“The Illusion of Absence: How a Common Feature of Magic Shows Can Explain a Class of Road Accidents”, Vebjørn Ekroll, Mats Svalebjørg, Angelo Pirrone, Gisela Böhm, Sebastian Jentschke, Rob van Lier, Johan Wagemans, Alena Høye2021 (illusion-of-depth bias, sight)⁠:

The purpose of the present note is to draw attention to the potential role of an [amodal completion] recently discovered visual illusion in creating traffic accidents.

The illusion consists in a compelling and immediate experience that the space behind an occluding object in the foreground is empty. Although the illusion refers to a region of space, which is invisible due to occlusion (a blind spot), there is evidence to suggest that it is nevertheless driven by visual mechanisms and that it can be just as deceptive and powerful as ordinary visual illusions.

We suggest that this novel illusion can make situations involving blind spots in a road user’s field of view even more dangerous than one would expect based on the lack of visibility by itself. This could be because it erroneously makes the road user feel that they have actually seen everything there is to see, and thus has verified that the blind spot is empty.

This hypothesis requires further testing before definitive conclusions can be drawn, but we wish to make researchers and authorities involved in the analysis of traffic accidents and on-the-spot crash investigations aware of its potential role in order to encourage registration of relevant data and facilitate further research.

[Keywords: perception, road safety, magic, A-pillar obstruction, blind zone, illusion of absence, amodal completion, LBFTS accidents, improper lookout, view obstructions]

…Here, we spell out how this illusion may be relevant for our understanding of traffic accidents involving blind zones, such as those created by the roof supports next to the windshield in cars. We review preliminary evidence from basic vision research suggesting the illusion of absence may render drivers “mentally blind” to the perils of certain blind zones, thus inhibiting appropriate caution and heightening the risk of traffic accidents. We argue that more basic research into the critical stimulus conditions triggering the illusion of absence may have important implications for evaluating the relative effectiveness of different countermeasures against accidents involving blind zones. Further research on the illusion of absence may also have important implications for the legal questions pertaining to driver negligence and culpability. Awareness of the potential role of this novel and counterintuitive illusion may also guide future applied research in road accident analysis and prevention.

…Another interesting way in which something may seem to appear out of nowhere has only recently been described in cognitive science (Ekroll et al 2017; Svalebjørg et al 2020; Øhrn et al 2019). When an object seems to appear out of thin air in a magic show, it is often produced from a nearby hiding place, such as behind the magician’s thumb or palm. But when the spectators try to figure out what just happened, they almost invariably fail to consider this rather mundane and nominally obvious possibility and instead have the impression that something impossible (ie. magical) just happened (Svalebjørg et al 2020). Why are people so easily and consistently fooled by such simple tricks? Ekroll et al 2017 have proposed that this is because they are victims of a previously unknown and highly counter-intuitive visual illusion, which makes them immediately and automatically experience the objectively invisible space behind an occluder (such as the magician’s thumb) as empty, although the soon-to-appear object is actually hidden in it. When the object is pulled out from this perceptually empty space, it seems to materialize out of nowhere.

…Informal demonstrations of the illusion of absence: The top panels in Figure 1 show a static demonstration of the illusion of absence. All the objects on the table visible in Figure 1a are hidden behind a violet “bubbled” occluder in Figure 1b, but notice how difficult it is to imagine that they are really there. [cf. “Mormon porn”] Obviously, there is no direct visual evidence for or against the objects hidden behind the occluder, but we nevertheless experience an illusion which is reminiscent of the well-known cognitive fallacy of taking absence of evidence as evidence of absence.

Many magic tricks may owe much of their impressive deceptiveness to this “illusion of absence” (Ekroll et al 2017; Svalebjørg et al 2020): By moving objects out of the perceptually empty space created by the illusion of absence, magicians can create the illusion that they appeared “out of nowhere”.

Movie 1 in Øhrn et al 2019 shows a simple example, where the magician makes a coin apparently appear out of nowhere by pulling it out from the perceptually empty space behind his thumb. Richard Wiseman’s YouTube videos “The Mystery of the Red Cards” and “The Ball” show some further relevant examples. The Tom Scott Youtube video “Why This British Crossroads Is So Dangerous” contains a “virtual reality” simulation of a bicyclist suddenly appearing right in front of a car from the blind zone behind the roof support next to the windscreen (the so-called “A-pillar”). Note how surprising the sudden appearance of the bicyclist is, and how the experience of the event is very similar to the experience of many magic tricks.

…Preliminary theoretical explanation of the illusion of absence: It is currently not established what perceptual mechanisms and principles underlie the illusion of absence, but it appears plausible to speculate that the mechanisms operate according to the generic view principle [cf. Gestalt completion] (Albert2001; Albert & Hoffman2000; Freeman1994; Koenderink & van Doorn1986; Nakayama & Shimojo1992). According to the generic views principle, the visual system assumes that the structure in the retinal image is qualitatively stable with respect to small changes in viewpoint. [ie. Bayesian inference on objects, which fails in the presence of skilled adversaries or in rare (and evolutionarily novel) situations like automobile accidents.]

…The generic view principle readily explains why the broomstick in Movie 2 in Øhrn et al 2019 (pg2) is experienced as a ball rather than as the stick it actually is. It also readily explains why the space behind the illusory ball is experienced as empty. Actually, all the demonstrations of the illusion of absence we have described above are readily explained by appealing to this principle. Magic tricks relying on the illusion of absence (Svalebjørg et al 2020) involve a very special alignment of the occluder and the hidden object along the line of sight. If the tricks were viewed from a somewhat different viewing position, the hidden object would have been visible, and the trick ruined. This is why magicians take care to “watch their angles” (eg. Bobo1982; Macknik et al 2010_14ya). Similarly, creating the static demonstration of the illusion of absence in Figure 1 required careful alignment of the bubbled occluder so that it would cover all the objects on the table.

A straightforward prediction of the hypothesis that the illusion of absence is due to mechanisms operating according to the principle of generic views is that the illusion of absence should be more likely to occur or be stronger for a very small (or narrow) occluder than a bigger (or broader) one. The reason for this is that if a narrow occluder is to be occluding another object, however small, the occluder and the occluded object must be very narrowly aligned along the line of sight (except when the occluded object is very close to the occluder). A broader occluder, on the other hand, allows for many more possible positions of the occluded object relative to the occluder. Øhrn et al 2019 tested this prediction using both a narrow occluder and a wider one. As predicted by the generic view principle, the illusion of absence (as measured indirectly via the floating illusion) was weaker for the broader occluder.

…As already mentioned, it appears plausible that the A-pillar will provoke the illusion of absence since it is relatively narrow. Based on the principle of generic views, one would also expect that the tendency to experience the illusion of absence is particularly strong for moving occluders such as the A-pillar, because prolonged total occlusion of an object behind a moving occluder requires a higher degree of coincidence than prolonged total occlusion in static situations such as the one studied by Øhrn et al 2019. Some observations made by Wade & Hammond2002 in a virtual reality experiment investigating the present kind of traffic scenario suggest that an illusion of absence is indeed evoked by the A-pillar. For instance, they noted that “participants sometimes expressed mild anger at being tricked or fooled into a collision” (ibid, pg7, our emphasis) and that the “comment was often made that the car just “appeared” at the intersection” (ibid, pg8). The illusion of absence may have several important consequences in this and similar scenarios.