PERCEPTION II:Depth Perception, Relative Height, Linear Perspective

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Introduction to Psychology PSY101

Lesson 18

PERCEPTION II

Depth Perception

· Depth perception is the perceptual tendency/ ability to see objects in three dimensions,

although the image that falls on the retina of the eye is two-dimensional; thus enabling us

to perceive distance.

· "Depth Perception" is the skill to perceive depth and distance e.g. we are able to judge the distance

of the incoming car, height of the cliff or of a roof top, size of an object, weight of a sand bag etc,

in a glance, just by having a look at it.

· This sort of perception is largely due to the fact that we have two eyes which are slightly distant

from each other, so the brain integrates the two slightly different images and combines them into

one consolidated view; However the differences in images or `Binocular Disparity' is not ignored

by the brain.Eleanor Gibson and Richard Walk discovered this phenomena in 1960 by using the

miniature cliff with a drop- off covered by sturdy glass.

· Placed the young infants of 6- 14 months at the edge of visual cliff. Their mothers motivated them

to crawl on the glass, but most of them refused to do so, indicating that they could perceive

depth__ this may be due to the fact that they learned to perceive depth in the crawling age.

Physiological depth cues are formed by our visual system with the help of muscular movements or

adjustments. It is yet difficult to tell how these cues contribute to the depth perception. The

adjustments, which the eyes and eye muscles make, are assumed to be of weak nature. These include:

accommodation and convergence.

The psychological depth cues: are based on the interpretation and analysis of the retinal image that is

caused by the working of the visual cortex in the brain. Depth Cues to Perception

There are two important cues for the perception of depth. These include:

I. Monocular cues for depth perception.

II. Binocular cues for depth perception.

I.Monocular Cues for Depth Perception

· Also known as " pictorial cues" because painters use these cues in order to tell about

depth; these lead to three- dimensional information.

· These generate the ability to judge distance and depth such as linear perspective and

interposition with only one eye.

· Depth and location can be perceived with single eye also.

i. Relative motion

A monocular cue for perceiving depth and distance in which when

we move, the objects at different distances change their relative

positions with the visual image___ with those that are closest seem

to be moving faster.

Relative Size

The monocular cue for depth perception in which we assume that the two objects are similar in size, the

one that make the smaller image appears to be more distant.

Interposition

A monocular cue for perceiving depth in which the nearer objects partially block/ hinder our image of the

more distant objects.

Relative Height

A monocular cue to depth perception and distance in which higher objects appear to be more

distant.

Can be explained by doing practically as we are moving in car, train, bus etc. Fixate your gaze at

some point (fixation point)___ say a tree, __ the objects that are closer than the tree seems to be

moving backwards and also seem to move faster. The objects that are beyond the fixation point

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seem to move along but at lower speed and seem as farther away. Brain has the capability to

compute these speed and distance clues in order to perceive distances.

Linear Perspective

The monocular cue for the perception of depth and distance in which two parallel lines seem to be

converging at some point indicating increasing distance.

Rail road tracks, highway tracks etc appear to be converging at some distance and so, contribute to the

rail- crossing accidents by making people to over estimate the train's distance: a massive train size

makes the perception that it is moving slowly.

Relative Brightness

A monocular cue for depth perception in which the dimmer objects seem to be more distant.

Nearby objects seem to reflect more light than the farther ones. When going for a walk in a thick-fog

morning, one may judge distance wrongly as due to fog the objects may be perceived to be farther than they

do on the clear shiny morning. That is why they contribute to increasing accidents.

Light and Shadow

Perceptual phenomenon for the perception of depth and distance in which when light strikes an irregular

object, certain parts are brightly illuminated whereas others lay in shadow. These shadowed parts tell us

about the depth of the parts concerned___ painters use this phenomenon when portraying something on

the canvas such as human face and its various structures.

Texture Gradient

An American psychologist, James J. Gibson was the ever first individual who emphasized the

importance of texture gradient for perceiving depth.

Mainly applied to textures (structures) of surfaces and arises when we observe the surface when in slant

rather than from a straight angle or from above. Pictorial Cues for Depth Perception

Monocular cues are used for the perception of depth and distance

· We can get an extensive 3-D impression in 2-D pictures on a flat surface

Atmospheric Perspective

Particles and vapors in the atmosphere result/ cause the scattering of light that makes a very distant

surface appear hazy.

The other techniques they use are;

a. Occlusion: near objects overlap far surfaces.

b. Relative height and size: objects further away from the horizon seem nearer and larger

objects seem closer.

c. Linear perspective: provides a strong cue to distance that can effect perception.

d. Shading: provides a cue for shape rather than distance.

Motion Parallax

The change in the position of the retinal image with the side-to-side movement of the head; providing a cue

to the distance.

Occurs when objects are at different distances and we are also moving at different rates when in motion.

The object nearer to the person appears to move backward but the more distant objects appears to be static

as we move. The rate of an object's movement provides a cue to its distance.

Although motion plays an important role in depth perception, the perception of motion is an important

phenomenon in its own right. It allows a baseball outfielder to calculate the speed and trajectory of a ball

with extraordinary accuracy. Automobile drivers rely on motion perception to judge the speeds of other cars

and avoid collisions. A cheetah must be able to detect and respond to the motion of antelopes, its chief

prey, in order to survive.

How does your brain know which movement on the retina is due to your own motion and which is due to

motion in the world? Understanding that distinction is the problem that is faced by psychologists who want

to explain motion perception.

One explanation of motion perception involves a form of unconscious inference. That is, when we walk

around or move our head in a particular way, we unconsciously expect that images of stationary objects will

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move on our retina. We discount such movement on the retina as due to our own bodily motion and

perceive the objects as stationary.

Psychologist James J. Gibson's explanation of motion perception was too complicated. He reasoned that

perception does not depend on internal thought processes. He thought, instead, that the objects in our

environment contain all the information necessary for perception e.g. aerial acrobatics of a fly. Clearly, the

fly is a master of motion and depth perception, yet few people would say the fly makes unconscious

inferences. Gibson identified a number of cues for motion detection, including the covering and uncovering

of background. Research has shown that motion detection is, in fact, much easier against a background.

Thus, as a person moves in front of you, that person first covers and then uncovers portions of the

background and this reflects movement.

People may perceive motion when none actually exists. For example, motion pictures are really a series of

slightly different still pictures flashed on a screen at a rate of 24 pictures, or frames, per second. From this

rapid succession of still images, our brain perceives fluid motion--a phenomenon known as stroboscopic

movement.

Binocular Cues for Depth Perception

· The ability to judge distance and depth such as retinal disparity and convergence with two eyes,

which are slightly apart from each other.

· Our eyes are slightly apart from each other having a distance of about 2 ½ inches or 6 cm, so the

images that fall on the two retinas are slightly different.

· These two slightly different images are then integrated and processed by the brain, but it does not

ignore the possibility of difference in images__ known as " binocular disparity".

i. Retinal/ Binocular Disparity

· The cue to depth perception, the greater the disparity (difference) between the two

images the retinas receive of an object, the closer the object seems to us___ this

disparity allows the brain to judge distance.

· This can be proved with the help of simple experiments done by you. i.e. hold a

pencil directly in front of your nose; the retinas of yours will receive different

views/ images. Now make them closer firstly to one eye and then to the other. At

greater distance, the disparity is smaller.

· This discrepancy of the two images that falls on the retinas varies with respect to

the distance from which the object is being perceived___ used for determining

distance e.g. if we see two objects in which one is considerably closer than the

other, the retinal disparity will be greater and we perceive the greater depth

between the two.

· On the contrary, if the two objects were at similar distance, the retinal disparity

will be minor/ smaller and we perceive the objects at similar distance.

· Movie directors use these phenomena in order to create depth illusions by using

two cameras, spaced highly apart, to produce slightly different images, each image

for each eye.

· In 3-D movies, the two images are presented simultaneously which produces a

double image; special glasses are worn for this purpose to provide us with the

genuine sense of the depth phenomena.Convergence and Accommodation

A binocular cue for depth perception that illustrates, that when we assume that the two objects are

of same size, the one that produces a relatively smaller image will be perceived as distant.

It is a neuromuscular cue in which the more the eyes converge inwards, the nearer the object seems.

The eye lens becomes thicker due to the activation of ciliary muscles to focus on the nearer object____ the

degree of activation of ciliary muscles gives us the cue to depth.

Convergence and accommodation are only effective at close distances and can tell only the distance to a

single object in the visual field.

Selective Attention

· Perceptual process in which the person chooses the stimulus which he is interested in; paying

attention to only the stimulus of interest.

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Humans give attention to the objects that are exceptionally bright, loud, novel, or high in

contrast___ we are also motivated to give attention to the objects that are meaningful as well as

relevant e.g. if we are thirsty, we will give more attention to water and the like.

Advertisers extensively use this phenomena of selective attention using bright contrasts, high

volumes and more rapid speech than usual__ and most importantly broadcast at that time when

people are particularly sensitive to their content e.g. the food- related ads are shown at lunch or

dinner times.Stroop Task

Difficult and frustrating exercise in which one is confronted with two powerful and competing

stimuli___ the meaning of the word and the colors in which they are written e.g. I LIKE YOU, if

written in red ink.

In these cases, it has been observed the people pay more attention to the reading/ content of the

stimulus rather than the color from which it has been written (because they are experienced with

reading than with naming colors).

b. Dichotic Listening

· A procedure in which individual wears earphones in which different messages are

sent to each ear at the same time.

· After hearing the stimuli, the individual is asked to reproduce them aloud as it

comes to one ear: " shadowing".

· In this process, individual can easily identify the talking person as man or woman

and whether change in voices takes place during the message or not.

· Experiments suggest that although people pay full attention to one stimulus at a

time, they also pay some level of attention to the other stimuli as well; it shows the

possibility of learning something although being unaware of it.The phenomenon

of selective attention is of particular importance for the people who have to constantly monitor e.g.

such as pilots, traffic controllers, rescue workers, and firemen.

Form-Perception

· A perceptual phenomenon in which we perceive the shape, form or pattern of any

object___ give name to objects as house, tree, table, chair etc

· Mainly it involves two important principles:

· Figure- ground relationship

· Contours

Figure- Ground Relationship

· Our perceptual tendency to see objects with the foreground as well as the

background___ the object is being recognized with respect to its back ground e.g.

Black board and chalk, car parked in front of a wall, painting against the wall etc.

· Contrasting figures and their grounds are early and quickly perceived

· It is a vise- versa relationship i.e., figure cannot be observed without a ground and

ground cannot be recognized without having a figure.

Contours

· Perceptual phenomenon in which we are able to maintain a difference of the form from its

background due to the perception of contours e.g. In observing the paper, which has two colors,

white and black__ there is no contour at all. But as it becomes lighter rather than becoming dark, a

person can simply identify the difference. And when the difference is much apparent, we simply

divide into two parts as light and dark and skip different shades as lighter or darker____ where

brightness changes abruptly, we perceive contours.

Motion Perception

· Motion simply means the relative/ progressive change in the position of the person in space with

time. Objects cannot be perceived fully when in motion. It is also difficult due to the fact that our

eyes cannot follow the moving object with great precision and efficiency all the time.

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Relative Motion

· While looking at moving automobiles, the ones that are nearer seem to be moving more rapidly

than those at a moderate distance, and those that are more distant seem to be moving along.

· Relative motion can also be interpreted through experience, when one can fairly tell the speed of a

train or a bus by noticing outside the window as to how rapidly the nearby objects are passing.

Radical Motion

· A movement directly towards or away from the observer. Continuous and radical motion is being

perceived when the retinal image continuously changes.

· The change in size of the retinal image gives the perception of motion.

Perceptual Constancy

A perceptual tendency to perceive object as unchanging in size, shape, color, lightness etc., even though

changes in illumination and retinal image do take place.

There are a number of constancies identified by psychologists

Lightness Constancy: Means that the object's lightness or brightness remains the same in

spite of changes in illumination.

Lightness constancy illustrates an important perceptual principle: Perception is relative. Lightness

constancy may occur because the white piece of paper reflects more light than any of the other objects

in the room--regardless of the different lighting conditions. Another explanation, proposed by 19th-

century German physiologist Hermann von Helmholtz, is that we unconsciously take the lighting of the

room into consideration when judging the lightness of objects.

II .Color Constancy: Closely related with lightness constancy and refers to the perception of color of

the object remaining the same in spite of changes in lighting conditions.

Color constancy can be seen if one has worn a pair of sunglasses with colored lenses. In spite of the

fact that the colored lenses change the color of light reaching your retina, you still perceive white

objects as white and red objects as red. The explanations for color constancy parallel those for lightness

constancy. One proposed explanation is that because the lenses tint everything with the same color, we

unconsciously "subtract" that color from the scene, leaving the original colors.

III. Shape Constancy: Means the shape of the object remains the same in spite of some changes in its

orientation.

To understand shape constancy, hold a book in front of your face so that you are looking directly at the

cover. The rectangular nature of the book should be very clear. Now, rotate the book away from you so

that the bottom edge of the cover is much closer to you than the top edge. The image of the book on

your retina will now be quite different. In fact, the image will now be trapezoidal, with the bottom edge

of the book larger on your retina than the top edge. (Try to see the trapezoid by closing one eye and

imagining the cover as a two-dimensional shape.) In spite of this trapezoidal retinal image, you will

continue to see the book as rectangular. In large measure, shape constancy occurs because your visual

system takes depth into consideration.

IV. Size Constancy: refers to our ability or tendency to perceive objects as remaining of the same size

despite having distance from the observer.

When an object is near to us, its image on the retina is large. When that same object is far away, its

image on the retina is small. In spite of the changes in the size of the retinal image, we perceive the

object as of the same size. For example, when you see a person at a great distance from you, you do not

perceive that person as very small. Instead, you think that the person is of normal size and far away.

Similarly, when we view a skyscraper from far away, its image on our retina is very small--yet we

perceive the building as very large.

Visual Illusion

Also known as optical illusion.

Illusion is misperception, or false perception.

It is when the physical stimulus constantly and persistently produces error in perception.

There are various types of illusion of which the most famous are as follows;

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i.Muller- Lyer Illusion

The visual illusion in which the two lines of the same lengths appear different because of the change in

position of arrows at each end of two lines__ arrows pointing out appear shorter than the arrows

pointing inwards.

Causes of Illusions

Sensory deficits and defects

Readiness and expectation

Atmospheric variables

Effect of drugs

Artistic manipulation

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