Mueller-Lyer Experiment

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Introduction
Design
Data Analysis
 References


Introduction
The Mueller-Lyer experiment is designed to give students the opportunity to determine the effect of fin angle on perceived line length. Fin angle has been intensively studied, but there is disagreement on whether the effect of angle is linear (e.g., Lewis, 1909; Dewar, 1967; Presey & Martin, 1990) or curvilinear (e.g., Restle & Decker, 1977). Rather than study the effect of fin angle using the classic Mueller-Lyer arrangement shown below on the right, this experiment uses just one of the illusory figures, as shown on the left. That is, one of the red lines has fins attached, but the other does not. In the experiment, the red line without fins attached is adjustable, and the research participant's task is to adjust the plain red line to the same length as the red line with fins attached.
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Design

The independent variable is fin angle, which varies from 15 degrees to 165 degrees in 15 degree steps. There are, then, 11 levels to the independent variable. Obtuse angles designate values that create "fins in" stimuli. Acute angles are "fins out" values.  The figures below illustrate.

Four sets of 11 trials are conducted. In each set the angle used on any one trial is chosen at random from the set of 11, but once the angle has been used, it does not reappear until the next set of trials.

The red line of the illusory figure (i.e., the red line with blue fins attached) has a random length of between 100 and 150 pixels on the 384 X 512 pixel screen that is used by PsychExps. The adjustable line is randomly set to either 90 or 160 pixels at the start of each trial. Thus sometimes the adjustable line must be made longer and sometimes shorter in order to create a perceptual match with the illusory line.  When it is smaller, the error is reported as negative; when it is made longer, the error is reported as positive.

The number of trials is fixed at 44 so that each of the 11 angles can be judged 4 times. « Return to Top»


Data Analysis

 

 

The data for the experiment can be graphed to judge the effect of fin angle. Figure 1 is a plot of an individual data set to which a regression line has been added to indicate the best linear fit. The individual data points represent the average adjustment error on the four trials for each fin angle.

An analysis of variance on these data would not be interesting but a regression analysis is of interest, as is a correlational analysis to measure the degree of linear relationship. The same analyses performed on individual data sets can be repeated on the class data.

 

 

Deviation in Degrees from 90 Degrees
Angle relative to shaft

75

60

45

30

15
Acute          
Obtuse          

Another analysis of interest would address the question whether the illusory effects of fins with acute angle values (fins out) are the same as the illusory effect of angles with obtuse angle values. A 2 X 5 analysis of variance could be used to address this question. There is some evidence that fins creating obtuse angles with the shaft produce more perceptual distortion than complementary angles that form acute angles with the shaft (e.g., Adam & Bateman, 1983; Christie, 1975).« Return to Top»

Data Format for Downloadable csv file
The data obtained via the Download data link on the PsychExperiments homepage are comma delimited.  The first entry (up to the first comma) is the affiliation of the research participant.  Typically this is the class to which the participant belongs.  Non-affiliated participants use the default entry of Interested Person. Next is the randomly assigned ID code for the research participant. The third and fourth entries are the data and time (24-hour format) on the server when the data were received.  Next is a text entry for gender. F is for female and M for male. After gender comes an entry for handedness, which in current data will be a null entry (a comma) because we no longer gather handedness data.  Downloading older data will produce an entry in the handedness field of R for righthanded, L for left-handed, or M for mixed handedness. Age in integer format is the next data entry. Raw data from the experiment follow age.  The data consist of 44 trials.  The first data entry for each trial is a trial number (1-44).  This is followed by an integer designation of the fin angle (15 to 165 in steps of 15). Next is the magnitude of the error in length estimation in pixels.  Negative numbers indicate that the user manipulated line was shorter than the comparison line. Positive numbers indicate the opposite.  

 The table below gives the data format (excluding user information) The text line below that shows a complete data set for one research participant.

Trial Number Fin Angle Magnitude of error (pixels) Trial Number Fin Angle Magnitude of error (pixels) etc.
1, 150, 10, 2, 30, 22, etc. 
Acadia University--Sullivans Psyc2023,ZWW034,3/9/2001,15:10,F,,46,1,150,10,2,30,22,3,60,8,... 42,120,-9,43,135,-7,44,150,-8, 


References

Adam, J. & Bateman, L. (1983). A correlational analysis of symmetry between the arrowhead and featherhead Mueller-Lyer illusions. Perception, 12, 119-129.

Christie, P.S. (1975). Asymmetry in the Mueller-Lyer illusion: Artifact or genuine effect? Perception, 4, 453-457.

Dewar, R.E. (1967) Stimulus determinants of the magnitude of the Mueller-Lyer illusion. Perceptual and Motor Skills, 24, 708-710.

Lewis, E.O. (1909). Confluxion and contrast effects in the Mueller-Lyer illusion. British Jounral of Psychology, 3, 21-41.

Presey, A.W. & martin, N.S. (1990). The effects of varying fins in Mueller-Lyer and Holding illusions. Psychological Research, 52, 46-53.

Restle, F. & Decker, J. (1977). Size of the Mueller-Lyer illusion as a function of its dimensions: Theory and data. Perception & Psychophysics, 21, 489-503.

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Last revised:November 01, 2003 07:01:31 PM
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