Psychology 1XX3 Notes – Depth, Distance and Motion – Mar 7 , 2010
Introduction to Depth and Distance:
We can make depth/distance decisions because we can create a mental image of a
three-dimensional world, based on a combination of tap- down and bottom-up
Remarkably, new visual input from a 2-dimensional retinal surface is processed
along with our existing knowledge of the environments objects and surfaces to
create this 3-D world.
This essential skill is aided by the many cues that have been incorporated into
evolution of depth perception.
Monocular and Binocular Cues:
How do we transform flat, two- dimensional visual images projected onto the
retina into an accurate a 3-d mental image of the environment?
Psychologists have identified two main classes of cues that we use to perceive
depth in our environment: binocular cues, which are depth cues that require two
eyes, and monocular cues, which are cues to depth that you can get using one
Def’n of Convergence: A cue to depth resulting from the way our eyes turn
inwards to fixate on a specific point
There are two types of binocular cues to depth perception. The first is called
convergence, which results from the way our eyes turn inwards to fixate on a
You can feel this yourself if you look at your finger at arm's length and then
slowly bring it towards you while still focusing on your finger. As your finger
approaches your face you will feel your eye muscles straining. The feedback that
we receive from these eye movements gives us information about depth.
However, convergence as a depth cue only works for objects that are relatively
close, because with objects that are far away, the eyes don't have to turn in at all
to fixate on the same point, and the path of sight for your two eyes becomes
Def’n of Retinal Disparity: A cue to depth resulting from the fact that our eyes
will each see a slightly different visual scene.
Retinal disparity is caused by the fact that our eyes, which are located about 6 cm
apart, will each see a slightly different visual scene.
You can see this yourself if you point to an object in the distance and then open
and close one eye at a time. You will see your finger jumping around and pointing
to a different object when you look with each eye, and these are the two different
scenes that each eye is receiving.
When these two scenes are combined in the brain, the resultant perception is
depth. In fact, our visual systems are equipped with a class of neurons that fire
maximally only when each retinal image is slightly different.
Three Types of Monocular Cues:
There are three main types of monocular cues: accommodation, motion, and
pictorial cues. Accommodation:
Accommodation involves changes in the shape of the lens as you focus on objects
at different distances.
When objects are near, we make a different accommodating response than when
objects are farther away. Just like the binocular cue of convergence, information
about an object's distance is coming from the feedback that we get from our eye
However the lens can only change so much in shape, and because of this,
accommodation is only an effective cue for depth up to about 2 meters.
For all points beyond this distance, the lens is a constant shape, and so we have to
rely on other depth cues. (See image below.)
Motion: Two Types Motion Parallax and Optic Flow
The second main type of monocular cue arises from motion, and we can use two
different motion cues to perceive depth.
The first is motion parallax, which refers to the fact that when we pass by a scene,
objects in the scene pass by us at different speeds, depending on how far away
these objects are relative to us.
Objects that are close, appear to speed by much faster than objects that are farther
away. You can see this for yourself when you drive in a car and look out the
window. The fence posts that are right beside the road seem to whiz by, but the
buildings in the distance look like they’re standing still.
We can use the speed of the objects going by us in the scene as we move as a cue
to how far away these objects are from us.
The second motion cue to depth is optic flow, which refers to the changing optical
projection of a scene that is caused by the motion of the observer, as well as
motion of objects within the scene.
Obviously, as you get closer an object will get bigger, and it will get smaller as
you move away from it. Not only does the size of the object that you’re focusing
on change as you move toward or away from it, so does the entire visual scene.
As with motion parallax, objects that are close to you will seem to move more in
the visual scene and change more in size than objects that are farther away, and
these changes in size and motion can give cues to depth. Pictorial: Seven Types Interposition, Relative Size, Linear Perspective, Texture and
Haze, Shading and Elevation
A third type of monocular cue are pictorial cues, of which there are seven
different kinds. These are the cues that artists expertly use to give the impression
of depth working on a two-dimensional canvas.
For example, interposition provides a good clue to depth. When you have an
object that partially blocks another, it is perceived as being in front of the other
Interposition is most effective when the objects are familiar and you know what
their shapes should be, although it can still provide information about depth if the
objects are unfamiliar because of the Gestalt principle of closure.
For example, if you have two circles that are partially overlapping, you will tend
to see the partially blocked shape as a circle and not as a crescent because of the
principle of closure. As a result, you will perceive the full circle as being in front
of the partially blocked circle instead of seeing a full circle beside a crescent
Relative size is another pictorial cue that can provide information about depth.
If you have two objects that are the same shape but different sizes, then the larger
shape will be perceived as closer.
Another way that relative size can provide depth information with familiar objects
is by their size. For example, you know what size a car should be, so if you see an
image of a tiny little car you will know that the car is very far away.
This relates to the concept of optic flow; one way we avoid collisions with other
cars is by monitoring the rate of expansion of the approaching car on our retina
and keeping this rate constant.
Obviously, when an object is closer to us, it projects a larger image on our retinas
than if the same object is farther away, and we can use this information to gauge
Example: if you are pulling up to a red light and you get closer to the car in front
of you, it will project a larger image on your retina and appear to expand in size.
It turns out that you rely very heavily on this information to gauge when to stop
by keeping this rate of expansion constant. (See image on next page.) Linear Perspective:
Another pictorial depth cue is linear perspective, which is really obvious if you’ve
ever looked down a railroad track.
Even though you know that the tracks are parallel, they appear to converge at a
single point on the horizon. This provides a cue to depth because objects that are
farther away decrease in size and spacing between objects.
Artists use these cues to achieve depth with anything that is rectangular, square or
cube-shaped, like buildings, shelves, doorways, or roads. The artist should draw
lines that converge to capture the farthest point away from the viewer’s eye.
Texture and Haze (Aerial Perspective)
Texture and haze, which are sometimes called aerial perspective, are other
pictorial cues that both provide information about depth by the fact that objects
that are farther away from us will have fewer details.
For example, if you're looking at a gravel road, then you will easily be able to see
the texture of the rocks immediately under your feet, but as you look in the
distance, you will just see the same rocks becoming a uniform grey colour with
The same applies with haze. When you are looking at objects that are farther
away, it will be harder to see the outline and texture of the object compared to
objects that are closer to you.
Shading is another depth cue that artists use to their advantage. Although it cannot
give us information about how far away the object is from us, it can tell us what
part of the object is close to us and what is farther away.
Take a close look at this picture of dents and bumps. On the left hand side, the
bumps are shaded on the bottom and the dents are shaded on top.
What happens when you turn this picture upside down. It now appears that what
was previously a bump now looks like a dent, and conversely, what was
previously a dent now looks like a bump. (See image below.)
Why is this so? We are so used to light coming from above, like the sun that we
automatically use the pattern of light striking an object to tell us whether the
object surface is coming toward us or receding away from us.
If the light is striking the bottom of the object, then it is receding away from us,
and if the light is hitting the top of the object then it’s coming toward us. Elevation:
Elevation is a pictorial depth cue that tells us how far away an object is by how
close it is to the horizon.
Objects that are higher up in a picture, or closer to the horizon, are seen as farther
away than objects that are lower in a picture.
Evolution of Depth Perception:
Since depth perception is an important adaptive mechanism critical to survival,
you may expect that it to be present in species that can move about in their
We can test this ability with the visual cliff experiment. When a large variety of
species were tested in this paradigm, including turtles, birds, small and large
mammals, and different primate species, there was no doubt that these animals
were responding to the available depth cues by avoiding what was perceived to be
the edge of a cliff.
This suggests that a wide variety of animal species are capable of depth
perception even with different types and placement of eyes.
Effect of Eye Placement on Depth Perception:
The type of cues that an animal can use to perceive depth depends a lot on where
the animal’s eyes are placed on its head.
Prey animals typically have eyes that are on the side of their heads, like rabbits
and fish; these animals have very limited depth perception from binocular cues,
and instead must rely on monocular cues.
Predator animals typically have both eyes facing front, like cats and primates;
these animals are able to use both binocular and monocular cues.
What is the evolutionary advantage of being able to use both monocular and
binocular depth cues? In addition to providing information about distance
between predators and prey, the binocular cue of retinal disparity is excellent at
Retinal disparity allows you to group toge