Lecture 20- image forming properties of the eye: Optics and the retina Flashcards
How many different classes of cone photoreceptors in the retina ?
S L M cones
3
What is the long wavelength ?
red
What is the middle wavelength ?
green
What is the short wavelength ?
blue
What do rods do ?
respond at very low light levels
What does a diagram show where you can see single photoreceptors in the retina ??
many more long wavelength photoreceptors than middle and short photoreceptors
How much cones does the retina contain ?
5 to 6 x 10^6 cones; (5 to 6 million )
How much rods does the retina contain ?
110 x 10^6 rods in each retina (110 million )
What are the M and L cone thresholds (smallest single needed to discriminate red green colour differences ?
0.4% in L cones
0.8% in M cones
2 L cones for every M cone
What are the S and L cone thresholds (smallest single needed to discriminate yellow blue colour differences ?
- much larger scale contrast
- due to lower density
how many S cones are there for every L and M cones ?
1 S cone for every 20 to 100 L and M cones
where is there no S cones ?
no S-cones at the very centre of the fovea
what is the best S-cone colour thresholds ?
7%
how many L cones for each M cone ?
2 L cones for every M cone
what happens when you look at an image ?
- 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
what is the function of photoreceptors ?
- 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
how many signals do we end up with at retina ?
- 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
what do we do when we have too many signals to be able to transfer to the cortex ?
- we need some processing of visual signals within the retina which is carried out in these neural stages
what does the processing that occurs in the retina result in ?
- results in a number of vision channels
what are the vision channels ?
- 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
compare the amount of light in the long wavelength region and the middle wavelength region with short wavelength region ?
we end up with yellow-blue chromatic channel
what is red - green channels
L M channel mediates red green colour vision
what happens if you sum up signals generated in L and M cones ?
we end up with luminance contrast channel which tells us how much light there is at particular point of image
what is the spectral composition of luminance contrast channel ?
sum of twice the long wavelength sensitivity plus middle wavelength sensitivity
what is the diameter of rod photoreceptors ?
- rod photoreceptors have small diameter
what happens at rod photoreceptors ?
- 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
what does the luminance contrast channel provide us with ?
the luminance contrast channel which provides us with the highest acuity , higher spatial resolution.
the ability to see fine detail particularly at low contrast
what is luminance contrast channel based on ?
the luminance contrast channel is based on long wavelength and middle wavelength receptors
what is the difference in peak spectral responsivity of L and M cones ?
under 30 nm
what is the actual change of refraction or chromatic aberration based on ?
is a result of the shift in the spectral responsivity
what do S-cones respond to ?
respond to short wavelength
what do S-cones respond to ?
respond to short wavelength
what are the properties of photopic luminance efficiency channel ?
- 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)
what is the scotopic luminance efficiency channel ?
- 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
what is the red-green (L-M) chromatic channel
- based on differencing L and M cone signals
- low temporal frequency response (10Hz)
- isolated using “isoluminant” stimuli buried in dynamic luminance contrast
what is the yellow-blue , S-(L+M) chromatic channel ?
- enables us to see yellow blue colour differences
- based largely on S-cone signals
- very low temporal response
- very poor spatial resolution
what do luminance contrast channels do ?
- they extract photopic contrast and scotopic contrast
what do colour channels do ?
extract red/green and yellow/blue signals
what does the contribution of these channels make to stimulus detection and its suprathreshold “ effective contrast” depend on ?
- stimulus and background characteristics
- location in the visual field
- state of light adaptation of the retina
what receptors do we have at high light levels ?
mostly cone photoreceptor signals - we use mostly photopic luminance contrast channels
what happens when light levels drop ?
- cone photoreceptors begin to lose sensitivity
- we start using rod photoreceptor signals and rely on the scotopic contrast channel
what happens when object field angle is 0?
object is on axis like in spherical aberration
when is spherical aberration minimised ?
when pupil size is small and that affects the quality of the image in central vision
what happens when we move to the peripheral visual field where we have large amounts of astigmatism , field curvature and distortion ?
the spatial resolving power of the retina is very poor at large eccentricities
what is the visual axis ?
- 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
what is the optical axis ?
- optical axis intersects the retina 1.25 mm away from the fovea on the nasal side
what is the optical axis ?
- 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
what does the size of pupil depend on ?
- 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 )
what is scotopic ?
low light levels
rod
what is mesopic ?
intermediate light levels -
rod and cone
what is photopic ?
high light levels
cone
how do we sense the amount of light on retina ?
- the photopic luminance contrast channel
what are retinal ganglion cells ?
- 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
what is light flux channels ?
- 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
