Figure 1. The D2 is a light-training reaction device. It consists of a board (4 ft. × 4 ft.) that can be raised or lowered relative to the height of the operator. It contains 64 target buttons arranged into 5 concentric circles surrounding a center screen that can be illuminated to serve as a stimulus for the participant. Reaction time is measured to the nearest 1/100 of a second via attached computer software.