Lecture 20: Vision

Question 1

Describe the cornea

Answer 1

• Curved, transparent front layer.
• Does most of the focusing (~70% of refraction).
• Works with the aqueous humor to bend incoming light.

Question 2

Describe the lens

Answer 2

• Located behind the iris.
• Focuses light precisely onto the retina by changing shape (accommodation).
• Accommodation is controlled by ciliary muscles

Question 3

Describe the pupil

Answer 3

• The opening that allows light into the eye.

Question 4

Describe the iris

Answer 4

• Muscular ring controlling pupil diameter.
• Constricts in bright light, dilates in dim light.

Question 5

Describe the retina

Answer 5

• Light-sensitive layer at the back of the eye.
• Contains photoreceptors (rods and cones), bipolar cells, ganglion cells.
• Converts light into electrical signals sent to the brain.

Question 6

Describe the fovea

Answer 6

Where photoreceptors (cones) are most densely packed in the retina

Question 7

Vision isn’t static; the eye scans the subject to bring the image into the fovea.

Answer 7

Vision isn’t static; the eye scans the subject to bring the image into the fovea.

Question 8

The image is inverted and smaller than reality. The brain is important in interpreting the retinal image that we actually see.

Answer 8

The image is inverted and smaller than reality. The brain is important in interpreting the retinal image that we actually see.

Question 9

Roughly how many rods are there in the retina?

Answer 9

~120 million

Question 10

Roughly how many cones are there in the retina?

Answer 10

~6 million

Question 11

Describe rods

Answer 11

• High sensitivity to light → used in night (scotopic) vision.
• No color detection.
• Poor spatial resolution (not good for detail).
• Absent in the fovea; dense in the periphery.

Question 12

Describe cones

Answer 12

• Require bright light (photopic vision).
• Detect color (trichromatic: red, green, blue cones).
• High spatial resolution.
• Densely packed in the fovea (central vision).

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Question 13

Describe the signal transduction pathway

Answer 13

• Light enters the eye and is focused by the cornea and lens onto the retina.
• Phototransduction in photoreceptors:
• Light changes the shape of retinal (a vitamin A derivative).
• Triggers a cascade that hyperpolarizes the photoreceptor.
• Less glutamate is released, which is a key signal change.
• Signal passed to bipolar cells:
• ON bipolar cells depolarize with light (inhibited by glutamate).
• OFF bipolar cells hyperpolarize with light (excited by glutamate).
• Then to ganglion cells, which fire action potentials.
• Axons of ganglion cells form the optic nerve.

Question 14

Describe the optic nerve

Answer 14

• Carries signals from the retina to the brain.
• Each eye has an optic nerve.

Question 15

Describe the primary visual cortex (V1)

Answer 15

• Located in the occipital lobe.
• Processes:
• Orientation
• Edges
• Motion
• Color
• Builds an initial “image” from the raw signals.

Question 16

Describe the Dorsal Stream (“Where” Pathway):

Answer 16

• Projects to parietal lobe.
• Handles motion, depth, spatial awareness.
• Important for navigation, locating objects.

Question 17

Describe the Ventral Stream (“What” Pathway):

Answer 17

• Projects to temporal lobe.
• Identifies objects, faces, colors.
• Critical for recognition and memory association.

Question 18

Explain how lesions at different points in the visual pathway cause distinct field deficits:

Answer 18

• Optic nerve damage → complete blindness in one eye.
• Optic chiasm lesion → bitemporal hemianopia (loss of peripheral vision).
• Optic tract lesion → homonymous hemianopia (loss of same visual field in both eyes).

Question 19

Describe the pupillary reflex

Answer 19

• Bright light → pupil constriction.
• Involves optic nerve (afferent) and oculomotor nerve (efferent).

Question 20

Describe the accommodation reflex

Answer 20

• Constriction of pupil, lens thickening, eye convergence for near vision.

Question 21

Describe the adaptation to dark

Answer 21

• Regeneration of rhodopsin in rods.
• Takes ~20–30 minutes to fully adapt to darkness.

Question 22

Describe the adaptation to light

Answer 22

• Faster adjustment.
• Reduces rod activity, increases cone function.

Question 23

Summary

Answer 23

• Vision depends on precise optical structures and highly organized neural pathways.
• The retina acts as a sophisticated processor before signals even reach the brain.
• Different regions of the brain reconstruct the visual scene based on electrical signals.
• Disruptions at any stage (retina, optic nerve, brain) lead to characteristic visual deficits.

Question 24

What is the photopigment?

Answer 24

Rhodopsin, which is activated by light and is a GPCR

Question 25

What is the G protein that is associated with rhodopsin?

Answer 25

Transducin

Question 26

How is the receptive field formed?

Answer 26

When many sensory receptor cells converge and form synapses with a single neurone, they collectively form the receptive field of that cell.

Question 27

Ganglion cells have concentric receptive fields on centre and off centre. What are these designed to do?

Answer 27

designed to detect contrasts

Question 28

Is it true that one photoreceptor can contribute to the receptive field of many different ganglion cells and bipolar cells?

Answer 28

Yes

Question 29

What cells are the only cells that fire action potentials in the retina?

Answer 29

Ganglion cells