PERCEPTION

Outline for Lecture 6: Feb 27, 2001

1) Announcements:

Exam results: Average was 67.5%. Top mark 70/75, lowest mark 21/75. Remember you must pass both tests to pass the course (your bonus marks will not lift you to a pass if you fail both tests.)

Will the following students please see me at the break - you are not on the official class list, so make sure the registrar knows you are in the course. Black, Kierra: Gough, W. Bryan: Jilesen, Andrea: Larsh, Mike: Mangru, Vickram: Wang, Tim: Williams, Adwin: Wilson, Heather.

2) Perception (Chapter 6)

2 Videos: The Brain, 2nd edition # 9: Visual Information Processing II: Perception (9 min.) The Brain, 2nd edition, #10: Perception: Inverted vision (5 min.)Class Demonstration - the Stroop effect

PERCEPTION

Making sense of the external world

e.g., orange - Distal Stimulus is the apple - has 3 dimensions

Proximal Stimulus is the image in 2 dimensions that the orange casts on our retina

EMPIRICISTS - John Locke, Hermann von Helmholtz

- sensations are modified and reinterpreted based on experience.

NATIVISTS - Immanuel Kant(18th C)

James G. Gibson (20th C)

many aspects of perceptual experience are part of our natural endowment.

A. PERCEPTUAL ORGANIZATION

Max Wertheimer (1880-1943) founder of Gestalt psychology.

Organization basic to all mental activity

Figure/ground relations
Grouping

Laws of Perceptual Organization

proximity

similarity

continuity

closure

connectedness

B. DEPTH PERCEPTION - is it innate or gained through experience?

Example: visual-cliff experiment with children (picture in text p 215.)

Binocular cues - more for close objects

retinal disparity

convergence

Monocular cues - more for objects farther away

linear perspective

relative size

interposition or overlap

texture gradients

relative brightness

relative height

Video: The Brain, 2nd edition # 9: Visual Information Processing II: Perception (9 min.)

C. MOTION PERCEPTION

- stroboscopic movement - we interpret a rapid series of still pictures as movement (basis of moving pictures)
- phi phenomenon - if two visual stimuli are briefly flashed one after the other, it can appear as if one stimulus is moving back and forth (see webpage link - apparent motion). The timing of the flashes is important, though, and there's a direct relation between the spacing of the stimuli and the inter-stimulus interval (ISI).
- induced movement - if one object frames another, and the frame moves, we perceive the thing being framed as having moved

This illusion explains how we perceive the moon as moving behind clouds, when really it's the clouds that are moving.

D. PERCEPTUAL CONSTANCY

Our ability to see as unchanging an object whose proximal stimulus is changing.

Shape constancy (door opening), tops of boxes illusion
Size-distance relations (overheads shown last time) - interplay explains many illusions
Lightness (or brightness) constancy - we perceive things as having a constant brightness even when illumination changes. Perceived brightness depends on relative luminance, so the surrounding context really matters (come up and see my Nature Neuroscience cover page after class).

E. PERCEPTUAL ADAPTATION

Video: Inverted vision (5 min.)

Susannah's visual world is turned upside down by special lenses she wears for a week. She manages to adapt, but it takes a full week - the brain is very adaptable. It only takes her an hour to re-adapt to the normal world when the experiment ends.

F. PERCEPTUAL SET

Our prior experience strongly influences how we perceive things.
Overhead examples: young/old woman
Past experiences (schemas) help us interpret ambiguous sensations.

G. AUTOMATIZATION OF PERCEPTION

(Class Demonstration)

• with practise, some perceptual tasks can become automatic

• this can be useful, but also it can get in the way

• a good example of obligatory automatic processing is the Stroop interference effect (named after J. Ridley Stroop, 1935)

The idea is that we can't help reading, and it interferes with other cognitive tasks. For example if the task is to name the colour of colour adjectives that are not the same as colour of the ink in which the adjectives are written, it's quite difficult. If the word 'green' was written in brown ink and the task was to say 'brown', one couldn't help reading 'green', so colour naming would be slowed by the automaticity of reading.

• this test is sometimes used as a tool to measure the extent to which a child reads automatically