Spatial Vision (M2) Flashcards
What refers to how the visual system responds to patterns of light and dark across space?
spatial vision
What do low-contrast acuity charts tell the doctor about the patient?
know less than normal but not a measure of contrast sensitivity for the patient
What allows for Vernier acuity to be so “hyperacute”?
responses to different stimuli overlap or blur occurs
What acuities are impaired by strabismic amblyopia? 1. Which are not impaired? 2
- Vernier and optotype acuities
2. grating acuity
What is the type of hyperacuity that is the ability to use the slightly different images in the two eyes to see in depth? 1. Is this better or worse than Vernier hyperacuity? 2
- stereopsis
2. slightly worse
What are the limits on grating resolution acuity?
- Nyquist limit
- optical blur
- light level
- position on retina
- rods vs. cones
- infants vs. adults
What are the disorders that affect Vernier acuity?
- refractive amblyopia
2. strabismic amblyopia
For human CSF, what value is usually the peak sensitivity?
5 cycles/degree
What are the differences in brightness (and color) of an object with respect to other objects in the field of view called?
contrast
What visual disorders can lead to reduced CSF?
- cataracts
- corneal conditions (especially LASIK)
- multiple sclerosis
- Parkinson’s
- cortical vision impairment
- albinism
Which comes faster for infants: Vernier acuity or resolution acuity?
Vernier acuity (4-6months)
What is the reason for low spatial frequency roll-off?
bright bars stimulate whole receptive field so lateral inhibition from the surround cancels excitation in the center
Is the logMAR or Vernier acuity better in the visual periphery? 1. In the fovea? 2
- logMAR better
2. Vernier
How many arc sec is the minimum Vernier acuity?
10 arc sec
For sine-wave grating, what is the contrast referred to? 1. What is the formula for this? 2
- Michelson or grating contrast
2. grating contrast =(Lmax - Lmin)/(Lmax + Lmin)
What is a discrimination that is finer than the spacing of the rods and cones or by widely-spaced detectors?
hyperacuity
What is the overall ability of infant vernier acuity?
it is poor but not that poor once their poor contrast perception is taken into account
What is one disorder that does not affect Vernier acuity very much but does affect resolution and optotype acuity?
cataracts
What does pre-cataract surgery logMAR predict about post-op logMAR?
doesn’t predict post-op logMAR as well as a pre-op hyperacuity
What is the shape of the contrast threshold for sine waves?
u-shaped
What are the well known low-contrast letter charts?
- Pelli-Robson chart
2. Bailey-Lovie chart
How many arc sec is the minimum resolution acuity?
30 arc sec
What are the ways to tell if cataract surgery is going to be helpful to the patient?
- helpful if no other ocular pathology (but sometimes hard to tell if it’s there)
- use hyperacuity to see if beneficial because poor hyperacuity before the surgery implies some ocular pathology other than catract
What is the numerator of the Snellen fraction? 1. The denominator? 2
- distance at which test was taken
2. distance at which the smallest optotype patient can resolve subtends 5 arcmin
What is the minimum detectable offset of two nearly-colinear marks?
Vernier acuity
How many arc sec is the minimum optotype acuity?
60 arc sec
What is the shape of the contrast sensitivity for sine waves?
n-shaped
What are examples of Vernier hyperacuities?
- leveling
- centering/bisection
- spatial frequency discrimination
- orientation discrimination
- telling time
- measuring length
In the clinic, what is the minimum angle of resolution of an adult usually determined by?
optotypes rather than gratings
What are used as a tool to assess the sensitivity of the visual system to spatial changes in luminance?
sine-wave gratings
What is a person’s ability to appreciate the locations of stimuli relative to each other?
Vernier acuity
What causes the losses in the ocular media, making the Nyquist limit hard to measure? 1-4. What do these all lead to? 5
- defocus
- higher-order aberrations
- scatter in the ocular media and retina
- stray light not absorbed by the visual pigments or the RPE
- reduced contrast
How do you measure contrast sensitivity?
have subject look at sine-wave gratings of different spatial frequencies and adjust until barely visible
How does light affect visual acuity?
more light, more sight until maximum VA achieved
Is the Vernier acuity limited by the photoreceptor mosaic? 1. What does blur do to the acuity? 2. Is it better or worse int he periphery? 3
- no
- relatively immune to blur (like cataract)
- worse
What is the highest spatial frequency the patient can see at 100% contrast called? 1. What are the common values of this? 2. What is this called overall? 3
- high-frequency cutoff
- 30-60 cycles/degree (30-50 in fovea)
- grating resolution acuity
What is the reciprocal of the contrast threshold (the lowest contrast a patient can detect) called? 1. How is this typically measured? 2
- contrast sensitivity
2. take log of the inverse of the threshold
What stops the retina from seeing the strange artifacts that form at above-Nyquist gratings?
blur
Do loses in the ocular media reduce contrast more at high or low frequencies?
high frequnecies
At the youngest ages, which is better: Vernier acuity or resolution acuity?
resolution acuity
What is the highest spatial frequency that can be distinguished from gray at 100% contrast?
grating resolution acuity
What is the optically maximum possible resolution acuity? 1. How many stripes per cones is this? 2
- Nyquist limit
2. one stripe per cone
Why is the Nyquist limit not easily measured?
losses in the ocular media
Is our contrast better or worse at low spatial frequencies? 1. What is this called? 2
- worse
2. low-frequency rolloff
What is the problem with current common visual acuity charts?
only at high frequency resolution but reduction in low and moderate spatial frequencies can be handicapping
What is the characteristic human contrast sensitivity function?
inverted U-shape
What is the center to center spacing of cones in the fovea in stripe widths? 1. In spatial frequency? 2. For the Snellen equivalent? 3
- 30 arc sec (one stripe width)
- 60 cy/deg
- 20/10
