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Do illusions distort perception? Practical report of experiment based on Muller-Lyer Illusion

Abstract

Twenty four Muller-Lyer tests were administered to one research participant to assess if visual illusions distort perception. It was hypothesised that errors in perception can be manipulated by providing feedback on performance to the research participant during the experiment. Enhanced knowledge of how to solve Muller-Lyer tests did result in a marked improvement in the research participant’s performance, confirming Gregory’s (1997) theory of errors in perception.

Introduction

This report will examine how illusions distort visual perceptions using the Muller-Lyer test. The Muller-Lyer test is characterised by misperceptions of the horizontal line, and the end-point location (Binstead and Elliot, 1999). Furthermore, Otto-de Haart, Carey and Milne (1999) have suggested, “the Muller-Lyer illusion is actually an illusion of extent, rather than position” (p 1437). Research has shown that there are individual differences in susceptibility to the illusion based on cognitive and social factors (Koch and Hayworth, 2003), as well as gender differences (McGraw, 1999). Many explanations have been offered for the distortive effects of the Muller-Lyer illusion, with varying degrees of empirical evidence (Binstead and Elliot, 1999). However, Gregory’s (1990) misapplied size constancy theory is well-researched (McGraw and Standford, 2001). This theory claims that subjects perceive the mid-point of the horizontal line as projecting from a more distant position when the arrows are pointed outwards, compared to inwards arrows. Therefore, arrows pointed inwards means that the observer perceives the line as being shorter.
Rationale
Gregory (1997) has argued that errors in perception maybe attributable to the individual’s knowledge being either inappropriate, or misapplied to the visual task. Millar and Al-Attar (2002) have shown how instructing subjects to ignore the arrows, when responding to the Muller-Lyer test, improves performance. The purpose of this study is to assess if providing feedback to a research participant on their performance on the Muller-Lyer battery after they have completed each block of tests improves subsequent performance based on enhanced knowledge of the illusional effects.

Method

The experimental study was of a case-study design using one adult participant, of normal visual ability, who completed 24 Muller-Lyer tests. After completing eight tests, each was marked by an independent adjudicator who recorded the incidence of errors, and the size of errors for each test, as measured in millimetres. Brief feedback was offered to the research participant on their performance, giving an indication of how many of the tests had been wrongly estimated, and by how much. However, the test script was then removed from view. The research participant was then asked to continue with a further eight of the tests at which point further feedback on overall performance was given. Then, the research participant completed the final eight tests.


Results

The results of the experiment are presented in Table 1. Formal statistical methods were not used since this was a case study design using one research participant, with low statistical power. The research participant made an error of estimation of the midpoint on each test completed. Nevertheless, Figure 1 clearly shows an improvement in the overall performance of the research participant since the size of errors on the second block, and the third block of tests was lower than for the first block of tests. Descriptive statistics were obtained using SPSS version 10, and the output is located in the Appendix. The overall mean error in millimetres for all 24 tests was -4.08 (SD 3.106). However, when the test performances were considered in three blocks, an interesting pattern emerged. The mean error size for the first 8 tests was – 7.12 (SD 2.23), lowering to -4.25 (SD 1.28) for the second block of 8 tests and lowering again to -88 (SD 1.64) for the final 8 tests.


Figure One: Performance of one research participant

Table 1: Number of errors and size of errors
Test no. Size of error of estimation from line midpoint (mm)*

1A -7mm
1B -10mm
1C -10mm
1D -9mm
1E -5mm
1F -5mm
1G -6mm
1H -5mm
2A -5mm
2B -5mm
2C -6mm
2D -5mm
2E -4mm
2F -3mm
2G -2mm
2H -4mm
3A -3mm
3B +2mm
3C +1mm
3D -1mm
3E -1mm
3F -2mm
3G -1mm
3H -2mm
* Negative figure indicates an error in direction of the tail of the Muller-Lyer test. Positive figure indicates an error in direction of arrow-head of the Muller-Lyer test.


Discussion

Concurrent with Gregory’s (1997) theory, this study shows that individual perceptual response to illusion is dependent on the appropriate application of knowledge. Where feedback is made available to the research participant, they are able to adjust and improve their subsequent performance based on newly acquired knowledge on how to ‘solve’ perceptual illusion tests more efficiently. In conclusion, this experiment, and previous studies, have clearly demonstrated that illusions can distort visual perceptions but further research is required with a larger sample size.

References

Binstead G and Elliot D (1999) The Muller-lyer illusion as a perturbation to the saccadic system. Human Movement Science 28, pp 103 -127.
Dewar M and Carey D (2006) Visuomotor immunity to perceptual illusion. Neuropsychologia (In Press).
Gregory R (1990) Eye and brain. (Princeton, New Jersey).
Gregory R (1997) Knowledge in perception and illusion. (University of Bristol, UK).
Koch C and Hayworth E (2003) Examining the relationship between need for cognition and the Muller-Lyer Illusion. North American Journal of Psychology 5.2, pp 149 – 156.
McGraw K (1999) Large sample, single experiment estimates of the side of gender differences on visual illusions. (University of Mississippi, USA).
McGraw K and Stanford J (2002) The apparent distance of interior and exterior corners. Journal of General Psychology 121.1, pp 19 -16.
Millar S and Al-Attar Z (2002) The Muller-Lyer illusion in touch and vision. Perception & Psychophysics 64.3, pp 353 – 365.
Otto-de Haart E, Carcy D and Milne A (1999) More thoughts on perceiving and grasping the Muller-Lyer illusion. Neuropsychologia 37, pp 1437 – 1444.


Appendix

SPSS Output
Descriptive statistics for all 24 responses

Descriptive statistics for block 1 (first 8 tests)

Descriptive statistics for block 2 (second set of 8 tests)

Descriptive statistics for block 3 (last set of 8 tests)