K3 indicator

The K3 indicator (second from the left)

The K3 indicator was a measurement marker used by the 23rd century body function panel. This marker, not normally active, measured the level of pain experienced by a patient.

While treating Spock for a flying 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!")

In the final draft script of "Dagger of the Mind", this indicator was termed a "brain wave index" (in stage directions) and was said (in McCoy's dialogue) to show "brain pattern, delta waves."
The Star Fleet Medical Reference Manual describes the K3 indicator as displaying the "electrical activity" of the brain of a patient, while conceding that extreme activity may indicate high levels of pain. This means that it would not be considered a true "dolorimeter," or pain-measurement device.
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