To illustrate how different people have described
the relationship between the physical world and our
experience, i.e., illustrate different proposed
psychophysical laws
To collect some data to allow us to test some of
these laws
To build on the foundations of the last two labs
Magnitude Estimation
All of the methods so far have measured something
about perception at or near our limits to either
detect a stimulus or a change in the stimulus.
There was a need for a method to try to learn
something about stimuli that are easily detectable or
the difference between two stimuli that are easily
told apart, i.e., is one stimulus twice as bright as
another stimulus?
Harvard psychologist, S.S. Stevens pondered this
question and basically developed magnitude estimation
out of an elevator conversation with another Harvard
professor (not a psychologist).
Simple basic idea. Present a stimulus, have
participants give the stimulus a number that they they
indicates the sensory strength of the that stimulus.
Modulus: In some versions, a standard stimulus
is used, call the modulus. This stimulus is
given a standard number, whatever the researcher
wants, say 50. Then the participant assigns
numbers to the other stimulus that takes the modulus
into account. For example, if the participant
thinks the stimulus just presented is twice as strong
as the modulus and the modulus is 50 then the subject
should give the stimulus a 100.
More in Chapter 2 of the text.
What is a scientific law?
A description of a regularity observed in the data
They do not imply an explanation for the
regularity
Weber's Law
Developed to describe how JND's change as the
background intensity changes
This is an attempt to describe how our sensitivity
to change relates to the intensity of the background
stimulus
Questions:
Are all difference thresholds the same size
regardless of the intensity of the background
stimulus?
If not, is there any systematic way that the
size of these difference thresholds change?
Weber answered these questions as:
No, thresholds get larger as the background
intensity gets larger
More than that, the JND increase at the same
proportional rate as the background
He summarized his statements with the
following equation (it says exactly the same
thing as above):
k
=
ΔI I
k is a constant.
I is the background intensity
ΔI is
the difference between the intensity of the JND
stimulus and background intensity
Fechner's Law (Not in text)
Review logarithms for yourself
Observations:
Fechner starts with Weber's Law
Weber's Law suggests that we get progressively
less sensitive to stimulus change as intensity
increases
Conjecture:
What is Weber's Law refers not just to our
ability to detect change but our overall
sensitivity to stimuli
In essence, threshold perception and
suprathreshold perception are the same
Leads to the following Law
S=klog(I)
S is the strength of our sensory experience
k is a constant and the same k as in Weber's Law
I is the intensity of the stimulus
Stevens' Power Law
Starts with his development of Magnitude
Estimation
This is a direct measure of suprathreshold
perception which is the object of Fechner's Law
In many cases Fechner's Law is a reasonable fit
for magnitude estimation data, say brightness or
loudness
In some cases, Fechner's Law does not fit at all,
e.g., pain
Stevens wanted a general law to cover all of the
situations - that is one equation not a set of
several
Science likes to use as general a description as
is possible
Stevens developed the following law, also called
just the Power Law
S=cIb S and I
are the same as for Fechner's Law
c is a constant and can be anything
b is the exponent that changes the shape of
the function. See the text for more.
Weber's Law Experiment: Use Forced Choice:
Frequency Discrimination: ISLE 2.8b
Will Run 6 times
On the Stimulus Settings Tab Change (once per
run):
Standard Frequency:
Lab 1 (AM): 100, 200, 400, 800, 1600,
& 3200 Hz (This will be your I)
Lab 2 (PM): 200, 400, 800 1600, 3200,
& 6400 Hz (This will be your I)
Change no other settings
On the Method Settings:
Leave all the settings except:
Maximum value of Frequency Difference
(Hz): 75
This is a Forced Choice method (Remember how
to calculate these thresholds)
Magnitude Estimation: Tone Loudness &
Magnitude Estimation: Line Length from Chapter
2 Media
Number of Levels to Test: 10
Number of Repetitions: 7
Use the modulus (leave checked which is the
default). Note the value.
Problems:
For Weber's Law Experiment : Method of Constant
Stimuli: Frequency Discrimination
Put data into the report.
Calculate the JND for each
standard frequency
Use the Forced Choice threshold equation
Your JND will already be ΔI
Convert to ΔI/I
Plot on a bar graph (What Excel calls a column
graph)
X axis = I (Standard Frequency (Hz))
Y axis = ΔI/I
So 6 bars
Answer the following question: Does the data
fit Weber's Law or not? Why or why not?
Using the Magnitude Estimation Data from Week 2
Paragraph describing procedure
Plot the result from both Magnitude Estimation
experiments on the same graph.
Does the graph fit better with Fechner's or
Stevens' predictions? Look at the
shape. Consider all of the data
together. Do not answer for each data set
separately but the answer must apply to both
results together.
Worth 25 points
Point: Develop your skills at interpreting graphs,
in particular, in how data relate to theoretical
ideas.