Beyond the penalty for the choice, we also notice that Shaibu had a faster reaction time using his preferred (left) hand than his non-preferred hand in both the simple and choice reaction time conditions. In the simple reaction time condition the penalty was about 50 milliseconds, while it was only about 40 milliseconds in the choice reaction time condition.

So, we expect the fastest reaction times when we are detecting a response, rather than classifying it, and responding with our preferred hand, rather than our non-preferred hand. There is a small penalty for responding with a non-preferred hand, but a larger one for having to classify the stimulus and choose our response.