Christopher has a photographic memory and he tells us now what that's like. For one thing, it allows him to recall pretty vivid memories of his mother.
He also uses his photographic memory when he meets someone, to figure out if he's met them before.
And one last thing: it helps him remember how to act in certain situations, recalling how he acted in the past. Makes sense to us.
He compares these images in his mind to what the rest of us might call "imagination."