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”?

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

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

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“

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)

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

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

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

Perceptual pathways Can examine how visual information is gathered and processed

Rods -- brightness Cones -- fovea, color

(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

CONTRAST -- crucial aspect that visual system uses for clues

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”

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

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

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 (25-100 msec)

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?

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

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

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.

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)

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.

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

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