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.