Chapter 10: Perceiving Depth and Size
Dec 15, 2015
Chapter 10: Perceiving Depth and Size
Oculomotor Cues
• Oculomotor cues are based on sensing the position of the eyes and muscle tension
– Convergence - inward movement of the eyes when we focus on nearby objects
– Accommodation - change in the shape of the lens when we focus on objects at different distances
Monocular Cues
• Monocular cues come from one eye
– Pictorial cues - sources of depth information that come from 2-D images, such as pictures
• Occlusion - when one object partially covers another
• Relative height - objects below the horizon that are higher in the field of vision are more distant
–Objects above the horizon lower in the visual field are more distant
Monocular Cues - continued
• Relative size - when objects are equal size, the closer one will take up more of your visual field
• Perspective convergence - parallel lines appear to come together in the distance
• Familiar size - distance information based on our knowledge of object size
Figure 10-3 p229
Figure 10-4 p230
Monocular Cues - continued
• Atmospheric perspective - distance objects are fuzzy and have a blue tint
• Texture gradient - equally spaced elements are more closely packed as distance increases
• Shadows - indicate where objects are located
– Enhance 3-D of objects
Figure 10-6 p231
Figure 10-7 p231
Figure 10-8 p231
Figure 10-9 p232
Motion-Produced Cues
• Motion parallax - close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly
• Deletion and accretion - objects are covered or uncovered as we move relative to them
– Covering an object is deletion
– Uncovering an object is accretion
Binocular Depth Information
• Stereoscopic depth perception
• Differences between 2D and 3D movies
– “Stereo Sue”
– Strabismus
Binocular Disparity
• Binocular disparity - difference in images from two eyes
– Difference can be described by examining corresponding points on the two retinas
Disparity (Geometrical) Created Stereopsis (Perceptual)
• Stereopsis - depth information provided by binocular disparity
– Stereoscope uses two pictures from slightly different viewpoints.
– Random-dot stereogram has two identical patterns with one shifted in position.
Figure 10-19 p239
Disparity (Geometrical) Created Stereopsis (Perceptual) - continued
• Three types of 3D TV
– Passive – polarized glasses
– Active – electronic shutter glasses
– Lenticular – mini lenses on screen, no glasses needed
Figure 10-20 p239
Figure 10-21 p240
The Correspondence Problem
• How does the visual system match images from the two eyes?
– Matches may be made by specific features of objects.
– This may not work for objects like random-dot stereograms.
– A satisfactory answer has not yet been proposed.
The Physiology of Binocular Depth Perception
• Neurons have been found that respond best to binocular disparity.
– These are called binocular depth cells or disparity selective cells.
• These cells respond best to a specific degree of absolute disparity between images on the right and left retinas.
– Disparity tuning curve
The Physiology of Binocular Depth Perception - continued
• Experiment by Blake and Hirsch
– Cats were reared by alternating vision between two eyes.
– Results showed that they:
• had few binocular neurons.
• were unable to use binocular disparity to perceive depth.
The Physiology of Binocular Depth Perception - continued
• Experiment by DeAngelis et al.
– Monkey trained to indicate depth from disparate images.
– Disparity-selective neurons were activated by this process.
– Experimenter used microstimulation to activate different disparity-selective neurons.
– Monkey shifted judgment to the artificially stimulated disparity.
Perceiving Size
• Distance and size perception are interrelated
• Experiment by Holway and Boring
– Observer was at the intersection of two hallways.
– A luminous test circle was in the right hallway placed from 10 to 120 feet away.
– A luminous comparison circle was in the left hallway at 10 feet away.
Perceiving Size - continued
• Experiment by Holway and Boring
• On each trial the observer was to adjust the diameter of the test circle to match the comparison.
• Test stimuli all had same visual angle (angle of object relative to the observer’s eye).
– Visual angle depends on both the size of the object and the distance from the observer.
Figure 10-26 p243
Perceiving Size - continued
• Part 1 of the experiment provided observers with depth cues.
– Judgments of size were based on physical size.
• Part 2 of the experiment provided no depth information.
– Judgments of size were based on size of the retinal images.
Size Constancy
• Perception of an object’s size remains relatively constant.
• This effect remains even if the size of the retinal image changes.
– The changes in distance and retinal size balance each other
Visual Illusions
• Nonveridical perception occurs during visual illusions.
• Müller-Lyer illusion:
– Straight lines with inward fins appear shorter than straight lines with outward fins.
– Lines are actually the same length.
Müller-Lyer Illusion
• Why does this illusion occur?
– Misapplied size-constancy scaling:
• Size constancy scaling that works in 3-D is misapplied for 2-D objects.
• Observers unconsciously perceive the fins as belonging to outside and inside corners.
• Outside corners would be closer and inside corners would be further away.
Figure 10-36 p249
Figure 10-37 p249
Müller-Lyer Illusion - continued
– Since the retinal images are the same, the lines must be different sizes.
• Problems with this explanation:
– The “dumbbell” version shows the same perception even though there are no “corners.”
– The illusion also occurs for some 3-D displays.
Figure 10-38 p250
Ponzo Illusion
• Horizontal rectangular objects are placed over railroad tracks in a picture.
• The far rectangle appears larger than the closer rectangle but both are the same size.
• One possible explanation is misapplied size-constancy scaling.
Figure 10-41 p251
The Ames Room
• Two people of equal size appear very different in size in this room.
• The room is constructed so that:
– The shape looks like a normal room when viewed with one eye.
– The actual shape has the left corner twice as far away as the right corner.
Figure 10-42 p251
Figure 10-43 p251
The Ames Room - continued
• One possible explanation - size-distance scaling
– Observer thinks the room is normal.
– Women would be at same distance.
– Woman on the left has smaller visual angle (R).
– Due to the perceived distance (D) being the same her perceived size (S) is smaller
The Ames Room - continued
• Another possible explanation - relative size
– Perception of size depends on size relative to other objects.
– One woman fills the distance between the top and bottom of the room.
– The other woman only fills part of the distance
– Thus, the woman on the right appears taller
Moon Illusion
• The moon appears larger on the horizon than when it is higher in the sky.
• One possible explanation:
– Apparent-distance theory - horizon moon is surrounded by depth cues while moon higher in the sky has none.
– Horizon is perceived as further away than the sky - called “flattened heavens”.
Moon Illusion - continued
– Since the moon in both cases has the same visual angle, it must appear larger at the horizon.
• Another possible explanation:
– Angular size-contrast theory - the moon appears smaller when surrounded by larger objects
– Thus, the large expanse of the sky makes it appear smaller
• Actual explanation may be a combination of a number of cues.