Beyond illusions : on the limitations of perceiving relational properties

Abstract

Explaining the perception of our visual world is a hard problem because the visual system has to fill the gap between the information available to the eye and the much richer visual world that is derived from the former. Perceptual illusions continue to fascinate many researchers because they seem to promise a glimpse of how the visual system fills this gap. Illusions are often interpreted as evidence of the error -prone nature of the process. Here I will show that the opposite is true. To do so, I introduce a novel stance on what constitutes an illusion, arguing that the traditional view (illusion as mere discrepancy between stimulus and percept) has to be replaced by illusion as a manifest noticed discrepancy. The two views, unfortunately, are not necessarily related. On the contrary; we experience the most spectacular illusions where our perception is pretty much on target. Once our interpretation of the sensory data is off the mark, we usually no longer experience illusions but live happily without ever noticing the enormous perceptual and conceptual errors we make. The farther we move away from simple pictorial stimuli as the subject of our investigations, the more commonplace a discrepancy between percept and reality does become —and the less likely we are willing to call it illusory. Two case studies of our perception of relational properties will serve to illustrate this idea. The case studies are based on the conviction that perceiving is more than mere sensation, and that some degree of (unconscious) judgment is a necessary ingredient of perception. We understand little about how to balance objects and we make fundamental mistakes when perceiving the slipperiness of surfaces. All the while, we never experience illusions in this context. Thus, when dealing with simple percepts, illusions may be revealing. But when it comes to percepts that involve relational properties, illusions fail to arise, as perception is not concerned with veridicality but appears to be satisfied with the first solution that does not interfere with our daily activities.