Spatial vision Flashcards
Spatial vision
The study of visual mechaisms used for the detection and discrimination of objects
Visual acuity
Visual-based measure of the threshold to detect or discriminate the details comprising a specific target
Recognition acuity
Infers resolution by requiring the identification of targets
Resolution acuity
Resolve the minimum separation
Localization acuity
Detect the minimum change in location or relative position of two or more targets
Detection acuity
Minimum luminance or contrast required to see a stimulus
Absolute measure
The resolving capactiy of the eye
Relative measure
Whether or not the threshold is normal
Resolving capacity
Inferred from the size of the smallest letter than cen be recognized orrectly
Optotypes
Specially designed letters
What can a normal observer see?
Correctly identify an optotype that subtends 5 arc minutes
What is the gold standard of visual acuity?
Landolt C
How to tell is two letters are equally legible?
Share similar probabilities of confusion
What are the two standardized groups of optotypes?
Sloan letters and British standard
Critical distance
Distance at which the optotype will habe a total angular subtence of 5’ min
How to get equivalent snellen from actual snellen?
20 x Cirtical distance/actual test distance
Minimu angle of resolution
Stroke width of the optotype or the reciprocal of the snellen fraction
When is LogMAR negative?
When the MAR is less than 1 or when visual acuities are greater than 20/20
Visual acuity rating
100-50*LogMAR
At what rating of VAR is there considered profound vision loss?
0
M-notation
Used for near visual acuity, distance in meteres at which a near optotype will subtend 5 arc minutes
What are the 4 design flaws of the snellen chart?
Number of optoypes per acuity line is notequal
Legibility of optotypes is not equal
Spacing between optotypes
Progression of acuity sizes
ETDRS
Early treatment of diabetic retinopathy study
What are the 5 advantages of the Bailey-Lovie chart?
Equal number of optotypes in each row
Letter spacing scales with letter size
Letter size follows a logarithmic progression
Letter legibility
Adatable for use at non-standard distances
Point spread function
The area the retinal image is spread over resulting from a point source target
5 factors that affect two point threshold
Magnitude of defocus Magnitude of aberrations Magnitude of diffraction Contrast sensitivity Subjective criterion
What creates the contrast signal to the eye?
A dip between the two PSF functions
What is the peak contrast threshold for the eye?
1%
What is the diameter of a foveal cone?
30arc seconds
How does the PSF change with increasing magnitudes of defocus, aberrations and diffraction?
It becomes wider and shorter
Why is the two point resultion acuity not normally used?
Criterion of doubleness
Worsens with increasing luminance
Astigmatism creates differential meridional blur
What is the airy disc?
Pattern of light distribution characterized by peak intensity followed by a minimum and smaller peaks
Rayleigh’s criterion
Resolution of two point targets within a diffraction limited eye occurs when the peak of the airy disc of one target coincides with the first minimum of the airy disc of the other target
What is the MAR of a perfect optical system?
31.45 arcseconds
Rayleigh’s criterion formula
theta= (1.22 x wavelength)/area
Grating acuity
The highest spatial frequency that can be reliably detected by an observer
What are the three things affecting grating acuity?
Optical factors
Receptor sampling
Contrast sensitivity
How do you go from spatial frequency to snellen denominaotr?
30/SF * 20
What does spurious resolution enable?
Higher spatial frequencies to be visible in defocused image with contrast reversal
Types of localization acuity
Dot vernier
Line vernier
Spatial interval acuity
Bisection acuity
Hyperacuity
Acuity thresholds that are several factors smaller than the receptive field limit of 30 arcseconds
Factors limiting use of localization acuity
Time
Electronics
Lateral judgements
Stimulus design
What does detection acuity measure?
The sensitivity to detect the contrast of the line or dot target
What factors affect VA?
Refractive error Retinal eccentricity luminance contrast contour interaction
What happens to VA with increased refractive error
Progressive decrease in letter based acuity
Retinal eccentricity
distance relative to the fovea expressed in angular units
What happens to VA with increased retinal eccentricity
Acuity decreases progressively
What happens to VA with increased luminance
Better VA, until plateau at 100-300cd/m2
Where is the peak VA for scotopic luminance levels
5 degrees
What happens to VA with an increase in contrast
Increases until plateaus at 80%
What is the Pelli-Robson chart for?
Attempts to robe contrast detection sensitivity using acuity charts in order to detect any abnormalities
What is contour interaction?
How closely the letters are spaced
What happens with a decrease in contour interaction?
Declines past 5*MAR
