L2 - Light, Eye, and Retina

Question 1

What is the nature of light?

Answer 1

• Big spectrum of electromagnetic waves that we are sensitive to
• Light source and light travels in straight lines from the source
• When it hits an object, light can be absorbed, refracted, or reflected

Question 2

How does light carry info?

Answer 2

• How light is reflected depends on reflectance and orientation of surfaces
• Env structure leads to a spatial light pattern of differential spectral composition and intensity
• Organisms can exploit reflected light to gain info about the structure of the env

Question 3

How does light travel into the eye?

Answer 3

• Light travels through cornea, then the iris (coloured tissue), then into the pupil and into the eye
• Hits the lens and helps the eye to focus onto the retina - sheet of sensors that picks up light (refracted)

Question 4

What are image forming eyes?

Answer 4

• Light coming from one point in space is mapped onto one point on the retina
• Lens refracts things to certain points on the retina and inverts the image

Question 5

What happens when light hits the retina?

Answer 5

• Light has to travel to a range of different neurones before it reaches photoreceptors: molecules change configuration when they pick up light, can become or reduce activity

Question 6

What is the structure of the neuron?

Answer 6

• Photoreceptors: rods (longer and when lights are dim), cones (shorter and fatter and when lights are bright)
• Interneurons: horizontal cells, bipolar cells, amacrine cells
• Axons to brain via optic nerve: ganglion cells

Question 7

Describe rods?

Answer 7

• Many
• One type of photopigment
• Very sensitive to light - scotopic vision
• Poor spatial resolution
• High temporal resolution

Question 8

Describe Cones?

Answer 8

• Fewer
• 3 types of cones with peak sensitivities at different wavelengths - colour vision
• Less sensitive to light
• High spatial resolution
• Poor temporal resolution

Question 9

What is Duplex retina?

Answer 9

Two systems for seeing

Question 10

Why do rods have lower spatial resolution?

Answer 10

• Only one rod has to pick up light out of the 4 to transfer that info to the brain,
• System is more sensitive, because 4 rods go to one interneuron, so spatial resolution is bad because the brain cannot tell which rod it came from

Question 11

Why do cones have higher spatial resolution?

Answer 11

The cone needs to pick up light because it is only connected to one, so is less sensitive to light but has a higher spatial resolution

Question 12

What is the distribution of photoreceptors across the retina?

Answer 12

• Fovea: point on the retina that lies directly behind the opening of the eye.
• At the fovea (centre of visual field) there are lots of cones (peaks) but no rods
• Places further along the retina = lots of rods and fewer cones

Question 13

What is the blind spot?

Answer 13

• No cones or rods on the retina
• All axons on photoreceptors leave the eye to go to the brain
• Also allows blood vessels to get into the eye to allow functioning
• The blind sport is covered up by the other eye because we have two eyes

Question 14

What is a receptive field?

Answer 14

• Specific to each ganglion cell
• Has to receive light in that region or in the receptive field to activate

Question 15

What are the spatial and stimulus aspects in the retina?

Answer 15

• Spatial aspect = vertical connectivity in retina
• Stimulus aspect = horizontal connectivity

Question 16

How are ganglion cells like a ballot box?

Answer 16

• Cylinders are like photoreceptors
• Directly linked ti a bipolar cells which links to a gangoln cell
• Other cylinders connect to a horizontal cell which transfers info to a bipolar cell
• Whenever main photoreceptor is active, it activates the bipolar cells this activates ganglion cell (seen with + sign)
• Those linked through horizontal cells have an inhibitory effect
• Leads to a centre-surround receptive field (peripheral activation inhibits ganglion cell)
• Will take in both positives and negative and then ganglion sums up, is more positive then the ganglion becomes active, if equal, nothing happens, if more negative = reduces baseline performance

Question 17

What does neural activity look like for ganglion cells?

Answer 17

• If light does not hit the photoreceptors, it will not activate = no activity
• If light hits directly in the centre (+) = neural activity becomes higher
• If light hits only the periphery = reduces neural activity
• If light hits everywhere, centre and periphery = neural activity stays the same

Question 18

What is the differences between on-centre or off-centre receptive fields?

Answer 18

• On-centre = positive in middle and negatives around it
• Off-centre = Negative in middle, and positives around it

Question 19

What do centre-surround receptive fields allow?

Answer 19

• Ganglion cells indicate local differences in light intensities = monitoring contrast
• Independent of overall changes in light intensity = efficient code to represent amount of light in the env as you code fewer pieces of information

Question 20

Why do perceive the same grey scale as two different colours?

Answer 20

NORMAL DIFFERENT GREYS:
- If you put the ganglion cell in the lighter grey, neural activity is at baseline as same amount of light hitting receptive field. SAME for darker grey because although there is less light, the light is spread across equally
- If you place the stimulus in the middle where some of the periphery is on the dark grey, one part of the surround gets less light so inhibition does not cancel the facilitation = higher than baseline neural activity
- Lighter grey = MORE LIGHT, so when stim is moved to dark grey but one periphery is in the dark grey = lower than baseline
SAME COLOUR GREY:
- Same grey value and same amount of light emitted, but in the middle, grey level goes up, then down and then baseline
- Place ganglion cell on left = baseline, place on right = baseline