"The cognitive neuroscience of constructive memory: Remembering the past and imagining the future"
Q&A
[Q = Question, A = Answer, C = Comment]
Q: Common problem in experimental retrieval task is to how to distinguish self-generation vs retrieval of memory.
A: Hard to say whether hippocampal activations are a reflection of recombining details or in fact a reflection of creating novel episodes. Our design doesn’t allow us to make that distinction. We need other kinds of evidence to speak to that.
Q: Do you plan to do anything with more with temporal acuity?
A: We have a potential study, similar to this study, that tries to look at temporal course of activity, related to the past and future, in addition to neural imaging study. In terms of describing these activations, instead of asking future>past, we can ask whether “imagining > remembering”. Our results don’t require that simulating, remembering events. We’re hoping to do a follow-up study trying to distinguish imaging future and remembering (imagine retrospectively) the past. So far, it seems that the two imaginative activities hang together, so the functional side seems to relevant to our hypothesis.
Q: What if people imagine impossible or trivial events (e.g. events in outer space)?
A: Good question. We’ve thought about it, but still haven’t taken this into the scanner. Spooner & McDermann did this study behaviorally. They found that impossible/imagined events are much less detailed than familiar ones. I suspect that neural correlate studies will show something similar. Although we would need to find a way to show that the amount activation is not a reflection of the amount of detail that is projected.
Q: Autobiographical nature—would it make a difference if the memory is about what you are doing as opposed to about someone else?
A: Interesting question that that requires more data. Imaging study by Spooner & McDermann: Subjects showed a number of areas showing greater activation for imagining what they did in the future as opposed to Bill Clinton doing something in the future. They didn’t get the full network as we did, so it is possible that some of the regions will be the same (regardless of whether you are imagining your own future or someone else’s). More studies are needed.
C: Regarding your result that younger people are able to imagine their immediate past/future more vividly and in more detail than younger people: My grandmother may have a difficult time imagining/describing her own future compared to youngsters, but may be able to imagine someone else’s future (e.g. imagining the future of “a girl with an unfortunate background”) in a richer more detailed way compared to people who have lived less and experienced than her.
Q: Is it possible within science we conduct today to give any kind of evidence to claim that “memory is for the sake of the future”? This is what Aristotle considers to be a teleological cause rather than an efficient or material cause.
A: This is part of what we’re struggling. It’s certainly going to be impossible to do one experiment to show something that broad. The kind of data that we are getting at least seems to suggest that some aspect of that is plausible.
C: Instead of thinking of memory as being useful for thinking about the future, what about the brain structures that mediate the kinds of memory you are talking about are also responsible for mediation/thinking about the future in an adaptive sense. It is adaptive for us to be able to think about the future. So it’s not necessarily memory that’s important for thinking about the future.
C: Right, but the key point is, I think, how you would be able to show that it is “useful for the future”. That part would be very difficult to “prove” with correlation-based data.
A: I would agree with that—it would be very difficult to support that kind of idea from purely correlational imaging data. Certainly it would be impossible to get that from one fMRI experiment.
Q: Remembering is about the past, whereas memory distortion is a function of flexible recombinations/variance that happens in/about the future. But people recognize memory distortions as happening in the past. So doesn’t this indicate that memory will shift to the future/imagining the future? Couldn’t this suggest that memory is for the future?
A: That sounds similar to our line of reasoning. We’re trying to come up with an experimental design to test the hypothesis that: “individuals who are more susceptible to certain kinds of memory distortions are better at projecting the future.” This would be one way to link the two (i.e. seemingly undesirable memory distortions in recollection are actually desirable for predicting the future), but we don’t have evidence for that yet. The age experiment is as closest we have gotten.
Q: How do these ideas connect with philosophy of perception? Do you think that our limits to imagine will also limit our ability to perceive novel events? Also, can you share with us what you think about visual chunking (mechanism & how it relates to memory)?
A: I agree that they are related but I haven’t thought about it in much depth. It is a possible direction one can take in understanding memory.
Q: What about novelty? If our past memories limit the ways in which we can envision the future, does it also limit our ways to recognize novelty? Specifically, would amnesics be better at perceiving something that is very new to them than someone with a very rich memory?
A: No, I don’t think they would. The distinction between the novel and familiar are more blurred, if anything, in these experiments. Amnesics they false alarm less on gist-related lure words but they do tend to false alarm more to totally unrelated items, i.e. they can’t distinguish between the novel and familiar.
Q: Is there a difference between the near and distant future.
A: There is. We’ve done a followup analysis. We’ve found a difference in hippocampal response between recent and remote, past and future. Hippocampus modulates stronger to recent than to remote past (reflection of detail). It looks and behaves differently than recent and remote future, where it (hippocampus) seems to be more engaged with more distant future events (requires more combination of events projecting into the future).
C: There was a study in which people were given the same choices. Depending on whether the choice was placed in the near or distant future, people made different rational decisions.
A: Yes, these are the kinds of paradigms that may be necessary in order to distinguish more clearly imaginative activity that is applied to the past and the future when there may be differential consequences of events at different time points in the future (as opposed to our experiments—you can imagine any experience at any time).
Q: If you are able to get past information about a family member, and tell them a story that actually happened to them, but tell them “imagine you were in this place ten years from now…” and tell the story until they realize at some point that it was something that actually happened to them. Do you think the hippocampal differences will be the same as your results? Is this even possible to do?
A: If they imagined in the future, something that actually happened to them? I would expect certainly that they would get the same kind of overlap that we saw. The interesting question to me would be the “future activity greater than past activity” in the three regions—that effect might be mitigated because we are doing less recombining activities and already projecting what already happened in its current form. We may be able to get at the core of the extent to which this recombination activity is really critical.
There was an interesting paper on Nature Neuroscience about temporally encoded memory where you get something like a replay of the temporal steps, but you will also get a “pre-play” past memories when the animal is about to go down a remembered path.
Q: Could it be the case that depressed patients have decreased cognitive capacity in general, or is this something specifically related to memory?
A: There is a lot of research on trying to understand whether the reduced specificity effects past and future are due to a general working memory decline as opposed to something restricted to memory. I think that evidence suggests that it is more general, that it is not “just memory”. I don’t know of any imaging studies off-hand, that have looked for activation of autobiographical memory in depressed patients. There will be an interesting study coming out in Nature Neuroscience: Positive and negative events, and were how the areas are used differently in people who are more and less optimistic about the future. Their experiment asked about positive and negative events, and project ahead. They found activations in addition to the same regions that we did, in the amygdala (emotional), and part of the rostral interior singulate. What they regions showed an interesting dampening down when you projected negative events (vs positive events, or past -/+ events), moreover, they showed that the dampening down correlated with trait-optimism of the individual measured by a separate scale. What’s interesting is that the rostral interior singulate (i.e. the region that correlates with optimistic behavior) shows an abnormality in depression.
Q: How do you think about the underlying neural representations and operations of the recombination operations (aside from populations being activated/suppressed)?
A: I don’t have any novel ideas.
Q: Do you think computational models will aid in understanding?
A: I think they will provide a stylistic enhancement.
Q: What about computational representations/approaches towards memory?
A: Certainly many classical computational models do a good job explaining certain kinds of memory distortion, though none have been proposed that allows prediction of the future events. I think that it would be an interesting exercise to pursue.
Q: It is often assumed that memory is just a feature that goes along with other kinds of processes (e.g. sensory, motor processes), which makes it difficult to conceive of what exactly the hippocampus does—as it apparently does “just memory”. This seems to be an answer to the question, in that the processing that the hippocampus does is relational, recombining processing and what we call episodic memory is the outcome of this processing.
A: Yes. One outcome of this processing.
C: Need not be in the sense that we’re turning everything on its head. Experiments of future processing says that it’s not memory, it’s a control for memory. The fact that you get more activation in the future than the past suggests that it’s not memory, it’s the application/processing of the memory than just the relational recombination itself.
A: Yes, it does kind of reverse the relationship between memory and imagination/future planning. That’s ahead of the train and memory is a helper.
Q: Could photographic memory be considered as the extreme case of highly inefficient (unable to recombine) memory (and the other extreme would be highly imaginative memory)? And perhaps it is the saliency of the experience that influences where the memory is placed on the continuum (e.g. we seem to remember more details about emotionally charged experiences).
A: I prefer to avoid photographic memory. No convincing documentation. However, we can think of a continuum you proposed. It would be interesting to investigate the contributing factors to where the memories fall on this continuum. It may be age related (e.g. older people at the cost of less specific memory and less specific future imagining, may have more “wisdom”). It would be interesting to map out individual differences in
Q: If memory recombination can happen almost in the way that you “want it to happen”, it seems that memory is highly subjective, and unreliable as a source for determining “objective” phenomena.
A: I wouldn’t go quite as far. There is no question from eye-witness literature, that one must be very careful (high confidence =/= high memory). We must strike appropriate balance between trusting memory too much (because it does have vulnerabilities in various contexts) and trashing memory as being useless because under many conditions it is reasonably accurate.