Explain why a patient with anterograde amnesia can appear to behave completely normally during the affected period, despite having no later memory of it at all.

Anterograde amnesia is specifically a disturbance of forming new, lasting memories, not a disturbance of moment-to-moment awareness, attention, or the capacity to respond appropriately to what is happening. A person can hold a normal conversation, answer questions sensibly, and act entirely appropriately using their intact short-term working memory and existing knowledge and skills, while the process that would normally convert that experience into a lasting memory simply fails to occur. This is precisely why the finding can be so unsettling for those around the patient afterward — everything about their behaviour at the time looked ordinary, yet none of it was retained, which is exactly the gap between behaviour and memory formation that defines the phenomenon.