A parietal bone was a bone in the skull found in most humanoid species, notably Humans and Vulcans. In 2373, The Doctor attached a neurocortical monitor to Lieutenant Tuvok's parietal bone in order to record a complete encephalographic profile in case he suffered another mental breakdown. Tuvok had experienced one while the USS Voyager encountered a class 17 nebula in the Delta Quadrant. It was later discovered that Tuvok was suffering from a memory virus unknowingly transmitted to him from his crewmate from the USS Excelsior, Lieutenant Commander Dimitri Valtane, in 2293. (VOY: "Flashback")