The Muller

THE MULLER-LYER MULLER-LYER ILLUSION

Which of the two vertical line segments is longer? Although Although your visual system tells you that the left one is longer, a ruler would confirm that they are equal in length. The Muller-Lyer Muller-Lyer illusion is one of the most famous of illusions. It was c reated y !erman  "sychiatrist #ran$ Muller-L Muller-Lyer yer in %&&'. (ne role of an e)"erimental "sychologist is to find e)"lanations for "sychological  "henomena li*e the Muller-Ly Muller-Lyer er illusion, and then to "erform "erform e)"eriments to show show whether or not the e)"lanations are valid. Let us loo* at some "ossile e)"lanations for the Muller-Lyer illusion and some ways to e)"erimentally test their validity.

THE DEPTH EXPLANATION In the three-dimensional world, de"th "erce"tion concerns +udging distance. The closer an o+ect is to the retina, the larger it is on the retina. owever, in the twodimensional world of the Muller-Lyer illusion, our rain ma*es assum"tions aout the relative de"ths of the two shafts ased on monocular "ictorial cues. We We are used to seeing outside corners of uildings as near to us with the to" and ottom of the corner slo"ing out and away li*e the outward slanting fins of the Muller-Lyer illusion. illusion. We are used to seeing inside corners of uildings as farther from us with the to" and  ottom of the corner slo"ing slo"ing in somewhat towards towards us li*e the inward slanting slanting fins of the Muller-Lyer illusion.

The retina is saying that the two shafts are the same length ut the rain is inter"reting the Muller-Lyer as a de"th issue, with the shaft that loo*s li*e an outside corner eing closer and the shaft that loo*s li*e an inside corner eing farther away. In other words, the retina is saying /two shafts equal/ and the rain is saying /outside shaft shorter than inside shaft/. The rain usually wins differences li*e this. Thus, the rain sees as longer than . 0sychologists have attem"ted to su""ort this theory that the Muller-Lyer illusion is caused y our e)"eriences with outside and inside corners, y showing the illusion to an African trie that lived in circular huts and therefore had no "erce"tual e)"eriences with corners. 0eo"le in this trie didn1t seem to e fooled y the illusion thus su""orting the /e)"erience with corners/ e)"lanation of the illusion. A counter-study concerned a man who was com"letely lind e)ce"t for light sensitivity from the of age 2. 3ecently this man received a successful corneal trans"lant. 4tudies have shown that he is im"ressively free from geometrical illusions that are associated with a suggestion of de"th such as the 4he"ard Tales illusion shown elow -- the two tale to"s are the same si$e.

owever, he shows roughly normal susce"tiility to the Muller-Lyer illusion. This finding suggests that the Muller-Lyer illusion does not de"end on "rocesses associated with de"th "erce"tion. It should e noted that neither the /African trie/ study nor the /lind man/ study are EXPERIMENTAL  studies. To e e)"erimental, you must have random assignment. It is not "ossile to randomly assign "eo"le to e /African trie/ or /lind/. Is an e)"erimental test of the /de"th/ theory "ossile? #or e)am"le, would doing the Muller-Lyer illusion y sustituting circles or squares for the slanting fins e a "ro"er e)"erimental test of the illusion?

4ome "sychologists would argue /yes/. 0artici"ants could e randomly assigned to either a Muller-Lyer with fins or a Muller-Lyer with circles. Then, the results could e com"ared to see if there was a difference in the "erformance of the two grou"s. 4ome "sychologists would argue /no/. They would say that, even though the MullerLyer with the circles loo*s li*e the Muller-Lyer with the fins, they are not the same. #or e)am"les, circles are "rocess y /curve/ detectors while fins are "rocessed y /angle/ or /corner/ detectors.

Innate feature detectors have een found in the visual system 3esearchers uel and Wiesel in wor* on vision in frogs and cats have found innate feature detectors in the visual system. 5etailed study revealed three asic *inds of feature-detecting cells. 4im"le cells res"ond to a "articular stimulus a""earing in a circumscried area of the field for e)am"le, a "oint of light in the u""er-left quadrant. Thus, sim"le cells re"ort location as well as feature. 6om"le) cells res"ond to a "articular stimulus e.g., a "oint of light a""earing anywhere in the field7 thus, they re"ort only the "resence of a feature, not its location. y"ercom"le) cells res"ond to cominations of sim"le features, such as form corners, curves, and angles. 8)"erimental "sychologists have to e careful that they are testing theory, and not +ust testing differences. (n the other hand, would the /de"th/ theorists e logical in arguing that even though their theory is ased on /learning/, the most critical com"onent of the theory is /unlearned/ iological feature detectors. It is required that e)"erimental "sychologists ma*e logically consistent arguments in su""ort of their "oint of view. As an alternative, is it "ossile to e)"erimentally test the de"th theory y using only the fins and no shafts? Is the shaft an essential "art of the learning e)"erience? Is it an essential "art of the e)"erience of de"th?

If you did this study, and you found that the illusion was the same for the grou" that did the illusion with the shaft "resent and for the grou" that did the illusion with the shaft asent, would this result invalidate the /de"th/ theory, or would it only show that shafts are not necessary for de"th "rocessing? What ways can you thin* of to test the /de"th/ theory?

THE EYE MOVEMENT EXPLANATION

This e)"lanation suggests that the shaft ending in the inward slanting fins causes  "eo"le to "erceive it as shorter ecause the "erce"tion of the shaft is "ulled ac* y the /turning ac*/ of the fins. In other words, our eyes go out toward the "oint and then come ac* as they follow the fin shafts ac*. This turning ac* of our eyes or  "erce"tion ma*es the shaft seem shorter. 6onversely, the outward slanting fins draw our "erce"tion on farther ma*ing that shaft seem longer.

(ne e)"erimental way to test this theory is to see if flashing the illusion faster than our eyes can move will still "roduce the illusion.

LIMITED VISUAL ACUITY THEORY 9isual Acuity is our aility to distinguish details in the visual field. We have good visual acuity at the center of the fi)ation "oint, ut in the "eri"heral region our sight is highly lurred. In a lurred image, neighoring "oints or line-segments a""ear to move closer together. According to this theory, when we loo* at the Muller-Lyer illusion, we tend to fi)ate on the center of the shaft etween the two end"oints. Therefore, the fins are in our "eri"heral or lurry vision. This means that the fi)ation in the fins moves away from the center of the fins. As follows:

The result is that for the outward fins, the shaft loo*s longer and for the inward fins, the shaft loo*s shorter.

4ince this theory a""ears to de"end on the si$e and length of the fins e.g. longer fins would move the illusion farther from the center of the fins

and not on the distance se"arating the two fin heads, it is "ossile to e)"erimentally test the theory y com"aring short se"aration com"arisons of of inward and outward fin heads with long se"aration com"arisons while *ee"ing the head si$es the same.

INTERTIP DISPARITY THEORY The Interti" 5is"arity Theory says that the illusion is created ecause "eo"le  "erce"tually measure the illusion from the ends of the ti"s of the fins. Therefore, for the inward fin "art of the illusion, the ma)imal illusion should occur at $ero interti" distance where fins meet to form a diamond sha"e and then decrease with increasing interti" distance. 3esearch has shown this to e the case.

AVERAGING THEORY The there are two *inds of Averaging theory. The first concerns the fins, only, and claims that the fin "air affects the "erce"tual system1s aility to measure the shaft or s"ace distance. 4"ecifically, it says that the Muller-Lyer +udgment is ased on the

average of distances enclosed y the fin "air. The average distance enclosed y the inward fins is less than the average distance enclosed y the outward fins. Therefore, the inward fin s"ace loo*s shorter than the outward fin s"ace. The second averaging theory claims that the ratio of fin length to shaft length determines the strength of illusion effect. #in length would e sutracted from shaft length in the inward fin "art of the illusion and added to the shaft length in the outward fin "art of the illusion, thus creating the illusion. (ne way to test this theory would e to have two inward half-fins on the same side oth right or oth left of the shaft, com"ared with two inward half-fins on o""osite sides of the shaft one left and one right. The ratio of fin length to shaft length would  e the same in oth cases. If these different configurations "roduced different magnitudes amounts of illusions when com"ared to some standard, then the averaging theory would not e su""orted.