Figure 1.

Schematic decision tree illustrating why categorical perception is at odds with normative decision theory. The example draws on the “graveyard spiral” illusion in which pilots under conditions of poor visibility may perceive their aircraft as banked when it is actually flying level. To decide which corrective action to take, an ideal observer computes the posterior probability of being in one or another state of the world (banked or flying level) and uses it to weight the expected utility of the actions available in each world state. Here, the plane is in fact flying level and trying to correct for an illusory bank will result in disaster. The ideal observer incorporates the small subjective probability of being in the alternate state and selects the best action (fly level) given the potentially catastrophic consequences of correcting the bank. A categorical perceiver, in contrast, settles on a single (illusory) state of the world, thus closing off the actions contingent on the nonperceived state (in gray). This will lead to disaster if, as in this example, there is a large cost associated with the alternate state.