Visual Pathway

Question 1

What is the range of wavelengths in visible light?

Answer 1

400 - 700 nm

Question 2

Where does light become neural pathways?

Answer 2

optic nerve

Question 3

What is considered the “relay station”?

Answer 3

The LGN (lateral geniculate nucleus) in the thalamus

Question 4

What are the two systems that occur once information is passed from the thalamus to the cortex?

Answer 4

1) The “What” System: identifying objects, understanding the different patters and structures
2) The “Where/How” System: spatial processing

Question 5

What is transduction?

Answer 5

The process of physical energy from light being translated into neural signal

Light energy –> Chemical reaction –> Electrical signals

• mediated by receptors (cones and rods)
• “bleaching” in photopigments

Question 6

When does light not move in a straight line?

Answer 6

When it travels through objects that bend it (REFRACTION)

e.g. done by the cornea and the lens

Question 7

Why is refraction important for vision

Answer 7

The lens must focus and perfectly converge light onto the retina
(ACCOMODATION)

Question 8

How does the lens manipulate refraction?

Answer 8

Through accommodation, which involves changing the shape of the lens by contracting or relaxing the ciliary muscles that are attached to the lens

(under control of the motor system in the brain)

Question 9

What are the two ways that images on the retina are inverted?

Answer 9

1) upside down

2) left to right

Question 10

What is isomerization?

Answer 10

• receptors contain many pigment molecules
• one photon is absorbed by a pigment molecules, causing it to become ISOMERIZED and change shape
• a chain reaction occurs where other pigment molecules are also isomerized
• leads to the receptor generating an electrical/neural signal

Question 11

What is another word for isomerization?

Answer 11

bleaching

photopigments are bleached by photons

Question 12

How many photons are needed to activate a receptor?

Answer 12

one photon

Question 13

Why is the retina considered inverted?

Answer 13

Receptors are located at the very back

Question 14

What structures are located at the center of our visual field?

Answer 14

• Fovea (point of central focus)

* Macula

Question 15

What is the area outside of the central visual field called?

Answer 15

Periphery

Question 16

Human field of view: horizontally

Answer 16

180º

Question 17

Human field of view: binocular vs monocular

Answer 17

~140º binocular vision; ~ 40º monocular

Question 18

Human field of view: nasal vs temporal

Answer 18

each eye is ~60º nasally; ~100 degree temporally

Question 19

Human field of view: vertically

Answer 19

~60º above the horizontal meridian
~75º below the horizontal meridian

*upper visual field is restricted b/c of the eyebrows

Question 20

What is the rule of thumb?

Answer 20

the width of our thumb occupies two degrees of visual angle

Question 21

What is the visual field of the fovea?

Answer 21

~2º diameter of central vision

note:

• no rods, only cones
• 1% retina but ~50% of visual cortex

Question 22

What is the visual field of the para-fovea?

Answer 22

area immediately surrounding the fovea

~4-5º around the center

Question 23

What is the visual field of the periphery?

Answer 23

the area beyond 8º of the center

note: much fewer resources devoted to this huge portion of the visual field; this is why we must orient to our central field of view (fovea)

Question 24

What is eccentricity?

Answer 24

a measure of how far something is from the center

Question 25

What is the blind spot?

Answer 25

~3º area where there are no receptors, therefore we can not take in light information

*axons of many ganglion cells forming the optic nerve and leaving the eye

Question 26

What are our four dark adaptation mechanisms?

Answer 26

1) constriction of the pupil:
changing the pupil size controls the amount of light that enters the eye

2) throwing away photons:
in bright conditions, there are too many photons
reaching the retina and bleaching the pigments –> most pigments are not available and do not respond to
photons

3) ganglion cells:
their receptive field mostly encode contrast and not overall level of light

4) the duplex system of receptors:
rods and cones enable us to operate well in both dim and bright conditions

Question 27

In ganglion cells, what happens when light hits the ON portion of the receptive field?

Answer 27

firing is excited above baseline

Question 28

In ganglion cells, what happens when light hits the OFF portion of the receptive field?

Answer 28

firing is inhibited below baseline

Question 29

What happens if both ON and OFF are hit by stimulus (uniform light)

Answer 29

excitatory and inhibitory cancel each other out, and the cell remains at baseline

–> ganglion cells only care about CONTRAST (light vs dark)

Question 30

How does “pooling” differ between rods and cones?

Answer 30

Rods: many rods tend to project to the same ganglion cell (pool)

Cones: different cones tend to project to specific ganglion cells 1:1 (more picky, no pooling)

Question 31

What is the acuity-sensitivity tradeoff?

Answer 31

RODS
(Sensitivity): Pooling allows rods to have higher sensitivity, bc there is a higher probability of ganglion cells receiving info and being activated
(Acuity): Pooling decreases acuity in rods b/c it prevents stimuli patterns from being differentiated (the same ganglion cell is still being activated)

CONES:
(Sensitivity): Not-pooling decreases the probability of ganglion cells being activated, because cones may or may not be excited by a photon and they are directly attached to a single ganglion cell
(Acuity): Not-pooling increases acuity b/c different neurons are being activated and can be distinguished from one another

Question 32

What is a scotoma?

Answer 32

an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is surrounded by a field of normal – or relatively well-preserved – vision

*a blindspot is a type of scotoma

–> brain uses a filling-in process