Cue approach to depth
An approach that explains depth perception by identifying information in the retinal image (2-D), and also information provided by aiming and focusing the eye on an object that is correlated with depth in the scene.
Depth cue in which one object hides or partially hides another object from view, causing the hidden object to be perceived as being farther away.
According to cue theory how do we learn about the connection between cues and depth?
Through our previous experience with the environment. These associations are then made automatically by the brain.
3 major groups of cues
Depth cues based on our ability to sense the position of our eyes and the tension in our eye muscles. Indicates when an object is close.
How are oculomotor cues created? (what are the 2 oculomotor cues)
The inward movement of the eyes that occurs when we look at nearby objects
The change in the shape of the lens that occurs when we focus on objects at various distances.
Cues that work with only one eye
3 major types of monocular cues
2) Pictorial cues
3) Movement based cues
Depth cues that can be depicted in pictures.
Relative height cue
Objects that have bases below the horizon appear to be farther away when they are higher in the field of view. Objects that have bases above the horizon appear to be farther away when they are lower in the field of view.
Relative size cue
When two objects are of equal size, the one that is farther away will take up less of the field of view
Perception that parallel lines in the distance convege (come together) as the distance of that line increases.
Familiar size cue
Using prior knowledge about the sizes of objects to make judgements about their distance
William Epstein (1965)
Showed that under certain conditions, our knowledge of an object’s size influences our perception of that object’s distance.
What stimuli did Eptsein use in his familiar size experiments?
Equal sized photogtaphs of different coins.
What happened in Epsteins experiment when observers used two eyes?
The coins no longer gave the perception of depth.
Atmospheric perspective cue
When distant objects appear less sharp and have a slgiht blue tint.
What creates the atmospheric persepective?
The farther away an object is, the more air and particles (dust, air pollution) we have to look through. Ex. Mountains that look 3 hours away in Philly will look 6 hours away in Montana due to different atmospheres.
Texture gradient cue
Elements that are equally spaced in a scene appear to be more closely packed as distance increases. In other words, objects equally spaced will appear closely packed at a farther distance.
Why are shadows useful?
-Shadows help determine the location of objects.
-Shadows enhance the 3-D of objects.
Depth cues created by movement
2 types of motion-prodced cues
1) Motion parallax
2) Deletion and accretion
Motion parallax cues
Occurs when, as we move, nearby objects appear to glide rapidly past us, but more distant objects appear to move more slowly. Ex. objects that speed by while looking out a moving car appear as blurs
What creates the perception motion parallax cues?
Images of near objects travel large distances across the retina. Images of far objects travel smaller distances across the retina.
Deletion (motion-produced) cue
Provides info about the relative depth of two surfaces. Occurs when a farther object is COVERED by a nearer object due to sideways movement of the observer relative to the objects.
Accretion (motion-produced) cue
Provides info about the relative depth of two surfaces. Occurs when the farther object is UNCOVERED by the nearer object due to sideways movement of observer.
Cues that depend on two eyes.
The difference in the images in the left and right eyes. Ex. The position of your finger in front of you using just the left eye changes when you use just the right eye (it moves slightly off to the side)
Corresponding retinal points (Binocular disparity)
The points on each retina that would overlap if one retina were slid on top of the other. Receptors at corresponding points send their signals to the same location in the brain
Horopter (corresponding retinal points)
Imaginery surface that passes through the point of fixation and indicates the location of objects that fall on the corresponding points in the two retinas.
Two points, one on each retina, that would not overlap if the retinas were slid onto each. Since the object is NOT located on the horopter the image falls on the noncorresponding points.
Angle of disparity (noncorresponding points)
The visual angle between the images of an object on the two retinas. When images of an object fall on corresponding points, the angle of disparity is zero. When images fall on noncorresponding points, the angle of disparity indicates the degree of noncorrespondence.
Why is absolute disparity important?
It provides info about the distances of objects. Greater disparity = greater distance from horopter.
The difference between two object's disparities. This disparity info remains the same no matter where the observer looks as long as the objects stay in the same positions.
Absolute disparity vs. Relative disparity
Absolute disparity always changes.
Relative disparity (diff. between two objects) always stays the same as long as the objects stay in the same location relative to the observer.
The impression of depth that results from infromation provided by binocular disparity.
Stereoscope (Charles Wheatstone)
Device that produces the illusion of depth using two slightly different pictures.
A pair of stereoscopic images made up of random dots. This stimulus contains no pictorial cues (ie. occulsion, relative height etc).
What does the random-dot stereogram prove?
Shows that depth can be perceived with just disparity alone (without depth info.)
How does the visual system match the image on the left eye with the corresponding image on the right eye to determine binocular disparity
Eyes located in front of the head. Overlapping field of view
Eyes located on opposite sides of the head. Do not have an overlapping field of view.
Locating objects by sending out high-frequency pulses and sensing the echo created when pulses are reflected from objects in the environment.
Ken-Ichino Tsutsui research on neurons that respond to pictorial depth
Using monkeys, he studied neurons in the parietal cortex that repsond to depth indicated by texture gradients. He showed that those neurons respond to both pictorial depth cues and binocular disparity.
Binocular depth cells/disparity-selective cells
Neurons in the striate cortex(VI) that respond to absolute disparity.
Randolph Blake & Helmut Hirsch (1975) connection between binocular depth cells and Depth perception
Selectivly reared cats so they 1) had few binocular neurons 2) could not use binocular disparity to percieve depth = removal of binocular neurons eliminates stereopsis.
The insertion of a small electrode into the cortex and sending electrical charges to activate the neurons.
Gregory DeAngelis (1998) work with microstimulation
Used microstimulation on monkeys to activate a different group of disparity-selective neurons. Results showed that they shifted their depth judgement toward disparity signaled by the stimulated neurons.
Where are neurons senstive to absolute disparity located?
Primary visual receiving area
Where are neurons sensitive to relative disparity located?
Temporal lobe and other areas higher in the visual system.
A. H. Holway and Edwin Boring (1941) experiment on misperceving size in absence of depth information.
Observers change the size of the comparison circle in the left corridor to match his or her perception of the test circle on the right
Angle of an object relative to the observers eye.
Holway and Boring (1941) visual angle experiment
Found that eliminating depth information = difficulty in judging the physical sizes of the circles + size estimation is influenced by objects visual angle
Our perception of an objects size stays relatively constant even when viewed from different distances. Ex. solar elcipse.
Hypothesized mechanism that maintains size constancy by taking percieved distance into account.
Size of an after image depends on the distance of the surface its viewed on. Farther away the surface, the larger the afterimage appears.
Size-distance scaling equation
S = K (R x D)
(S): objects percieved size
(R): size of retinal image
(D): percieved distance of object
Muller-Lyer illusion (size illusion)
Two lines of equal length appear to have different lengths due to the "fins" at the end of the lines.
Rich and Gregory (1966)
Misapplied size constancy scaling
Explains the Muller-Lyer illusion. Mechanism maintaining size constancy when applied to 2D images produces illusions.
R.H. Day (1989, 1990)
Conflicting cues theory
Our perception of line length dpends on two cues
1) Actual length of vertical line
2) Overall length of figure.
This means that in the Muller-Lyer illusion, the line with the outward fins is percieved as larger because its overall length is larger than the line with the inward fin.
Gregory vs. Day (illusion of size)
Gregory believes illusions are affected by depth info.
Day disagrees with Gregory and believes cues for length produce illusions.
Ponzo (railroad) illusion
Two objects of equal size postioned between two converging lines appear different in size. According to misapplied scaling the top image appears larger due to depth info provided by the converging lines.
Distorted room that makes two people of equal size appear different in size. The person with the smaller visual angle (smaller retinal image) is perceived as shorter.
When the moon appears larger near the horizon then when it is high in the sky.
Apparent distance theory (moon illusion)
Idea that the horizon moon (viewed across a field of terrain) should appear farther than the higher moon(viewed in empty space) b/c the terrain contains depth info whereas empty space does not.
angular size contrast theory (moon illusion)
Idea that the moon appears smaller when it is surrounded by larger objects. Therefore, the night sky makes the elevated moon appear smaller b/c the sky is huge.