MEMORY, Blake wrote, enables us to “traverse times and spaces far remote”. It constitutes mental time travel, with which we can recollect, in vivid detail, events that took place many years ago. We have known, for the best part of a century, that memory is reconstructive rather than reproductive. That is, recollection involves piecing together specific details of the event, and mixing these with our own biases and beliefs. While not being completely accurate, our memories are, in most cases, reliable enough.
It is because of the reconstructive nature of memory that we are able to travel forward in time as well as back into the past. Research carried out in recent years has shown that imagining future events and recalling those that we have already experienced are dependent on the same core network of brain regions. It seems that both involve the same cognitive processes – when we look forward to something that might happen in the future, the brain generates a simulation of that event using fragments of memories of past events.
However, the evidence for this is indirect, and it is possible that what are thought to be simulations of future events are in fact merely memories of past events being “recast” into the future. But a new study, due to be published in the September issue of the journal Neuropsychologia, now confirms that these simulations are indeed novel constructions, and also shows that remembering actual experiences and imagining possible future events depend on distinct subsystems within the common core network.
A number of studies published in the past few years have shown that remembering past events and imagining future events involve similar cognitive processes. Compelling evidence for this comes from behavioural studies of amnesics – as well as severe memory deficits, these patients also have great difficulty envisioning the future. Thus, it is suggested that fragments of past experiences provide a source of details which can be flexibly recombined to simulate future events. There is, however, no direct evidence for this; it is possible that imaging future events involves merely recasting past experiences, such that simulating the future involves retrieving a single memory of the past and projecting it forward in time, and the studies carried out to date have not yet distinguished between these possibilities.
Neuroimaging has revealed that remembering and simulating the future are now known to depend on common neural substrates; the core network activated in both cases includes the hippocampus, poserior cingulate gyrus, inferior parietal lobule, and medial frontal and lateral temporal cortices. Interestingly, activity in this network is greater during simulation of future events than during remembering. This may reflect processes that do not occur during remembering, such as the recombination of memory fragments or, if imagining the future involves merely recasting the past, the addition of a new “timestamp” to the memory of a past experience.
The new study, led by Donna Rose Addis of the University of Auckland in New Zealand, was designed to address these issues, and to investigate whether imagining the future has a neural signature that is distinct from remembering the past. 23 participants – all of them college students – were first asked to complete a spreadsheet detailing memories of nearly 200 personal events they had experienced within the past 5 years, each of which had occurred at a specific time and in a specific place. A few days later, they underwent a brain scanning session, during which they were asked to recall some of the events that had actually happened and also to imagine past and future events.
For the trials involving imagining events, the participants were presented with cues consisting of details which had been randomly extracted from different memories and then combined. Each cue consisted of details of a person, object and place taken from multiple episodes provided in the first session, and the participants were instructed to imagine these details in a single novel episode. In this way, the participants were prevented from recasting past experiences as future events; they also confirmed, in descriptions provided after the scanning sessions, that they were able to combine the separate memory elements into coherent representations of imagined future events.
As expected, the researchers found that remembering past events and simulating future ones activated overlapping regions of the core network of brain structures. But closer examination revealed that each was associated with a distinct subsystem within the network. For example, extensive regions of the medial prefrontal cortex, parietal lobe and the anterior portion of the hippocampus were activated during imaging future events, but not during retrieval of memories. On the other hand, remembering, but not imagining, led to activation of parts of the visual cortex, likely reflecting the imagery associated with memory retrieval. Furthermore, each subsystem was found to be activated over a different timescale, with the imagining subsystem becoming active earlier (at between 2-4 seconds) and peaking for longer, than the remembering subsystem.
- Anatomy of a false memory
- Confabulatory hypermnesia, or sever false memory syndrome
- Reconstructive memory: Confabulating the past, simulating the future
Addis, D., et al (2009). Constructive episodic simulation of the future and the past: Distinct subsystems of a core brain network mediate imagining and remembering. Neuropsychologia 47: 2222-2238. [PDF]
Szpunar, K. K. & McDermott, K. B. (2008). Episodic future thought: Evidence from ratings of subjective experience. Conscious. Cogn. 17: 330-334. [PDF]
D’Argembeau, A. et al (2008). Neural correlates of envisioning emotional events in the near and far future. Neuroimage 40: 398-407. [PDF]