We are time surfers, riding a wave made of ephemeral moments toward a future of our own imagining.
“The Brain—is wider than the Sky—
For—put them side by side—
The one the other will contain
With ease—and You—beside— ”
Emily Dickinson penned this famous verse around 1862. The stanza touches on the expansive nature of human imagination to encompass the experience of the boundless sky, but all of time could just as well be included.
If I call your attention to it, you can perceive and estimate the passage of time required to read this sentence. I remember what I did this morning, yesterday, a year ago, even in my earliest childhood memory lying on an ironing board as my mother changed my diaper. I learned to trust – tentatively as a child but more firmly as I grew older – that the next moment in the flow of consciousness would happen without incident, and would be followed by numerous moments, too many to count. I can remember the past with pride or regret. And I can imagine a future with either joy or anxiety.
We can even extend this trick beyond the boundaries of our own lives. We can mentally travel billions of years into the past to the beginning of the universe, or forward to imagine the death of our sun as it consumes the earth – and further, stretching across an endless ocean of time when the universe finally grows cold and winks out. The brain is indeed “Wider than the sky”, in space and time.
The brain seems to build a model of time that is tied to the narrative structure of human existence. The components of this biographical model include the brain’s memory and attention networks.
Memory ties most naturally to the elements of time contained in the past. Whether yesterday or a moment ago, the past only exists when the memory of it is encoded by our senses, then laid down on the imperfect recording medium that is the human memory circuitry, to be accessed – and imperfectly reconstructed – at a later moment. Most would agree that those personal experiences we label “past” are also labeled “memories”. Would we also agree that our sense of time is dependent on memory? That would seem less intuitive.
The brain takes a few milliseconds to convert stimulus to response, so the best we can do is to live in the immediate past, a wave of existence that is always frozen at the moment just before it crashes – but that gap in time is so brief that we do not notice it. From an evolutionary perspective that little blink of a delay isn’t large enough to matter.
The future is more complicated. As we are poised there, helpless before the rogue wave of time, we see the inevitability of the weight of falling water and imagine that we hear the crash, anticipating it before it happens. We are Hokusai’s helpless boatmen, frozen for eternity at that moment as past becomes future. The predictive power of the brain, immersed in a hospitable pocket of an orderly universe, has taught us what to expect – even to the point that those expectations become routine. We tend to ignore time because it seems both predictable and familiar. That familiarity is due to our remembered experiences and ability to weave them together to create a model, not only of what was, but what will be.
Has anyone escaped the confines of this mental model of time? None have done so completely, but a few have come close – their journeys provide important clues to time’s secret.
Henry Molaison, the ultimate neuroscience test subject, is best known for his severe memory deficits after surgery to removed both of his hippocampi, part of the brain’s storage medium for memories. Henry possessed a profound anterograde amnesia, and he took part in one of the longest running human experiments in brain science.
Most of the tests Henry took were of his memory, but a lesser known paper published by Whitman Richards in 1973 tested Henry’s perception of time. He asked Henry to estimate intervals in a simple test: the experimenter would just say “go” and then, … “stop“ at different timed intervals – and then asked Henry to reproduce what he heard, up to an interval of 300 seconds. He then plotted the estimated time versus the actual length of the interval. Most people can do this pretty well, leading to a diagonal line (which I drew in red). But Henry’s data looked different:
Henry did well at repeating short intervals (fig.3), but for longer intervals his sense of time began to fray, and – importantly – while the errors could have occurred in any direction, he consistently underestimated the duration of the presented interval (which I’ve shaded with the blue ellipse). Henry’s memory window (the longest time he could retain newly acquired declarative information) was about 20 seconds – and his time errors increased as his mind was tasked with stepping outside of this window. It would seem memory is a necessary component of our sense of passing time, or to put it another way, we know what a minute “feels like” in part because we remember what previous minutes felt like.
This notion of the involvement of memory in the perception of time at first seems at odds with time flying “when you’re having fun”, and the apparent slowness of “watched pots” that are coming to a boil. A fun activity with simple demands may be no less fun than a fun, but complex activity. But the difference is that in the case of the watched pot, attention is on the elapsed time, and not a fun distractor. Tasks that take the mind away from the passage of time lead to a distortion of this perception. We become too distracted to plant the flags that mark the moments.
Tomorrow appears to be a mental model based on an extrapolation of yesterday into the future, and imagining ourselves placed in its narrative space. Endel Tulving and Daniel Schacter examined the role of memory in this process. They studied another amnesic patient known by the initials K.C. (whose hippocampus and other brain regions were damaged in a motorcycle accident), who provided an opportunity to examine how profound deficits in episodic memory could erase the ability to imagine the future. When asked to imagine his future, K.C. literally drew a blank.
Studies of individuals with more precise hippocampal damage have refined the role of the hippocampus in generating future imagined scenarios. In 2007 Hassabis and colleagues elicited future imaginings in subjects with bilateral hippocampal damage by posing a future scene to them, and observed that the test subjects could anticipate fragments of the scene, but they found it difficult to arrange them into a coherent whole – a narrative frame.
It’s a bit dangerous to extrapolate from special cases, because we know that the hippocampus is extensively connected to the rest of the brain – damage or removal of the hippocampus not only affects the hippocampus, but also the areas connected to it. Interestingly, neuroimaging studies have implicated many of the same hippocampal and prefrontal networks involved in imagining the future as are evoked when subjects recalled prior events. There was an exception – the right hippocampus was apparently more strongly activated when imagining the future.
This involvement of the hippocampus has interesting implications beyond what brain regions “light up” during the complex activity of imagining the future. One of the major functions of the hippocampus appears to be encoding the memory of “place”, as arrays of cells within its structure encode the memory of specific positions. Strong hippocampal activation when imagining the future raises a number of questions in light of this spatial function. Could it be that one consequence of amnesia is not only loss of episodic memory, but also deficits in the ability to “place” ourselves in a future narrative frame? Older adults also show changes in the precision of their episodic memory, which appears to coincide with reduced fidelity of future imagining. Is one consequence of age related memory loss a subtle loss in the ability to imagine ourselves in our own future?
The hippocampus is like a compass defining our place in the immediate world. These studies, and others, suggest that it may also be the GPS for mental time travel.
In a parallel with the quote from Emily Dickinson, Albert Einstein once said, “Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world…”. While it’s always treacherous to disagree with Einstein, I’ll go out on a limb and suggest that this famous quote presents a false choice – the key elements of imagination appear to depend intimately on the retention and reworking of prior knowledge to generate the narrative frame of an imagined future.
Age begins to sneak time from us with each passing year. Just as the rise of the world’s oceans as the globe warms may be imperceptible in the short term, our sense of time may also shift as we age. In our youth, a passing year is a large percentage of the total time we have existed (for example, at the age of 4 a year is fully a quarter of your entire life to that moment). The events that populate our young lives are infused with a special novelty and meaning as our brains learn to make sense of an alien planet. And they are populated with novel events (birthdays and holidays) that serve as signposts.
As we age, it’s a bit harder to encounter the unfamiliar. That expansive youthful year becomes a tiny fraction as we approach 80, closing in on a single percent. This may underlie the cognitive benefits of the novelty of travel, of making new friends, changing up our jobs, and engaging hobbies. Not so much to live longer – but to provide the signposts of living and change, of building the sense of our “place” in time, that make it seem longer. The trick to a longer seeming life, and the ability to imagine a rich future may simply be to create wonderful memories, and dwell on them enough to create those essential signposts (Could this mean that a little social media narcissism isn’t such a bad thing?). If we can devise ways to make life seem longer and richer, isn’t that just as important as efforts to extend it?
Our reliance on memory to structure our sense of time, including our future “sense”, also suggests that imagination is bounded by the experiences we remember – even if they are remembered imaginings of things we have never experienced. And in the case of imagined things, like the beginning of the universe, things we will never experience: the stuff of dreams.
Thanks for your time.