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

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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

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.