1. Perceiving Form – Part I
Kimberley A. Clow

2. Outline Perceiving Patterns & Edges Texture Cues Ambiguous Figures
Pop-Out Effects
Ambiguous Figures
Gestalt Laws
Theories
Template Theories
Feature Theories

3. Form Defined by Texture

4. Perceiving Form Change as a requirement for perceiving form
Snow Blindness
Stabilized Retinal Images
Organization occurs in the mind, not the retina
Top-Down Processing
Bottom-Up Processing
Gestalt Laws of Perception
BOTTOM-UP PROCESSING
combine individual elements OF THE STIMULUS (line form, etc.) and combine them into a unified perception.
TOP-DOWN PROCESSING
use of knowledge, expectancies, schemas, and concepts to interpret what we see
- based on learning
- active, creative process
We need local variations in the intensity of light and variation in the pattern of illumination on the retina over time in order to see. Snow blindness occurs partly because of the constancy of the light (it reflects off of the snow the same, so you get the same information no matter where you look, and thus you can’t discriminate form). Another way to lose vision due to static information is to stabilize an image on the retina. This can be done with special contacts that projects an image onto the retina. Then, no matter where the eye moves to, the same image is being reflected onto the same part of the retina. Eventually, the image fades. The contours disappear in chunks and the colour fades away. You don’t see the image anymore. If you flicker the image on and off very rapidly, however, the image returns and will not fade.
Gestalt psychologists were interested in the processes that cause certain elements to seem to be part of the same whole. They formulated several principles of perceptual organization that govern the emergence of a visual figure. Their basic observation was that elements within a pattern do not seem to operate independently and that the whole is more than merely the sum of its parts.
The whole is greater than the sum of its parts
Laws of Perception
Figural Organization
Proximity
Similarity
Good Continuation
Closure
Symmetry
Textural Contours
Subjective Contours and Illusory contours

5. Figure-Ground GESTALT ORGANIZATIONAL PRINCIPLES FIGURE-GROUND
Tendency to perceive an object against a background
The central figure is usually on top of what we perceive to be the background.
We perceive borders or contours wherever there are distinct changes in colour or brightness, but we interpret these as part of the figure rather than part of the background.
We pay more attention to what is being treated as figure than ground. We notice changes in the “figure” more than changes in the “ground”
Figure-Ground Relations
Ambiguous Images
Organization occurs in the mind, not on the retina
Smaller shapes more likely to become figure than ground

6. What is Figure? What is Ground?
M.C. Escher

7. What Do You See? Organization occurs in the mind, not the retina
Top-Down Processing
Bottom-Up Processing
These images are all based on perceiving edges

8. Gestalt Laws of Perception
How are they organized?
The Gestalt psychologists formulated a number of principles relating to the segregation of figure and ground and the grouping of objects.
Objects appear as they do because of the relation of component parts to one another
“the whole is greater than the sum of its parts”…
Proximity
Nearby objects tend to be grouped together

9. Similarity
Things that look similar tend to be grouped together

10. 2
Similarity of size
Similarity of shape
Similarity of orientation 3 1

11. Violation Good Continuation LAW OF CONTINUITY
GESTALT ORGANIZATIONAL PRINCIPLES
LAW OF CONTINUITY
tendency to link together
individual elements in
a way that makes sense.
Good Continuation

12. Due to law of continuity most people see this as one REALLY long dog
Really two different dogs
GESTALT ORGANIZATIONAL PRINCIPLES

13. Symmetry Humans prefer symmetrical objects-- aesthetic appeal
Most biological objects have the property of symmetry
Detecting regions that differ from their immediate surroundings is an early step in detecting symmetry
The visual system must also register similarities in the spatial arrangement of regions in the visual field

14. LAW OF CLOSURE
tendency to close open figures & fill in gaps in an incomplete figure

15. Subjective Contours Textural Contours
Subjective Contours and Illusory contours
In this image, a curved line is perceived where the alignment of the lines is discontinuous

16. Template Theories
We identify objects by matching the current stimulus to a template (or model) in memory
Stimulus
Templates
Template Theories
We have stored models (templates) to which incoming information is compared
Many computer programs function in this way
Too much real world variability for this to be possible
Think of all the different ways a ‘g’ might be written, in all the different fonts and handwritings… we’d need millions of templates!
Every template would have to be learned before it could be used to interpret new stimuli
We can recognize partial pictures and stimuli out of focus
Do we have blurry templates?

17. Feature Theories
So, if we don’t store an exact copy of every visual stimulus we can recognize, then how do we recognize patterns? Perhaps we break patterns down into their constituents, into the pieces they are made up of, into their features or components.
One of the more popular feature theories is called pandemonium.
Pandemonium: called that because the original theory used the analogy of groups of demons shouting out the results of their analyses at each level.
Image demons encode the contents of the retinal image and pass that on to the feature demons. Feature demons are each responsible for looking for a specific feature. If the feature they are responsible for is in the image, they start shouting. If the feature they are responsible for is partially present, they will shout, but not as loudly. These shouts are listened to by the cognitive demons, each of which is listening for a particular combination of shouts from the feature demons. If they hear the pattern they are responsible, the cognitive demons will yell, and they will yell more loudly the closer the pattern they detect is to their own pattern. A decision demon listens to this pandemonium caused by the cognitive demons and chooses the loudest demon as the one most likely to be the pattern presented to the sensory system.
Pandemonium
Data-driven feature extraction process
Four different stages
Image demons encode the contents of the retinal image
Feature demons shout when they detect their feature
Cognitive demons shout if they hear a particular combination of shouts from the feature demons
Decision demon listens to this “pandemonium” and chooses the cognitive demon that is making the most noise
Accounts for errors made identifying letters
Doesn’t account for context effects
What is considered a feature or pattern is arbitrary

18. These Theories Are Not Enough
These theories are all data driven
bottom-up processing
Need to allow for the effects of context as well
top-down processing
In real life, we have expectancies and these exert an influence on what we perceive
It is truly humbling to realize that understanding the simple act of recognizing that you are looking at a pencil, and that the pencil is not part of the desk on which it rests but is a separate object in its own right, remains a problem about which there are many theories but still no final answer
And to make the process even more difficult, there are effects like perceptual constancy, development, experience, and individual differences to account for
These theories are all data driven
bottom-up processing
Need to allow for the effects of context as well
top-down processing
In real life, we have expectancies and these exert an influence on what we perceive
The pile of clothes that looks like a person out of the corner of our eye
The school classmate you don’t recognise in the mall

19. Can You Identify This Shape?
Law of Pragnanz: the organization of the visual array into perceptual objects will always be as “good” as the prevailing conditions allow
Based on regularity, simplicity, and symmetry