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Perception. We have previously examined the sensory processes by which stimuli are encoded. Now we will examine the ultimate purpose of sensory information PERCEPTION - the conscious representation of the external environment. Perceptual Organization.

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  • We have previously examined the sensory processes by which stimuli are encoded.

  • Now we will examine the ultimate purpose of sensory information

  • PERCEPTION - the conscious representation of the external environment.

Perceptual organization
Perceptual Organization

  • Some of the best examples that perception involves organization of sensory input was provided by the Gestalt Psychologists.

  • Gestalt psychologists hypothesized that “the whole is greater than the sum of the parts.”

  • They were interested in showing the global nature of our perceptions

Gestalt grouping principles
Gestalt Grouping Principles

  • Gestalt theorists argued that our perceptual systems automatically organized sensory input based on certain rules.

  • Proximity

  • Similarity

  • Closure

  • Good Continuation

  • Common Movement

  • Good Form

Figure and ground
Figure and Ground

  • Gestalt Psychologists also thought that an important part of our perception was the organization of a scene in to its:

  • Figure - the object of interest

  • Ground - the background

Depth perception
Depth Perception

  • One of our more important perceptual abilities involves seeing in three-dimensions

  • Depth perception is difficult because we only have access to two-dimensional images

  • How do we see a 3-D world using only the 2-D retinal images?

Depth perception cues
Depth Perception Cues

  • Cue - stimulus characteristics that influence our perceptions

  • We are able to see in 3-D because the visual system can utilize depth cues that appear in the retinal images.

Types of depth cues
Types of Depth Cues

  • Depth cues are usually divided into categories, we will consider two types of depth cues:

  • Monocular- depth cues that appear in the image in either the left or right eye

  • Binocular - depth cues that involve comparing the left and right eye images

Monocular depth cues
Monocular Depth Cues

  • Occlusion

  • Relative image size

  • linear perspective

  • Texture gradient

  • Position relative to Horizon

  • Differential lighting of surfaces (Shading)

Relationship between perceived size and perceived depth








Relationship Between Perceived size and Perceived Depth

  • To perceive the size of objects accurately we must also perceive their distance accurately.

  • Thus, many visual illusions occur simply because a particular image lacks sufficient depth cues.

This figure shows that image size depends upon both object size and distance

Ames room
Ames Room

  • The Ames room is designed so that the monocular depth cues give the illusion that the two people are equally far away

Other size distance illusions

(a) Müller-Lyer illusion

(b) Ponzo illusion

Other Size-Distance Illusions

  • In each of these examples, the top and bottom lines are actually the same length.

  • In each case the top line looks longer.

  • Why?

Muller lyer illusion
Muller-Lyer Illusion

  • Perceptual psychologists have hypothesized that the top horizontal line looks longer because it also looks farther away.

  • Specifically, the inward pointing arrows signify that the horizontal line is closest to you, and the outward pointing arrows signify the opposite case.

Ponzo illusion
Ponzo Illusion

  • Converging lines indicate that top line is farther away than bottom line

Binocular depth cues
Binocular Depth Cues

  • Monocular depth cues allow us to see in 3-D with the view of only one eye, but our best depth perception occurs if we look through both eyes.

  • This is because our right and left eyes see a slightly different view of the world.

  • This difference between the image in the two eyes is know as Binocular Disparity.

Binocular stereopsis
Binocular Stereopsis

  • Binocular Stereopsis - the perception of depth based on retinal disparity

  • Psychologists typically study binocular stereopsis with the use of stereograms.

  • Stereogram - stimulus consisting of a left eye and a right eye image. When these two images are fused, the Illusion of depth is created.


  • Another way to create the illusion of depth through binocular stereopsis is with an Autostereogram.

  • An autostereogram is formed by superimposing two repeating patterns

  • The two patterns are slightly offset, and when viewed properly, this offset is seen as a binocular disparity.

Perceptual constancy
Perceptual Constancy

  • When viewing conditions change, the retinal image changes even if the objects being viewed remain constant.

  • Example: as a person walks away from you their retinal image decreases in size.

  • important function of the perceptual system is to represent constancy in our environment even when the retinal image varies.

Size constancy

Point B

Point A

Size Constancy

  • Cylinders at positions A and B are the same size even though their image sizes differ

  • The depth cues such as linear perspective and texture help the visual system judge the size accurately

Shape constancy
Shape Constancy

  • It is hard to tell if the figure on the upper right is a trapezoid or a square slanted backward.

  • If we add texture, the texture gradient helps us see that it is actually a square.

Attention combining the elements of perception
Attention: Combining the Elements of Perception

  • We can not be consciously aware of all the perceptual information available to us at any one time.

  • Thus, we have the ability to enhance some aspects of our perception while ignoring others.

  • This selective process is know as attention.

Preattentive processing
Preattentive processing

  • Some aspects of our perception are under our conscious attentive control.

  • Example: In a large crowd, we can concentrate on listening to some people and ignoring others.

  • However, other perceptual processes occur automatically and we can not consciously control them.

Feature integration
Feature Integration

  • Feature detection is an example of a preattentive perceptual process, and can be done in parallel

  • Feature Integration, seems to require attention and most be performed serially.

Feature integration1

Stage 1




Stage 2


Line slant







of stimulus

(a red X)





Feature Integration

Feature pop out



Feature Pop Out

  • The slanted line amongst vertical lines Pops Out, but the cross amongst horizontal and vertical lines does not pop out.

The stroop effect learned automaticity
The Stroop Effect: Learned Automaticity

  • Some abilities which once required attention can become automatic through practice.

  • An example of such learned automaticity is found in the Stroop Intereference Effect.

  • Stroop found that the act of reading could interfere with your ability to perform simple perceptual distinctions like naming colors.

Stroop effect



































Stroop Effect

  • In Stroop’s experiment, observers were asked to look at a word and name the color of the ink used to write that word.

  • Stroop found that if the meaning of the word was different than the color of the ink, observers were less successful at naming colors.