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What is a depth discrimination threshold?
the smallest depth difference that we can perceive
What is the approximate stereothreshold for humans?
4-5 arcsec; 95-98% of population achieves stereoacuity of better than 40 arcsec
Why are many clinical tests unable to test an individual's actual stereoacuity?
because acuity may be better than smallest target in the test
T or F: Our brains combine the vernier acuities of each eye in some way to achieve stereoacuity.
False.
use of disparity for depth judgements is better than using only monocular cues to depth by a factor of 20
Does better stereoacuity mean better stereopsis?
Not necessarily
-depth matching may be poor because of a depth perception bias that affects accuracy
-some people may demonstrate asymmetries in the ability to see crossed vs. uncrossed disparities
How can we calculate angular disparity for small disparities?
n=(2a(Δd))/d^2
n= angular disparity in radians
2a= inter pupillary distance
Δd= linear separation between fixation target and second target
d=fixation distance
What is the relative disparity equal to?
difference between the absolute disparities of the two points
What is the absolute disparity of a fixated point?
zero; it is located in the horopter
What does disparity direction depend on?
which target is the reference
What are factors that affect stereoacuity?
-geometric factors
-retinal eccentricity
-target separation
-luminance
-exposure duration
-practice
-color
-optical blur/defocus/decreased contrast
-stimulus motion
How does PD affect stereoacuity?
the same linear depth between two points will have a larger angular disparity for someone with a larger PD; they will perceive greater depth for a given linear depth difference
What is true about angular disparity at near?
a small linear depth difference can have a large, easily detectable angular disparity
What is true about angular disparity at distance?
the same linear depth difference will have a much smaller angular depth
When does the geometric limit for disparity occur?
when Δd=d because you cannot have a disparity larger than your viewing distance
When is Stereoacuity best?
When the reference target is on the horopter and has zero absolute disparity
Where is the stereoacuity better: at the fovea or peripheral locations?
At the fovea. when observer fixates target, depth discrimination is better when made relative to the fixation target
Can the retinal periphery detect fine positional differences?
No because it has larger receptive fields therefore the peripheral retina is less sensitive to binocular disparity
What are characteristics of central stereopsis?
-operates over +/- 0.5° range at the fovea
-specialized for detecting small disparities
-specific for matching similar patterns in each eye
What are characteristics of peripheral stereopsis?
-operates over a range of +/- 7-10°
-selective for large disparities
-monocular images need not precisely match
T or F: peripheral stereopsis may still be intact when central stereopsis is absent.
True
What are fine disparities processed by?
small receptive fields (high spatial frequency)
What are coarse disparities processed by?
large receptive fields (low spatial frequency)
Stereothresholds decrease (get better) as a function of ______________ spatial frequency. (increasing or decreasing)
increasing (skinnier bars)
How big is the region at the fovea for detecting disparity?
4-6'
When do upper limits of stereopsis decrease?
with increasing spatial frequency (go diplopic faster with high spatial frequency targets)
Why does the range of stereoscopic vision decrease with increasing spatial frequency?
the decrease in stereothresholds and the decrease in the upper limit of stereopsis do not change at the same rate (asymptote at 2.4 cpd)
What is processed first: coarse or fine disparities?
coarse; at short exposure durations, only coarse disparities can be seen
How are coarse disparities best seen?
with temporal depth modulations (oscillations)
How are fine disparities best seen?
with static stimuli
Fine stereopsis is mediated by the _______________ system.
parvocellular; coarse is mediated by magnocellular
In terms of target separation, when is stereoacuity threshold the lowest?
reference and test targets are laterally separated by 15-50 arcmin
What is true if reference and test targets are widely separated?
if greater than 50 arcmin separation, their depth difference becomes difficult to compare and thresholds will be elevated
What is true if reference and test targets are too close together?
if <15 arcmin, stereoacuity is degraded
What is the result of dimmer background luminance on stereoacuity?
less sensitivity to disparity and the perception of depth from disparity (very noticeable under scotopic conditions)
Can we see depth with short exposures?
Yes but thresholds are elevated; the best stereothreshold is achievable if the observer is given at least 100 msec exposure to the target
Why can stereoacuity improve with practice?
maybe improved visual attention
What is the correlation between stereoacuity and color?
stereo thresholds are worse for blue targets than for red or for green targets due to larger receptive fields for blue cones ( the blue cone system has lower resolution and lower contrast sensitivity)
Stereoacuity is _____ affected by binocular blur than is spatial resolution. (less or more) Why?
less; stereoacuity is much more adversely affected by monocular blur because it causes a significant decrease in stereoacuity
What is true at extremely low contrast (<5%)?
stereopsis is difficult to appreciate as stereo thresholds are elevated; observers can determine direction of disparity but not magnitude
What is the relationship between contrast and stereo acuity?
as contrast increases, stereoacuity rapidly improves
What is true when contrast is <10x threshold?
stereoacuity is inversely proportional to the square of the contrast
What is true when contrast is >10x threshold?
stereoacuity is inversely proportional to the cube of the contrast (so threshold improves more rapidly with higher contrast)
What is true if the contrast between the two eyes images is unequal?
stereoacuity is worse than when both eyes have equal contrast even if the equal contrast is the lower value of the unequal contrast pair
T or F: Stereo acuity is better with natural fixation than with stabilized retinal images.
True
What is the stereomotion threshold?
36"
What is stereomotion detection?
the binocular detection of motion in depth in the absence of monocular cues
How does the visual system detect stereomotion?
-serial processing (process the rate of change in disparity overtime)
-parallel processing (compare the speed of the monocular half images)
Are there neurons that respond specifically to stereomotion?
there is little evidence to support this
What is patent (quantitative) stereopsis?
-stereopsis from disparities within panum's fusional range
-both direction and magnitude of depth differences can be accurately perceived
What is latent (qualitative) stereopsis?
-stereopsis from a range of diplopic disparities within which the direction but not the magnitude of depth differences can be accurately perceived
T or F: Panum's fusional range decreases with retinal eccentricity.
False; it increases
What is the Pulfrich phenomenon?
An object swinging in the frontoparallel plane appears to be traveling in an elliptical pattern toward and away from the observer in depth when there is a lower luminance contrast in one of the observer's eyes
What are clinical conditions that can initiate the Pulfrich effect?
-optic neuritis
-anisocoria
-monocular cataract
-cornea scar
-any ocular pathology that affects on eye to a much greater extent than the other
What is the classical explanation for the Pulfrich effect?
temporal delay is geometrically equivalent to spatial disparity so the Pulfrich stimulus activates mechanisms which usually process spatial disparity (doesn't work on the stroboscopic effect)
What does the classic explanation of the pulfrich effect not work for?
stroboscopic pulfrich effect
