A neuro-synaptic relay on the end of an ultrasonic extractor
A neuro-synaptic relay was a type of implant that recorded neuroelectric responses in humanoid brains.
In 2374, Luther Sloan implanted a neuro-synaptic relay behind Doctor Julian Bashir's right ear. He later asked Bashir to remove it with an ultrasonic extractor so that he could confirm the doctor's innocence. (DS9: "Inquisition")
According to a subtle script note, it can be observed in the teaser, after Bashir wakes indicating that he'd already been tagged by Sloan, that "[h]e idly scratches behind his right ear. (NOTE: This seemingly innocent gesture will eventually pay off in Act Five.)"