Inevitability of Illusions

Abstract

This chapter primarily concerns a very general constraint on brains: that they take time to compute things. This simple fact has profound consequences for the brain, and vision in particular. I will put forth evidence that it is the visual system’s attempting to deal with this computing delay that explains why we experience the classical geometrical illusions. Figure 2.1 shows a sample such illusion; basically, the illusions are those found in any introductory Psychology course. I will also, along the way, briefly discuss a general approach to modeling brain computation: that approach is decision theory, wherein the brain, or some portion of it, is modeled as an ideal rational agent acting to maximize its expected utility on the basis of probabilities concerning the nature of the uncertain world. This is referred to as the Bayesian framework for visual perception, and with it researchers have made some important breakthroughs. We will need to understand it, and its shortcomings, to understand how the visual system copes with the time it takes to compute a percept. I also discuss the difficulties of one of the older and more established inference-based theories of the geometrical illusions. Before proceeding, it is important to understand why there may be computing delays in perception.