Perception: Getting Started

April 24, 2003

Perception is a key topic in Cognitive Science

• Cognitive Science regards the mind as an information processing device.

• Information is about something, and the processor must have inputs.

• Perception provides this: the input comes through the senses and is about the world outside the mind.

• One reading suggested that this is where the real action is in cognition. Which one?

Many Aspects to the Study of Perception

• At least 5 senses: sight, hearing, smell, taste, and touch (plus balance, temperature,…?)– All are quite different, with specialized properties– We will deal almost exclusively with sight & hearing

• Perception can be studied behaviorally, physiologically, or modeled computationally

• There is substantial literature on perception in at least biology, psychology, philosophy, computer science, & linguistics

Reasoning vs. Perception

REASONING• Characteristically

human• Computational

modeling straightforward

• Little known about biological mechanisms

PERCEPTION• Similar among

higher animals• Computational

modeling difficult• Quite a bit known

about biological mechanisms

Overview of Today’s Class

• Continue with general remarks on perception

• Some examples from language

• Some perceptual (mostly visual) illusions

• Discussion of what we learn from the examples

Superficial Historical Remarks

• Until relatively recently, perception could only be studied through introspection & casual observation

• Biological investigations were limited to examining people with perceptual deficits, largely post mortem

• Controlled psychological experiments only developed in 19th century

• New instrumentation greatly advanced both behavioral & biological studies in past 50 years

• Techniques for studying live brains in action developed in the past 10-20 years are revolutionizing this field.

Some General Questions About Perception

• How objective are our perceptions? Is it true that “Seeing is believing”? Should it be?

• How do our senses work?• How does the information from our senses get

processed by the central nervous system?• How is perceptual information integrated with

other knowledge?– Is there a common “language of thought” into which

perceptual information is translated?– If not, how can we model perception

computationally?

How do we understand speech?

• First guess: it’s like reading -- identify sound segments, piece them together into words.

• Would be remarkable, if true– very subtle differences in sound differentiate

speech sounds– we can identify words even with very

incomplete or degraded information

pa vs. ba

• The difference is in when vocal cords begin moving -- about 60 msec (1/16 of a second) earlier in ba

• We have no problem hearing the difference, even under noisy conditions

Unfamiliar contrasts are harder

Examples from the Mon language of Burma & Thailand:‘dog’

‘to cross’

‘to understand’

But speech isn’t like reading

• Speech sounds change, depending on the other sounds around them:– comfort vs. comfortable– spill vs. pill

• Word boundaries don’t always correspond to breaks in the speech stream

Another linguistic example: color terms

• The color spectrum is a continuum, but languages divide it up by naming regions

• Do speakers of languages that divide the spectrum differently perceive color differently?

• This has been studied extensively by anthropological linguists Berlin & Kay

Berlin & Kay’s Findings

• Color-name boundaries are highly variable, not only across languages, but across individuals and even across time for a given individual.

• There is wide agreement on focal colors -- that is, the best exemplar of a given color name.

• Languages have between 2 and 11 basic color terms -- that is, terms that are neither: – restricted to particular types of referents (e.g.,

blond), nor– special cases of some other color (e.g., cardinal)

Berlin & Kay, continued

• The basic colors are partially ordered:

black and white

red

yellow or green

blue

brown

grey or orange or pink or purple

Questions about the Berlin & Kay work

• Did they get the facts right: putative counterexamples in some languages (but lots of supporting evidence).

• Is the ordering based on the physiology of the visual system?

• More generally, what does this tell us about vision and categorization?

And now some visual illusions…

Why does the perspective switch?

Parallel Illusions (Hering)

Kanisza’s Triangles

Two Figures

• Why does one look like a cube, but not the other?

Which dark vertical line is longer?

Distance and Retinal Image Size

The Right Stuff

Would illusions involving straight lines and right angles work if our society weren’t full of straight lines and right angles?

Down, Down

• What trick did Escher use to make the top staircase look monotonic?

• What trick is being used in the audio version?

Why do the intersections seem splotchy?

The inner squares are all the same shade of gray.

All of the blues are the same shade. Why do they appear so different?

Each stripe is uniform, but the middle ones appear to be darker toward the left. Why?

Which circles are bumps, and which are indentations?

This is the same figure as on the previous slide, but upside down.

Rotating Mask

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Shepard’s Rotation Studies

Rotation Study Results

• Time to answer that two figures were the same was proportional to amount of rotation

• Suggests that visual perception involves manipulable mental images

• Raises questions for cognitive science about the “language of thought”

Conclusions

• Perception is highly sensitive to context

• Perception is more than the passive reception and composition of stimuli

• The mind interprets sensory stimuli based on past experience

• How information from different senses is integrated is a hard problem