Perception of Depth
Perception of Depth
Cues to depth:
1. Oculomotor
2. Monocular
3. Binocular
The Retinal Image is 2-D, but what information is
available to construct the 3-D environment?
Oculomotor Cues
a) Convergence
b) Divergence
Oculomotor Cues
c) Accommodation
The shape of the
lens changes as a
function of the
distance of an object
Monocular (pictorial) Cues
a) Occlusion (overlapping object is closer)
b) Size in the field of view (bigger is closer)
c) height in the field of view (higher base is farther away)
d) familiarity (sizes that we “know”)
Monocular (pictorial) Cues
a) Occlusion (overlapping object is closer)
b) Size in the field of view (bigger is closer)
c) height in the field of view
(higher base is farther away)
Pictorial depth cues: occlusion, “relative” size &
height, shadows
Which coin is closer? Which one is looks bigger?
Which coin is closer? Effect of “Familiar Size”
Pictorial Cues
Linear Perspective
“Perceptual convergence”
Linear perspective: lines perceived as
parallel that “travel” toward a
“vanishing point”
Pictorial Cues
texture gradient
Relates to “ground”
All other cues relate
(size, height, linearity,
occlusion, shadows,
familiarity)
Relative height: objects higher (& smaller) in the picture are perceived as
farther away
If the object is smaller and lower in the frame, the object is perceived as
near, but relatively smaller
Pictorial Cues
atmospheric perspective
(aerial perspective)
The higher & hazier contours are
“hazier” because they are farther
away (more air, moisture, dust to
look through)
Pictorial cues: atmospheric cues
Higher & hazier
Lower & sharper
Pictorial Cues
Implied lighting & shadows: shading
Motion Cues: reconfiguration of the visual field
b) Deletion (covered)/Accretion
Motion Cues: reconfiguration of the visual field
b) Deletion/Accretion (uncovered)
Motion Cues: Motion Parallax
Motion Cues
a) Motion parallax
-When an observer moves, closer object moves a greater number of
degrees of visual angle on retina than further objects
-Subjective impression: objects nearer the observer move faster than
more distant objects
One Eye: Position 1
TA
HA
Position 2
HB
TB
Monocular (pictorial) Depth Cues
•
•
•
•
•
•
•
•
•
Occlusion
Size
Height
Familiarity
Linear Perspective
Texture Gradient
Atmospheric (aerial) Perspective
Shading: Lighting & Shadows
Movement Cues: (i) Deletion/Accretion; (ii) Motion
Parallax
Test the size of your monocular and
binocular visual fields
• Close one eye at a time
• Move your thumb across the visual field of each
eye individually
• About how wide is your monocular field?
• About how wide is your binocular field?
• How different is the perspective of any one
point in your binocular field when you switch
from one eye to the other?
Binocular Disparity for Stereopsis
Depth cues with Bi (two) nocular (eyes)
disparity
• Binocular disparity: difference (“disparity”)
between the two points of view of the left and right
eyes (retina)
• Stereopsis: Experience of depth from the joining
together of left/right views at the level of the higher
brain areas (LGN & cortex)
• Corresponding retinal points: specific location on
each retina that communicates up to the identical
receptive area of the cortex
Binocular Disparity for Stereopsis: The Horoptor
and corresponding retinal points
fixate blue object: image falls on corresponding
points (identical locations) on both left and
right retinas
horopter – imaginary
“circle” that passes through
the point of fixation
all images that are on the
horopter fall on the exact
same corresponding
points on both retinas
Horopter
Images on the horopter share a corresponding retinal point
Images not on the horopter are on different points on the retina
Binocular Disparity for Stereopsis
images of objects not on the horopter fall
on non-corresponding points on the retina
angle of disparity
Fovea
Binocular Disparity for Stereopsis
the further away from the
horopter, the greater the
angle of disparity
Binocular Disparity for Stereopsis
things behind the horopter are
in uncrossed disparity (you’d
have to “uncross” or diverge
your eyes to see it clearly)
things in front of the horopter
are in crossed disparity (you’d
have to “cross” or converge
your eyes to see it clearly)
Binocular Depth Cells in Visual Cortex
(striate cortex, V1) are disparity specific
Disparity selective cell
Also called “Disparity Detectors”
Perceiving relative size
Perceiving Size
“Whiteout” is one of the most treacherous weather
conditions possible for flying. Frank pilots his
helicopter across the Antarctic wastes, blinding light,
reflected down from thick cloud cover above, and up
from the pure white blanket of snow below, making
it difficult to find the horizon, or to know “up” from
“down.” He thinks he can make out a vehicle on the
snow below and he drops a smoke grenade to check
his altitude. To his horror, the grenade falls only
three feet before hitting the ground. Realizing that
what he thought was a truck was actually a discarded
box, Frank pulls back on the controls and soars up…
drenched in sweat, he realizes how close he just
came to a whiteout fatality…
Holway and Boring (1941): relation between size & distance
(depth cues)?
comparison circle
test circles
Where the
subject was
standing
1 deg
Task for subjects: match the size of the “comparison” light exactly
to the same size (diameter) of the test light.
Condition 1: do matching with lots of depth cues
Condition 2: do matching with fewer depth cues
Note: the test stimulus light is always the same retinal size (1 degree)
Holway and Boring (1941)
comparison
circle
Where the
subject was
standing
test circles
1 deg
With depth cues:
Subjects accurately matched the physical size of the “comparison” and “test”
stimulus lights
Made match regardless of how far away (and despite the fact that retinal images
were the same sizes)
Without depth cues:
Subjects routinely got the literal size of the stimulus wrong
Matched the visual angle so that “comparison” and “test” lights were always
the
same size on the retina
All “test” stimuli were believed to be the same size regardless of how close they
were (all at 1-degree angle), so they looked the same
Conclusions from Holway & Boring (1941): Perceived
Depth and Size of objects MUST Be codependent
A
A
- We
need depth information to accurately make
judgments about size
- In
the absence of depth information we determine the size of
objects by the size of the image that they cast on our retina
Emmert's Law: size-distance scaling equation
- Our perception of size equals the size of the Retinal
image times the perceived distance away
S
p
=
(R X D )
p
S = perceived size
R = size on the retina
D = perceived distance
Experiencing Emmert's Law:
Experiencing Emmert's Law first hand:
- the farther away an afterimage appears, the larger we
perceive its size
If we look for the afterimage
against a far wall the image
looks much larger
If we look for the afterimage
on a piece of paper right in
front of us, it looks smaller
Size of the afterimage, determined by how far away we look
(or we think we look) to see the image
Look against
a far wall
Look at a
piece of
paper on
your desk
Bleached out
cones in fovea
(after-image)
Look against a
near wall
Emmert's Law: perceived size equals retinal size times perceived distance
Emmert's Law: size-distance scaling equation
- Our perception of size equals the size of the Retinal
image times the perceived distance away
S
p
=
(R X D )
p
S = perceived size
R = size on the retina
D = perceived distance
Size Constancy:
We perceive most object's physical size accurately
regardless of it's distance from us
A
<A
We can do this because of depth information
-size-distance scaling mechanism
- AND… FAMILIARITY
Most common index of size? Familiarity
Perception of Size (back to depth)
Cues to depth:
1. Oculomotor
2. Monocular
3. Binocular
The Ponzo Illusion
Which cylinder in the “hallway” is perceived as larger?
Optical Illusions of Distance: Ponzo Illusion
If object is perceived as further away, but it has same retinal size, it
must be larger
Perceptual Illusions
The Ponzo Illusion
Which monster appears larger?
Perceptual Illusions
The Ponzo Illusion
Ames Room: “throwing off” monocular depth cues
Ames Room: “throwing off” monocular depth cues
Ames Room
• The Ames room is designed so that the monocular
depth cues give the illusion that the two people are
equally far away
All roads lead back to Emmert's Law:
size-distance scaling equation
- Our perception of size equals the size of the
Retinal image times the perceived distance away
S
p
=
(R X D )
p
S = perceived size
R = size on the retina
D = perceived distance
The Ames Room
Our perception of size equals the size of the Retinal
image times the perceived distance away
Ames Room: “throwing off” monocular depth cues
Moon Illusion
Apparent-Distance Theory: Horizon v. Sky and
the “flattened heavens” theory (Kaufman &
Rock, 1962)
All roads lead back to Emmert's Law:
size-distance scaling equation
- Our perception of size equals the size of the
Retinal image times the perceived distance away
S
p
=
(R X D )
p
S = perceived size
R = size on the retina
D = perceived distance