1
Q
How do distance and depth bring back the inverse problem?
A
How does the visual system recover information about the 3D spacial structure of the environment if it has to get information about an object’s distance and orientation of the surface in order to do so?
Visual system must compute the orientation and distance of each surface reflecting light to the retina.
2
Q
What are the pieces of information necessary for orientation and distance of surfaces?
A
- Slant
- Distance
3
Q
Slant
A
angle between line of sight to the surface patch and its normal (the direction perpendicular to the surface)
4
Q
Tilt
A
direction of a surface’s depth gradient (orientation)
5
Q
Any stimulus is ambiguous (can come from distal point at any distance), so how does visual system determine depth and distance of objects in visual field?
A
(among other things)
- accomodation
- convergence
- binocular disparity
- motion parallax
6
Q
What is accomodation?
A
- ocular, monocular, absolute D info
- lens shape changes when fixated object changes in distance.
- lens is flat at greater distances and curved at short distances, which serves to focus image
- Focal point: the point behind the lens where light rays passing through it converge.
- If accomodation fails, image is blurry. Elderly have accomodation problems because the elasticity of the lense decreases with age.
7
Q
What is convergence?
A
- ocular, binocular, absolute
- the angle formed by the direction of gaze of two eyes when both are fixating on an object.
- The closer the object, the larger the convergence angle. The furhter away the object, the smaller the convergence angle.
- Reduces double images of objects.
8
Q
What is binocular disparity?
A
Disparity that occurs because our eyes are a few inches apart, so things are viewed from two different distances in space and there is an imperfect overlap.
- Visual system disparity into depth, so it is a relative depth cue.
- Direction of disparity says what’s in front of what.
9
Q
What is a horopter?
A
a notional curved surface that delineates corresponding retinal points
10
Q
What is a stereoscope/stereoscopic depth?
A
- stereoscope: contraption that allows each eye to view a different image and allows for artificial depth to be viewed.
- Stereoscopic depth: the disparity that arises from a difference in widths between images transmitted to the eyes, signaling depth.
- We are sensitive to disparity up to about 100 feet.
11
Q
What is the correspondence problem?
A
Which element in the right eye’s image correspond to what element in the left eye’s image? Shape may play a role in allowing the visual system to create a correspondence between these two images.
12
Q
What are the two theories regarding temporal shape analysis?
A
- Shape first theory: The left image and right image exist, then each eye individually analyzes the shape of the image. Once that shape analysis is completed, stereopsis can occur.
- Stereopsis first theory: The left image and right image exist, then they are fused and stereopsis occurs. Once that happens, shape can be analyzed.
13
Q
Random Dot Stereogram
A
- no monocular shapes
- shows that disparity analysis precedes shape analysis since there’s no shape in either eye’s view.
- Thus, shape is derived from and follows disparity analysis and the resolution of the correspondence problem.
14
Q
What is binocular rivalry?
A
- grossly mismatched images are projected to each eye, and rivalry occurs, since one image is seen while the other image is suppressed.
- suppressions alternate
15
Q
Why do we experience singularity of vision, even though we always have two different images on our retinas?
A
May be:
- alternating suppression of one image by the visual system
- fusion: the images on the retina fuse together to form one cohesive image.
16
Q
What are the rules governing binocular rivalry?
A
- figure dominates ground
- figure carries area of ground with it
- meaningful image dominates
- evidence for suppression theory: if you close one eye, light doesn’t appear dimmer, and if you’re looking at a light grey stimulus in one eye and a dark grey stimulus in the other, the percept isn’t medium grey.
17
Q
What is motion parallax?
A
- monocular, dynamic, relative to depth
- perceived differences in movement between near and far objects
- caused by changes in the angle of the stimuli on the retina as one moves further across the retina than the far object, which makes closer object appear to move faster than further object.
- Fixated point does not displace
- points closer than fixation displace in the opposite direction
- good for relative depth information at great distances, unlike disparity
18
Q
Looming
A
If something on a screen increases in size but maintains its dimensions, it looks like it’s moving toward the observer. Babies show this, as well.
19
Q
What are the configural (pictorial) depth cues?
A
- perspective (linear and size)
- texture density gradients
- height in plane
- occlusion
- shadow (cast and attached)
- familiar size
Information that is available from just looking at a picture
20
Q
What are the ocular cues?
A
- accomodation
- convergence
- binocular disparity
- motion parallax
- accretion and deletion of texture
21
Q
Perspective
A
- linear perspective is strongest source of configurational depth information.
- parallel lines project converging parallel lines at same vanishing point onto the retina
- 3D scenes project a 2D image onto flat surface, but is recoverable from configurational depth cues (inverse problem addressed)
- size perspective: if it’s smaller, it’s further away, like Gibson cylinders
22
Q
What is the Kinetic Depth Effect?
A
- the three-dimensional structural form of an object can be perceived when the object is moving
- if a shadow is cast onto a screen by a rotating wire shape, a viewer can readily perceive the shape of the structure behind the screen from the motion and deformation of the shadow.
- Assumption of rigidity: The visual system prefers to see a rigid object rotating than the 2D object morphing.
23
Q
Height in plane
A
position relative to the horizon line. The further above it is, the higher it is. Requires perceiving ground plane. Not super powerful, but can be accompanied by another type of perspective.
24
Q
Texture density gradients
A
- Gibson
- Density of texture increased with distance (field of flowers)
- The steeper the gradient, the greater the slant in depth, sharp discontinuity gives information about edges.
- direction of gradient gives direction of slant
25
Foreshortening
circles looking elliptical instead of like circles implies a change in depth or position. Objects appear more oblong or distorted from their true shape.
26
Aerial Perspective
Things are blurred and bluish at greater distances (mountains in distance). Not as powerful as other types of perspective.
27
Interposition
* partial occlusion is seen as depth
* the occluding shape says it's in front, but there's no information about how far in front it is.
* does not require familiar figures
28
Two types of shadows
1. Cast: person's shadow reflected on sidewalk
2. Attached: shadows from contours on the object itself (noses, indentation of eyes). When light falls on convex object, it casts shadow below it. When light falls on a concave object, it casts shadow above it. Shadow location gives strong information about depth of an object.
29
Familiar and relative size
* familiar size requires familiarity with object. If dime appears the same size as a quarter, it must be closer because we know it's a smaller object in real life.
* relative size: the smaller the image, the farther away it is
30
What supports the idea that depth perception is innate?
* Spalding: put piglets on a chair to see if they would take a step and fall. They didn't go anywhere.
* Lashley and Russell: used neonate rats raised in the dark and put them on jumping stand. When shown a flat surface, the rat jumped to it, and the jumping stand measured the force of the jump, suggesting that rats could calculate how much force was needed to reach the other flat surface.
* Gibson's visual cliff: only aquatic babies crossed the cliff
* shadow is an innate depth cue
31
Arguments for learned depth perception
* Transactionalists: proposed that we learn about depth from interactions with the environment.
* Trapezoidal window: window-shaped object that appears to be viewed from an angle so that one side is larger/closer. As the trapezoidal window rotates, it appear to oscillate, since we know the far side is never the larger side.
* Ames room: our experience with rooms iis that they are rectangles and the dimensions do not vary wildly (depth/length/width look the same but are not)
* we don't have to learn to see in the third dimension (it's innate) but experience and pictorial cues can inform our perceptions.
32
What are the frames of reference for direction?
* retinocentric (the retina)
* egocentric (the self)
* exocentric (the visual scene)
33
Retinocentric direction
* when the object's direction is defined relative to the center of the retina
* given by the momentary positions of elements in the retinal image where the image at the fovea is in the center of the retina
* doesn't determine what's straight ahead, since the eye's position changes constantly
34
Egocentric direction
* the location of an object with respect to the self
* determined both by subject (eye plus image location information) and object (roelofs effect) information.
* This relationship is modifiable through perceptual learning.
35
Exocentric direction
the location of an object with respect to another object
36
Direction constancy
perceiving an object as having a constant, unchanging direction despite the position of its image on the retina (due to the change of the motion of our own eyes)
37
Subject-relative direction constancy
* Retinal image information coupled with eye position information may account for perceived direction constancy
* If the eye is looking 20 degrees to the left and an image is 20 degrees to the right of the center of the retina, the brain calculates that the object is straight ahead.
* Hill experiment: asked participants to set a light to directly in front of them while eyes were fixed 30-degrees off center. Underconstancy occurred (2 degrees short of center, underestimate how much our eyes are turned). But Ps could set the light in front of them when looking straight ahead, so there's some form of subject-relative information.
38
Object-relative determination of egocentric position
* Roelofs effect: when frame of reference is offset from directly ahead of observer, they place a light off center by a few degrees in the direction of the center of the frame.
* Frame of reference does not affect open-loop pointing (where participant can not see hand), pointing is accurate despite visual mislocalization.
* Speaks to the idea of separate dorsal (sensorimotor) and ventral (perceptual) visual streams
39
Adaptation to altered radial direction
* If eyes are straight ahead and an image is directly on the fovea, does it mean the object is directly ahead? Not necessarily.
* Prisms displace object image toward the base
* Shows visual system can adapt and recalibrate the relationship between image location, eye position, and perceived position of object
40
Adaptation Experiments
1. Helmholtz: Prisms, made things appear 20 degrees off center and saw if people would point to object in front of them (full adaptation) or if they would point off center. This experiment did not affect open loop pointing, still pointed (the real) ahead.
2. Held: Adaptation requires recalibration through perceptual learning of the relationship between eye position and image location.
3. Harris: proprioceptive adaptation, no perceptual learning, but rather visual capture of where you feel your arm, which recalibrates proprioception. Adaptation occurred with observed arm, but not unobserved arm.
41
Size Constancy (over/under)
* Overconstancy: like the trapezoidal window, nothing looks larger when it's farther away. Not a real perceptual phenomenon, more likely a judgmental phenomenon arising from the size-distance paradox
* underconstancy: objects look smaller at a distance, perhaps because of inadequate distance info. Real perceptual phenomenon, since distance information may be inadequate at great distances.
42
Size perception
* retinal image size is ambiguous indicator of object size
* image size varies inversely with distance from the retina (law of visual angle)
* prior knowledge does not affect size perception (gibson cylinders)
43
Misplaced Size Constancy
* Muller Lyer an Ponzo illusions may be explicable in terms of size constancy.
* Imagine Muller Lyer is part of a room and the corner of one wall has inward fins and the corner of the other wall has outward fins. The one with the outward fins would look far away naturally, so since the retinal images are the same size, the one that';s farther away must be considered longer.
* These cues are implicitly telling us about distance, which we are then taking into account.
44
Moon illusion
* horizon moon appears to be much larget than at its zenith
* size-distance paradox: since moon looks smaller at zenith, it's judged to be further away than on horizon.
* eye elevation had no effect on size judgment
* Rock & Kaufman: take moon away and ask to judge the distance to the horizon. They judge it farther than they do the zenith.
* Moon on horizon looks about 50% larger
45
Two modes of constancy perception
1. Subject-relative constancy perception: indirect, taking distance into account, as well as other info that is not relevant. Other information is often extra-retinal
2. Object-relative constancy perception: direct, perceived size is based on the object's relationship to the surround/ground. Ponzo and Gibson cylinders are examples. Since we think ponzo is receding in distance, the further line occludes more of the ground, so it appears larger. If lines turned vertical, people have no problem seeing that they're the same length.
46
Relational Determination of Size
* participants were shown a line inside frame of reference, then they were shown another larger frame of reference and asked to match the length of the shown line
* 50% of participants drew the line to have the same ratio as the comparative image. Other 50% showed partial ratio effect.
* Ratio effect explains size constancy. If we see a picture of a man next to a house, we will think it's the same sized man no matter how far away the picture is taken. The ratio between man and house remains the same.
47
Frames of reference in size perception
* Frames of reference are very important and can create illusions.
* Ebbinhaus illusion shows two circles of the same size. One is surrounded by larger circles and the other surrounded by smaller circles. The one surrounded by smaller circles appears larger, though they're the same size. The frame of reference has changed, though.
48
Occlusion illusion
A figure bounded by an occluding edge appears larger than the same figure not bounded by an occluder.
49
Size illusions
* Ebbinghaus circles
* Muller-Lyer
* Ponzo
* Vertical/horizontal
* Titchner illusion
50
Innateness of size constancy
* Bower: Babies conditioned to make certain response to a stimulus (30 cm cube at 1 m) at 2 weeks old.
* then tested for constancy with 30 cm cube at 3 m (size constancy match)
* Tested 90 cm cube at 1 m (distance match)
* Tested 90 cm cube at 3 m (retinal image match)
* Retinal image match had fewest responses
* Size constancy and distance matched conditions had roughly the same number of head turns
* Size constancy appears innate
51
El Greco Fallacy
* El Greco was a painter who painted people tall and thin (elongated figures)
* There was a belief that he painted like this because he had an astigmatism that stretched his images in a vertical direction
* Discredited because only the people were stretched. If all vision was elongated, then all of the painting would be elongated (thus appearing proportional)
* Thus, it was likely just an artistic decision
52
Size Constancy (theories)
* Helmholtz: (indirect): subject-relative perception of size, computational, relationships between object and observer
* Gibson (direct): object-relative, size perceived as a result of relationship between objects or objects and their textured surround (ground)
53
Motion
* perceived continuous change of position over time
* motion of an image on the retina is not the main determinant of perceived motion
54
Paradoxical Motion
motion perceived without the change of position
55
"Image motion is neither necessary nor sufficient for the perception of motion"
1. Images of stationary objects move on the retina (our own eye motion), but if we move our eye 10 degrees one way and the position of the image is 10 degrees the other way, they cancel out.
* Despite image motion, appears to be stationary (vector addition)
2. If you follow a moving object with your eyes, it does not move on the retina, but all the stationary objects around the followed object do move on the retina.
* No motion of tracked image on the retina, but object motion is perceived
3. Illusions of movement: Apparent motion, Autokinetic Motion, Induced Motion
56
What Is The Source of Eye Movement Information?
•Afferent: Feedback from joint receptors to central nervous system (CNS) •Efferent: Outflow from CNS to the brain
* No joint receptors in eye muscles, therefore no afferent information about eye movements
* Information must be **Efferent**. Eyes are being commanded by CNS to move, which is coded in the CNS and available to the perceptual system to know what's moving and what's stationary
57
Comparator Mechanism
* Comparator Mechanism (hypothetical): compares efferent copy or corollary discharge information with image motion
* a process comparing image with observer motion in determining whether an object is moving or stationary and, if moving, how fast and in what direction (subject-relative determination)
58
Early evidence for comparator mechanism
* Ernst Mach: Putty around the eye\>\>try to move eye\>\>apparent motion of scene. (efferent eye motion information not matched by image displacement. Visual field jumps, creating a mismatch that the visual system can't explain.
* Helmholtz: push eye with finger\>\> Visual field jumps; there is no efferent information from the CNS telling the eye to move even though the eye is physically moving, causing images to change position on the retina.
* Brindley & Merton: curarized eyes, eyes were paralyzed but then tried to move them, which caused the visual field to jump (efferent info not matched with image displacement)
59
Optokinetic drum
* Holst and Mittelstadt
* Reafference (afferent feedback associated with efference)
* Exafference: (afferent feedback, not associated with efference)
* vertical stripes rotated inside apparatus, causing fly to fly in the direction opposite motion in an attempt to stabilize the motion of the stripes (optokinetic response)
* fly's head flipped upside down, so when tried to fly in opposite motion it actually doubled the motion of the stripes and the fly flew faster and faster until it died
60
Losses of Position Constancy
* Aubert illusion: underconstancy, if you track a light with your eyes (slow pursuit), you underestimate how fast it's going. If tracking accurately and eyes are moving so it remains on the fovea, you estimate it moving more slowly than if it were moving at the same speed but eyes remained stationary (underestimation of eye velocity).
* Filhene illusion: When a moving object is pursued by eye, stationary objects in scene perceived to move in opposite direction. This may be due to an underestimation of speed of eye motion (underconstancy)
61
Object-relative explanation of position constancy
* Direct (Gibson)
* visual proprioception
* Position constancy not based on extra-retinal information but on behavior of dynamic optic array
62
subject-relative explanation for position constancy
* indirect
* based on relation between eye movement info and image motion
* eye movement information derived from efference, not afference
63
Stoboscopic/apparent/phi motion
* illusions of motion
* no motion on the retina, but motion perceived
* color doesn't destroy stroboscopic motion
* Kaufman: evolutionary value because stroboscopic motion can explain why we see motion when it is very fast and what would otherwise be on the retina would be a blur.
* when light moving 17 degrees/sec, Ps saw motion when middle was occluded, but when ends were ocluded, all they saw was a blur
* Phi innate in babies and guppies
64
Does apparent motion depend on the fact that the retina is stimulated in two different locations?
no
* gestaltists said yes, thought it was isomorphic process in the brain
* Not excitation of two different locations on the retina, but rather the perception that light is coming from two different points in space
* perceiving change of direction, since if you foveate each point of light, there will be no motion on the retina
65
Correspondence problem (motion)
How does the visual system know which item to pair with what?
* Wagon wheel effect: spokes appear to go wrong direction on TV because if the wheel is turning clockwise 250 degrees per frame, it appears to be going 10 degrees counterclockwise. If four spokes moving at 90 degrees per frame, it will appear that there's no motion at all.
* proximity can account for this pairing, shape can not
66
Types of stroboscopic motion
1. Short range: occurs over small distances and small temporal increments, element motion
2. Long range: degrees apart, long ISIs, very influenced by cognitive processes, group motion (ternus effect/phenomenal identity, group maintains its identity with motion)
67
Autokinetic Motion
* if you fixate on single light in the dark, it appears to dance.
* not because of motion of the eyes, but because there's no frame of reference and there's uncertainty about one's eye movements in the dark
* Another case of apparent motion when there is no image motion on the retina
68
Induced Motion
* seeing motion without any image motion (duncker)
* in the dark, look at screen with one light that moves very slowly (subject-relative motion threshold)
* if you add another light and make one move and one remain stationary, motion will be detected at a lower threshold (object-relative motion threshold)
* motion is ambiguous since there are no frames of reference, so the stationary one may appear to be the one in motion (two point induced motion)
* if you put frame around one point of light and move the frame, the light appears to be moving while the frame appears stationary (even if the frame is moving faster than the subject-relative threshold)
* object-relative motion dominates visual system
69
Induced motion of the self
* When you're inside the frame of reference
* inside train and train across the platform moves, you feel like you're the one moving
* vestibular system responds as if you were actually moving (visual proprioception)
* can cause an individual to lose his balance
* Gibson believed in visual capture of proprioception
* "swinging room" where individual is stationary causes people to teeter or fall
70
Velocity Constancy
1. Subject-relative: takes distance into account
2. Object-relative: looks at rate of displacement relative to a surrounding frame of reference. Even if objects are far away and the displacement is less, it still displaces the same amount of background as it would if it were close to the observer
71
Velocity transposition
* object in the larger frame of reference will have to move more quickly than object in smaller frame to look like they are moving at the same rate
* has to do with rate of displacement of the frame of reference. At half the speed, an object will cover as much of a frame of reference half the size as an object moving twice as fast in a frame of reference twice as large
* minor underconstancy
Visual Perception Final
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