Humans do not evidence choice biases in simple discrimination tasks

Abstract

The contribution of sensory and decisional processes to perceptual decision making is still unclear, even in simple perceptual tasks. When a decision maker needs to select an action from a set of balanced alternatives, any tendency to choose one alternative more often- choice bias-is consistent with a bias in the sensory evidence, but also with a preference to select that alternative independently of the sensory evidence. To decouple sensory from decisional biases, here we asked humans to perform a simple perceptual discrimination task with two symmetric alternatives under two different task instructions. The instructions varied the response mapping between perception and the category of the alternatives. In two experiments, we found that 30 out of 32 participants exhibited sensory biases and 15 decisional biases. The decisional biases were consistent with a criterion change in a simple signal detection theory model. Perceptual decision making, thus, even in simple scenarios, is affected by sensory and decisional choice biases.