While treating Spock for a neural parasite infiltration in 2267, Doctor McCoy activated this indicator to illustrate to Captain Kirk how much pain the creature was inflicting. The indicator rose variably between 75% to 85% of its maximum; McCoy commented that it was "no wonder the poor devils go mad," implying that the pain indicated was severe. When Spock continued to speak, the indicator increased to nearly 100%. (TOS: "Operation -- Annihilate!" )