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.

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 than, 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

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' 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 the Power Law.

S=cI^{b
}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.

No new data needs to be collected. Use the data from both of last week's
magnitude estimation experiments

Weber's Law Experiment

Standard Motion Size: 3, 6, 12, 24, 48, 96

Number of Trials Per Level: 10

Width of Grating Cycle: 512

Grating Contrast: 100

This is a Forced-Choice method

Problems:

Using the Magnitude Estimation Data from Week 2

Paragraph describing procedure

Use graph from last lab. 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.

For Weber's Law Experiment

Calculate the JND for each standard motion size. Use the
Forced-Choice threshold equation

Your JND will already be ΔI.

Convert to ΔI/I.

Plot on a bar graph. X axis = I (standard motion size). Y axis = ΔI/I. So 6 bars.

Answer the following question: do the data fit Weber's Law or not?
Why or why not?

Worth 25 points

Point: Develop your skills at interpreting graphs, in particular, in how
data relate to theoretical ideas.