It is a truism that time seems to expand or contract depending on our circumstances: In a state of terror, seconds can stretch. A day spent in solitude can drag. When we're trying to meet a deadline, hours race by.
A study published this month in the journal Psychophysiology by psychologists at Cornell University found that, when observed at the level of microseconds, some of these distortions could be driven by heartbeats, whose length is variable from moment to moment.
Psychologists fitted undergraduates with electrocardiograms to measure the length of each heartbeat precisely, and then asked them to estimate the length of brief audio tones. The psychologists discovered that after a longer heartbeat interval, subjects tended to perceive the tone as longer; shorter intervals led subjects to assess the tone as shorter. After each tone, the subjects' heartbeat intervals lengthened.
A lower heart rate appeared to assist with perception, said Saeedeh Sadeghi, a doctoral candidate at Cornell and the study's lead author. "When we need to perceive things from the outside world, the beats of the heart are noise to the cortex," she said. "You can sample the world more -- it's easier to get things in -- when the heart is silent."
The study provides more evidence, after an era of research focusing on the brain, that "there is no single part of the brain or body that keeps time -- it's all a network," she said. "The brain controls the heart, and the heart, in turn, impacts the brain."
Interest in the perception of time has exploded since the COVID pandemic, when activity outside the home came to an abrupt halt for many and people worldwide found themselves facing stretches of undifferentiated time.
A study of time perception conducted during the first year of the lockdown in Britain found that 80% of participants reported distortions in time, in different directions. On average, older, more socially isolated people reported that time slowed, and younger, more active people reported that it sped up.
"Our experience of time is affected in ways which mirror, generally, our well-being," said Ruth S. Ogden, a psychology professor at Liverpool John Moores University and author of the lockdown study. "People with depression experience a slowing of time, and that slowing of time is experienced as being a worsening factor of the depression."
The new Cornell study addresses something different: how we perceive the passage of microseconds. Understanding those mechanisms may help us to manage trauma, in which instantaneous experiences are remembered as drawn out, Ogden said.
When trying to assess the importance of an experience, she said, "our brain just looks back and says, Well, how many memories did we make?" She added, "When you have this really rich memory, richer than you would normally get in a 15-minute period of your life, that's going to trick you into thinking that it was long."
Research into perception of time has focused, until recently, on areas of the brain, said Hugo Critchley, a professor of psychiatry at Brighton and Sussex Medical School who has studied how heartbeats affect memory for words and fear responses.
"I think there's much greater appreciation that cognitive functions are intimately linked, perhaps even grounded in, the control of the body, whereas most of the psychology up to the 1990s dismisses the body as being something controlled at the level of the brain stem," said Critchley, who was not involved in the Cornell heartbeat study.
Previous research has investigated how physical arousal is connected to stress processing, and emotional states such as anxiety and panic, Critchley said. The new study expands on that by focusing on the role of the heart in a nonemotional function, the perception of time, which can be linked to larger distortions in thinking.
"You can't look at cognitive function in isolation," he said. "Even understanding how the brain develops and starts representing internal mental states, people are looking at the primacy of the inescapable internal information you need to control to keep alive."
One reason that the body may be closely involved in the perception of time is that time is closely related to metabolic needs, said Adam K. Anderson, a professor of psychology at Cornell and a co-author of the new study.
"Time is a resource," Anderson said. "If the body was a battery, or a gas tank, it's trying in the moment to say, How much energy do we have? We're going to make things seem shorter or longer in terms of time based on how much bodily energy we have."