Vision

Question 1

What is the role of the cornea and lens?

Answer 1

• Focus light
• Refraction is the bending of light, and it’s done in the cornea

Question 2

How does the lens do accommodation (focusing the lens)?

Answer 2

Ciliary muscles in the eye adjust focus by changing the shape of the lens

Question 3

What controls light entering the eye?

Answer 3

The pupil, an opening in the iris

Question 4

What controls eye movement?

Answer 4

Extraocular muscles

Question 5

What types of cells does the retina contain?

Answer 5

• Visual processing begins here
• Photoreceptor cells - rods and cones
• Bipolar cells - receive input from the photoreceptors and synapse on ganglion cells, whose axons form the optic nerve
• Horizontal cells - contact photoreceptors and bipolar cells
• Amacrine cells - contact bipolar and ganglion cells

Question 6

What does inside-out processing mean?

Answer 6

Light comes all the way back to reach the retina, where the processing of vision begins

Question 7

What are the two functional systems for photoreceptors?

Answer 7

• Photopic system (cones) requires more light and allows for color vision - good visual acuity
• Scotopic system (rods) works in dim light - low visual acuity
  We have more rods than cones

Question 8

How does the visual system deal with a wide range of light intensities?

Answer 8

1) Adjusting pupil size
2) Range fractionation - different receptors handle different light intensities - Low thresholds (rods), high thresholds (cones)
3) Photoreceptor adaptation- individual photoreceptors adjust sensitivity to the prevailing level of illumination (photoreceptors stop responding after being activated for too long)

Question 9

What is visual acuity, and where is it sharpest?

Answer 9

• Measure of how much detail we see
• Sharpest in the fovea (center of visual field)
  –> high density of tightly packed cones
  –> receives direct light input that doesn’t pass through other cells
• Best where eyes converge
• Acuity falls off towards the periphery of the visual field (where only one eye brings information)

Question 10

What is the optic disc?

Answer 10

• Where blood vessels and the optic nerve leave the eye
• No photoreceptors
• Blind spot
• We don’t see a hole in our visual field because saccades (eye movement) constantly shift the eyes so that adaptation doesn’t cause the scene to disappear

Question 11

Where are rods found?

Answer 11

• Absent from the fovea
• Numerous in the periphery - that’s why the pupil dilates under low illumination, to help let in light to activate the rods

Question 12

What is lateral inhibition?

Answer 12

A process where interconnected neurons inhibit neighbors and produce contrast

Question 13

What are the axons of the optic nerve called once they pass the optic chiasm?

Answer 13

optic tract

Question 14

Where do nasal hemiretina (projections close to nose) and temporal hemiretina (lateral portion of the retina) project their axons to?

Answer 14

• Nasal hemiretina - contralateral side
• Temporal hemiretina - ipsilateral side of the brain

Question 15

Where do axons in the optic tract synapse?

Answer 15

1) Lateral geniculate nucleus (LGN) of the thalamus - post-synaptic axons in the LGN form optic radiations
2) Optic radiations terminate in the primary visual cortex (V1) or striate cortex, of the occipital lobe

Information from the left visual field is processed on the right side and vice versa

Question 16

What happens if there is damage to the retina vs damage in the cortex?

Answer 16

• Retina: prevents information about light from coming into the brain (you wouldn’t see anything)
• Cortex: light comes in, but sense can’t be made (you might see an image but not know what it means)

Question 17

What is the visual field?

Answer 17

The whole area you can see without moving your head or eyes

Question 18

What would be the visual trade offs between a predator and a prey?

Answer 18

• Prey - wider visual field
• Predator - better acuity (higher overlap in visual field) so best binocular input

Question 19

What is the trichromatic hypothesis of color vision?

Answer 19

• Three types of cones
• Each had a different type of opsin (photopigment) and responds to a different part of the spectrum
  –> short (S) (peak at 420nm)
  –> medium (M) (peak at 530nm)
  –> long (L) (peak at 560nm)

Question 20

Is each types of cone activated by one color only?

Answer 20

• No
• It’s likely the relative activity of each type of cone that informs our brain about color

Question 21

What type of cones are missing in color blindness?

Answer 21

• M cones (sensitive to medium wavelength light)

Question 22

Why are men more color blinded than women?

Answer 22

• Sex linked gene
• Women have two copies so they would need two mutated X chromosomes to be color blind

Question 23

What happens in myopia?

Answer 23

• Nearsightedness
• Occurs if the eyeball is too long
• This causes the image to be in focus in front of the retina
• Might be due to overuse of ciliary muscles
  -Glasses correct the focus so that the image hits the retina (fovea)

Question 24

What is Amblyopia?

Answer 24

• Reduced visual acuity, not caused by optical or retinal damage
• Usually due to misalignment of eyes
• Lazy eyes
• If untreated –> we ingnore information from the eak wye
• If weak eye is used regularly with the good eye covered, vision can be preserved in both eyes