“Time is passing too fast!” Many of us use that phrase every day when we feel like our kids are growing up fast or when a deadline sneaks up on us. When Virginie van Wassenhove hears that phrase, it conjures an entirely different point of view. She goes straight to consciousness, musing on how we perceive reality.
“When it comes to time, we tend to use linguistic shortcuts that may abuse the state of reality and fundamentally bias the way we think about time and the way scientists conceptualize issues related to time,” she says. “I am interested in understanding how the slow time scales of squishy matter afford us to assign meaning to reality.”
A cognitive neuroscientist at CEA and INSERM in Paris, van Wassenhove is working to understand the neural underpinnings of time. She has organized a symposium on the topic at the Cognitive Neuroscience Society (CNS) annual meeting in San Francisco this month — featuring scientists who are exploring evidence of how we construct mental models of time.
“Our work suggests that the conscious arrow of time — thinking about the past or the future — is calibrated on where we imagine ourselves to be, what the brain represents to be the ‘here and now.'” -Virginie van Wassenhove
Traditionally, she explains, research on time has focused on how the brain quantifies duration and responds to external rhythms. Now, comparisons of mental time and space at the computational level are offering promising venues to explore new questions on how we perceive time in the brain — questions, van Wassenhove says, like “do high-level representations of time, unlike those of space, show an obligatory orientation past to future?”
“Our work suggests that the conscious arrow of time — thinking about the past or the future — is calibrated on where we imagine ourselves to be, what the brain represents to be the ‘here and now,’” she says. Van Wassenhove’s team’s work thus suggests that how we sequence time is determined by internal representations of ourselves.
For Marc Howard, a cognitive neuroscientist at Boston University, his fascination with time dates back to childhood. “The brain’s estimate of the past and the future is foundational to our experience,” he says. “We don’t experience a series of isolated moments. We experience a stream of memories receding gradually towards the past and a stream of predictions approaching from the future.” Our memory is thus a timeline that we must construct, he says.
What is exciting about the field now, Howard says, is that scientists are finding neurophysiological evidence to support what was once only theoretical mathematical models. “Over the last several years we’ve seen growing evidence that there’s a record of time in various parts of the brain,” Howards says. “These ‘time cells’ behave like we would expect from a compressed record of the past and have been observed in many brain regions, e.g. hippocampus, prefrontal cortex, and the striatum.”
“The field of neuroscience will have to further mature and embrace the fact that it will not be possible to understand the human mind without describing how the brain tells, represents, and conceptualizes time.” -Dean Buonomano
In even newer work he’ll be discussing at the CNS meeting, researchers have now found evidence to support a mathematical construct of time called a “Laplace transform.” Last summer, Albert Tsao in the Moser lab at the Kavli Institute for Systems Neuroscience and Centre for Neural Computation, which first discovered time cells, published a paper showing evidence that suggested the lateral entorhinal cortex maintains the Laplace transform of time.
Howard described the Laplace transform as a fun-house mirror reflection of an image that shows the pattern of events in time that lead to the present. “The reflection doesn’t look much like the image you started with,” he says. “But different images would give different reflections. If you knew the mapping between each image and its reflection, you could look at a reflection and guess what image caused it.”
For several years, Howard and colleagues have argued that the brain constructs time as a Laplace transform and a corresponding inverse. (The inverse in the funhouse analogy, he says, is the shape of the funhouse mirror.) In this construct, time cells estimate the past but with a systematic distortion predicted by the Laplace equations; the estimate gets more blurry for events further in the past.
Howard’s lab is working to replicate the Tsao data in monkeys to show that the Laplace transform equations govern this construct. “If this result holds up across tasks and species, it suggests that the equations are telling us something relatively deep about the way the brain estimates time,” he says. “It suggests we can work out a real theory of memory and maybe cognition more broadly.”
Indeed, Howard’s ultimate goal is to create “simple, elegant, and powerful descriptions of cognition.” While it’s still early, he says, “these new results convince me that this goal is at least possible.”
Indeed, van Wassenhove quotes Dean Buonomano (Your Brain is a Time Machine, 2018): “The field of neuroscience will have to further mature and embrace the fact that it will not be possible to understand the human mind without describing how the brain tells, represents, and conceptualizes time.”
-Lisa M.P. Munoz