1. Perception Unit
How can we so easily perceive the world? Objects may be moving, partially hidden, varying in orientation, and projected as a 2D image on our retina.
Why do perceptual illusions work? What are we doing besides “just looking”?

2. PERCEPTION UNIT
What is vision for?
Perceptual pathways
Characteristics of Perception
Context effects on Perception
Change blindness

3. What are the goals of vision?
What is it?
Where is it?
How is it moving?

4. What is it? Agnosia – “without knowledge” -
impairment in visual recognition
Cannot recognize common objects
Prosopagnosia
- inability to recognize faces
(but can recognize objects)
“Man who mistook his wife for a hat“

5. Want to identify objects, not colors or lights or edges or…
Why is it hard to go from retina to object?
Changing orientation
Occlusion (partially hidden)
Distance
Changing shapes (for objects with parts)

6. Where is it?
Spatial neglect -- pay attention to only part of the world.

7. Person with spatial neglect copied the drawing of the house while it was still present

8. How is it moving?
Damage to motion processing areas -- can see objects but not motion
Can see objects change their position, but not their motion -- snapshots rather than motion

9. Perceptual pathways
Can examine how visual information is gathered and processed

10. Rods -- brightness
Cones -- fovea, color

12. (I) Perception: Definition & characteristics
Perception - The process through which sensations are interpreted, using knowledge & understanding of the world, so that they become meaningful experiences.
(Sensations - immediate & basic experiences generated by isolated,
simple stimuli.)
Characteristics: -- not just seeing what is there
Knowledge-based - perception matches incoming stimuli with previously learned information.
Inferential - Stimuli are not always perceived as they are, but infer what should be there.
Automatic - many perceptual processes operate rapidly, without
conscious awareness.
Relational - perception is influenced by relations among stimuli in the
environment.
Adaptive - perception is flexible and can accommodate changes in the environment

14. CONTRAST -- crucial aspect that visual system uses for clues

16. What is vision for? (Marr)
Vision is a process that produces from images of the external world, a description that is useful for the viewer and not cluttered with irrelevant information
Very active process -- not just “looking”

17. Context effects in object perception
Bottom-up processing: low-level information used to build up shapes for object perception
Top-down processing: a person’s concepts and expectations (knowledge) influence object perception

18. Incoherent scene Coherent scene
Easier to recognize fire hydrant in coherent scene

19. Change blindness What is our visual experience like?
Seems to be smooth
What do our eyes do as we look at object or scene?
Fixation -- lasts for 200 msec (1/5 second)
Saccade -- eye movement ( msec)

20. How do we integrate across fixations?
If we are getting succession of fixations, why is our experience smooth?
Possibility: we have a rich image from previous fixation and we align it with the current one to give impression of continuity (like panorama mode)
Prediction: last fixation is well-remembered (since need to combine it with current one)
Is it?

21. Change blindness - flicker paradigm (e. g
Change blindness - flicker paradigm (e.g., Rensink, O’Regan, & Clark, 1997)
Test memory for previous fixation by having new fixation differ and see if people can tell where difference is.
Take a scene, make two versions, A and B, with one difference between them. (marginal or central interest of object changing)
A for 240 msec. -- Blank screen for 80 msec
B for 240 msec. -- Blank screen for 80 msec
A for 240 msec. -- Blank screen for 80 msec, and so on

22. Change blindness - flicker paradigm (e. g
Change blindness - flicker paradigm (e.g., Rensink, O’Regan, & Clark, 1997)
Results -- seconds to identify the change
Change in marginal interest 11.0 seconds
Change in central interest 4.5 seconds

23. Change blindness - naturalistic (Simons & Levin, 1999)
Perhaps effect due to strange flickering paradigm?
Experimenter approaches S and asks for directions.
During conversation, two workman carry a door and walk between them.
Experimenter switches places with one workman (note they are wearing different clothes)
Conversation continues.

24. Change blindness - naturalistic (Simons & Levin, 1999)
What % of subjects notice change of person??
50%!
Why? Unsure, but likely that classified at abstract level, not with details of person
(other study showed that if both construction workers, then even worse)

25. Implications of Change blindness
Possibilities
Only an incomplete perceptual record is retained across fixations
Not comparing across fixations
In either case, Goal relevance is crucial.

26. Perception Unit
Perception complex mixture of innately determined processes that become adapted to the world from our experiences
Much of perception is making assumptions to simplify a too complex environment and making inferences to fill in missing information

27. Readings and Key terms Bernstein et al., Chapter 5, 143-170 Perception
Grouping factors
Agnosia
Context effects
Bottom-up processing
Top-down processing
Change blindness