Muller-Lyer Illusion Experiment Software
Overview
The program MULLER presents stimuli that demonstrate one form of the classic Muller-Lyer illusion. This illusion
consists of two line segments with arrowheads on the ends. The arrowheads can affect the perceived lengths of the
line segments. In the present experiment the subject is presented with a form similar to that illustrated below. 
The
central set of "wings" is movable--and the subject's task is to move the wings so that the left portion of the central
horizontal shaft appears to be equal in length to the right portion of the shaft. The left-arrow key moves the wings to
the left, and the right-arrow key moves them to the right. When the subject perceives the two portions of the shaft
to be equal in length, pressing the up or down arrow (or the space bar) will record the current setting and advance to
the next trial. From trial to trial, the angle that the wings make with the shaft might vary, and this information is
saved. By default, the wings will make a 45 degree angle with the shaft on some trials, and a 90 degree angle
(straight up and down) on other trials. Note that responses to wings at the 90 degree angle should not be affected by
the illusion.
Raw data from the experiment are placed in the file DMULLER.DAT. The data saved include (a) the angle between
the wings and the shaft, (b) the overall length of the central shaft, (c) the initial length of the shaft that was to the left
of the central wings, (d) the length of the wings, and (e) the length of the shaft to the left of the central wings when
the subject responded. All lengths are provided in arbitrary units that can range from 0 to approximately 32,000. An
analysis program (MULLANAL) reads the data file and computes the mean proportion of the central shaft that was to
the left of the central wings at the moment that the subject indicated their response at the end of the trial. Separate
means are reported for each different wing-angle that was studied. The output of the analysis is placed in the file
AMULLER.ANL.
Running the experiment
To run the experiment, type MULLER and hit "Enter". When asked to "Press any key to begin",
pressing the 'P' key will allow you to change various parameters that determine the design of the experiment and
details of the events on a trial. Pressing a key other than 'P' will cause the program to begin the first trial
immediately. The parameters that can be safely changed are listed below, along with their probable default values.
1. Total number of trials per block (must be multiple of 4):
(default=32)
2. Total number of blocks per session:
(default=5)
4. Number of practice blocks:
(default=1)
5. Inter-trial interval (msec):
(default=500)
7. Wing angle # 1 (degrees):
(default=45)
8. Wing angle # 2 (degrees):
(default=90)
9. Wing angle # 3 (degrees):
(default=45)
10. Wing angle # 4 (degrees):
(default=90)
11. y location of stimulus (video units):
(default=16000)
13. Total length of shaft (video units):
(default=16000)
14. Length of wings (video units):
(default=4000)
15. Mean X location of center of stimulus:
(default=16000)
16. range of change in initial x location of stimulus:
(default=3000)
17. Amount to move central wings with each change:
(default=100)
19. Color of lines (white, red, green, blue, yellow):
(default=1)
Note. Each of the four wing angles occurs equally often in a block, in a random order.
Analyzing the data (You will need analysis program available below)
Simply type MULLANAL and hit "Enter" from the directory in which the experiment was run. When asked
"Number to trim from each end" the user has the option to exclude the desired number of extreme judgments in each
condition (wing angle) from the analysis. Hitting "Enter" will not exclude any data. To view the means from each
condition TYPE or PRINT the file AMULLER.ANL (i.e., issue the command: TYPE AMULLER.ANL "Enter").
If you want to view the raw data, TYPE or PRINT the file DMULLER.DAT. Each time that MULLER is run, the
program will destroy all previous DMULLER.DAT data files. Each time that MULLANAL is run, it will destroy all
old AMULLER.ANL files. Therefore, be sure to look at the data before the experiment is run again.