Some people remember dreams, others don't. The same person can wake up with dream recall one day and without on other days. I know that the association between REM sleep and dreaming was initially established by awakening test subjects during REM and asking them what they remember.
My question is about the dream recall phenomenon – imagine a person is woken up from a dream while his brain is being scanned. What is the difference in brain region activations shortly before awakening between the case where a dream is recalled and a dream is not recalled? In both cases polysomnographic readings would conclude that REM is experienced and probability of dreaming is high.
I'm asking this question in relation to this question about working memory of the memory recall patient HM. As I heard about his case, I could not help but think that his "living in the present" seems very similar to the experience of dreaming. While dreaming, one does not have a clear memory of the past events, or any desire to even access this memory. I'm trying to understand: can the difference in dream recall for the same test subject be localized to some inhibition of hippocampus or another brain structure, similar to what HM had surgically removed?
Dreams are prone to be "episodic", where a continuous dream of say 20 minutes may contain several very distinct scenes (2–5 minutes each) that have no relation to each other. I'm interested if such dream changes involve the same mechanism that HM experienced when he was distracted.