Cruise Scientific Visual Statistics Studio Visual Statistics Illustra Mueller-Lyer Apparatus Anyone who attended college could not have missed seeing the Mueller-Lyer perceptual illusion. It typically occupies the front pages of introductory psychology textbooks, next to the alternating vase-human face figure, the Necker cube, and in old psychology textbook next to alternating pictures of human skull and two young ladies. Its typical rendering shows two lines of the same length, one terminated by arrows and the other by the inverted arrows, where the second line appears to be longer than the first one. Few people know that the original Mueller-Lyer illusion was not a drawing, but an apparatus. The author first encountered the original Mueller-Lyer apparatus at the University of Muenster, Germany. The instrument was located in a rather dusty display window. Mounted on a dark brown oak board, its movable arrow was operated by a pair of worn out strings. The eerie feeling the contraption conveyed was heightened by the surroundings. Muenster is an old Hanseatic city. The medieval walls of its university still echo with the footsteps of mendicant friars and goliardic verses composed by its student body. Nevertheless, this encounter with the real Mueller-Lyer apparatus was disquieting. Prior to visiting Germany, the author saw hundreds of Mueller-Lyer figures: horizontal and vertical, aligned on a line or separated by white space, with parallels or shifted edges. Some were printed in black on white, others were colored or redesigned by graphic artists. But all were petrified in two-dimensional fields, immovable and static. The story of the Mueller-Lyer illusion is included here to illustrate that sometimes, the plausibility of narratives changes over time. The apparatus The apparatus was mounted on a wooden block and consisted of a brass plate, a beam, and vertical bars. The middle pair vertical bar was attached to a pair of strings. Subjects, sitting at a distance from the apparatus, could move the bars by pulling the strings. On the backside of the brass plate were engravings of protractors and rulers. During the experimental phase of the experiment, the angular separation of the vertical bars was typically set to 45 and 135 degrees. During the control phase of the experiment, the angular separation of all vertical bars was 90 degrees. During both phases of the experiment, the researcher recorded the discrepancies between the physical and estimated middle of the beam. This technical description of the Mueller-Lyer device may leave the reader baffled as to why is it discussed here. The point we try to make is that the original meaning of the Mueller-Lyer illusion was quite different from the meaning subsequent generations ascribed to this illusion. Individual differences At the time of inception of the Mueller-Lyer apparatus, one of the issues generating attention of the scientific community was the problem of variability of subjective estimates while matching lighted surfaces to the brightness of starts. These practical difficulties lead Bessel (1784-1846) to formulate the personal equation problem that is basically the problem of measurement of individual differences. Fechner (1801-1887), using the data provided by Mueller-Lyer apparatus, introduced the notion of constant and variable errors and provided a general solution to the problem of measurement of individual differences. Idealists and realists Around the turn of the century, the Mueller-Lyer illusion was used in debates between philosophical idealists and realists that continued the classical controversy between John Locke and George Berkeley. Locke maintained the view that material objects exist independently of the mind. Berkeley believed that the Lockean view leads to atheism and postulated that nothing exists apart from the mind. His doctrine is frequently summarized by the expression esse est percipi, (to be is to be perceived), frequently illustrated by parable of a falling tree in a forest with no-one perceiving its fall. Bishop Berkeley further reasoned that since a given object is likely to be perceived by different individuals as being the same, the individuals' mind had to be created by a divine being. Realists asserted that objects in the external world exist independently of what is thought about them. Idealists' counterargument was that this view fails to explain perceptual mistakes and illusions, and supported this contention by using the Mueller-Lyer illusion. ...And beyond In the 1920s, Gestalt psychologists saw the perceptual phenomena reflected by the Mueller-Lyer figure as illustrating their doctrine of the primacy of the whole. Gestalt translates from German approximately as ‘pattern,’ ‘form,’ or ‘configuration.’ Gestalt psychologists such as Kurt Koffka, Wolfgang Kohler, and Max Wertheimer argued that relationships among components of an object rather than properties of its components determine what is perceived. During the ascendancy of Skinner’s operant conditioning theory in the 1950s, conditioned stimuli provided by the Mueller-Lyer configurations of bars and arrows led to the conclusion that even ‘chickens react in a manner which suggests that they experience an illusion. In the 1960s, the figure was interpreted within the context of research pertaining to perceptual differences between ‘civilized’ people growing up in the rectangular, ‘carpentered’ world and ‘primitive’ people growing up in an oblique world of thatched huts and hogans. It was hypothesized that rectangular environment provides different cues than oblique environment and the Mueller-Lyer illusion was used to support this argument For a while it seemed that the Mueller-Lyer illusion was finally laid to rest when, in the 1970s, the American Psychologist published the article: ‘Liar’s Illusion’. Measuring actual lengths of the Mueller-Lyer arrows in about a dozen introductory psychology textbooks, the line bordered by arrows was actually shorter, this difference in length being statistically significant. Over the time, graphic artists repeatedly rendering this illusion for countless PSY101 textbooks also succumbed to its deceptiveness.
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