Response biases in simple decision making: Faster decision making, faster response execution, or both?

When people are biased to use one response more often than an alternative response in a decision task, they also make the preferred response more quickly. Sequential sampling models can accommodate this difference in response time (RT) by changing the relative amount of evidence that must accumulate to decide in favor of one versus the other response, but nondecision processes might also play a role, such as the amount of time between selecting and executing a response. We investigated the influence of decision and nondecision processes in two experiments. In Experiments 1a and 1b, arrows appeared on the screen, and participants were asked to move a joystick in the direction of the arrow or make a keypress as quickly as possible. Results showed that motor execution times were faster for expected directions than unexpected directions. In Experiments 2a and 2b, participants decided whether a high or low number of asterisks was displayed on the screen. Decision times were faster for the stimulus class that was more likely to appear, and this effect was larger when participants could anticipate both the likely stimulus class and the motor response needed to identify it than when they knew the likely stimulus class but the associated motor response changed probabilistically from trial to trial. These results show that both decision and nondecision factors contribute to bias effects on RT.