A neurocortical monitor was a device used to record a complete encephalographic profile. It was placed on the parietal bone and could be programmed to send an alert to sickbay if it detected any unusual neurological activity.
In 2373, The Doctor had Lieutenant Tuvok wear a neurocortical monitor in order to record a complete encephalographic profile if he had another episode related to his mental breakdown. (VOY: "Flashback")
In 2377, after Seven of Nine began having disconcerting dreams about Unimatrix Zero, The Doctor gave her a cortical monitor to track her REM cycles. (VOY: "Unimatrix Zero") He also had her wear one later that year, when her cortical node was malfunctioning, as an alternative to remaining under observation in Sickbay. (VOY: "Imperfection")