The Persian scientist, Ab? Rayh?n al-B?r?n?  was the first person to describe the concept of reaction time:

Not only is every sensation attended this by a corresponding change localized in the sense-organ, which demands a certain time, but also, between the stimulation of the organ and consciousness of the perception an interval of time must elapse, corresponding to the transmission of stimulus for some distance along the nerves.

The first scientist to measure reaction time in the laboratory was Franciscus Donders. Donders found that simple reaction time is shorter than recognition reaction time, and that choice reaction time is longer than both.Donders also devised a subtraction method to analyze the time it took for mental operations to take place.By subtracting simple reaction time from choice reaction time, for example, it is possible to calculate how much time is needed to make the connection.