WHAT IS AN OPTICAL ILLUSION? - visual perception of a real object in such a way as to misinterpret its actual nature. EXAMPLE MULLER LYER - created by German psychiatrist Franz Muller-lyer in 1889 - components o two lines, at the end of each lines contain a pair of arrows that either point inward or outward.. o in the illusion, the line with the arrows pointing inwards seems to be longer than the line with the arrows pointing outwards o If there is a third line with no wings being compared, it will look shorter than the line with the arrows pointing inwards and bigger than the line with the arrows pointing outwards. - Real world Relevance o This depth illusion is often seen in paintings. Using lines sloping either up or down from corners gives the impression of depth on a 2 dimensional medium. - Our experiment o Main focus is on measuring perception. o We know that the muller lyer illusion exists, but we want to know HOW much longer the muller lyer line with the outturned arrows looks o Using constant stimuli- where the stimuli is fixed (cannot be altered by the subject) and there is no time limit for answering. o We will compared a Test line with no arrows to a Muller lyer line with arrows facing inward. The Muller lyer line was always 100 pixels long, while the Test line varied in length. o Our hypothesis is that the subjective equality of the Test line and the Muller Lyer line will show the Test line to be objectively longer than the Muller Lyer line. o Method 8 participants UBC students attending psych 309 Total of 225 trials. Press the space bar for a new trial. Press the I key if the line without arrows seems longer than the line with arrows. Press the k-key if the line seems to be shorter. Computer will verify your response. There is no time limit o Results Data showing the proportion of the ‘bigger’ responses In the left column we have the objective pixel size of the test line In the right we have proportion of bigger responses compared to the 100 pixel muller lyer line The results showed us a few things. 1) as you can see, if we take 50% mark of bigger responses. This is the length of subjective equality since half the time people said it was ‘bigger’ and the other half ‘smaller’ o Results show that subjective equality is at 110 pixels 2) we see a positive linear correlation for the length of the Test line vs. the proportion of times people reported the line being ‘bigger’ o when the line was at 85 pixels no one stated the line to be ‘bigger’ than the 100 pixel muller lyer, but at 127 pixels nearly everyone reported the test line being ‘bigger’ o Conclusions As for the linear correlations we have Statistically significant findings. Using the one tailed test and a confidence interval of 95%, we can be sure that there is less than 1 in 20 chance that the findings are incorrect. We can also reject the null hypothesis, and conclude that the point of subjective equality shows the Test line to be 110 pixesl (10%) longer than the 100 pixel Muller Lyer line. We have also successfully measured perception. WHY THE ILLUSION ? - many theories have been suggested - 1) Linear Perspective Theory o Two dimensional line junctions are interpreted as linear perspectives of three dimensional corners. o Relates to size constancy mechanism Size constancy mechanism Is a heuristic we use that provides stability in our perception of the world by giving our conception of particular objects in consistent set of properties despite variations in the retinal image size. For example, if we were eating a hamburger, as we are moving the hamburger closer to our mouths, it appears to become larger. However, because of our knowledge of size constancy, we know the actual burger isn’t getting bigger, it is simply closer to our face and therefore the retinal image is larger. EXAMPLE (burger getting bigger and bigger) Mostly useful, but can be used incorrectly which leads to illusions such as the muller lyer. We generally associate the line with the inward turning arrows as being further away, therefore ‘larger’ in reality, while lines with outward turning arrows are actually closer so therefore are ‘shorter’ in reality. EXAMPLE (buildings) o The lines display depth cues for distance that are strong enough to evoke the size constancy mechanism but that are not sufficiently strong enough to evoke the conscious apprehension of distance. o EXAMPLE (line building) o On one level we are treating it as 3 dimensional, but on the other as 2 dimensional. (conscious confusion) - 2) Stimulus Averaging Theory o perceived size reflects the size as a whole rather than partial. o In the real world, we see objects as a whole rather than in parts. o Relates to selective attention and filtering: the ability to ignore irrelevant stimuli in the environment while more task-relevant stimuli are being processed. o So what is going on in the muller lyer is that if measuring the entire stimuli, the line with the outward arrows IS indeed shorter than the line with the inward facing arrows. o This theory is supported by research conducted on children. Since children have the most trouble inhibiting irrelevant information, we should see a stronger affect in them Results show that this is the case. There is an inverse relationship to strength of illusion and age up until 25 years old where it levels off. CROSS CULTURAL DIFFERENCES CARPENTERED WORLD HYPOTHESIS - certain aspects of our environment make us more or less responsive to certain patterns of depth cues appearing in pictures such as the muller lyer. - study conducted by Segall et al. in 1966. o Subjects From Africa who live in the shrubs From urban areas such as large cities o Results Showed that the average Mueller-lyer illusion was greater for the more urban groups. Reasons? The hypothesis observes that in westernized countries, people are surrounded by rectangular shaped buildings, right-angled corners, and straight streets. In contrast, the Zulu, a people from Africa live in spherical huts and engage in agriculture that follow the land rather than ploughing straight lines. Absence of experience with certain types of depth cues impairs certain perceptual functions such as size constancy. IS THE MULLER LYER ILLUSION JUST A VISUAL PHENOMENON? - Christipher Hook at Oxford university 2003 studied the phenomenon by modality of touch. - Hypothesized that like in vision, people would over estimate the line with arrows facing inward and underestimate the line with arrows facing outwards. - Used the method of limits o Stimuli was presented in size sequence, in both ascending and then descending order. Participants had to state whether the test line was longer, shorter, or same as the muller lyer line. The point of equality was determined when the subjects consecutively went straight to longer from a shorter trial. - Method o 30 participants o 2 sets of trials. 1) was in tactile modality. Presented with embossed muller lyer cards and where blindfolded. They were asked to touch using only 1 finger on their dominant hand the muller lyer line, then the test line. Touching the lines was only allowed once and they were not allowed to touch them at the same time. 2) in the control trials, the muller lyer was presented visually. In the same way. Only allowed to see the lines once and not at the same time. - Results o Objective equality was 7cm. o As you can see, as predicted, there were significant under and over estimations of the muller lyer lines in both visual and touch modalities. o There was no statistical significance between the modalities suggesting that the muller lyer is just as strong in the tactile modality. - Discussion? o What does this mean? Can we reject the theories of linear perspective and stimulus averaging theories? NO! o Then what is going on? Truth is as of yet we are not sure. o Perhaps the illusion is true in both modalities because both sense send their information to the sensory association cortex. o There is a possibility that the muller lyer illusions is so strong and common that when we feel it on a card, we are able to visualize it. Through the visualization we encounter the illusion. Perhaps further study needs to be conducted on the congenital blind to find out whether they too experience this illusion. If they do, then we can rule out internal visualization since the born blind do not visualize.