Such is one result of the tests carried out on No.6 during the episode of 'Checkmate.' No.2 responds that he'd never be able to fake that 'a negative reaction to pain,' to which the doctor agrees "Its would take superhuman will power!"
Well it would seem that by the time of the next episode No.6 has lost that "superhuman" will power. Because while undergoing an interrogation session with No.2 in his office, No.2 pokes No.6's forehead with the point of his sword "Ah! You react" No.2 comments. No sign of a 'negative reaction to pain there!'
Be seeing you