`Lecture 4: The eye

Question 1

What are the 5 stages to how we see?

Answer 1

1) Light reflects off objects and enters the eye.
2) It passes through the cornea, pupil and lens.
3) The lens focuses the light on the retina
4) Photoreceptor cells convert light waves into electrical impulses
5) The electrical signals travel through the optic nerve to the brain.

Question 2

What is the electromagnetic spectrum between?

Answer 2

400 and 700nm

Question 3

What are photoreceptors?

Answer 3

=specialist cells that are sensitive to light

Question 4

What do the cornea and lens do?

Answer 4

Help ensure that object being viewed will fall into focus 16.8mm behind the centre of the lens i.e centre of retina

Question 5

What do the ciliary muscles do?

Answer 5

Relax and contract to change the shape of the lens according to distance of the object we need to focus

=accommodation

Question 6

What does the pupil vary from?

Answer 6

2 to 8 mm

-as light increases, pupil diameter decreases restricting amount of light entering the eye

Question 7

Where are photoreceptors?

Answer 7

At the back of the retina

Question 8

why are photoreceptors at the back of the retina?

Answer 8

As the eye has evolved they have been pushed back and additional layer of processing cells have been piled on top

Question 9

How many photoreceptors is the retina made up of?

Answer 9

100 million

Transduction= convert light energy into neural activity

Question 10

Two varieties of photoreceptors

Answer 10

– Rods more sensitive to light levels (luminance) than cones and more of them (120 million).
– Cones are colour sensitive (only 6-7 million) and are more concentrated in the fovea.

Question 11

What is ratio of L and M to S

Answer 11

100:1

Question 12

What is the job of the photoreceptors?

Answer 12

convert this light energy into neural signals that can be understood by the brain. Light causes a chemical cascade of reactions to occur in the photoreceptor which causes these cells to send a signal to the next layer in the retina.

Question 13

Which is the only part of the we can see colour?

Answer 13

Fovea

-can detect luminance in the periphery

Question 14

Where are rods and where are cones

Answer 14

rod=periphery

cones=centre of fovea

Question 15

Structure of retina

Answer 15

*learn this

Question 16

What potentials do photoreceptors and bipolar transmit?

Answer 16

graded potentials

Question 17

What happens in a graded response?

Answer 17

Releases a graded amount of neurotransmitter dependent on the amount of light intensity

Question 18

Bipolar cells on and off

Answer 18

• Two types of bipolar cells ON (activated by an increase in photon catch on the receptors) and OFF (activated by a decrease).
– (i) cone bipolars (have both ON and OFF bipolars) and transmit to ganglion cells
– (ii) rod bipolars only have ON variants and transmit to amacrine cells

Question 19

Amacrine cells

Answer 19

• more numerous than horizontal and ganglion cells
• relatively unknown
• appear to form alternative route in linking bipolar cells to ganglion cells

Question 20

Biplexiform ganglion cell

Answer 20

directly connected to rod photoreceptors but are influenced by both rods and cones? Measures ambient light levels and may have a role in controlling pupil diameter and body rhythm.

Question 21

Bistratisfied ganglion cells

Answer 21

only response at high light levels and shows spectral opponency (blue-yellow).

Question 22

Midget ganglion cell

Answer 22

most numerous and respond only at high light levels and shows both spatial and spectral opponency (red-green).

Question 23

Parasol ganglion cell

Answer 23

remains active at low light levels and shows spatial opponency but no colour opponency.

Question 24

Receptive fields

Answer 24

• A single ganglion cell receives input from several inputs from several photoreceptors.
• These make up the centre versus surround receptive fields.

Question 25

On centre cell vs off centre

Light on centre only

Answer 25

-ganglion cell fires rapidly

Off centre
-ganglion doesn’t fire

Question 26

On centre cell vs off centre

Light on surround only

Answer 26

-cell does not fire

off centre
-cell fires rapidly

Question 27

On centre cell vs off centre

No light on centre or surround

Answer 27

-cell doesn’t fire

off centre
-cell doesn’t fire

Question 28

On centre cell vs off centre

Light on centre and surround

Answer 28

-weak response

off centre
-weak response

Question 29

What does ganglion cells o centre mean?

Answer 29

**

Question 30

What does ganglion off centre mean?

Answer 30

**

Question 31

Receptive fields: Spectral

Answer 31

**

Question 32

From retina to LNG

Answer 32

• From the ganglion cells neural signals travel along the optic nerves of each eye to reach the optic chiasm.
• At the chiasm information from the left visual field in both the left and right eye, join and travel onto the right LGN and Visual cortex (V1).
• Likewise the right visual field from both eyes go to the left LGN and left V1.

Question 33

LNG: Magocellular layers

Answer 33

have large cell bodies and receive input from the parasol ganglion cells ( i.e. luminance).

Question 34

LNG: Parvocellular layers

Answer 34

contain small cells and receive input from the colour sensitive midget ganglion cells.

Question 35

LNG: Loniocellular layers

Answer 35

orm a sub-layer at the bottom of each M and P layer and receives input from the bistratified cells

Question 36

What do cells in layers 1, 4, 6 receive?

Answer 36

input from the contralateral (opposite side) eye (grey circles).

Question 37

What do cells in layer 2, 3 and 5 receive?

Answer 37

input from the ipsilateral (same side) eye (black circles).

Question 38

Functions of the LNG

Answer 38

• Sharpen spatial responses by modifying the centre surround receptive fields, possibly be increasing surround inhibition.
• Begin integrating information across larger retinal areas.
• To enhance the salience of sudden changes in stimulation by modulating the firing patterns in the LGN.