This post is about an article by Schacter et al (pdf) regarding how the constructiveness of memories may crucially be due to the need to simulate future scenarios. But before I go to the main course, I would like to touch upon a starter: Jeff Hawkins Heirarchical Temporla Memory (HTM) hypothesis. I would recommend that you watch this excellent video.

As per Jeff Hawkins, we humans are basically prediction machines, constantly predicting the external causes and our responses to them. Traditionally, the behaviorist account has been that we are nothing but a bundle of associations- either conditioned pavlovian associations between stimuli and stimulus-response or a skinerrian association between our operant actions and environmental rewards. Thus every behavior we indulge in is guided by our memory of past associations and the impending stimulus. Jeff Hawkins refines this by postulating that we are not passive responders to environmental stimuli, but actively predict what future causes (stimuli) are expected and what our response to those stimuli may be. Thus in his HTM model, the memory of past events not only exerts influence via a bottom up process of responding to impending stimulus; but it is also used for a top-down expectation or prediction of incoming stimulus and our responses to it. Thus, we are also prediction machines constantly using our memory to predict future outcomes and our possible responses.

Now lets get back to the original Schacter article. Here is the abstract:

Episodic memory is widely conceived as a fundamentally constructive, rather than reproductive, process that is prone to various kinds of errors and illusions. With a view toward examining the functions served by a constructive episodic memory system, we consider recent neuropsychological and neuroimaging studies indicating that some types of memory distortions reflect the operation of adaptive processes. An important function of a constructive episodic memory is to allow individuals to simulate or imagine future episodes, happenings, and scenarios. Because the future is not an exact repetition of the past, simulation of future episodes requires a system that can draw on the past in a manner that flexibly extracts and re-combines elements of previous experiences. Consistent with this constructive episodic simulation hypothesis, we consider cognitive, neuropsychological, and neuroimaging evidence showing that there is considerable overlap in the psychological and neural processes involved in remembering the past and imagining the future.

As per the paper the same brain areas and mechanisms are involved in both remembering a past event and imagining a future one – and the regions involved include the hippocampus. These findings in itself are not so fascinating, but the argument that Schacter et al give for , as to why, the same regions are involved in both memory retrieval and future imaginings, and how this leads to confabulations and false recognitions is very fascinating. As per them , because we need to simulate the future events, and as the future events are never an exact replica of past events, hence we do not store the past events verbatim, but store a gist of the event, so that we can recombine the nebulous gist to create different possible future scenarios. Due to this fact (the need for simulation of future events), the memory is not perfect, and in normal individuals it is possible that they confabulate (attribute the source of their memory erroneously) or make false recognitions on memory tests like the DRM.

Fisrt a bit of background on DRM paradigm. In this test, a list of related words are presented to a subject: eg yawn, bed, night, pillow, dream, rest etc. All of these relate to the theme of sleep. Later in a recall test, when this thematically related word is presented to normal subjects, they most often say that they had encountered the word sleep earlier. However given an unrelated word like hunger, most are liable to recognize that the word was not encountered previously. What Schachter et al found was , that in those subjects that had damage to hippocampus/ other memory areas and were amnesics, this effect of confabulating the gist word was reduced. In other words, those with brain damage to memory areas were less likely to say that they had encountered the related word sleep during the original trial. this, despite their poor performance in overall remembering of old list items as compared to controls. This clearly indicates that remembering the gist vis-a-vis details is very important memory mechanism.

I believe that we should also take into account the prototype versus exemplar differences in categorization between the males and females into account here. I would be very interested to know whether the data collected showed the expected differences between males and females and hopefully the results are not confounded due to not taking this gender difference into account.

Anyway , returning to the experimental methodology, another sticking point seems to be the extending of results obtained with semantic memory (like that for word lists) to episodic memory.

Keeping that aside, the gist and false recognition data results clearly indicate that the constructive nature of memory is an adaptation (it is present in normal subjects) and is disrupted in amnesics/ people with dementia.

Thus, now that it is established that memory is reconstructive and that this reconstruction is adaptive, the question arises why it is reconstructive and not reproductive. To this Schacter answers that it is because the same brain mechanism used for reconstructing memory from gist are also used for imagining or simulating future scenario. They present ample neuropsychological, neuroimaging and cognitive evidence on this and I find that totally convincing.

The foregoing research not only provides insights into the constructive nature of episodic memory, but also provides some clues regarding the functional basis of constructive memory processes. Although memory errors such as false recognition may at first seem highly dysfunctional, especially given the havoc that memory distortions can wreak in real-world contexts (Loftus 1993; Schacter 2001), we have seen that they sometimes reflect the ability of a normally functioning memory system to store and retrieve general similarity or gist information, and that false recognition errors often recruit some of the same processes that support accurate memory decisions. Indeed, several researchers have argued that the memory errors involving forgetting or distortion serve an adaptive role.

However, future events are rarely, if ever, exact replicas of past events. Thus, a memory system that simply stored rote records of what happened in the past would not be well-suited to simulating future events, which will likely share some similarities with past events while differing in other respects. We think that a system built along the lines of the constructive principles that we and other have attributed to episodic memory is better suited to the job of simulating future happenings. Such a system can draw on elements of the past and retain the general sense or gist of what has happened. Critically, it can flexibly extract, recombine, and reassemble these elements in a way that allows us to simulate, imagine, or ‘pre-experience’ (Atance & O’Neill 2001) events that have never occurred previously in the exact form in which we imagine them. We will refer to this idea as the constructive episodic simulation hypothesis: the constructive nature of episodic memory is attributable, at least in part, to the role of the episodic system in allowing us to mentally simulate our personal futures.

I’ll finally like to end with the conclusions the author drew:

In a thoughtful review that elucidates the relation between, and neural basis of, remembering the past and thinking about the future, Buckner and Carroll (2007) point out that neural regions that show common activation for past and future tasks closely resemble those that are activated during “theory of mind” tasks, where individuals simulate the mental states of other people (e.g., Saxe & Kanwisher 2003). Buckner and Carroll note that such findings suggest that the commonly activated regions may be specialized for, and engaged by, mental acts that require the projection of oneself in another time, place, or perspective”, resembling what Tulving (1985) referred to as autonoetic consciousness.

This Seems to be a very promising direction. The ‘another time and place’ can normally be simulated withing hippocampus that also specializes in cognitive maps. We may use the cognitive maps to not only remember past events, but also simulate new events. In this respect the importance of dreams may be paramount. Dreams (and asleep) may be the mechanism whose primary purpose is not memory consolidation; rather I suspect that the primary function of dreams is to work on the gist of the memory from the previous day, simulate possible future scenarios, and then keep in store those memories that would help and are likely to be encountered in future. Thus, while dreaming we are basically predicting future scenarios and sorting information as per their future relevance. Not a particularly path-breaking hypothesis, but I’m not aware of any thinking is this direction. Do let me know of any other similar hypothesis regarding the function of dream as predictors and not merely as consolidators.