What is the difference between the two conditions? The simple reaction time task is one where the only requirement is to detect the stimulus. But the choice reaction time condition requires us to classify the simuluus in addition to detecting it: is it the 'go' stimulus, or the 'no go' stimulus? This classification takes time, so we see an increase in the reaction time.

In addition to classification, we have to program or select a response: do I respond with my left hand, or my right? In the simple reaction time task we can be prepared to respond, while in the choice reaction time task we also have to choose the response to make.