Perception, 1975, volume 4, pages 107-109. Building the Devil's Tuning Fork. Brooks Masterton, John M Kennedy^. Scarborough College, University of Toronto, ...
Perception, 1975, volume 4, pages 107-109
Building the Devil's Tuning Fork
Brooks Masterton, John M Kennedy^ Scarborough College, University of Toronto, West Hill, Ontario, Canada Received 14 April 1975
Figure 1 is a curious kind of display, known as the Devil's Tuning Fork or a three stick clevis and generally classified as an 'impossible figure' (Kennedy 1974; Schuster 1964; Gregory 1970). It is not ambiguous like a Necker cube, alternating from one ecological appearance to another. It is not an incomplete figure where parts of the outline and silhouette appear to be absent (Kennedy 1974). Accounts of the display's appearance of impossibility have stressed that: (a) the middle prong appears to be in two places at the same time (Gregory 1970); (b) the display involves incompatible surface depth cues linked as though they were compatible ('counterbalanced') (Gibson 1966; Kennedy 1974). Rules for the construction of line figures that employ the Devil's Tuning Fork pattern have been described by Baldwin (1967). Hay ward (1968) and Robinson and Wilson (1973) employed these rules to make many intriguing figures of this type. Here we will try to distinguish two different kinds of criteria for defining this display as impossible in the first place, and then take issue with one of these criteria. The first criterion asserts that an object is impossible if there is no solid object (made of surfaces rather than wires or drawn lines) which, if it were to be depicted by drawing an outline of it, would result in a line pattern like that of figure 1. In other words, to be 'possible', it must be able to exist as a solid object and a subject could draw a picture of it. Conversely, if it is impossible, it could not exist as surfaces whose edges correspond to the lines in the picture.
Figure 1. H The first author designed and tested the fork, and the second helped with writing this note.
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B Masterton, J M Kennedy
The second criterion that might be employed is simpler. It asserts that the depth relations generally perceived by a subject on first exposure to figure 1 are incompatible. As Penrose and Penrose (1958) have suggested, in describing impossible objects of this type individual parts of the object may appear to be normal and easily interpreted in three dimensions; however, when we try to connect these parts and perceive the whole object (depicted by the drawing) the incompatible depth cues cause an anomalous perceptual effect and the object 'looks impossible'. This second criterion is 'weaker' than the first, for it relies on what is usually seen, rather than asserting that no perceivable object could fit the depiction. The first criterion is inconvenienced by figure 2, a photograph of an object made of planar surfaces. This object has been shown to subjects viewing from the same angle as the camera's view shown here and subjects drew the same pattern as in figure 1. The object is shown from another angle in figure 3, where it can be seen that certain surfaces project upwards, although in viewing photographs and line drawings the corresponding surfaces are generally perceived ap projecting downwards. It might be said in tune with Gregory's analysis of perception that when we see the problematical (impossible) object depicted in figures 1 and 2, we are experiencing the 'likely' or 'probable' depth hypothesis evoked by these figures. In order to see the possible object depicted by figures 1 and 2 we must adopt the 'highly unlikely' depth hypothesis that the centre area of the fork projects upwards. The second criterion of impossibility is not inconvenienced by the object shown in figures 1 and 2. In Gregory's terms again, it can be said that the depth relations which are likely to be perceived involve incompatibilities. It is interesting and important to note that subjects who drew figure 1 on viewing the object shown in figure 2 did not appear to notice any impossibility initially. Neither viewing the actual object nor drawing it and looking at the drawings resulted in any of the usual comments normally evoked by figure 1. Only after the unusual impression of the figure was described did any subjects seem to see that the figure had any peculiar characteristics. This contrasts sharply with our general experience, for usually subjects mention finding figure 1 odd immediately, and it shows that the depth cues are not compulsory—they are only 'usually' read in the impossible way.
Figure 2.
Building the Devil's Tuning Fork
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It is also worth mentioning that we have tried perceptually reversing the veridical depth relations of the object in question (viewing from the camera angle shown, of course) and the result is an impression of a problematical (impossible) object. Thus it is not necessary to have a flat picture to get an impression of the impossible object. Gregory (1970, p 57) distinguishes between impossible objects and impossible figures. An impossible object has no possible 'object hypothesis' that will solve the contradictory features of the depicting drawing, and therefore it cannot be built in three dimensions with the use of surfaces. An impossible figure is one that, seen normally, is impossible; but it does have an unexpected object hypothesis which will reconcile all the features of a drawing so that it may be interpreted as a possible three-dimensional object, and it can be built in three dimensions using surfaces. Because there is an object hypothesis for figure 1, it should be classified as an impossible figure.
Figure 3. In sum, it is now apparent that: (a) figure 1 is usually seen as a picture of an impossible object (by our second criterion of impossibility), (b) figure 2 shows a real object which is an unlikely, yet perceptually correct version of figure 1, and (c) because there is an object hypothesis (although highly unlikely) that resolves the incompatible depth cues that usually occur to a person perceiving figure 1, the Devil's Tuning Fork is an impossible figure which can be physically constructed using surfaces. References Baldwin H, 1967 "Building better blivets" Worm Runner's Digest 9 104-106 Gibson J J, 1966 The Senses Considered as Perceptual Systems (Boston: Houghton Mifflin) Gregory R L, 1970 The Intelligent Eye (New York: McGraw-Hill) Hayward R, 1968 "Blivets—research and development" Worm Runner's Digest 10 89-92 Kennedy J M, 1974 A Psychology of Picture Perception (San Francisco: Jossey-Bass) Penrose L S, Penrose R, 1958 "Impossible objects: A special type of illusion" British Journal of Psychology 49 31-33 Robinson J O, Wilson J A, 1973 "The impossible colannade and other variations of a well-known figure" British Journal of Psychology 64 363-365 Schuster D H, 1964 "A new ambiguous figure: A three stick clevis" American Journal of Psychology 11 673
