CogLab Experiment Mullery Lyer Stephanie Kubota CAS PS336 March 8 2013 Introduction The Muller Lyer Illusion is a classic optical illusion presented by Franz Carl Muller Lyer during the late 1800 s McCarthy The optical illusion demonstrates how the visual perception is tricked into perceiving whether or not a line appears longer or shorter depending on the orientation of wings it is presented with a line presented with wings that are oriented outwards appear longer while a line with inward oriented wings appear shorter One of the simplest ways in demonstrating the effects of the Muller Lyer illusion on perceived lengths of lines is through implementing the method of constant stimuli wherein randomly presented variations of a stimulus is presented against a constant comparison to develop a psychometric function curve Tan and Pizlo Therefore the CogLab Muller Lyer experiment measures how the Muller Lyer illusion affects the participant s perception of the length of a constant stimulus The following H1 Wing orientations in Muller Lyer lines changes the perceived length of the given line hypothesis is presented Methods and Procedure The experiment is a two alternative forced choice task where two presented lines can either be perceptually equivalent or not perceptually equivalent A vertical line measuring 100 pixels and with wings oriented outwards is presented as the constant stimulus In addition several other vertical lines in varying lengths and without an oriented set of wings are randomly presented and juxtaposed against the constant stimulus There were at total of 150 randomly presented vertical lines without wings displayed against the constant stimulus Participants were 1 Stephanie Kubota CAS PS336 March 8 2013 asked to decide which of the two presented lines is longer than the other on each given trial With that in mind the following variables are presented Control variable the line with wings oriented outwards constant stimulus Independent variable the length of the presented line without oriented wings comparison line Dependent variable the frequency in which the constant stimulus is reported longer than the comparison line Empirical Description The results of the Muller Lyer experiment produce a psychometric function The function describes the relationship between the presentation of a variety of physical stimulus and the subjective response of the participants Boring In this experiment the frequency at which participants reported that the comparison line appeared longer than constant stimulus can be displayed as a curve that exponentially increases at its point of subjective equality The point of subjective equality is determined when the constant stimulus and the comparison line is reported to appear the same length CogLab Since the constant stimulus is always presented 100 pixels long the effectiveness of the Muller Lyer illusion is demonstrated if and when the perceived comparison line is reported perceptually shorter than that of the constant stimulus even if it is measurably the same length The graph on Figure 1 shows that on global average approximately only 7 of the participants perceived that the comparison line is longer than that of the constant stimulus when it is presented 100 pixels long Furthermore participants only perceived that the comparison line was longer than the constant stimulus when it was presented 115 pixels long Thus the point of subjective equality the point at which one detects that the comparison line is longer than the constant stimulus is when the former is 15 pixels longer than the latter 2 Stephanie Kubota CAS PS336 March 8 2013 Figure 1 Global Average Data The group average is similar to the global average in that around only 6 of the participants perceived that the comparison line is longer than the constant stimulus when it is presented 100 pixels long Moreover participants only perceived that the comparison line was longer than the constant stimulus when it was presented 112 pixels long this makes the point of subjective equality 12 pixels longer than the constant stimulus Figure 2 Group Average Data On the other hand the individual average does not closely resemble the global average as much as the group average The individual participant reported that she never perceived that the 3 Stephanie Kubota CAS PS336 March 8 2013 comparison line is longer than the constant stimulus when it is presented 100 pixels long However she perceived that the comparison line was longer than the constant stimulus when it was presented 109 pixels long which marks the point of subjective quality 9 pixels longer than the constant stimulus Regardless even though the group or global averages may present a more consistent and continuous data than the individual average displayed on Figure 3 the latter still demonstrates the visual illusory effect found in Muller Lyer lines Figure 3 Personal Average Data Methodological concerns The experiment was designed to measure how the Muller Lyer illusion affects participants perceive the length of a constant stimulus The experiment is validated through the operationalization of the variables controlling the constant stimulus to 100 pixels and randomly changing the lengths of the comparison line this allows the changing lengths to directly affect the way in which participants perceive the length of the constant stimulus Furthermore because each variable is identified and controlled the experiment can be repeatedly executed and easily modified 4 Stephanie Kubota CAS PS336 March 8 2013 The large number of subjects who participated in the study also makes the given results more reliable and applicable to the general population of college students Furthermore since the results are purely empirical issues such as inter rater reliability and researcher bias become irrelevant However just like in any other experiment researchers cannot control extraneous confounding variables Because the experiment is administered online there is no way researchers can predict the environment at which participants take the given test Internal distractions such as multitasking hunger stress or lack of sleep can affect how much effort participants give in answering each trial questions properly Participants can also familiarize themselves or lose patience for the material after several trials Moreover external noise and factors such as loud sounds temperature and the angle at which the participant views the computer monitor can also affect his