images in peripheral vision

My son is 12 years old and has been suffering with Anxiety problems since he was 6, many unrealistic worries and fears. We did see a psychiatrist and she diagnosed him with Anxiety disorder and said he had a very vivid imagination & would most likely grow out of it. As he aged the worries became less frequent. The one thing he told me lately that I don't understand is that if he concentrates hard enough he is able to see (usually) a person (sometimes whatever or whoever he is concentrating on, but usually a total stranger) in his peripheral vision. He is able to do this at will. He said the image is a little transparent, but he can see it clearly in his peripheral vision. I know he isn't hallucinating because he realizes the person isn't really there and he is fine other wise & I know he's not pranking me. Can you give me any insight as to how this is possible. Thanks.
Brigitte