The irrationality of categorical perception.

Perception is often categorical: the perceptual system selects one interpretation of a stimulus even when evidence in favor of other interpretations is appreciable. Such categorization is potentially in conflict with normative decision theory, which mandates that the utility of various courses of action should depend on the probabilities of all possible states of the world, not just that of the one perceived. If these probabilities are lost as a result of categorization, choice will be suboptimal. Here we test for such irrationality in a task that requires human observers to combine perceptual evidence with the uncertain consequences of action. Observers made rapid pointing movements to targets on a touch screen, with rewards determined by perceptual and motor uncertainty. Across both visual and auditory decision tasks, observers consistently placed too much weight on perceptual uncertainty relative to action uncertainty. We show that this suboptimality can be explained as a consequence of categorical perception. Our findings indicate that normative decision making may be fundamentally constrained by the architecture of the perceptual system.