L08 - Retinal Mosaic & Sampling (AA) Flashcards
___ cones/mm^2 at fovea centre?
_ x max at 0.2 degrees away
_ x max at 1 degree away
___ cones/mm^2 at edge of retina
200k
0.5
0.1
5 k
Where is the physiological blind spot?
+ve 10 to 20 degrees (nasally) of eye where optic nerve exits and no photoreceptors are present
What techniques are used to create L, M, S- cone mosaics?
Adaptive optics photographs cones
Retinal densitometry identifies LMS cones
-Flood eye with intense light of x colour -> cones sensitive to x colour absorbs it and bleaches out -> shine light and see what is reflected back (slight diff present dep on light absorbed which allows us to work out LMS cones)
LMS cone arrangement
Random. More LM cones.
What is the resolution limit of the eye?
40 to 60 cycles per degree
Snellen fraction
A representation of visual acuity in the form of a fraction (e.g. 6/6 (20/20)) in which the numerator is the testing distance, usually expressed in m, and the denominator is the distance at which the smallest Snellen letter read by the eye has an angular size of 5 minutes (1 degree has 60 minutes).
*6/6 means that at six meters test distance the patient could correctly identify a letter that a ‘normal’ sighted person should see at six meters i.e. ‘normal’ vision. 6/60 means that the patient could only see at six meters what a ‘normal’ sighted person should see at 60 meters.
Interference fringe
A bright or dark band caused by beams of light that are in phase or out of phase with one another.
Interferometry
2 laser spots in eye, bypasses optics (measures retina ONLY)
CSF (Contrast sensitivity function)
It tells us how sensitive we are to the various frequencies of visual stimuli. If the frequency of visual stimuli is too high we will not be able to recognize the stimuli pattern any more. Imagine an image consisting of vertical black and white stripes. If the stripes are very thin (i.e. a few thousand per millimeter) we will be unable to see individual stripes. All that we will see is a gray image. If the stripes then become wider and wider, there is a threshold width, from which on we are able to distinguish the stripes.
Does Interferometry or CSF have better resolution? What happens when you go beyond 5 eccentricity and what does this suggest?
Interferometry
Result is identical - suggests resolution limit in retinal periphery is set by neural factors (retina/brain)
What is the retinal detector? PhotoR or GCs?
GCs - In fovea PhotoR : GC is 1 : 1 but in periphery GCs pool signals from multiple PhotoR
Would it be the better resolving capability (midget cells) or poorer resolving capability (parasol cells) that limits the resolving capability of the eye?
Better (midget)
*It’s like point 2 different megapixel cameras at an object, you would select the one with the higher megapixel
2 req to resolve finest details of an image
1) Detector must be small enough
2) Array of detectors must be fine enough
For a veridical percept, Nqyuist limit has to be under/over?
Under
If over - interpretation of image is low freq, distorted moire pattern
Would you expect aliasing centrally or peripherally?
Peripherally - (lower cone density), effect of
aliasing reduced by the disarray in sampling mosaic
= produces noisy pattern rather than plausible object
Not centrally as optics blur out info passed through it - hence it will be under Nyquist limit
