Stereopsis Flashcards
1
Q
Define Depth Perception
A
- The ability to see the world in 3D
- Mix of binocular or monocular cues
2
Q
Name the monocular & binocular cues?
A
3
Q
The cues that permit interpretation of depth with one eye alone are called..?
A
Monocular or empirical cues of depth
4
Q
Which one is hard wired vs learned (Monocular and Binocular)
A
Binocular depth cues = Hardwired
Monocular depth cues = Learned
5
Q
List the pictorial cues (cues seen in photographs) vs. Nonpictorial cues (Monocular cues)
A
Pictorial cues
- Retinal image size
- Emmert’s Law
- Size constancy (Familiar)
- Linear perspective
- Texture gradient
- Imposition (interposition)
- Aerial perspective
- Shading (Light & shadow)
Presented in 2D representation like a photograph or painting/ Seen monocularly
Nonpictorial cues
- Accommodation
- Motion parallax
- Angular declination
- Kinetic depth effect
6
Q
Relative (Retinal Image) Size
(Image: Which one is closer?)
A
- No reference = assume balloons are the same size
- When visual depth cues are removed
- smaller retinal image = perceived farther away
- larger retinal image = perceived as near
- Relative size is an important depth cue when viewing a scene that includes objects whose sizes can be compared to eachother
7
Q
Size & Perceived Distance (Emmert’s law)
A
-
Emmert’s law: The perceived size of the object producing a retinal image of a given fixed size is proportional to its perceive distance
- Greater distance = afterimage appears to increase in size
8
Q
Describe the moon illusion (Ex. Emmert’s Law)
A
- Moon on horizon = Larger (interpreted as more distance)
- Moon on Zenith = Smaller (Closer distance)
- Even though angular subtense is the same under both conditions
- Tree, houses, fields & other interposing objects causes the moon to appear farther away when viewed on the horizon and hence make it look larger
9
Q
Describe Size Constancy (Familiar Size) when distance cues are not available
A
- 2 objects with unequal retinal images
- Same size at different distances
- Size constancy = perceptual scaling of the perceived size of an object according to the estimate distance
- When distance cues are not available, the visual system defaults to using reintal image size as the main cue to stimulus size
10
Q
Describe size constancy when numerous cues are available (Natural Scenes)
A
- Cues occurs with objects of known size
- Ex. Taxi’s are larger than people
- So the visual system assumes that the taxi is far away because we rely on experience
- Even if other cues are removed, we can use familiar size to judge distance
11
Q
Define size constancy
A
- When people’s perception of a particular object’s sizes does not change regardless of changes in distance from the object, (even though distance affects the size of the object as it is projected onto the retina)
- Tendency to perceive an object as being of one size no matter how far away the object is
12
Q
What occurs when size constancy fails?
A
Size illusion occurs
13
Q
Linear Perspective
A
- Related to relative size, can be experienced by looking down a a long corridor or railroad track
- The retinal image size of the rail separation is smaller int he background than the foreground creating illusion of depth
- although they are physically the same distance from the eyes
14
Q
Describe Texture Gradient
A
- Bricks in the background = more densely packed
- Densely packed object = smaller retinal image size = more distance
15
Q
Interposition (Imposition)
A
- Occurs when view is partially obstructed
- Church is interposed between the viewer and mountains (perceived as lying distant to it)
- Nearer object occldues the distant object creating a sense of depth
16
Q
Describe Aerial Perspective
A
- form of interposition/clarity
- Fog, rain, smoke and smog may act as interposing elements-causing them to appear more distant and contributes to sense of depth
- Distance objects appear less sharper than near
17
Q
Lighting & Shading
A
- Direction of lighting & shading of an object can tell us about the object’s depth
- When lighting falls on an object = casts a shadow
- Shadow falls behind the object creating a sense of depth
18
Q
Describe Accommodation
A
- Increase accommodation to keep the object at near in focus can provide cue for depth
- Hypothesis: level of innervation of accommodation could provide distance information
- not used as much to judge distances
- Accommodation = weak cue for depth perception
19
Q
Describe Motion Parallax
A
- kinetic monocular depth cue
- Results when a moving observer fixates on an object while noticing the relative motion of surrounding objects
- Relative motion provides information regarding distance
- Distance = With motion
- Near = Against motion
20
Q
A
21
Q
Kinetic Depth Effect
A
- Shape from motion
- Rotating wire 2D - differential motion = give rise to 3D percept
- Looming - rapid expansion (or shrinking) in the size of any given image
22
Q
Angular Declindation below the horizon
A
- Monocular observer stands upright in an empty room viewing an object at 10 feet
- Despite the absence of pictorial depth cues the observer is able to correctly judge the object’s distances
- The object makes an angle with the horizon referred to as angular declination below the horizon
- The visual system uses this angle to determine object distance
23
Q
BV Disadvantages
A
- They require more neurons and neural activity
- Two eyes MUST work together with little error
- More prcise controlled system required
Coordination Entails
- Eye movement coordination
- Equal VA in both eyes
- Binocular fusion
- Binocularity without suppression
- Healthy eyes without anisometropia, aniseikonia
24
Q
Misalignment between the two eyes incorrectly can produce
A
- Strabismus (eye turn)
- Amblyopia (reduced spatial vision in one eye)
- Suppression or
- diplopia
25
Binocular vision (Advantage)
* Spare eye
* **Wider field of eye** (Major benefit)
* Binocular summaiton
* **Stereopsis** (Major benefit) - important basisi for
* figure-ground segregation
* Avoidance of collision with objects
* Navigation through environment
* Stereopsis also improves performance on near visual tasks
* We are more accurate in perceiving depth (stereopsis) = Major advantage!!
* Figure-ground segregation - spot camouflaged objects in some surroundings
* Better navigation through our environment, and prevention of collisions
26
Describe ground visual segregation
* Depth perception would allow these columns to stand out against the natural void of the dark bg
* Pt w/ alzheimer's disease have issues w/ figure-ground discrimination
27
List the binocular cues of depth
* Stereopsis (Retinal disparity)
* Convergence
28
Describe stereopsis (Retinal Disparity
* Combining info of both eyes to perceive a 3D percept
* most accurate when images are on **non-corresponding points** that are close together (ex. within panum's fusional area)
* When fixating at distance object, the monocular images of the **near object will NOT fall on corresponding points**, but will have disparity
* **near object = 3D**
* Strongest depth cue
* Stereopsis: lateral (horizontal binocular disparity) difference between the fixated and nonfixated object
* allows visual system estimate what degree the object is removed from the horopter
* Is automatic or preattentive (basic feature of visual perception)
29
* Fixating at the center ball, the observer is
* able to judge the relative distances of the **three balls even if monocular cues** (size, interposition) **are eliminated**
* Light rays eminating from the distant ball strike the retinas nasal to the foveas, giving rise to retinal disparity
* **Uncrossed disparity** - images from nasal to the foveas = **ball is further**
* **Crossed disparity** - images form temporal to the foveas = **ball is closer**
30
**Stereoposis (Retinal disparity):** The binocular system processes ____ \_\_\_\_\_. (Tells us if the object is closer or further away from us compared to other objects)
* **Relative Disparity** NOT absolute disparity (not absolute position of the object)
* Stereopsis is very important contributor to depth perception **at near distances** not so much for far objects
* The threshold for retinal disparity requires such objects to be separated by great distances
* **Small disparity** = fusion (stereopsis)
* **Large disparity** = physiological diplopia
31
\_\_\_\_\_ is the most robust depth cue
What does the stereopsis test tests for?
**Stereopsis**
* Stereopsis is achieved automatically without conscious effort (**preattentive**)
* Ex. Stereo-circles pop out quickly without effort
* **Stereopsis test:** tests for suppresion
32
Which distance is crossed vs uncrossed (nearer/farther) and they lie on which side of the retina? (Temporal/nasal)
* Farther/nearer determined by relative image displacement OD/OS
* **Nearer** (crossed disparity) = images on temporal retina
* **Farther** (uncrossed disparity) = image on nasal retina (**relative to fixation**)
33
Convergence
* Convergence is the **weakest** binocular depth cue
* Degree of convergence can influence perceived **distance & size**
* Ex. Judge distance by how much one converges (level of innervation for convergence)
34
What is the **SILO effect**? (Convergence)
* **SILO effect** - measurement of pt ranges of convergence & divergence
* **smaller** = in
* **larger** = out
* **SILO effect** refers to the presumed change of the perceived size of a test object that a patient experience, while maintaining fusion when convergence or divergence is varied (**Prism effect in new glasses**)
* **Increased convergence** = BO prisms (plus lenses), object may appear to become **smaller & nearer**
* **Increased divergence** = BI prisms (minus lenses), object may apper **larger & further**
35
When will you experience SILO effect?
* **SILO effect** - prism effect in new glasses and adjustment to new glasses
* Natural consequence of size constancy
* Pt need to be cautioned with new glasses
* The response is used in visual therapy as a feedback mechanism to pt about their performance
* In general the visual system is poor at using convergence cues, but as with SILO effect can modify our judgement
36
Describe SOLI effect
* **Smaller OUT; Larger IN**
* Opposite of SILO effect can happen
* Due to subject prior experience that near objects are larger
37
Why is vergence important? Describe micropsia & macropsia.
* **Vergence** is necessary to our ability to process large absolute disparities
* useless for small disparities
* **Micropsia** - objects look smaller w/ prism = causes smaller object appearances as if further away,
* **Macropsia** - objects look larger w/ prism = laerger object appears **closer**
* **Long term adaption can improve this effect**
38
Define **relative depth**
* comparison of relative distance between objects (which one is closer & which one is farther away)
* Stereopsis is a **relative measurement !!**
39
Define relative disparity
* encoding the realtive depth between objects as the difference between the absolute disparities of the 2 objects
40
Stereopsis is a ____ measurement
* Stereopsis is a relative measurement
* **Absolute depth/Absolute disparity** = difference in angular distance from the fovea in 2 eyes
* **Relative depth/Relative disparity** = differences in angles subtended in object space (a relative to b)
* The binocular visual system processes **relative disparity** NOT absolute disparity
* Binocular disparity does not tell us the absolute position of any particular object
41
Define orientation disparity
* Orientation disparity - differences in the **orientation of targets** presented to each eye might yield a percept of depth (results from gradual change in horizontal disparity)
* **Diffrequency** - phenomenon where **stereoscopic tilit\*** (percept) is created by 2 sine wave grating targets
