Following recovery if consciousness after cerebral trauma caused by a head injury a person is typically dazed, confused, and imperfectly aware if his whereabouts and circumstances. During this state it is not possible to store new memories. On recovery the person may be unable to recall this period (post-traumatic amnesia) and may exhibit memory failure concerning brief or long periods into the past (retrograde amnesia). Subsequently memories may gradually return and be interrelated in an appropriate time sequence. Analogous phenomena may occur in the case of collective memory as a result of natural disaster or major social upheaval (war, revolution, etc), or as a result of damage to some particular repository of collective memory.