An opticalsensor is located underneath the focused light source. The ratshould be positioned such that its tail obscures the focused lightsource from the sensor. When the system is started， either usingthe supplied footswitch or front panel mounted start key the lightsource illuminates and a timer starts counting in tenths of asecond. When the rat’s tail flicks， indicating its pain threshold，it uncovers the sensor. This tail movement turns off the timer andlight source. Reaction time can be read directly from the displayin seconds and tenths of a second.

A standard parallel port permits connection to a printer to recordthe trial number， energy level and reaction time. A calibrationfacility allows the light source to be set to the desired levelbefore commencing with the experiment.