Imagination and memory

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A recent paper in PNAS is getting some press. Patients with hippocampal damage and amnesia (the normal symptom) are also impaired in imagining future scenarios. The authors contend that this fits with a view of the hippocampus as necessary for creating the context in which we have rich inner experiences. The data hinge on how well you think the questions asked by the researchers reflect “imagining”. The patients were given a general scenario and asked to imagine an experience there. They were usually short on descriptive richness and “spatial coherence”. But they were also generally more brief, and I’m not sure this was controlled for. Some schizophrenia patients suffer from alogia (poverty of speech). They need something like a richness/detail measure.

If the hippocampus is important for vivid, rich recollection of past experience and for making up future experiences, it seems more like a setting for memories to play out in rather than a memory storage structure per se. This doesn’t really sit with the systems consolidation or the multiple memory trace as far as I can see, but Nadel and Moscovitch have jumped on it as a challenge to systems consolidation. (Refresher: Systems consolidation = over time memories become less and less dependent on the hippocampus; multiple memory trace = the reduced effect of hippocampal lesions over time is due to propagation of memory traces within the hippocampus). I think it’s interesting that imagination and memory recollection might have the same substrate. In efforts to eschew confabulation I often demure when asked to recollect particular details of an experience, while I have seen others in the process of storytelling give very rich, but erroneous details.

This passage from a patient’s attempt to imagine himself in a museum struck me as sort of tragic. I wonder if the patient becomes as frustrated and depressed as I would failing at this task:

[pause] There’s not a lot as it happens. So what does it look like in your imagined scene? Well, there’s big doors. The openings would be high, so the doors would be very big with brass handles, the ceiling would be made of glass, so there’s plenty of light coming through. Huge room, exit on either side of the room, there’s a pathway and map through the centre and on either side there’d be the exhibits [pause] I don’t know what they are [pause]…there’d be people. [pause] To be honest there’s not a lot coming. Do you hear anything or smell anything? No, it’s not very real. It’s just not happening. My imagination isn’t… well, I’m not imagining it, let’s put it that way. Normally you can picture it can’t you? I’m not picturing anything at the moment. So are you seeing anything at all? No.


  1. This paper fits with the notion I’ve always had about the hippocampus, which is that it participates in the sequential linking, and subsequent retrieval of, episodic memories. This property provides the so-called “remembered present” quale of consciousness, and may be the mechanism by which people produce spatio-temporally extended imaginings.

  2. So why should patients with hippocampal damage have difficulty encoding new non-episodic information? Names and the like. Is it because experiences have to be saved in an episodic buffer and then slowly stripped down to their components to be stored extra-hippocampally?

  3. I remember hearing once that the right hippocampus is involved in encoding semantic info, the left, spatial-episodic. So I should have said the left hippocampus. Your point is well-taken though.

  4. earlier today i was thinking that maybe it would be okay if we never had a grand unifying theory of the hippocampus. what if it actually does more than one thing? it’s pretty complicated circuitry, it can have different oscillatory states, and it has differentiated subregions along every axis. still you’d like to be able to predict the effects of perturbations. it’s gonna take a pretty wide-ranging integrative mind/model to get the thing under control.

  5. An important thing to remember about the HF is that its connectivity and neurochemistry is very different along the dorso-ventral axis. The differential connectivity of the dorsal and ventral hippocampi has been very well elucidated by LW Swanson in his work with Risold. One neurochemical difference that comes to mind is the differential expression of estrogen receptors. In the dorsal CA1, nuclear estrogen receptors are found primarily within inhibitory interneurons. In the ventral CA1, they are found predominantly within pyramidals. Consequently, the level of ERalpha expression is much higher within the ventral HF. And this fits quite well with the connectivity, since the ventral HF projects mainly to the ventral forebrain and hypothalamus– the home of many other steroid-sensitive nuclei.

  6. The reason I mention this is because it agrees with your point– that unless you take into account the massive differences in connectivity between the north and south HF, there never will be a unified account of the entire hippocampus. Most people, including myself, think of it as a region involved in declarative memory consolidation. Yet I doubt that the ventral HF’s connectivity with the hypothalamus has much to do with that!

  7. Considering the well-demonstrated role of the hippocampus in spatial processing and navigation (in addition to memory, both spatial and non-spatial), I wouldn’t be surprised if some of the lack of detail in this imagination of the museum has more to do with a lack of being able to set down a mental “coordinate system” on which to draw the layout of the museum, than a lack of imagining events per se. I think it’s useless to say “hippocampus=memory” when these other roles of that brain region are very plausible confounding factors. 
    I would find it frustrating to be in that patient’s position, but in fact I have the opposite situation. I find it very easy to imagine complex scenes that look nothing like what I’ve seen before. As a demonstration, my dreams are full of stunning landscape layouts, interesting buildings, etc. On the other hand, they often don’t have much plot and frequently lack many people (besides as sort of disembodied voices I hear). I also have a good sense of direction, and a good memory for certain (but not all) things, so maybe part of my hippocampus is on the overactive side.