Spatial Vision Flashcards
Define Spatial Vision. Provide an example of the clinical application of spatial vision
- Defined: the ability of visual system to detect (or discriminate) and resolve (luminance-defined) stimuli of various size & contrasts
- Clinical assessment of spatial vision is part of routine eye care
- Example: VA measurement – determination of spatial resolution a highly sensitive measure of visual function
- Concerned with variations in luminance across space
- Measured with sine-wave gratings of varying spatial frequency and contrast
- Clinically assessed using VA & Contrast sensitivity tests
- The visual system is evolved to detect & recognize objects from patterns of light and dark on the retina
Is absolute luminance or relative luminance more important in spatial vision?
- Absolute luminance is less important in spatial vision than relative luminance (contrast) levels because luminance levels vary greatly in the environment, but relative luminance’s (contrast) for visual stimuli do not
- The visual system responds to the luminance differences (contrast) as the boundaries between objects and their background, which is the basis for brightness constancy
Define Visual Acuity.
- Measures the smallest detail that can be resolved & recognized
- It measures our ability to see objects/targets of different size at high contact (i.e., of different spatial frequency)
- One point on the CSF
Define Contrast Sensitivity.
- A psychophysical measure used to assess the sensitivity of the visual system to spatial luminance changes of various spatial frequencies
- Provides a more comprehensive test of spatial vision compared to VA
- CS is measured by finding the lowest contrast needed to see light/dark gratings of varied fineness or spatial frequency
Define luminance (L) and its unit.
- The physical amount of light emitted by a source of reflected from an illuminated object
- Unit of measure: candela per square meter, cd/m2
Define brightness. What affects brightness perception?
- Aka. Intensity
- The perception of a luminous object by the human visual system
- Perception of brightness can be affected by:
- Adaptation
- Aftereffects
- The presence or absence of other objects in the visual field
Define Contrast.
- Aka. Contrast Threshold
- Refers to the difference in luminance between an object (Lmax) and its background (Lmin) where the object is typically brighter than the background
- Contrast can be defined by: ∆I / Laverage
- ∆I = difference between peak and average luminances (Lmax – Lavg)
- Laverage = the average luminance of the grating (the average of light peaks and dark troughs)
- Though useful, not practical for the measurement of contrast
Define modulation (M). How is it calculated?
- The contrast of a repeating pattern, commonly referred to as grating, where it is not apparent which part of the pattern is the “object” and which is the “background”
- Example: a series of light (Lmax) and dark (Lmin) stripes, as in the zebra
- Calculated as the ratio of the difference of the luminances to the sum of the luminances
- M = (Lmax – Lmin) / (Lmax + Lmin)
- Where the luminance is measured across the spatial extent of each component part of the pattern
- M = (Lmax – Lmin) / (Lmax + Lmin)
- Alternatively, modulation can be expressed by the mathematically equivalent ratio of the difference between maximal and average luminances to the average luminance (Lavg), i.e. modulation (contrast) = ∆I / Lavg
- M = (Lmax – Lmin) / Lavg, where Lavg = (Lmax + Lmin) / 2
Describe CS Test designs.
- Most manufacturers of CS tests refer to their charts and instruments as presenting stimuli at certain “contrast” levels, also mention modulation
- Correct usage: modulation values
- Chart 5016 designated for Low Vision: uses the upper 12 contrast levels ranging from 0 – 1.65 log CS (100% to 2.2% Weber)
- Chart 5017 designated for Peak Contrast Sensitivity: uses the lower 12 contrast levels ranging from 0.6 – 2.25 log CS (25% - 0.56% Weber)
What is a logarithm? Describe the use of Logarithm in CSF.
- The logarithm of a number represents that value as a power of 10. Mathematically, it is much easier to characterize the range
- For CSF, because of the potentially large ranges of responses for both spatial frequency and sensitivity, the data is commonly plotted on logarithmic scales on both axes
- This results in the characteristic inverted-U shape of the function
Contrast Threshold (CT)
- The least amount of contrast that can be seen (or resolved by the patient) in a chart (expressed in %) = Weber contrast in letter chart
- The maximum contrast is 100% contrast (Snellen compared to 10% Bailey-Lovie chart)
- The optotypes of the visual acuity charts are close to the maximum contrast
Contrast Sensitivity (CS)
- The reciprocal of contrast threshold (CS = no units)
- CS = 1/CT
Log Contrast Sensitivity (Log CS)
- “Log of CS” converts the CS values to a linear scale
- Suitable for comparisons between levels of contrast sensitivity
- Smaller the log CS values = poorer the patients CS
Spatial Frequency
- Refers to the number of light/dark cycles per degree (c/deg) of visual angle (i.e., components of a repeating pattern, or grating, occur within a given area or space) on the retina
Gratings
- Usually defined by gradual sine-wave (i.e., sinusoidal) variations in luminance
Contrast Threshold (CT) to Contrast Sensitivity (CS)
CT = 1/CS
Contrast Sensitivity (CS) to Contrast Threshold (CT)
CS = 1/CT
Contrast Sensitivity (CS) to Log Contrast Sensitivity (logCS)
LogCS = log of CS
Log Contrast Sensitivity (logCS) to Contrast Sensitivity (CS)
CS = 10logCS
Name and describe the 3 types of contrast.
- Weber Contrast is preferred for Letter Stimuli
- Michelson Contrast is preferred for Gratings
- RMS Contrast is preferred for Natural Stimuli
Weber Contrast
- Weber Contrast = (Lb – Lt) / Lb
- Lb = luminance of background
- Lt = luminance of target
- Commonly used in cases where small target/features are present on a large uniform background
- Weber contrast is preferred for letter stimuli
- the measure is also referred to as Weber fraction
Michelson Contrast
- Michelson Contrast = (Lmax – Lmin) / (Lmax + Lmin) = ∆L / Lavg
- Preferred for gratings
- Incorporates the maximum and minimum luminance
Describe the Scales of Weber vs. Michelson Contrast.
- On both scales:
- 0% indicates the absence of contrast
- 100% indicates the theoretical maximal contrast
- For values between the two scales differ
- In the range from 1% to 10%, which is the range most used for clinical measurements, the Weber contrast values are about 2x the Michelson values
- The log(CS) values differ by 0.3
- In the range from 1% to 10%, which is the range most used for clinical measurements, the Weber contrast values are about 2x the Michelson values
What is RMS Contrast?
- Root Mean Square (RMS) Contrast – does not depend on the angular frequency content or the spatial distribution of contrast in the image
- Defined as the standard deviation of the Pixel Intensities
- Not used clinically, but for image processing/analysis