The following personnel was working in the laboratory on Psi 2000.
Engineer
A Psi 2000 engineer
This Psi 2000 engineer was found dead from the chaos caused by polywater intoxication. (TOS: "The Naked Time")
The engineer was played by an unknown actor.
Scientist
A Psi 2000 scientist
A Psi 2000 scientist who suffered a bizarre death at the Psi 2000 laboratory was found by Commander Spock and Lieutenant Joe Tormolen. (TOS: "The Naked Time")
The scientist was "played" by a mannequin.