I'm guessing everyone thinks of it slightly different, for me cognitive impairment is short term memory problems, difficulty speaking or finishing sentences, muddling words, forgetting how to do routine things, difficulty typing, unable to read, write or watch tv etc.
Mental PEM for me is mental exhaustion, slipping into trances, inability to stay awake, lack of emotional buffer, not enough mental energy to think properly, sense of humour partially 'switched off' like a robot.