What brain regions are activated when a dream is remembered? 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.
Your memory of a dream would be an autobiographical memory, which is a memory system that is based on a combination of episodic memories and semantic memories. Autobiographical memories are memories of events that have happened to you, and thus are often retrieved from a first-person perspective. These are not the same thing as episodic memories, however, as episodic memories are contained in a separate memory system.

Your autobiographical memory knowledge base is distributed through neural networks in the frontal, temporal, and occipital lobes. The frontal lobes and anterior temporal lobes represent the more abstract or conceptual parts of the memory, and the occipital lobes/posterior temporal lobes represent the more sensory and perceptual details of specific events. These are predominantly represented in the right cortex. [1] Initially, a memory is formed in the left prefrontal region. Over time, this memory is 'moved' and held at a higher level in the posterior region.

However, there are more widespread activation patterns throughout the brain, and almost every region of the brain has been represented in at least one instance of autobiographic memory retrieval. [2] This lends itself to the belief that certain regions are used for certain types of retrieval, such as re-living experiences associated with the memory versus retrieving factual information from the memory.

Dreams themselves are not episodic memories. During REM sleep, there are specific brain regions that appear to be affected; in particular, there is increased activity in the formation of the hippocampus and decreased activity in prefrontal regions. There appears to be no temporal lobe activity, which implies that 'dream memories' are not true episodic memories, but instead representative of episodic fragments (at best). One study [3] even found that the majority of dreams (>80%) were comprised of primarily autobiographical memories. This suggests that dreams themselves are comprised of fragmented and selective personal experiences, perhaps to aid in the assimilation of memories into the autobiographical memory schema.



Sources used:

[1] Conway, M.A., Pleydell-Pearce, C.W., &amp; Whitecross, S.E. (2001). The neuroanatomy of autobiographical memory: a slow cortical potential study of autobiographical memory retrieval, Journal of Memory and Language. 45, 493-524.

[2] Svoboda, E.; McKinnon, M.C.; Levine, B. (2006). "The functional neuroanatomy of autobiographical memory: a meta-analysis". Neuropsychologia 44 (12): 2189–2208.

[3] Malinowski, J. E. and Horton, C. L. (2014), Memory sources of dreams: the incorporation of autobiographical rather than episodic experiences. Journal of Sleep Research, 23: 441–447.