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<h1>
<font face="Verdana"><font color="#000099">PSYCHOPHYSICS</font></font></h1></center>

<blockquote><font face="Verdana"><font color="#000000"><font size=-1>Experiments
in psychophysics explore the limits of our senses.&nbsp; Some of the earliest
questions in psychology centered on the question of our relationship to
the outside world.&nbsp; How real is reality?&nbsp; It often seems to depend
on who and how you ask.</font></font></font>
<p><font face="Verdana"><font color="#000000"><font size=-1>Are we influenced
by things we don't know we see or hear?&nbsp; Do subliminal stimuli really
influence behavior?&nbsp; Juggling the influence of the senses, transmission,
processing, and recognition has created a wealth of research.&nbsp; Is
an item subliminal if the reticular formation edits the information as
irrelevant?&nbsp; What roles do attention and conscious process have on
pure sensation?</font></font></font>
<p><font face="Verdana"><font color="#000000"><font size=-1>There are several
research methods which have been developed in psychophysics.&nbsp; People's
behavior changes depending on how they are asked to judge the stimuli,
how the stimuli are presented to them, and how the responses are collected.&nbsp;
Here are three classic methods which approach the problem in slightly different
ways.</font></font></font></blockquote>

<br>&nbsp;
<center>
<h3>
<font face="Verdana"><font color="#000099">The Methods</font></font></h3></center>

<blockquote><b><font face="Verdana"><font color="#660000">Method of Adjustment:</font></font></b></blockquote>

<blockquote>
<blockquote><font face="Verdana"><font size=-1>The subject sees a standard
line and is asked to adjust another line to match it.&nbsp; The subject
can lengthen or shorten the line until they are satisfied.</font></font>
<p><font face="Verdana"><font size=-1>Psychologists are interested in the
bias toward and against the original length of the adjustment line.&nbsp;
If the line was longer than the standard line, the subject is more likely
to say the two lines are equal when the adjusted line is a little longer.&nbsp;
This is called the <i>expectation error</i>.&nbsp; If the subject overshoots
the mark and makes the adjustment line a little shorter most of the time,
this is called the <i>habituation error</i>.</font></font>
<p><font face="Verdana"><font size=-1>The context of the sensation influences
the perception of the stimuli.&nbsp; Does this hold true if the subject
knows and understands the bias?&nbsp; Is it a tactical error resulting
from the eye movements used for comparing the two stimuli?&nbsp; Are there
subjects who habitually overshoot rather than undershoot the target?</font></font>
<br>&nbsp;</blockquote>
<b><font face="Verdana"><font color="#660000">Method of Limits:</font></font></b></blockquote>

<blockquote>
<blockquote><font face="Verdana"><font size=-1>The habituation and expectation
errors are also found in the Method of Limits.&nbsp; The subject sees a
standard line and a comparison line.&nbsp; They are asked to say if the
comparison line is longer, shorter, or the same as the standard.&nbsp;
The comparison line adjusts accordingly.&nbsp; If the comparison line is
longer, it will shorten if the subject says it is longer.&nbsp; It will
continue to shorten if the subject again says it is either longer or equal
to the standard stimulus.&nbsp; If the subject says that it is shorter,
the trial is over and the next trial begins.</font></font>
<p><font face="Verdana"><font size=-1>In both the Method of Adjustment
and the Method of Limits the habituation and expectation errors are counterbalanced
by including equal numbers of trials with longer and shorter lines.</font></font>
<br>&nbsp;</blockquote>
<b><font face="Verdana"><font color="#660000">Method of Constant Stimuli:</font></font></b></blockquote>

<blockquote>
<blockquote><font face="Verdana"><font size=-1>The subject sees a comparison
and a standard stimulus but cannot adjust them.&nbsp; The subject simply
indicates whether the comparison is longer or shorter.&nbsp; This method
is also influenced by a response bias; the tendancy to prefer to say shorter
or longer when unsure.&nbsp; The other two methods were developed to minimize
this type of response bias but brought in other bias errors.</font></font>
<p><font face="Verdana"><font size=-1>The method of constant stimuli requires
many more trials to obtain an estimate of the JND and PSE than the other
two.&nbsp; Because the subject is making a forced choice, longer <i>or</i>
shorter, you have to have a large number of trails to determine the level
of chance.&nbsp; The more trials you have for each comparison length, the
more accurate the data.</font></font>
<br>&nbsp;</blockquote>
</blockquote>

<blockquote>
<ol>&nbsp;
<br><b><font face="Verdana"><font color="#85645C"><font size=-1>Possible
variations include:</font></font></font></b>
<br>&nbsp;
<ol>
<ol>
<li>
<font face="Verdana"><font size=-1>Changing the length of the comparison
and/or standard lines.</font></font></li>

<li>
<font face="Verdana"><font size=-1>Changing the horizontal distance between
the two lines.</font></font></li>

<li>
<font face="Verdana"><font size=-1>Changing the vertical distance between
the two lines.</font></font></li>

<li>
<font face="Verdana"><font size=-1>Changing the number of trials.</font></font></li>

<li>
<font face="Verdana"><font size=-1>Changing the conditions or environment
-- instructions, setting, etc.</font></font></li>
</ol>
</ol>
</ol>
</blockquote>

<center>
<h3>
<font face="Verdana"><font color="#000099">Previous Studies by UMF Students</font></font></h3></center>

<ol>
<ul>
<li>
<font face="Verdana"><font color="#000000"><font size=-1>Contrasting performance
depending on the length of the line.</font></font></font></li>

<li>
<font face="Verdana"><font color="#000000"><font size=-1>Giving feedback
concerning performance to see if it increased accuracy.</font></font></font></li>

<li>
<font face="Verdana"><font color="#000000"><font size=-1>Contrasting the
different methods in a within subjects study to test for skill consistency.</font></font></font></li>

<li>
<font face="Verdana"><font color="#000000"><font size=-1>Testing Right
versus Left handed subjects to test to see if there were differences in
response bias.</font></font></font></li>
</ul>
</ol>

<p><br>
<hr SIZE=4 WIDTH="30%">
<hr SIZE=4 WIDTH="60%">
<blockquote><font face="Verdana"><font color="#000000"><font size=-1>We
have both the computer based visual Psycholophysics program and a set of
comparison weights which can be used -- or you can create a measure to
test another modality.</font></font></font>
<p><font face="Verdana"><font color="#000000"><font size=-1>You can find
the Psychophysics program in a white ring binder next to the computer in
my office and next to the computer on the desk on the landing.&nbsp; The
program is on a 5 1/4 inch disk.&nbsp; Instructions are on the first page
of the notebook.</font></font></font>
<p><font face="Verdana"><font color="#000000"><font size=-1>The program
includes complete instructions and a tutorial for you to use.&nbsp; It
will calculate your data for you and let you print the bar graphs.&nbsp;
To print a bar graph, simply hit the "Print Screen" button at the top right
of the keyboard.&nbsp; You should use the Epson dot matrix printer for
best results.</font></font></font>
<p><font face="Verdana"><font size=-1>There is an extensive literature
on psychophysics and there has been increased interest in the area over
the last five years.&nbsp; I have a book by Dember which will give you
some basic background.&nbsp; The Psychological Abstracts and Psych Info
will give you tons of references.&nbsp; When you do a search, start with
psychophysics and then limit it to your modality -- vision, touch, taste,
weight.</font></font></blockquote>

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<p><font face="Verdana"><font color="#666666"><font size=-2>&copy; <a href="mailto:mshea@maine.edu">Marilyn
Shea</a>, November 1999</font></font></font>
<br><font face="Verdana"><font color="#666666"><font size=-2>Department
of Psychology, University of Maine at Farmington</font></font></font>
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