From Halopedia, the Halo wiki
The neural marker is a human device that can effectively "read" and generate visualizations of an individual's thoughts. Sometimes used in interviews, the neural marker is placed on the right temple, and is able to project a direct feed of one's thoughts directly to an on-screen display, as well as monitor various brain functions. It is seemingly similar to the PQI, a "lie detector", though it is probably not the same device.
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