Chapter 1:
Sensation and Perception
Overview of this Lecture
Why study perception?
 What are the steps in the perceptual process
 How is perception measured?
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Why Study Perception?
Understanding how you perceive the world
◦ Language processing
◦ Color vision
Multitasking and attention
Mindfulness
Rehabilitation Counseling
◦ Devices to assist people with vision and hearing
losses
Your friend Ellen is helping you collect
bugs for your science class.
 Walk in the woods searching for bugs.
 If you don’t like bugs, imagine Ellen is
helping you shop for the latest, coolest
sneakers.
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Science project
Figure 1.2 (a) We take the woods as the starting point for our description of the perceptual process.
Everything in the woods is the environmental stimulus. (b) Ellen focuses on the moth, which becomes the
attended stimulus. (c) An image of the moth is formed on Ellen’s retina.
The Perceptual Process

Stimulus
◦ All objects in the environment are available to the
observer.
◦ Observer selectively attends to objects.
◦ Stimulus impinges on receptors resulting in internal
representation.
The Perceptual Process - continued

Electricity
◦ Transduction occurs which changes environmental
energy to nerve impulses
◦ Transmission occurs when signals from the
receptors travel to the brain.
◦ Processing occurs during interactions among
neurons in the brain.
Figure 1.3 (a) Transduction occurs when the receptors create electrical energy in response to the light. (b)
Tranmission occurs as one neuron activates the next one. (c) This electrical energy is processed through
networks of neurons.
Figure 1.4 Comparison of signal transmission by cell phone and the nervous system. (a) Cell phone #1
sends an electrical signal that stands for “hello.” The signal that reaches cell phone #2 is the same as the
signal sent from cell phone #1. (b) The nervous system sends electrical signals that stand for the moth.
The nervous system processes these electrical signals, so the signal responsible for perceiving the moth
is different than the original signal sent from the eye.
Figure 1.5 (a) Ellen has conscious perception of the moth. (b) She recognizes the moth. (c) She takes
action by walking toward the tree to get a better view.
The Perceptual Process

Experience and Action
◦ Perception occurs as a conscious experience.
◦ Recognition occurs when an object is placed in a
category giving it meaning.
◦ Action occurs when the perceiver initiates motor
activity in response to recognition.
Figure 1.1 The perceptual process. The steps in this process are arranged in a circle to emphasize that
the process is dynamic and continually changing. See text for description of each step in process.
Information that the perceiver brings to
the situation.
 Knowledge can affect a number of steps
in the perceptual process.
 Experience helps you better identify
stimuli.
 Example: Police knowing how to identify
evidence.

Knowledge
Take a look at the next two slides.
 How do you identify the stimulus?

Demonstration
Look at this drawing first, then close your eyes.
Did you see a “rat” or a “man”?
Take a look at the next two slides.
 How do you identify the stimulus?

Demonstration 2
.
What the perceiver brings to the situation.
 Experience and/or expectation
 Is perception bottom-up or top-down

Knowledge
Two Interacting Aspects of Perception

Bottom-up processing
◦ Processing based on incoming stimuli from the
environment
◦ Also called data-based processing

Top-down processing
◦ Processing based on the perceiver’s previous
knowledge (cognitive factors)
◦ Also called knowledge-based processing
Perception is determined by an interaction
between bottom-up processing, which starts with
the image of the receptors,
and top-down processing, which brings the
observer’s knowledge into play.
Both types interact
the image of the moth on Ellen’s retina initiates
bottom-up processing; and
(b) her prior knowledge of moths contributes to topdown processing.
(a)
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Depends on the tools we have available.
Early psychologist could only present
stimuli and ask subject to respond.
Ex. Rat/man demonstrations
Psychophysical approach (PP)
the stimulus-perception relationship
What you see is what you get.
How do we study perception?
PP: Psychophysical Approach
Present stimulus and get
subjects response of perception
PH1: Physiological Approach 1
Present stimulus and measure
neurons firing (brain response).
PH2: Physiological Approach 2
Present stimulus and measure
both brain response and
subjects response.
.
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How does the brain respond to visual
stimuli?
Visual cortex in back of brain.
Implant electrodes in visual cortex of cat.
Measure response from cells.
Nobel prize in 1981
Hubel and Wiesel
PH1 in Goldstein book
Physiological Response
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Newest techniques allow us to scan the
brains of awake, human subjects.
Subject can tell us what they perceive.
Brain scans show us which areas are
most active.
Assume those areas are important for
perception.
PH2 in book
Brain scans
(PP) links stimuli and perception,
(PH1) links stimuli and physiological processes and the
(PH2) links physiological processes and perception.
Begin with psychophysical approaches.
 First methods available to psychologists.
 Don’t require brain implant or scanners.

Measuring Perception
Psychophysics - Overview of Methods of
Measurement

Qualitative Methods (identify and sort)
◦ Describing
◦ Recognizing

Quantitative Methods (ratio scales)
◦ Detecting
◦ Perceiving Magnitude
◦ Searching
Qualitative Methods of Psychophysical
Measurement

Description
◦ Indicating characteristics of a stimulus
◦ First step in studying perception
◦ Called phenomenological method

Recognition
◦ Placing a stimulus in a category by identifying it
◦ Categorization of stimuli
◦ Used to test patients with brain damage
Description
Wisconsin Card Sorting Test
Where do you sort the top card on deck?
Quantitative Methods - Classical Psychophysics

Absolute threshold - smallest amount of
energy needed to detect a stimulus
◦ Method of limits
 Stimuli of different intensities presented in
ascending and descending order
 Observer responds to whether she
perceived the stimulus
 Cross-over point is the threshold
 Hearing Test
Figure 1.12 The results of an experiment to determine the threshold
using the method of limits. The dashed lines indicate the crossover
point for each sequence of stimuli. The threshold - the average of
the crossover values - is 98.5 in this experiment.
Classical Psychophysics - continued

Absolute threshold (cont.)
◦ Method of adjustment
 Stimulus intensity is adjusted continuously
until observer detects it
 Repeated trials averaged for threshold
Classical Psychophysics - continued

Absolute threshold (cont.)
◦ Method of constant stimuli
 Five to nine stimuli of different intensities
are presented in random order
 Multiple trials are presented
 Threshold is the intensity that results in
detection in 50% of trials.
Figure 1.13 Results of a hypothetical experiment in which the threshold for seeing a light is measured by
the method of constant stimuli. The threshold - the intensity at which the light is seen on half of its
presentations - is 180 in this experiment.
Is There An Absolute Threshold?
There are differences in response criteria among
participants
◦ Liberal responder - responds yes if there is the
slightest possibility of experiencing the stimulus
◦ Conservative responder - responds yes only if he
or she is sure that a stimulus was present
◦ Each person has a different response criterion but
the sensitivity level for both of them may be the
same
 Signal detection theory is used to take individual’s
response criteria into account.

Figure 1.17 Data from experiments in which the threshold for seeing a light is determined for Julie (green
points) and Regina (red points) by means of the method of constant stimuli. These data indicate that
Julie’s threshold is lower than Regina’s. But is Julie really more sensitive to the light than Regina, or does
she just appear to be more sensitive because she is a more liberal responder?
Quantitative Methods - Modern
Psychophysics

Magnitude estimation (scaling)
◦ Stimuli are above threshold.
◦ Observer is given a standard stimulus and a value
for its intensity.
◦ Observer compares the standard stimulus to test
stimuli by assigning numbers relative to the
standard.
Modern Psychophysics - continued

Magnitude estimation (cont.)
◦ Response compression
 As intensity increases, the perceived
magnitude increases more slowly than the
intensity.
◦ Response expansion
 As intensity increases, the perceived
magnitude increases more quickly than
the intensity.
Figure 1.15 The relationship between perceived magnitude and stimulus intensity for electric shock, line
length, and brightness. (Adapted from Stevens, 1962.)
Other Measurement Methods

Searching for stimuli
◦ Visual search - observers look for one stimulus in a
set of many stimuli
 Reaction time (RT) - time from
presentation of stimulus to observer’s
response is measured
Find Elmo