Perception

Some Questions to Consider
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Why can two different people experience different perceptions in response to exactly the same stimulus?
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How does the brain become tuned to respond best to things likely to appear in the environment?
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How does perception depend on a person’s knowledge about characteristics of the environment?
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Are there neurons in the visual system that might help us understand other people’s actions?

The Complexity of Perception
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Bottom-up processing
– Perception may start with the senses
– Incoming raw data
– Energy registering on receptors
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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
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Transduction: conversion of this energy to neural signals
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Attention: concentration of mental energy to process incoming information
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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)
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Accessory structure modifies energy
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Receptor transduces energy into a neural response
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Sensory nerve transmits the coded activity to the central nervous system
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Thalamus processes and relays the neural response
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Relayed to specialized areas of the cortex
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Perception of the world is created

Perception Is…
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The process of recognizing, organizing, and interpreting information from senses
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Not an exact copy of “the world”
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Based on our past experience and expectations

Approaches to Understand Perception
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Direct perception theories
– Bottom-up processing
– Perception comes from stimuli in the environment
– Parts are identified and put together, and then recognition occurs
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Constructive perception theories
– Top-down processing
– People actively construct perceptions using information based on expectations

Bottom-up Processing: Behavioral
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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
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Discriminability: geons can be distinguished from other geons from almost all viewpoints
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Resistance to visual noise: geons can be perceived in “noisy” conditions
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Distinct: 36 different geons have been identified

Top-down Processing
(Constructive Perspective)
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Top-down processing involves making inferences based on context, guessing from experience, and basing one perception on another
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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
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Perceived size is a function of both bottom-up and top-down processing
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Bottom-up processing
– the size of the image on the retina
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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)
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Top-down theory
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Some of our perceptions are the result of unconscious assumptions we make about the environment
– We use our knowledge to inform our perceptions
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We infer much of what we know about the world
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Likelihood principle: we perceive the world in the way that is “most likely” based on our past experiences

Perceptual Organization
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“Old” view – structuralism
– Perception involves adding up sensations
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“New” view – Gestalt psychologists
– The mind groups patterns according to laws of perceptual organization

Gestalt Laws of Perceptual Organization
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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
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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
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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

Gestalt Laws of Perceptual Organization
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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
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Law of proximity
– Things near each other appear grouped together
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Law of common fate
– Things moving in the same direction appear to be grouped together

Gestalt Laws of Perceptual Organization
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Gestalt laws often provide accurate information about properties of the environment
– Reflect experience
– Used unconsciously
– Occasionally misleading
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Gestalt laws are heuristics

Heuristics and Algorithms
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Heuristic: “rule of thumb”
– Provides best-guess solution to a problem
– Fast
– Often correct
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Algorithm: procedure guaranteed to solve a problem
– Slow
– Definite result

Other Perceptual Heuristics
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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
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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
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Some neurons respond best to things that occur regularly in the environment
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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
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What stream: identifying an object
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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”
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Damage to different areas of the brain cause very different deficits
– We can conclude that a specific area is necessary for a specific function
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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
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Single dissociation
– One function is lost, another remains
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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
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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
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Located in the premotor cortex
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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)