Lecture 7 and 8 Flashcards
– Relative size – Linear perspective – Texture gradient – Aerial Perspective – Lighting and shading – Interposition
monocular depth pictorial cues
– Accommodation
– Motion parallax
– Kinetic Depth Effect
monocular depth nonpictorial cues
Retinal Image size is inversely proportional to what?
distance
A decrease in retinal image size could be
caused by two things:
1) Object size remains constant but distance increases
2) Distance remains unchanged but the object shrinks.
This is the principle that, in vision, the perceived size of an object remains relatively constant, even if it’s retinal image size changes.
size constancy
similar to size constancy. The perceived shape of an object remains relatively constant even if the actual retinal image shape changes, as happens due to changes in viewing angle.
shape constancy (form constancy)
A projected after-image or eidetic image is altered in size in proportion to the distance of the surface on which it is projected.
emmert’s law
Perception of parallel lines or edges converging toward a distant vanishing point at the horizon.
It is based on the principle of size constancy.
linear perspective
distant objects are seen as more densely packed.
Also relates to size principle.
texture gradient
distant object appears less sharper than near object due to scattering of light.
aerial perspective (clarity)
the shadow of an object is interpreted as falling behind the object.
lighting and shading
the obstruction of the view of a distant object by a closer object.
interposition
more accommodation innervation needed for viewing nearer object
• A very weak cue
non-pictorial cue: accommodation
when an observer view a scene while moving, the relative position of objects change, depending on their locations relative to the fixation point .
motion parallax
Objects nearer than fixation causes what type of movement?
against
Objects beyond fixation causes what type of movement?
with
The relative distance of the object from the fixation point can be judged by the velocity of the motion.
The velocity is proportional to the separation of the object from the fixation point.
motion parallax
Can be used to determine whether an ocular opacity, seen during direct ophthalmoscopy, is located in front of or behind the pupil.
motion parallax
Fixate on iris and move slightly from side to side:
– Opacity in front of pupil shows what?
against motion
Fixate on iris and move slightly from side to side:
–Opacity behind pupil shows what?
with motion
The flat image of a rotating object acquires a strong perception of three-dimensional depth because of the motion in the image.
Related to motion parallax.
kinetic depth effect
The level of innervation of the two eyes required to fuse an object can be used as what type of cue?
depth cue
Increased convergence demand by BO prism makes the perceived image as what?
appears getting smaller and closer.
Increased divergence demand by BI prism makes the perceived image as what?
getting larger and farther
vectogram experiment is a test for what?
binocular depth cues: convergence
During convergence micropsia, the increased convergence innervation tells the brain that the object is getting closer, while the retinal image size remain unchanged. To maintain size constancy, observer thus perceive the object as getting smaller.
-SILO (small in, large out)
CONVERGENCE MICROPSIA
Convergence micropsia can only be observed during clinical testing condition, not in normal viewing condition. (T/F)
true
“SILO” is used as a feed back cue for clinician to monitor what the patient’s eyes are doing. (T/F)
true
What if observer reports SOLI?
- Just indicate the patient’s perceptual style, not necessarily means abnormal.
- when developing converging system, didn’t do it well. relies on other compensatory mechanisms to give depth cues.
the most robust depth cue.
A perception based mainly on horizontal relative
binocular disparity.
stereopsis
Crossed disparity stimulates stereoscopic _______.
nearness
Uncrossed disparity stimulates stereoscopic _______.
farness
Absolute depth perception tells the observer what?
how far objects are from themselves or other objects.
Relative depth perception tells the observer what?
about the comparison of depth.
Absolute depth is determined by what?
absolute disparity
the difference in the external longitudinal angles subtended by a object at each eye
absolute disparity
moon illusion can be explained by the principle of form constancy. t/f
false
b/c its based on size constancy
you are assessing a small media opacity using Direct ophthalmoscapy. while fixating on the pupillary border. when you move your head from l to r, you see the opacity move from r to l. where would this opacity most likely locate at?
cornea and anterior chamber
b/c we see against motion, the opacity should be located closer to us than fixation point
During perceived depth change in convergence micropsia, binocualr convergence cue and monocular accomoodation cue conflicts w/ each other. t/f
true
*when we close one eye to look at image, although its through polarized glasses, do we need to change accomm to make it clearer? nope b/c physical object didnt change. our monocular system tells us things didnt change in distance. when we are binocularly viewing, convergence tells us things are getting closer. this is a conflict.
stereopsis
Geometric disparity: The angular disparity in what?
object space
minimum displacement needed to see the difference in depth
stereoacuity
what is used to measure stereoacuity?
howard-dolman apparatus
Stereoacuity threshold in humans is what?
2-6 second of arc
*2 seconds of arc is very fine
you are using the howard-dolman apparatus to measure a pt stereoacuity.
testing distance= 3m
smallest distance b/w 2 rods that pt can differentiate: 1.8 cm
pt pd: 60 mm
what is his stereoacuity?
24 sec arc
*convert all to meters
pt stereoacuity: 10 sec arc
pd: 64 mm
target a: 4 meters away
target b: 1 cm further away from A
1.can he differentiate that A and B is separated in depth?
what is the minimum separation in depth b/w a and b that 2.the pt can appreciate the difference b/w them in depth?
1.n=8.3 sec arc
stereoacuity=10 sec arc
disparity is smaller than stereoacuity, therefore he cannot resolve!
- 1.2 cm
– The larger the PD, the larger the disparity seen
direct relationship
– the closer the objects, the larger the disparity seen
inverse relationship
viewing distance
Viewing Distance and Geometric Limit for Disparity
comparing to infinity, how close does the object have to be when you can tell that it is nearer than infinity, using only stereopsis?
ex: at 10 arc seconds, max distance is 1320 m
• Retinal eccentricity:fovea has the best stereoacuity
• Contrast:
– At contrast threshold (low contrast), stereopsis difficult
– Slight increase in contrast cause great improvement of stereopsis
– If contrast unequal between eye, stereopsis will be degraded
• Luminance level:dimmer–less sensitive to depth
• Color:blue cone system–poorer stereopsis
• Exposure duration:brain takes some time to process
Other Factors Influencing Stereoacuity
describe the lower limit of stereopsis: what is the smallest disparity we can detect
Stereoacuity
what is the largest disparity that can still give us depth sensation
upper limit
– High quality stereopsis, the perceived magnitude of object
depth is proportional to the disparity.
– This zone extends a short distance beyond Panum’s area. Right outside Panum’s area will see diplopia but still has patent stereopsis.
– Objects can be roughly perceived as being either nearer or farther than fixation, but the magnitude of depth cannot be perceived.
– This zone is beyond Panum’s area, will see diplopia.
– This violate Worth fusional hierarchy
• Principle: each eye sees a different image, image pair has disparity to stimulate stereopsis • Types: – Stereoscope – Anaglyph and vectogram – Liquid crystal shutter stereogoggles – Free fusion
Methods of Displaying Stereoscopic Images
Used in VT to expand convergence/divergence range
stereoscope: mirror stereoscope
Stereoscope: Brewster Refracting Stereoscope
a type of brewster stereoscope
anaglypy
Use polarized filter to separate two eye’s view.
vectogram
Alternating occlusion of one eye at high speed (15 Hz or higher).
liquid crystal shutter stereogoggles
Vision has 2 parallel neuronal pathways from retina to visual cortex:
- Parvocellularpathway–Fine stereopsis
* Magnocellularpathway–Coarse stereopsis
Fine stereopsis is more susceptible to what?
binocular anomaly.
small/ moderate strabismus.
Strabismus patients with small eye turns may have peripheral fusion/ peripheral stereopsis.
Vision can use 2 different types of stereoscopic image processing, depends on the environment. what are they?
local stereopsis
simple monocularly- visible objects, such as lines, edges, or contours.
Global stereopsis is more sensitive to binocular anomalies because
it is based only on fine stereopsis. Patient can not take advantage of monocular cues.
Clinical testing: random dot stereogram works better than local stereo testing to pick up reduced stereopsis.
patent stereopsis can be perceived even with slight diplopia?
true
b/c slightly outside of panums area can perceive stereopsis
what factors can degrade stereoacuity?
low stimulus contrast
testing in peripheral vf
blue colored stimulus
Fine stereopsis is more susceptible to any binocular anomaly, including anisometropia and strabismus. (t/f)
true
clinical stereo testing using random dot stereogram works better than local stereo testing to pick up mildly reduced stereopsis because global stereopsis is more sensitive to binocular anomalies. (t/f)
true
global stereopsis uses fine process pathway
the other is fine and course process
stereogram:
At 40 cm testing distance, No. 1 correlates to what? Perceived depth is what?
400” arc disparity; 4.8 mm
Stereogram:
At 40 cm testing distance, No. 10 correlates to what? Perceived depth is what?
20” arc disparity, 0.24 mm
Autostereogram
– A repetitive pattern
– The eyes must converge or diverge one repetitive width. Then the different left and right eye images will contain disparities.
– The disparities give the perception of stereoscopic depth.
contains repeating figures in different period.
Autostereogram
Over-converge by one period:
increased period produces what type of disparity
crossed disparity?
Under-converge by one period:
Increased period produces what type of disparity?
uncrossed disparity
Temporal disparity create stereopsis
color induced stereopsis
- Neutral density filter is placed in front of one eye.
- ND filter reduces the retinal illuminance to one eye, which causes the neural signal from that retina to be slightly delayed compared to the other eye’s.
Pulfrich Phenomenon
-
Potential causes of the Pulfrich effect
Try to draw out Pulfrich phenomenon with the filter in front of OD. What direction of movement do you expect? Clockwise or counterclockwise?
not sure
how is chromostereopsis caused?
- Transverse Chromatic Aberrations:
* Visual axis and optical axis of the eye slightly different.
whats the result of chromostereopsis?
• From the same fixation plane, blue light falls more
nasally on the retina than red light.
• Blue light creates relative uncrossed disparity– perceived as farther.
