Why We Need Methods
Our senses convey information we need to
interpret and respond to stimuli of light,
chemicals, mechanical pressure, and temperature.
How can we respond so quickly to the endless
flow of inputs from our environment?
What can be learned by investigating our sensory
mechanisms and their relationship to both
physiological and psychological phenomena?
The field of psychophysics examines this
relationship between physical stimuli in
our environment and our psychological
reactions or behavior to them.
Although the physical events around us
can be measured, they are perceived
subjectively in a way that is difficult to
Psychophysical techniques have served as keys to
unlocking the mysteries of human perceptual
The tools of the psychophysicist are independent
variables (e.g., light, sound, or mechanical
pressure), and dependent variables -- behavioral
responses of various kinds, such as vocalizations
(e.g., I saw it) or button presses.
To use an analogy imagine you have a
complex machine and you’d like to
examine it. There are three major ways:
1. Anatomic: open it up and determine its
structure, draw diagrams of the parts and
their connections. For example, take
apart a computer tower, you’ll find slots,
cards, chips, etc.
2. Neurophysiological: determine the
functions of the device, what each part
does. For example take a voltmeter to the
computer tower and measure different
areas, see how much electrical energy goes
where and in what directions.
3. Psychophysical: determine the operation
capabilities of the device. Input information,
then observe and measure the changes to the
machine. For example, a black box.
From the input/output relationship, the
operational characteristics of the system
Some Key Concepts in
The rest of this lecture will consist of key
concepts in moderate detail
These are methods used to obtain
The Concept of the Threshold:
How sensitive a sensory system is.
Determined by measuring how much of a
particular stimulus is required to reliably
detect that stimulus
A sensory threshold represents the entrance
of a stimulus into sensory existence.
The threshold for a particular light stimulus is
that intensity which allows it to be "just
Method of Limits
Stimulus is either gradually increased
(Ascending Series) or decreased
(Descending Series) in intensity
The subject indicates on each trial (on each
presentation) whether the stimulus was
"seen" or "not seen"
(or felt, or heard, or smelled, etc.)
Method of Adjustment (MOA)
Subject controls the intensity of the
The subject adjusts the intensity until the
stimulus is judged to be (in the case of a
visual stimulus) "just visible"
For example, you keep turning your stereo
up, louder and louder untill someone
complains or you’ve lost hearing. haha.
Method of Constant Stimuli
The order of presentation of the stimulus is
randomized, so the subject cannot
anticipate the intensity of the stimulus on
any given trial.
The percent (Y) responses can be plotted
as a function of stimulus strength, and a
psychometric function (math) can be
Forced-Choice Procedures (FC)
Subjects are presented with two or more
alternatives, and must select one on each trial
even if the stimulus was not clearly seen.
The choice can thus be coded as a criterion-
free "correct" or "incorrect".
Alternatives can be presented sequentially
(temporal forced-choice), or can be presented
simultaneously (spatial forced-choice).
There must be at least two alternatives, but
there can be up to four or five.
Here is an example of a three-
alternative spatial forced-choice
task (color discrimination).
Since there are more than 2
becomes an "oddity" task, that
is, the subject's taks is to
choose the "odd" (i.e., different
Absolute Thresholds and Difference
Absolute Threshold- the amount of a
stimulus required to simply detect it
against a background (e.g., detecting
light in an absolutely dark room).
Difference Threshold, defined as the
size of the difference between two stimuli
required in order to just tell them apart.
Another word for the Difference
Threshold is the Just-Noticeable
JND is not an absolute amount of
stimulus, but is a constant proportion of
the background "standard" stimulus, IO
The larger increments required for
them to be "seen" on "standard"
backgrounds of increasing intensity is
illustrated in the three figures.
Thus, the more intense (or larger) the
background stimulus, the larger the
increment needed to be in order for it to
be detected on top of the background.
Stimuli which exceed threshold are
referred to as suprathreshold.
Measuring and expressing the
relationship of suprathreshold stimuli to
each other is referred to as sensory
Magnitude Estimation Subjects assign
numerical values to the strength of stimuli
Magnitude Production subject adjusts the
intensity of a stimulus to equal a prescribed
Cross-Modal Matching subject might be
asked to adjust the brightness of a visual
stimulus until it is judged to be as bright as
an auditory stimulus is loud.
Signal Detection Theory
Signal Detection Theory: analyze the
performance of telecommunication
systems (which transmit and receive
information, as do nervous systems).
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