Chapter 3
Perception
Some Questions to Consider
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Why can two different people experience different
perceptions in response to exactly the same stimulus?
How does the brain become tuned to respond best to
things likely to appear in the environment?
How does perception depend on a person’s knowledge
about characteristics of the environment?
Are there neurons in the visual system that might help
us understand other people’s actions?
The Complexity of Perception
• Bottom-up processing
– Perception may start with the senses
– Incoming raw data
– Energy registering on receptors
• Top-down processing
– Perception may start with the brain
– Person’s knowledge, experience,
expectations
Definitions
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Sensation: absorbing raw energy (e.g., light waves,
sound waves) through our sensory organs
Transduction: conversion of this energy to neural
signals
Attention: concentration of mental energy to process
incoming information
Perception: selecting, organizing, and interpreting
these signals
Overview: Sensation and Perception
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Energy contains information about the world
(usually incomplete, full of noise, and distorted)
Accessory structure modifies energy
Receptor transduces energy into a neural response
Sensory nerve transmits the coded activity to the
central nervous system
Thalamus processes and relays the neural response
Relayed to specialized areas of the cortex
Perception of the world is created
Perception Is…
• The process of recognizing, organizing, and
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interpreting information from senses
Not an exact copy of “the world”
Based on our past experience and
expectations
Approaches to Understand Perception
• Direct perception theories
– Bottom-up processing
– Perception comes from stimuli in the environment
– Parts are identified and put together, and then recognition
occurs
• Constructive perception theories
– Top-down processing
– People actively construct perceptions using information
based on expectations
Bottom-up Processing: Behavioral
• Recognition-by-components theory (RBC)
– We perceive objects by perceiving
elementary features
– Geons: three-dimensional volumes
– Objects are recognized when enough
information is available to identify object’s
geons
Caption: (a) Some geons; (b) some objects created from the geons on the left.
The numbers on the objects indicate which geons are present. (Adapted from
“Recognition-by-Components: A Theory of Human Image Understanding,” by I.
Biederman, 1987, Psychological Review, 24, 2, pp. 115-147, Figures 3, 6, 7,
and 11, Copyright © 1987 with permission from the author and the American
Psychological Association.
Caption: An airplane represented (a) by nine geons and (b) three geons.
(Source: Adapted from “Recognition-by-Components: A Theory of Human
Image Understanding,” by I. Biederman, 1987, Psychological Review, 24, 2,
pp. 115–147, Figures 3, 6, 7, and 11, Copyright ˝ 1987 with permission from
the author and the American Psychological Association.)
Geons
• Discriminability: geons can be distinguished
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from other geons from almost all viewpoints
Resistance to visual noise: geons can be
perceived in “noisy” conditions
Distinct: 36 different geons have been
identified
Top-down Processing
(Constructive Perspective)
• Top-down processing involves making
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inferences based on context, guessing from
experience, and basing one perception on
another
Occurs quickly, automatically
Caption: “Multiple personalities of a blob.” What we expect to see in different contexts
influences our interpretation of the identity of the “blob” inside the circles. (Source:
Andrew Hollingsworth. 2005. Memory for object position in natural scenes. Visual
Cognition, 12, 1003–1016. Reprinted by permission of the publisher, Taylor &
Francis Ltd, http://www.tandf.co.uk/journals. Photographs courtesy of Antonio
Torralba.)
Perceiving Size:
Taking Distance into Account
• Perceived size is a function of both bottom-up
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and top-down processing
Bottom-up processing
– the size of the image on the retina
Top-down processing
– the perceived distance of the object
– the size of the object relative to other
objects in the environment
Helmholtz’s Theory Of
Unconscious Inference (~1860)
• Top-down theory
• Some of our perceptions are the result of
unconscious assumptions we make about the
environment
– We use our knowledge to inform our perceptions
• We infer much of what we know about the
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world
Likelihood principle: we perceive the world in
the way that is “most likely” based on our past
experiences
Perceptual Organization
• “Old” view – structuralism
– Perception involves adding up sensations
• “New” view – Gestalt psychologists
– The mind groups patterns according to
laws of perceptual organization
Gestalt Laws of Perceptual Organization
• Law of good continuation
– Lines tend to be seen as following the
smoothest path
Caption: We perceive this pattern as continuous interwoven
strands because of good continuation.
Gestalt Laws of Perceptual Organization
• Law of good figure (simplicity or prägnanz)
– Every stimulus pattern is seen so the
resulting structure is as simple as possible
Caption: Law of simplicity. We see five circles, as in (a), not
the more complex array of nine objects, as in (b).
Gestalt Laws of Perceptual
Organization
• Law of similarity
– Similar things appear
grouped together
Caption: Law of similarity. (a) This display
can be perceived as either vertical columns
or horizontal rows; (b) more likely perceived
as columns of squares alternating with
columns of circles, due to similarity of shape
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Gestalt Laws of Perceptual Organization
Law of familiarity
– Things are more likely to form groups if the
groups appear familiar or meaningful
Caption: The Forest Has Eyes by Bev Doolittle (1985). Can you find
13 faces in this picture? (Source: “The Forest Has Eyes” 1984 Bev
Doolittle, courtesy of The Greenwich Workshop, Inc.)
Gestalt Laws of Perceptual Organization
• Law of proximity
– Things near each other appear grouped
together
• Law of common fate
– Things moving in the same direction
appear to be grouped together
Gestalt Laws of Perceptual Organization
• Gestalt laws often provide accurate
information about properties of the
environment
– Reflect experience
– Used unconsciously
– Occasionally misleading
• Gestalt laws are heuristics
Heuristics and Algorithms
• Heuristic: “rule of thumb”
– Provides best-guess solution to a problem
– Fast
– Often correct
• Algorithm: procedure guaranteed to solve a
problem
– Slow
– Definite result
Other Perceptual Heuristics
• Light-from-above heuristic
– Light comes from above
– Is usually the case in the environment
– We perceive shadows as specific
information about depth and distance
• Occlusion heuristic
– When object is partially covered by a
smaller occluding object, the larger one is
seen as continuing behind the smaller
occluder
Caption: (a) Some of these discs are perceived as jutting out, and some are
perceived as indentations. The explanation for this perception is that light
coming from above will illuminate (b) the top of a shape that is jutting out
and (c) the bottom of an indentation.
Neurons and the Environment
• Some neurons respond best to things that
occur regularly in the environment
• Neurons becomes tuned to respond best to
what we commonly experience
– Horizontals and verticals
– Experience-dependent plasticity
Caption: Greeble stimuli used by
Gauthier. Participants were
trained to name each different
Greeble.
Caption: Magnitude of brain responses to faces and
Greebles (a) before and (b) after Greeble training.
The colored areas in the brain records indicate
brain activity. The FFA is located within the white
squares. (Source: Reprinted with permissions
from Gauthier, I., Tarr, M. J.,Anderson, A. W.,
Skudlarski, P., & Gore, J. C. 1999. Activation of
the middle fusiform “face area” increases with
experience in recognizing novel objects. Nature
Neuroscience, 2, 568–573.)
Perception and Action: What and Where
• What stream: identifying an object
• Where stream: identifying the object’s
location
Perception and Action:
Using Dissociation Logic
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If you are trying to understand a complex system, you
can logically deduce conclusions from “malfunctions”
Damage to different areas of the brain cause very
different deficits
– We can conclude that a specific area is necessary for a
specific function
Brain Ablation method allows scientists to damage
specific areas of otherwise normal brains (usually in
monkeys or cats)
– Controlled damage allows for clear conclusions to be
drawn
Perception and Action: Dissociation Logic
• Single dissociation
– One function is lost, another remains
• Example: Monkey A has damage to
temporal lobe. This monkey is no longer
able to identify objects (what) but can
still identify locations (where)
– Therefore, what and where rely on different
mechanisms, although they may not
operate totally independent of one another
Perception and Action: Dissociation Logic
• Double dissociation
– Requires two individuals with different
damage and opposite deficits
• Example: Monkey A with temporal lobe
damage has intact where but impaired what;
Monkey B with parietal lobe damage has intact
what but impaired where
– Therefore, what and where streams must have
different mechanisms AND operate independently of
one another
Caption: The two types of discrimination tasks used by Ungerleider and Mishkin.
(a) Object discrimination: Pick the correct shape. Lesioning the temporal lobe
(purple shaded area) makes this task difficult. (b) Landmark discrimination: Pick the
food well closer to the cylinder. Lesioning the parietal lobe makes this task difficult.
(From Mishkin, Ungerleider, & Macko, 1983.)
Caption: (a) Alice can’t name objects but can accurately reach for them; (b)
Bert can name objects, but has trouble accurately reaching for them. This
illustrates a double dissociation.
Mirror Neurons
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Neurons that respond the same way when actually
performing an act and when observing someone else
perform the act
Located in the premotor cortex
One function of the mirror neurons might be to help
understand another person’s actions and react
appropriately to them (Rizzolatti & Arbib, 1998;
Rizzolatti et al., 2000, 2006)