Perception: Getting Started April 24, 2003. Perception is a key topic in Cognitive Science Cognitive...
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Perception: Getting Started
April 24, 2003
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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?
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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
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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
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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
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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.
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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?
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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
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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
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Unfamiliar contrasts are harder
Examples from the Mon language of Burma & Thailand:‘dog’
‘to cross’
‘to understand’
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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
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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
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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)
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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
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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?
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And now some visual illusions…
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Why does the perspective switch?
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Parallel Illusions (Hering)
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Kanisza’s Triangles
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Two Figures
• Why does one look like a cube, but not the other?
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Which dark vertical line is longer?
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Distance and Retinal Image Size
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The Right Stuff
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Would illusions involving straight lines and right angles work if our society weren’t full of straight lines and right angles?
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Down, Down
• What trick did Escher use to make the top staircase look monotonic?
• What trick is being used in the audio version?
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Why do the intersections seem splotchy?
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The inner squares are all the same shade of gray.
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All of the blues are the same shade. Why do they appear so different?
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Each stripe is uniform, but the middle ones appear to be darker toward the left. Why?
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Which circles are bumps, and which are indentations?
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This is the same figure as on the previous slide, but upside down.
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Rotating Mask
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
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Shepard’s Rotation Studies
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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”
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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