Muller-Lyer illusion-how to write on Practical Note Book

Introduction

An optical illusion occurs when our perception of an object differs from physical reality. In the well-known Muller-Lyer illusion, two lines of equal length are perceived to have different lengths. The difference in perceived length occurs because the arrowheads on the two lines have different orientations. Another example of an optical illusion is the Horizontal-Vertical illusion. Lines of equal length but oriented differently can appear to have different lengths. The purpose of the experiment reported here is to study the interaction between the Muller-Lyer illusion and the Horizontal-Vertical illusion. In particular, we wondered whether it would be possible to find a combination of arrowhead angles and line angles so that the two illusions completely canceled one another.

ABSTRACT

There are three horizontal lines. Two of the lines contain a pair of "wings." The wings are drawn outward or inward from the end of the line. The illusion is that the line with the outward-drawn wings tends to look longer than the line with the inward-drawn wings. The line without wings tends to look smaller than the line with outward-drawn wings and bigger than the line with inward-drawn wings. It is an illusion because the lines are actually all the same length, which you can verify with a ruler.

METHOD

For the Müller-Lyer illusion, we will have observers compare the perception produced by a line with outward-drawn wings to the perception produced by lines with no wings. We will systematically vary the length of the line without wings to see when the perceived line lengths match. We can then look at the physical length of the matching line without wings and use that as a measure of the strength of the Müller-Lyer illusion. There are several ways to go about making such comparisons. One of the simplest and most powerful is the method of constant stimuli.

We will generate a large set of lines without wings of varying lengths and have the observer compare each one with a standard line with wings. For each comparison the observer notes whether the line without wings is perceived to be longer or shorter than the line with wings. Unlike some other psychophysical methods (like the method of adjustment), the stimuli are not changeable by the observer, thus they are constant stimuli. The observer's task is just to report on the perception.

RESULT

we will find the proportion of responses where the line without wings seemed bigger than the standard as a function of the physical length of the line without wings. With such a curve, you can often identify critical values, such as the point of subjective equality, where the line without wings seemed to be the same size as the line with wings (e.g., 50% of the time it is described as bigger and 50% of the time is described as smaller).

Two effects are clear from the data. First, as the arrow angle increases, the perceived line length increases. This is consistent with the basic Muller-Lyer illusion, since larger arrow angles represent outward pointing arrows. Second, the estimates for vertical lines are larger than those for horizontal lines, since the closed symbols generally lie above the open symbols. This is consistent with the basic Horizontal-Vertical illusion.