Task 6 Three dimensions Flashcards
Cue approach to depth perception
Explains how we get from the flat image on the retina to three dimensional perception of the scene
Occlusion
Cue that one object is in front of another. This cue is learnt through previous experience with environment and once it is learnt, we automatically associate particular cues and depth –> We experience world in three dimensions
Oculomotor cues
Cues based on our ability to sense the position
Convergence (Oculomotor cues)
Inward movement of eyes that occurs when we look at nearby objects.
Most effective cue of the two.
Accomodation (Oculomotor cues)
Change in shape of lens that occurs when we focus on objects at various distances.
These cues work over distances at close range (up to 2 meters).
Monocular cues
Cues that work with one eye. These include accommodation, pictorial cues and motion-produced cues.
Pictorial cues
Sources of depth information that can be depicted in a two-dimensional picture. These cues are stationary, meaning they are fixed.
Occlusion
Occurs when one object (partially) hides another from view.
Partially hidden object seen as being farther away.
Does not provide info about object’s distance.
Works at any range.
Relative size
When two objects are of equal size, the one farther away will take up less of our field of view than the one closer.
This cue depends on a person’s knowledge of physical sizes.
Works at any range.
Familiar size
Cue used when judging distance based on prior knowledge of size of objects.
Most effective when other information about depth is absent.
Texture gradient
Elements equally spaced in scene appear to be more closely packed as distance increases.
Increasing fineness of texture as distance increases enhances perception of depth.
Relative height
Objects with their bases closer to horizon are usually seen as being more distant. Works from 2m.
Perspective Convergence
Cue experienced when looking down parallel lines that appear to converge in distance.
Atmospheric perspective
Occurs when distant objects appear less sharp than nearer objects, with a slight blue tint. Works above 30m
Shadows
Decreases in light intensity caused by blockage of light that provide info about location of objects.
Motion produced cues
Cues that emerge when we start moving, that enhance our perception of depth. Both work at close and medium ranges (up to 20 meters).
Motion parallax
Occurs when nearby objects appear to glide rapidly past us, but more distant objects appear to move more slowly.
Example: Moving nearby cars appear to speed by in a blur, whereas those farther away appear to be moving slightly.
Explanation: Image of objects closer to us move farther across the retina than that of objects farther away.
Deletion and Accretion
As an observer moves sideways, some things become covered (= deletion), and others become uncovered (= accretion).
Binocular depth information
cue that relies on info from both eyes
Stereoscopic vision
Ability to use binocular disparity as a cue to depth.
Binocular disparity
Differences between two retinal images of same scene. Basis for stereopsis.
Difference with monocular vision – When watching a 2-D movie, both eyes receive same image, so depth is indicated by monocular pictorial depth cues. When watching a 3-D movie, both eyes receive different images, so stereoscopic depth perception occurs
Corresponding retinal points
Points on the retina that overlap if eyes are superimposed on each other.
Horopter
Imaginary circle that passes through point of focus. Surface of zero disparity
Non corresponding points
Images of objects that are not on horopter fall on noncorresponding points.
Absolute disparity
– Degree to which these objects deviate from falling on corresponding points. It is determined by measuring angle between where corresponding point would be located and where it is actually located.
Angle of disparity
Amount of absolute disparity. It provides information about object’s distance from horopter, with greater angles of disparity indicating greater distances from it
Crossed disparity
Sign of disparity produced by objects in front of horoptor. Objects located in front of horopter appear to be displaced to the left in right eye, and to the right in left eye
Uncrossed disparity
Sign of disparity produced by objects behind horoptor. Objects located behind horopter appear to be displaced to the right in right eye and to the left in left eye
Relative disparity
Difference in absolute disparities of objects in a scene that remains the same as an observer looks around a scene.
Example: When gaze is on woman, her image falls on observer’s foveas (= disparity is zero) and that of the man falls on noncorresponding points (= disparity). When gaze is shifted to man, his image falls on foveas (= disparity is zero) and that of the woman falls on noncorresponding points (= disparity).
DISPARITY (GEOMETRICAL) CREATES STEREOPSIS (PERCEPTUAL)
Disparity is related to geometry, the locations of images on retina, and stereopsis is related to perception, the experience of depth created by disparity
Stereoscope
Device for simultaneously presenting slightly different images to one eye and the other, producing an illusion of depth because it creates same binocular disparity that occurs when a person views the scene naturally.
Random-dot stereogram
Stereogram made of large number of randomly placed dots, that contains no monocular cues to depth. Aimed at showing that disparity alone can result in depth perception
Correspondence problem
Visual system must compare images of left and right retinas in order to calculate the amount of disparity.
Visual system matches images on left and right retinas on the basis of specific features of objects.
Still no answer as to how it works with random-dot stereogram
PHYSIOLOGY OF BINOCULAR DEPTH PERCEPTION
Neurons have been found that respond best to binocular disparity.
Binocular depth cells (disparity-selective cells)
Respond best when stimuli presented to left and right eyes create specific degree of absolute disparity.
Primary receiving area – Neurons sensitive to absolute disparity.
Temporal lobe and other areas – Neurons sensitive to relative disparity.
Depth perception begins in primary visual cortex and extends to different areas in ventral and dorsal streams.
Selective rearing
experiment by Blake and Hirsch where cats were reared by alternating vision between two eyes every other day during their first 6 months.
Results: Showed that cats had few disparity-selective neurons and they were unable to use binocular disparity to perceive depth.
Conclusion: Eliminating disparity-selective neurons eliminates stereopsis and therefore, disparity-selective cells are responsible for stereopsis.
Microstimulation
a procedure in which small electrode is inserted into cortex and electrical charge is passed through electrode to activate neurons nearby.
Procedure: Monkey observes random-dot stereogram while experimenters stimulate neurons in monkey’s cortex that were sensitive to particular amount of disparity. When neurons tuned to disparity different from that of images on retina, monkey shifted its perception of depth toward disparity signaled by stimulated neurons
Holway and Boring experiment
Experiment aimed at demonstrating that we can misperceive size when accurate depth information is not present.
Procedure: Observer changes diameter of comparison circle in left corridor to match her perception of the size of test circles presented in right corridor. Each test circle, presented separately, casts same image on retina and has visual angle of 1 degree.
Visual angle
WHAT IS A VISUAL ANGLE?
Angle of object relative to observer’s eye that is determined by extending lines from person to the lens of observer’s eye.
It depends on size of stimulus and on its distance from observer – When person moves closer, visual angle and size of image on retina become larger.
Small object near and larger object far can have same visual angle (= thumb method).
We perceive the sun and moon to be same size because we are unable to perceive their distance, therefore, we base our judgement on their visual angles.
Size constancy
Fact that our perception of object’s size is relatively constant, even when we view object from different distances.
Under conditions of poor depth information, size constancy decreases.
Size constancy remains even if size of object in retina changes.
Size distance scaling
Idea that perceiver combines the size of object formed on retina and its distance to perceive the actual size of an object.
Equation – S = K ( R x D ).
S Object’s perceived size; K Constant; R Size of retinal image and; D Perceived distance of object.
As person goes farther away from us and size of image in retina (R) becomes smaller, our perception of person’s distance (D) becomes larger. Since it creates a balance, it results in perception of size (S) staying the same.
Emmert’s law
The farther away an afterimage appears, the larger it will seem.
Size of afterimage is determined by distance of surface against which afterimage is viewed.
(R) remains the same, (D) increases and, therefore, (S) is larger.
Misapplied size constancy scaling
Explanation for this effect proposing that mechanisms that help us perceiving in 3-D world sometimes create illusions when applied to objects drawn on 2-D surface.
Fins on right line make it look like inside corner of room and fins on left line make it look like corner viewed from outside.
(R) remains the same and (D) is larger, therefore, (S) is determined by perceived distance.
Conflicting cues theory
Our perception of line length depends on (1) actual length of vertical lines and (2) overall length of figure.
Overall length of right figure is larger so, the vertical line appears larger.
Theory rejects idea that depth information is involved in determining illusions.
Ponzo illusion
Illusion that causes two people of equal size to appear very different in size.
Reason – Construction of room because it is shaped so that left corner is almost twice as far from observer as right corner, causing woman on left to have much smaller visual angle than one on the right.
Size-distance scaling – (D) remains same, (R) smaller for woman on left, therefore, (S) is smaller.
Relative size – Perception of size of two women is determined by how they fill distance between bottom and top of room.
Moon illusion
Illusion in which moon on horizon appears much larger than when it is higher in the sky.
Constant visual angle – Moon’s physical size and distance from Earth remain the same throughout the night.
Apparent distance theory – States that moon horizon appears more distant because it is viewed across filled space of terrain, which contains depth information, and when it is higher in sky, it appears less distant because it is viewed through empty space, with little depth information.
Horizon appears to be farther away than the sky overhead – (R) remains same, (D) is larger for moon on horizon and, therefore, (S) is larger.
Angular size contrast theory – States that moon appears smaller when surrounded by larger objects.
Other factors – Atmospheric perspective, color and oculomotor factors.
Two eyes, to views: Brain and depth perception
Visual-image processing from eye to brain happens in stages
Rudimentary features are extracted early on areas V1 and V2 before reaching next stages for more refined analysis.
These include orientation of edges, direction of motion and color.
Many pathways go back from stage to stage.
V1 – Contains cells signaling disparity. Binocular disparity starts here.
V2 – Contains cells that extract illusory contours. Contours process starts here.
Correspondence problem
In binocular vision, problem of figuring out which feature of image in left eye should match with that of right eye.
Brain solves this problem by recognizing forms and then comparing extended outlines of the forms, helping the brain to avoid false matches.
Is comparison done before object boundaries are recognized or after?
Stereopsis precedes detection of extended outlines and boundaries.
Random-dot experiment – Square emerges only a result of a stereoscopic fusion, stereo matching must be a point-to-point measurement of displacement. Outline of square emerges from this comparison.
Special circumstances – Sometimes form perception precedes stereopsis, which shows the flexibility of brain’s visual centers.
It occurs when dots defining squares are uncorrelated in two eyes and thus, recognized separately.
