A psi-wave device

Psi-wave transceiver were transceiver implants used by the Prytt for interrogation purposes. They were connected directly to the brain stem of an individual and slowly adapted to their psi-wave patterns, eventually allowing the Prytt direct access to the individual's memory engrams. The devices could also be calibrated to connect two individuals, making them unable to move too far from one another without causing both to experience nausea. While the main purpose of this was to keep prisoners from wandering very far from one another, it also had the side effect of allowing them to read one another's thoughts.

In 2370, the Prytt Security Ministry abducted Captain Jean-Luc Picard and Doctor Beverly Crusher and implanted them with psi-wave devices, under the false impression that the Federation was conspiring with the Kes against them. They hoped to extract information about this supposed plot once the devices were calibrated, but Kes agents were successful in providing Dr. Crusher with her tricorder and an escape route. As the two wandered Kesprytt III, they discovered that the devices were transmitting their thoughts to one another. Eventually, both learned they had each been carrying romantic feelings for one another but were too afraid to admit.

The devices were removed once they were rescued by the USS Enterprise-D. (TNG: "Attached")

Although Lorin stated that the psi-wave devices were implanted in the cerebral cortex, within the skull, they were clearly seen to be at the base of the neck, near the brain stem.
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