Lecture 4 - Vision II

Question 1

Name four main regions of the photoreceptors and how do these photoreceptors elicit changes in membrane potential

Answer 1

1. outer segment
2. inner segment
3. cell body
4. synaptic terminal

photoreceptors convert electromagnetic radiation into changes in membrane potential

Question 2

What happens to photoreceptor in response to flashlight?

Answer 2

Hyperpolarise

Question 3

What is the state of the rods cells in the dark (i.e. dark current) and what happens when light is shone on to it?

Answer 3

Rods are depolarised due to high levels of cGMP which keeps the cGMP-gated Na channel opens, high inward current = depolarisation

When light is shine onto it the cells repolarise

• Phototransduction causes cGMP levels to fall
• This leads to closure of the Na channel
• Hence there is a decline in inward current and INCREASED IN NEGATIVE MEMBRANE POTENTIAL

Question 4

Describe the steps in phototransduction

Answer 4

1. Na channels open in the dark because of high levels of cGMP
2. Photon capture switches cis-retinal to trans-retinal, dissociating the rhodopsin
3. Opsin activates transducin
4. In transducin, you have an exhange of bound GDP to GTP
5. Elevation of GTP activates phosphodiesterase-5 enzyme
6. This enzyme converts cGMP to GMP
7. cGMP conc. falls
8. Loss of cGMP results in closure of Na channels
9. Inward current falls so membrane repolarises (hyperpolarise)

Question 5

Name the factors affecting dark adaptation (slow process)

Answer 5

1. Dilation of pupil (although this does not contribute largely due to small dynamic range of pupil size, only contribute to increase sensitivity of light by a factor of 10)
2. Regenration: bleached rhodopsin -> unbleached rhodopsin. Providing more available rhodopsin
3. Adjustment of functional circuit in the brain

Question 6

Name the factors affecting light adaptation

Answer 6

1. Constriction of pupil - restricting light flow
2. Reduced Ca2+ flux (through closed Na channels) reset light threshold
3. Dis-inhibition of guanylyl cyclase incresaes cGMP, opens Na channel
4. Increased Ca2+ flux desensitise photopigment and PDE5

Question 7

What is deuteranopia and what causes it

Answer 7

deuteranopia is the inability to distinguish red and green

Results from loss of M cones

Question 8

What is colour constancy

Answer 8

Perceived colour of objects remain relatively constant under illumination of different light

Question 9

Explain colour constancy as in how it is demonstrated

Answer 9

1. you would have a bluish object and applied a photodetectors to detect the light emitted from the blueish object
2. this provides the wavelength of the bluish light
3. illuminate this bluish light under different illuminant (reddish)
4. although the wavelength changes the we still perceive it as blue due to colour constancy

Question 10

which area of the brain responsible for perception of colour constancy? what experimental evidence are there?

Answer 10

v4 of the visual cortex

• cortical V4 cells show colour constancy
• neurons fire in accordance with perception not based on wavelength
• neurons in the v4 region reflects our experience of colour not the energetic component of light

Question 11

name the stream responsible for colour perception

Answer 11

ventral stream

Question 12

name the area where we’re good at recognising faces

Answer 12

fusiform face area

Question 13

name the condition where people can see shapes in different colour

Answer 13

synesthesia