as what colours?

Answer 2

Short: blue
Medium: green
Long: yellow, orange, red

Question 3

Light reflected from objects in the environment enters the eye through the _____ and is focused by the _____ and _____ to form sharp images of the objects on the _____, the network of neurons that covers the back of the eye and that contains the receptors for vision.

Answer 3

Pupil, cornea, lens, retina.

Question 4

What are the outer segments with regards to rods and cones?

Answer 4

Part of the receptor that contains light-sensitive chemicals called visual pigments that react to light and trigger electrical signals.

Question 5

Signals from the receptors flow through the network of neurons that make up the retina and emerge from the back of the eye in what?

Answer 5

Optic nerve.

Question 6

Describe the fovea.

Answer 6

Contains only cones. When we look directly at an object, the object’s image falls on the fovea. Contains ~1% of 6 million cones in retina.

Question 7

What is distinct about the peripheral retina?

Answer 7

Contains many more rods than cones. There are 120 million rods vs. 6 million cones in the retina.

Question 8

What is macular degeneration?

Answer 8

Destroys cone-rich fovea and a small area that surrounds it. Creates blind region in central vision, so when looking directly at something, lose sight of it.

Question 9

What is retinitis pigmentosa?

Answer 9

Degeneration of the retina that is passed from one generation to the next. First attacks peripheral rod receptors, results in poor vision in peripheral visual field. Severe cases attack foveal cone receptors = complete blindness.

Question 10

What are two reasons why we are not aware of our blind spot?

Answer 10

Located off to the side of our visual field, where objects are not in sharp focus. Mechanism in the brain “fills in” the place where the image disappears.

Question 11

Regarding focusing images on the lens, describe the cornea and the lens.

Answer 11

Cornea: provides ~80% of focusing power, cannot change shape.

Lens: provides ~20% of focusing power. Thin lens = less light bending (far objects); fat lens = more light bending (near objects).

Question 12

Shape of lens is controlled by _____, which lens is attached to with _____.

Answer 12

Ciliary muscles, zonule fibers.

Question 13

Relaxed lens = _____

Contracted lens = _____

Answer 13

Thin lens, fat lens.

Question 14

What is presbyopia? What is the solution to it?

Answer 14

Lens get harder as we age, maximum contraction of ciliary muscles declines, near point moves farther away.

Solution: bifocal glasses.

Question 15

Disc membranes are covered in opsins. Rods and cones have what respective opsin?

Answer 15

Rods = rhodopsin
Cones = cone opsins (small, medium, large)

Question 16

Isomerization of retinal causes a conformational change on rhodopsin that does what?

Answer 16

Exposes a binding pocket which interacts with G protein transducin.

Question 17

What is the transduction cascade?

Answer 17

Light-activated opsin → transducin → phosphodiesterase → cGMP → closes CGMP-gated channels → hyperpolarization

Question 18

How can we detect a single photon of light?

Answer 18

Single isomerization of retinal starts an enzymatic cascade.

Question 19

Describe the two stage dark adaptation curve?

Answer 19

Sensitivity increases for 3-4min, then levels off. Sensitivity increases again after 7-10min.

Question 20

Describe cone adaptation vs. rod adaptation.

Answer 20

Cone adaptation: sensitivity increases for 3-4min, levels off.

Rod adaptation: sensitivity gradually increases over 20min.

Question 21

Why is the time course of dark adaptation of rods different from cones?

Answer 21

Pigment bleaching and regeneration: when retinal absorbs from light, separates from opsin. Before bleached molecule can absorb light again, retinal must reattach to opsin (enzyme dependent).

Cone pigment: 6min regeneration
Rod pigment: 30min regeneration

Question 22

William Rushton developed what in the 1950s? What did it find with regards to dark and bleached retinas?

Answer 22

Retinal densitometry: measure visual pigment in living subjects.

Dark retina: little light reflected out of eye
Bleached retina: more light reflected out of eye

Found time-course of dark adaptation and pigment regeneration was the same.

Question 23

Describe rods and cones in terms of sensitivity and intensity.

Answer 23

Cones: low sensitivity specialized for high intensity situations.

Rods: high sensitivity specialized for low intensity situations.

Question 24

What are the peak sensitivities of rods and cones?

Answer 24

Rods: 498-507nm
Cones: 555-560nm

Question 25

What is the Purkinje shift?

Answer 25

When we go from cone to rod vision (dark adaptation), we become more sensitive to short wavelengths.

Question 26

Why must there be a lot of convergence in visual perception?

Answer 26

120m rods, 6m cones, 1m ganglion cells.

Question 27

Between rods and cones, which is responsible for more convergence? Why

Answer 27

Rods. More sensitive than cone systems, less acute.

Question 28

Photopic vision is _____ dominated, mesopic vision is used by _____, scotopic vision is _____ dominated.

Answer 28

Cone, rods and cones, rod.