A main staple of research in cognitive science and especially vision science has been, and still is, the investigation of illusions. For one, it is just an amazing fact that although we think that our experience of the world is direct, we live by a subjective model of our environment. We feel that we perceive the world as it is, a naïve realism as we might call it, but we are just not aware that the world is only presented to us as a (re-)construction of our nervous system. In more philosophical terms, this fundamental property of our experience has been referred to as “phenomenal transparency” (Metzinger 2003a), the inability to recognize that our mental states are representations. This is probably the reason why we are baffled in cases when the subjective character of our perception becomes evident, although this rarely occurs under natural conditions.
At least in the context of our modern culture, many people will have had the experience that their train is leaving the station when in fact they have just watched the train on the opposite side of the platform taking off. This phenomenon is termed vection, and everybody who has had this experience will remember the moment of insight when a cue destroys the illusion of self-motion and we realize that our train hasn’t budged. A more historical example of illusions under natural conditions is the waterfall illusion—, a type of motion aftereffect. After looking at a waterfall or flowing water for a long time, static objects, e.g., the river bank or trees, seem to move in the direction opposite to the previously perceived water flow, probably due to adaptation effects in brain regions processing motion (Anstis et al. 1998). Early descriptions of the effect have been attributed to Aristotle (384–322 BCE) and Lucretius (99–55 BCE; Wade 1998). But apart from these few examples, it’s rarely the case that the constructive nature of our perception is noticeable in everyday life.
Illusions have become a part of our popular culture and have had a strong impact on art. A whole art movement in painting, Op Art, is based on using known and discovering new visual illusions. It is a cultural version of vision research, presenting the fascinating nature of illusions to the public in aesthetically appealing ways. Illusions also feature prominently in the work of surrealist painter Salvador Dalí and other modern artists. For such artists, the medium presented a way of expressing the constructive nature of perception and signalled a departure from realism. For painters in general, knowledge about optics and the basis of visual perception has always been important for guiding the construction process of paintings and the refinement of techniques in order to achieve certain effects in the eye of the beholder. The entwinement of science and art is scrutinized in recent work looking at the interaction between fields (Zeki 1999). Two other forms of art that were more or less invented in close interaction with science are photography and film-making. The very basis of TV and movie presentations is rooted in the fact that we are able to fuse a rapid sequence of static images to construct a natural impression of moving objects. TV displays, projectors, and computer screens work with a certain refresh rate at which subsequent images are presented; the rate can be as low as 24 Hz in cinematography. The basic phenomenon that allows us to create a natural perceptual flow from flickering images is referred to as apparent motion, a type of illusory motion.
Because of the fascination with illusions and its influence on culture, illusions have been guiding research on visual perception for a long time—and continue to do so. But this is not the only reason for the utilization of illusions in science. Illusions are a powerful tool for understanding mechanisms of sensory processing in the brain that are unexpected or counterintuitive. Many motion illusions where motion can be seen in static displays (often seen in the entertainment sections of magazines) depend on a specific configuration of color values in directly abutting picture elements. These configurations of picture elements are repeated and cover the entire display, in sum creating a striking motion impression. Psychophysical experiments showed that the key to the illusion is the configuration of neighboring elements, whose effects cannot be predicted by current models of visual processing. Additional neurophysiological measurements in the same study demonstrated that different picture elements were processed with different latencies in certain areas of the visual cortex, mimicking a motion signal (Conway et al. 2005). This suggested a neural explanation for the occurrence of the illusion and led to a revision of existing models of motion selectivity.
Another driving force for the use of illusions in research was a resurgence of interest in understanding conscious perception. At the beginning of the 1990s, Francis Crick and Christof Koch started to publish a sequence of conceptual papers advocating the investigation of consciousness with empirical, and especially neuroscientific methods (Crick & Koch 1990, 1995, 1998). Since then the number of papers on consciousness has grown steadily in the domain of cognitive neuroscience. Certain visual illusions lend themselves specifically to investigating the nature of conscious processing. Some of the most prominent paradigms display the characteristic of bistability or multistability: When presented to observers, conscious perception alternates between two (bistability) or multiple (multistability) interpretations although the physical characteristics of the display do not change. Rubin’s face-vase illusion and the Necker Cube are just the most prominent among a multitude of examples for multistability (Kim & Blake 2005). The promise of using multistability is that it allows for disentangling the neural representation of the physical stimulus characteristics from the processes giving rise to conscious perception. The logic of the approach is that changes in neural activity accompanying switches in subjective experience during constant physical stimulation provide a guide to understanding the neural underpinnings of consciousness.