In his target article “Beyond illusions: On the limitations of perceiving relational properties,” Heiko Hecht (this collection) begins with a discussion of the traditional concept of illusion and how it has been employed in the context of research on vision. In its most basic sense, an illusion refers to a difference between our representation of a given scene and its actual physical properties. In an interesting take on the utility of illusions in research, Hecht suggests that the mere discrepancy between our perception and the real world—what he calls illusiond (“d” for “discrepancy”)—is less useful than one might think. In simple terms, our perception is off to some degree in many cases. But still, on the other hand it is amazing how on-target it is most of the time: it is sufficiently accurate for an effective interaction with the world. For Hecht, the term “illusion” should be reserved for situations when discrepancies (illusiond) are manifest, i.e., when the error is part of the experience and we become aware of it. This is termed illusionm (“m” for “manifest”) and is supposed to be the more interesting case. The moment of insight for the train-ride illusion described above might be a good example. Interpreting relative motion between trains as self-motion is often an adequate interpretation, but the error is manifested in a striking fashion experientially when we spot a part of the platform that indicates unmistakably that we are still in the same place.
In addition to the distinction between illusiond and illusionm, Hecht is concerned with cognitive illusions in comparison to the well-known perceptual illusions. His interesting observation is that when we move away from perception, the discrepancies between the real world and our judgments become even larger, sometimes to an absurd level. Humans are notoriously bad at everyday physics. Hecht mentions that we see nothing wrong with fabricated scenes that glaringly contradict Newtonian physics, and even our spontaneous actions reveal the same degree of error. Nevertheless, they are hardly ever noticed, i.e., illusiond rarely becomes illusionm in the cognitive domain. That this is especially the case for relational properties Hecht demonstrates with a series of his own experiments on physics judgments by university students. Even participants that should at least have some theoretical knowledge about the laws governing the real world (physics students) are surprisingly bad at finding the right answers to quizzes on balancing beams made of different materials with different weight distributions (Experiment 1) and on the slipperiness of surfaces (Experiment 2). In these examples, the students’ judgments are in stark contrast to the actual, real-world outcomes, which were also empirically tested in addition to deriving predictions from the laws of physics. So even though the paradigms were chosen to be experientially accessible and ecologically relevant, it seems that our cognitive system does not care about correctness or even rough approximations that would point it in the right direction. Even the mere ordering of solutions without providing quantitative details is seldom correct.
To summarize, Hecht suggests that the small deviations of our perceptual representations are no match for the sometimes extreme discrepancies found in the cognitive domain. Illusiond is the norm rather than the interesting exception in sensory processing because—at least in vision—the full three-dimensional representation of the world has to be derived from a limited array of two-dimensional information on the retinae. Hecht (this collection) refers to this as the “underspecification problem.” For an efficient solution to the underspecification problem, the system employs a range of assumptions and constraints on the makeup of the world to guide the reconstruction process. For Hecht, perception is therefore always fraught with cognitive elements. This is even more so when discrepancy is detected; illusiond becomes illusionm. Then, cognitive judgments are involved, and an explicit comparison process is initiated that allows us to capture the discrepancy and which makes it experientially available.