Vision (Colour & adaptation)

Question 1

What are the different types of cones in the human retina?

Answer 1

• Long (L): Red (560 nm)
• Medium (M): Green (530 nm)
• Short (S): Blue (420 nm)

Question 2

What is the principle of univariance?

Answer 2

• A specific type of cone can absorb light of a range of different wavelengths, not just the one it is specific to.
• It has the greatest sensitivity towards to colour it is specific to.
• When an L cone is presented with red light and green light, the same response can be produced if green light has 10x intensity as red light.

Question 3

What are the disadvantages of having too many different types of cones?

Answer 3

• Too much overlap between different cones may make differences in absorption very small and so difficult to distinguish.
• Wiring of the cones may be too complex.
• Density of each type of receptor would be low (due to spatial limitation of retina), resulting in reduced visual acuity.

Question 4

How can a dichromatic colour line be interpreted?

Answer 4

• Angle = Colour
• Length = Magnitude

Question 5

How is a colour triangle interpreted?

Answer 5

• A point within the triangle represents a colour
• The angle of a locus through point from centre of triangle to edge represents a particular hue.
• The distance of the point from centre of triangle represents saturation.

Question 6

What are the features of a trichromatic colour triangle?

Answer 6

• Each corner of triangle represents a particular ‘primary’ colour (red, green, blue).
• Centre of triangle represents white light.
• Colours in region at green vertex are unobtainable as a result of there being no region along absorption spectrum where M cones alone are stimulated. These colours can only be obtained if blue/red cones are adapted out.
• Colours on the red-blue vertex are unobtainable as a result of there being no wavelengths of light that can stimulate L and S cones without also stimulating M cones.

Question 7

Where in the visual pathway does colour opponency first appear?

Answer 7

Ganglion cells

Question 8

What are the types of colour opponent channels?

Answer 8

1. Red-green colour opponent channels
2. Blue-yellow colour opponent channels
3. Brightness cells

Question 9

What are the properties of the colour opponent channels?

Answer 9

• Red-green: Receives excitatory inputs from red and inhiitory inputs from green cones.
• Blue-yellow: Receives inhibitory inputs from blue cones and inhibitory inputs from red and green cones (combining to give yellow).
• Brightness: Receives inputs from all cones. Carried by

Question 10

What are the receptive fields associated with the ganglion cells?

Answer 10

1. Parvocellular: R+/G-, R-/G+ centre-surround receptive fields.
2. Koniocellular: B+/Y- centre-surround receptive fields.
3. Magnocellular: On/off non-chromatic receptive fields (R+G+/R-G-).

Question 11

What are the main problems associated with single opponent receptive fields?

Answer 11

• There is ambiguity of response (e.g. in R+/G- field):
  1. Small white stimulus causes large response as it excites red cones in centre.
  2. Large red stimulus produces same response as small red stimulus.
  3. Small green stimulus stimulates central red cones (principle of univariance) and produces some response.

Question 12

Which level of the visual pathway does double opponency first appear?

Answer 12

Blob cells in V1

Question 13

How does double opponency resolve problems associated with single opponency?

Answer 13

1. Small white stimulus no longer produces response as R+ and G- in centre cancel out.
2. Large red stimulus no longer produces response as stimulation of central R+ and peripheral R- cancels out.
3. Small green stimulus produces inhibitory response as the G- overwhelms the small R+ produced by green light.

Question 14

How is a double opponency circuit produced for a R+G-/R-G+ receptive field??

Answer 14

R+/G- centre:

• R+/G- geniculate neurones with receptive fields in central region synapse onto blob neurone via excitatory synpase.
• G+/R- geniculate neurones synapse with inhibitory synapse.

R-G+ surround:

• R+/G- geniculate neurones with receptive fields in surround region synapse onto blob neurone via inhibitory synpase.
• G+/R- geniculate neurones synapse with excitatory synapse.

Question 15

What are the functions of double opponent cells?

Answer 15

• Colour opponency (absolute colour detection) within coloursed objects.
• Colour contrast at borders between different colours.

Question 16

What is the function of V4 in colour processing?

Answer 16

• Colour constancy.
• This is phenomenon responsible for allowing specific colours to be perceived despite environemnt being illuminated by lights of the same hue.
• V4 compares the spectral reflectances of different coloured objects in environment and only respond to those with highest reflectance for particular wavelength, indicating that object is truly that colour.

Question 17

What are the different types of vision?

Answer 17

• Scotopic: Low light intensity vision where only rods are active.
• Mesopic: Mid light intensity vision where rods and cones are active.
• Photopic: High light intensity vision where rods are bleached and only cones are active.

Question 18

What are the conditions used for the increment-threshold experiment and why?

Answer 18

Background colour: Orange (to adapt out cones)

Spot colour: Green (preferentialy stimulates rods due to 500nm maximum sensitivity)

Retina region: Parafoveal region (maximum rod density)

Question 19

What is Weber’s law?

Answer 19

ΔI/(I + I₀) = k

Where:

ΔI = Increment threshold

I = Background intensity

I₀ = Dark light

k = Weber fraction (constant)

Question 20

What is the molecular basis of dark light?

Answer 20

Spontaneuos isomerisation of 11-cis retinal to all-trans retinal and activation of rhodopsin when not exposed to light.

Question 21

What is fields adaptation?

Answer 21

Process whereby photoreceptors elicit different sensitivities depening on the background light intensity.

Question 22

What is the molecular basis of field adaptation?

Answer 22

1. In dark conditions, Ca²⁺ enters photoreceptors with Na⁺ and inhibits guanylyl cyclase.
2. When light falls onto photoreceptor, transducin is activated, and closes CNs by promoting PDE activity and cGMP breakdown, decreasing inward Ca²⁺ current.
3. Na/Ca/K exchanger (NCKX1) extrudes Ca²⁺ in ratio of 4Na:1Ca.
4. Reduction in intracellular Ca²⁺ relieves inhibition on guanylyl cyclase and increases cGMP production.
5. Greater PDE activity needed to achieve reduction of cGMP required to cause CN closure.
6. More light needs to fall onto receptor in order to facilitate greater PDE activity and produce same response, hence reduced photoreceptor sensitivity.
7. Ca²⁺ prolonges activation of metarhodopsin II. Reduced [Ca²⁺] decreases duration of activation.

Question 23

What are the different mechanisms that operate to bring about field adaptation at levels of illumination?

Answer 23

Scotopic:

• Ca²⁺ mechanism
• Neural mechanisms (possibly at receptor-bipolar synapses)
• Recoverin mechanism

Mesopic:

• Ca²⁺ mechanism
• Rod-cone transition

Photopic:

• Ca²⁺ mechanism
• Bleaching

Question 24

What is the significance quantal fluctuations?

Answer 24

• Quantal nature of light results in random fluctuations in background light intensity proportional to √I.
• At very low light intensities, the quantal flucutation threshold exceeds increment threshold imposed by Weber’s law, resulting in non-linear relationship.

Question 25

What are the mechanisms responsible for dark adaptation?

Answer 25

• Bleaching adaptation describes phenomenon that occurs when there is a sudden transition from light to dark.
• Field adaptation describes the actual increase in sensitivity of the photoreceptor cells needed to detect light in low light levels.

Question 26

What is bleaching adaptation?

Answer 26

Describes phenomenon whereby sensitivity is recovered when there is sudden transiton from bright environment to dark environment as a result of receptor bleaching?

Question 27

What are the stages of bleaching adaptation?

Answer 27

1. Sensitivity is poor initially as both rods and cones are highly bleached.
2. After 10 minutes, cones recover (due to quicker retinal recovery mechanism) but rods are still highly bleached (rod-cone break)
3. Rods only recover after ~30 minutes.

Question 28

What are the mechanisms behind reduced sensitivity in bleaching adaptation?

Answer 28

• Increased dark light: Bleaching photopigments increases frequency of spontaneous isomerisations, effectively increasing dark light (post-bleach light).
• Rhodopsin photoproducts may persistently excite phototransduction pathway and inhibit Ca²⁺ current, reducing intracellular Ca²⁺ and sensitivity of photoreceptor cell.

Question 29

What is the Purkinje shift?

Answer 29

Shift of maximum sensitivity from 550nm before rod-cone break to 500nm after.

Question 30

Why does visual acuity decrease in low light conditions?

Answer 30

Rods predominate in low light visions. Due to the large amounts of convergence in rod pathway, neural blur in introduced, reducing visual acuity.

Question 31

What is the change in critical fusion frequency from rod vision to cone vision?

Answer 31

• Low light intensities (rod vision): 15Hz
• High light intensities (cone vision): 60Hz

Question 32

What are the differences between scotopic and photopic vision?

Answer 32

Sensitivity:

• Photopic vision has low sensitivity and is used in high light levels while scotopic vision has high sensitivity and is used in low light levels.
• Stiles-Crawford effect takes place in photopic vision (light in periphery less effective than centre light) but not scotopic.

Spatial properties:

• Photopic vision has high acuity and greater contrast sensitivity while scotopic vision has low acuity and less contrast sensitivity (less lateral inhibition).

Temporal properties:

• Photopic vision has high flicker fusion frequency while scotopic vision has low.

Colour sensitivity:

• Photopic vision has trichromatic colour discrimination while scotopic vision is monochromatic.

Question 33

What is the contribution of the pupils towards visual acuity?

Answer 33

Factor of 16