A man recalls, in detail, everything that happened up to and including the moment of his accident, but has no memory of the several hours immediately afterward, during which witnesses say he was talking and behaving normally. Before revealing the answer: is this retrograde or anterograde amnesia, and what does the witnesses' account add?

This is anterograde amnesia — the gap concerns the formation of new memories for the period following the accident, not the retrieval of memories from before it, which he recalls intact. The witnesses' account that he was talking and behaving normally during the affected period is itself an important, characteristic feature of anterograde amnesia: the person may behave and respond appropriately during the affected period, despite having no later memory of it, since the disturbance is one of failing to lay down lasting memories, not of being obviously incapacitated at the time.