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Optics > Lecture 20- image forming properties of the eye: Optics and the retina > Flashcards

Lecture 20- image forming properties of the eye: Optics and the retina Flashcards

1
Q

How many different classes of cone photoreceptors in the retina ?

A

S L M cones

3

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2
Q

What is the long wavelength ?

A

red

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3
Q

What is the middle wavelength ?

A

green

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4
Q

What is the short wavelength ?

A

blue

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5
Q

What do rods do ?

A

respond at very low light levels

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6
Q

What does a diagram show where you can see single photoreceptors in the retina ??

A

many more long wavelength photoreceptors than middle and short photoreceptors

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7
Q

How much cones does the retina contain ?

A

5 to 6 x 10^6 cones; (5 to 6 million )

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8
Q

How much rods does the retina contain ?

A

110 x 10^6 rods in each retina (110 million )

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9
Q

What are the M and L cone thresholds (smallest single needed to discriminate red green colour differences ?

A

0.4% in L cones
0.8% in M cones
2 L cones for every M cone

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10
Q

What are the S and L cone thresholds (smallest single needed to discriminate yellow blue colour differences ?

A
  • much larger scale contrast

- due to lower density

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11
Q

how many S cones are there for every L and M cones ?

A

1 S cone for every 20 to 100 L and M cones

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12
Q

where is there no S cones ?

A

no S-cones at the very centre of the fovea

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13
Q

what is the best S-cone colour thresholds ?

A

7%

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14
Q

how many L cones for each M cone ?

A

2 L cones for every M cone

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15
Q

what happens when you look at an image ?

A
  • the optics of the eye form an image of the scene on the photoreceptor mosaic
  • every point in the scene corresponds to a point in the image which varies in spectral composition
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16
Q

what is the function of photoreceptors ?

A
  • to produce a signal which is proportional to the amount of light within a narrow spectral range in a long wavelength , middle wavelength and short wavelength
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17
Q

how many signals do we end up with at retina ?

A
  • we end up with a lot of signals which are generated at every point on the retina
  • too many signals to transfer them to the cortex
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18
Q

what do we do when we have too many signals to be able to transfer to the cortex ?

A
  • we need some processing of visual signals within the retina which is carried out in these neural stages
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19
Q

what does the processing that occurs in the retina result in ?

A
  • results in a number of vision channels
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20
Q

what are the vision channels ?

A
  • chromatic channels to analyse the spectral composition of the light
  • luminance contrast channels which report the amount of light at every point on the retina
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21
Q

compare the amount of light in the long wavelength region and the middle wavelength region with short wavelength region ?

A

we end up with yellow-blue chromatic channel

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22
Q

what is red - green channels

A

L M channel mediates red green colour vision

23
Q

what happens if you sum up signals generated in L and M cones ?

A

we end up with luminance contrast channel which tells us how much light there is at particular point of image

24
Q

what is the spectral composition of luminance contrast channel ?

A

sum of twice the long wavelength sensitivity plus middle wavelength sensitivity

25
Q

what is the diameter of rod photoreceptors ?

A
  • rod photoreceptors have small diameter
26
Q

what happens at rod photoreceptors ?

A
  • thousands of rods contribute to the same signal that is channelled to the cortex through ganglion cell
  • due to extensive spatial summation the scotopic luminance contrast channel cannot resolve fine spatial detail but has high sensitivity to light
  • so can detect smaller light levels
  • rod photoreceptors shifted towards shorter wavelengths
27
Q

what does the luminance contrast channel provide us with ?

A

the luminance contrast channel which provides us with the highest acuity , higher spatial resolution.
the ability to see fine detail particularly at low contrast

28
Q

what is luminance contrast channel based on ?

A

the luminance contrast channel is based on long wavelength and middle wavelength receptors

29
Q

what is the difference in peak spectral responsivity of L and M cones ?

A

under 30 nm

30
Q

what is the actual change of refraction or chromatic aberration based on ?

A

is a result of the shift in the spectral responsivity

31
Q

what do S-cones respond to ?

A

respond to short wavelength

32
Q

what do S-cones respond to ?

A

respond to short wavelength

33
Q

what are the properties of photopic luminance efficiency channel ?

A
  • based largely on summation of L and M cone signals.
  • mediates spatial resolution which our ability to see fine detail
  • high temporal resolution ( 30 to 50 HZ)
34
Q

what is the scotopic luminance efficiency channel ?

A
  • based on rod receptor signals
  • extensive spatial summation of rod signal - channel unable to detect fine detail but has high sensitivity to light
  • lower temporal frequency ( 16Hz)
  • isolated using threshold measurements and selective background adaptation techniques
35
Q

what is the red-green (L-M) chromatic channel

A
  • based on differencing L and M cone signals
  • low temporal frequency response (10Hz)
  • isolated using “isoluminant” stimuli buried in dynamic luminance contrast
36
Q

what is the yellow-blue , S-(L+M) chromatic channel ?

A
  • enables us to see yellow blue colour differences
  • based largely on S-cone signals
  • very low temporal response
  • very poor spatial resolution
37
Q

what do luminance contrast channels do ?

A
  • they extract photopic contrast and scotopic contrast
38
Q

what do colour channels do ?

A

extract red/green and yellow/blue signals

39
Q

what does the contribution of these channels make to stimulus detection and its suprathreshold “ effective contrast” depend on ?

A
  1. stimulus and background characteristics
  2. location in the visual field
  3. state of light adaptation of the retina
40
Q

what receptors do we have at high light levels ?

A

mostly cone photoreceptor signals - we use mostly photopic luminance contrast channels

41
Q

what happens when light levels drop ?

A
  • cone photoreceptors begin to lose sensitivity

- we start using rod photoreceptor signals and rely on the scotopic contrast channel

42
Q

what happens when object field angle is 0?

A

object is on axis like in spherical aberration

43
Q

when is spherical aberration minimised ?

A

when pupil size is small and that affects the quality of the image in central vision

44
Q

what happens when we move to the peripheral visual field where we have large amounts of astigmatism , field curvature and distortion ?

A

the spatial resolving power of the retina is very poor at large eccentricities

45
Q

what is the visual axis ?

A
  • the visual axis is the line passing through the centre of the fovea and the nodal point N’.
  • visual axis defines the direction in which the eye is looking at any given moment
46
Q

what is the optical axis ?

A
  • optical axis intersects the retina 1.25 mm away from the fovea on the nasal side
47
Q

what is the optical axis ?

A
  • optical axis intersects the retina 1.25 mm away from the fovea on the nasal side
  • this distance corresponds to 4 deg of visual angle
48
Q

what does the size of pupil depend on ?

A
  • the size of the pupil varies systematically with ambient light level and affects many aspects of visual performance
  • range 2mm ( high light levels ) to 8mm ( dark light levels )
49
Q

what is scotopic ?

A

low light levels

rod

50
Q

what is mesopic ?

A

intermediate light levels -

rod and cone

51
Q

what is photopic ?

A

high light levels

cone

52
Q

how do we sense the amount of light on retina ?

A
  • the photopic luminance contrast channel
53
Q

what are retinal ganglion cells ?

A
  • retinal ganglion cells which receive signals from many rod receptors -
  • high amount of summation at low light levels
  • as light levels increases , rod signals become more saturated the same channel receives signals from cone receptors
  • at high light levels channel responds to light from signals within ipRGC
54
Q

what is light flux channels ?

A
  • retinal ganglion cells which receive signals from many rod receptors -
  • high amount of summation at low light levels
  • as light levels increases , rod signals become more saturated the same channel receives signals from cone receptors
  • at high light levels channel responds to light from signals within ipRGC

Optics (34 decks)

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