Assessment title: Perception Lab Report
Müller-Lyer illusion laboratory: The role of local and global processing in the Müller-Lyer illusion
Background (adapted, in part, from http://opl.apa.org/Experiments/About/AboutMüller-Lyer.aspxand Goldstein, 2014)
In the Müller-Lyer illusion the right vertical line (see the figure above) appears to be longer than the left vertical line, even though they are both exactly the same length. The presence of the fins at either end makes the lines appear to be different in length, with the fins-in arrangement causing the left line to look shorter and the fins-out arrangement causing the right line to look longer. Müller-Lyer coined the term “confluxion” to describe this illusion (“Müller-Lyer,” n.d.). The exact nature of this effect has been studied extensively without consensus (c.f., Dewar, 1967; Presey & Martin, 1990; Restle & Decker, 1977) about which perceptual principles account for the illusion (“Müller-Lyer,” n.d.).
Gregory (1966) explains the illusion on the basis of a mechanism called misapplied size-constancy scaling. Size constancy normally helps us maintain a stable perception of objects by taking distance into account (remember the size-distance scaling equation covered in your readings this week). Gregory proposes that it is this mechanism, normally useful in a three-dimensional world, which creates illusions when it is applied to two-dimensional surfaces, like the figure on this page. Gregory suggests that the fins on the right line make this line look like part of an inside corner, thus appearing further away. According to the size-distance scaling equation (S= R x D) as the distance (D) is larger, the size product (S) is longer. The fins on the left line create the impression of an outside corner and therefore appear closer.
Do you really think the Müller-Lyer looks like the corners of a wall? How about this “dumbbell” version of the illusion:
There is no obvious perspective or depth information available here, yet the illusion is still there!
Day (1989) proposed the conflicting cues theory, which states that our perception of the line length in this figure depends on two cues: (1) the actual length of the lines; a ‘local’ feature and (2) the overall length of the figure; a ‘global’ or ‘whole’ representation. According to Day, these two conflicting cues are integrated to form a compromise of the perception of length. As the overall length of the left part of the figure is shorter than the overall length of the right, the line appears slightly longer. Do you think the visual system can be fooled that easily?
The Müller-Lyer illusion has been tested, challenged and tested again many times. There is still no consensus as to why it occurs. You might be wondering what you can contribute as a university student to this decades-old conundrum, what new can possibly be found?
There has only ever been one direct test of Day’s proposal (see Mundy, 2014), and that produced some tantalising data: What if we could encourage our visual system to process more globally than it does normally? To always see the ‘whole’ instead of its parts? Assuming Day is correct, the illusion should be strengthened by greater attention to the whole figure – biasing the compromise we make when processing length. On the other hand, what if we were to encourage our visual system to process more locally? In that case, the illusion should be weakened by focussing perception to just single features, that is the line itself – ignoring the ‘whole’ and making our perception of length more accurate. Mundy (2014) found a particularly effective way to influence and modulate global and local processing through the use of Navon (1977) stimuli. The beauty of such stimuli relates to the fact that participants can respond to the large letter shape of such stimuli (global processing) or the smaller letters that make up the shape (local processing) whilst keeping the physical stimuli identical:
Processed globally, the figure reads E.S. Processed locally, it reads A.H. If a series of such stimuli are presented in succession and a participant is instructed to focus on one form of response (either global or local) then a bias towards that type of processing is generated for a short time after the presentation (for example, see Macrae & Lewis, 2002). If the strength of the Müller-Lyer illusion was then measured following the induction of such a bias, we might see a change, according to Day’s hypothesis. Mundy (2014) used a relatively short exposure to Navon stimuli and found that the Muller-Lyer illusion was significantly stronger for participants with a global bias, and significantly weaker for those with a local bias, compared with the control condition, thus supporting Day’s hypothesis. However, further studies are required to examine the merit of of Day’s hypothesis for the Müller-Lyer illusion by biasing participants toward global or local visual processing.
In order to replicate and extend the findings of Mundy (2014), we have selected an alternative method of generating global and local bias – mood induction. It has been shown by a number of researchers (e.g., Gasper & Clore, 2002) that mood has a significant bearing on perception. Sad people tend to favour processing of local features, whereas happy people tend to prefer processing whole (global) stimuli. Moreover, it has also been shown that listening to music can alter your mood. So, if we listen to some happy music, do we become more susceptible to the Muller-Lyer illusion, since our perceptual system is now biased more globally? On the other hand, if we listen to sad music, do we see the illusion more weakly, due to enhanced local processing? You should do a literature search to find more examples of this mood-perception phenomenon – there are a number within the face processing literature (e.g., Hills & Lewis, 2011) and further afield.
The aims of this experiment were to carry out a novel and innovative study to examine Day’s hypothesis that the Müller-Lyer illusion is created by ‘conflicting cues’ created by global and local features, replicating and strengthening the original findings of Mundy (2014).
Global processing should enhance the illusion by causing the viewer to judge length based on the whole figure.
A bias toward more local processing should weaken it as the viewer is more likely to focus on the line itself instead of the fins.
The experimental procedure is very simple and is, for the most part, modelled on that of the early experiments in which the illusion was quantified, such as that reported by Dewar (1967). You will note that the original experiment was far less high-tech than the current computerised study, but the principles are exactly the same. However, using music stimuli to manipulate processing level before and during the Müller-Lyer test has never been attempted.
A personal computer running a custom designed Java applet from the Online Psychology Laboratory, authored by Mark Tew and Ken McGraw, University of Mississippi. The experiment can be accessed from the following URL: http://opl.apa.org/Experiments/Start.aspx?EID=12 Your class ID code will be given to you by your instructor.
Music (Mozart Jubilate and Exultate; Mozart Requiem) via YouTube links below
Headphones and a clock/stopwatch
The class will split into three groups or ‘levels of processing’:
Global processing condition (Happy Mood Induction) –
Local processing condition (Sad Mood Induction) –
Participants in groups 1 and 2 will listen to pre determined music 1 minute before and during the entire experiment. The Global participants will listen to Mozart Jubilate and Exultate to induce a happy mood; the Local participants will listen to Mozart Requiem to induce a sad mood (music was chosen from examples given in Hills, Werno and Lewis (2011)). The Control participants will not listen to music before and during the experiment.
All participants will now access the experiment URL and enter their class ID code according to which condition they are in.
The study contained in this URL is a variation of the original Muller-Lyer illusion, one which enables investigators to study the effect of changes in fin angle on the apparent length of lines. Participants in the study are presented with two lines, as in the standard Muller-Lyer illusion presentation, but one of the lines has fins and one does not. Moreover, the two lines are initially different in length. The participant’s task is to adjust the plain line (without fins) to make the lengths the same. The adjustment is made using a slider (arrow in the figure below) that can be dragged using the computer mouse. As the slider is moved up and down the scale, the adjustable line changes in length.
Within the Müller-Lyer illusion experiment the independent variable is fin angle, which varies from 15 degrees to 165 degrees in 15 degree steps (i.e., 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165). Thus, there are 11 levels to the independent variable. Angles from 15 to 75 degrees are used to create “fins in” stimuli. Angles from 105 to 165 degrees are used to create “fins out” stimuli.
Two 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 total number of trials, therefore, is 22 with each angle being used twice.
The dependent variable is the difference in length between the two lines. Lengths are measured in pixels. The red line with fins attached–call it the “illusory” stimulus–has a random length of between 100 and 150 pixels. 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. To compute the difference, the length of the illusory stimulus is subtracted from the length of the adjustable line following adjustment. A positive difference indicates that the illusory stimulus appears longer than it in fact is; a negative difference indicates that the illusory stimulus appears shorter than it in fact is. Because non-zero differences represent errors in judging the length of lines, the dependent variable in this part of the experiment is a measure of adjustment error.
The overall independent variable is level of processing (Global, Local, or Control), which is modulated by mood induction via music exposure.
The overall dependent variable is the slope of the regression line produced by plotting adjustment errors across fin angle (i.e., the strength of the illusion, see below).
4 Data analysis
Examination of data
Adjustment errors can be graphed to judge the effect of fin angle on adjustment error.The figure below is a plot of data points that represent how the average adjustment error changed as a function of fin angle for one small set of participants. Adding a regression line (otherwise known as a trend line or line of best fit) to the plot shows the general linear trend – a good measure of the strength of the illusion.
You should produce a plot containing each of the three conditions (i.e., one per group: Global, Local and Control). You can do this all on one graph. A steeper regression line compared with Control would provide confirmation that the visual illusion has become more severe, producing greater adjustment errors. A more shallow regression line compared with Control would indicate that the illusion has weakened, creating fewer errors. You may also wish to calculate the slope/gradient of the regression line for each individual participant (tip: use the SLOPE function in Excel) and use ANOVA to determine if any statistical differences exist between level of processing conditions. No difference would imply that perhaps Day’s hypothesis is incorrect and that other explanations of the illusion must be sought.
Data Download and Format
Data are downloadable in three formats (XML, Excel spreadsheet format, and comma delimited for statistical software packages like SPSS), from, the following URL:
Remember to download the data from all three IV level of processing conditions (there will be three different class ID codes for Monash PSY3051 – please only use data from 2016).
The five columns provide classification data (participant ID number, gender, the class ID number, age, and date of participation)
Experimental data are recorded in columns 7- 17, reflecting the 11 different angles or levels of the IV presented in the experiment. Each value recorded in the respective column is a mean that represents a bias value that was derived by averaging the four trials.
5 Write up & References
Write up your findings in the form of a laboratory report/journal article, with a length of 2000 words. The submission dates for on-campus, off-campus students, can be found in the PSY3051 Unit Guide. Further information on marking is available in the next chapter of this lab manual.
References (see Moodle for links)
Day, R.H (1989). Natural and artificial cues perceptual compromise and the basis of veridical and illusory perception. In D. Vickers & P.L. Smith (Eds.), Human information processing: Mechanisms and models (pp. 107-129). London:
Dewar, R.E. (1967). Stimulus determinants of the magnitude of the Muller-Lyer illusion. Perceptual and Motor Skills, 24, 708-710.
Gasper, K., & Clore, G. L. (2002). Attending to the big picture: Mood and global versus local processing of visual information. Psychological Science, 3(1), 34-40.
Goldstein, E.B. (2014). Sensation and perception (9th ed.). Sydney: Wadsworth, Cengage Learning.
Gregory, R.L. (1966). Eye and brain. New York: McGraw-Hill.
Hills, P. J. & Lewis, M. B. (2011). Sad people avoid the eyes or happy people focus on the eyes? Mood induction affects facial feature discrimination. British Journal of Psychology, 102(2), 260-274.
Hills, P.J., Werno, M.A., & Lewis, M.B. (2011). Sad people are more accurate at face recognition than happy people. Consciousness and Cognition, 20, 1502-1517.
Macrae, C.N. & Lewis, H.L. (2002). Do I know you?: Processing orientation and face recognition. Psychological Science, 13, 194-196.
Müller-Lyer. (n.d.). Online Psychological Laboratory. Retrieved from http://opl.apa.org/Experiments/About/AboutM%C3%BCller-Lyer.aspx.
Mundy, M.E. (2014). Testing day: The effects of processing bias induced by Navon stimuli on the strength of the Muller-Lyer illusion. Advances in Cognitive Psychology, 10, 9-14.
Navon, D. (1977). Forest before the trees: The precedence of global features in visual perception. Cognitive Psychology, 9, 353-383.
Pressey, A. & Martin, N.S. (1990). The effects of varying fins in Muller-Lyer and holding illusions. Psychological Research, 52, 46-53.
Restle, F. & Decker, J. (1977). Size of the Muller-Lyer illusions a function of its dimensions: Theory and data. Perception and Psychophysics, 21, 489-503.
6 Marking Guidelines
Word limit: 2000 words (not to exceed 1850 words, excluding the abstract, figures or tables of results, the reference list and appendix). The abstract has a separate word limit of 150 words. Word limits include in text citations and subheadings. There is no 10% leeway on the abstract or main report.
Estimated return date: 4 weeks from submission
Criteria for Marking:
Reports should be structured with a title page, abstract, introduction, method, results, discussion and references as per the specifications in Findlay (2012). Reports should each typed double spaced on A4. Reports will be marked out of 10, in 0.5 intervals:
8-10 An outstanding report showing extensive knowledge and understanding and an exceptional ability to employ analysis, synthesis, and evaluation in the development and justification of hypotheses. An exemplary method showing complete command of issues of design, procedure, stimulus materials, and sampling. Accurate and clearly presented statistical analysis. Outstanding interpretation and evaluation of the results coupled with relation to the literature and awareness of problems and limitations. Writing is fluent, clear, and grammatical.
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