How Does the Brain Alter your Perception of Time?
Have you ever sat in a lecture or meeting that dragged on forever? Perhaps you looked at the clock, tried to distract yourself for a while, then looked at the clock again and only a few minutes had passed. Under different circumstances, you might have experienced the opposite: losing track of time. Maybe you got lost in your favorite hobby or spending time with your best friend, and before you knew it, several hours had flown by. These experiences are common, but scientists are just starting to understand the cognitive and neural mechanisms that underlie them.
Psychologists have long made a distinction between metric time and episodic time: metric time is what a clock measures — it moves forward steadily and without variation; episodic time, on the other hand, refers to our subjective perception of time. Imagine yourself sitting in a boring meeting for an hour; now, compare that to speaking with your best friend for an hour. Although both of these events last the same amount of metric time, there seems to be something real about the difference in how long those experiences feel. This phenomenon has a particularly mysterious quality to it: are we really experiencing the passage of time differently? Or is it just an illusory product of our memories?
A recently study published in Nature may shed light on this issue. The research group led by Stanford neurobiologist, Albert Tsao, believe they may have found the neurobiological correlates of episodic time perception. For context, researchers have historically been able to identify neurons that code metric time: “time cells” in the hippocampus, for example, have been found to fire continuously like a metronome as an animal performs a task (Tsao, 57). Other neurons, such as those in the suprachiasmatic nucleus, serve as biological clocks to regulate our circadian rhythms in accordance with the light/dark cycle. Despite these advancements, Tsao et. al write that “our understanding of how the brain represents…episodic time is still in a nascent stage” (57). His group’s findings, though, may represent a major step in the right direction.
The Tsao-led team investigated a particular group of neurons in the lateral entorhinal cortex (LEC) — a structure that serves as a key point of contact between the hippocampus — the brain’s memory center — and the cerebral cortex. In their 2018 study, the researchers allowed mice to explore a box whose walls alternated in color back-and-forth from black to white every six minutes; there was a two minute period with no colors between each switch. Meanwhile, the researchers recorded the firing rates of the mice’s LEC neurons using brain-implanted microelectrodes.
The first clue that these LEC neurons were time-sensitive is that they displayed descending “ramping activity”; that is, when a trial started and the walls changed color, each neuron fired repeatedly at a high frequency, but this rate steadily decreased as the trial progressed. During the two minutes between trials, these neurons were silent, but when the next trial started again, the ramping pattern resumed. Taken alone, this suggests that LEC neurons function like a countdown timer for the trial periods. But what does this have to do with episodic time? Couldn’t these neurons just be coding metric time?
The researchers addressed this in a second experiment in which mice were given a continuous-alternation task. Specifically, mice were put on a track that had a left turn and then a right turn; the experiment had 40 repetitions or trials. If it were the case that LEC neurons only code metric time, we would expect to see another ramping pattern in this experiment. In other words, the LEC neurons would decrease in firing rate over the course of a run on the track, and be silent during the intertrial periods. The researchers found that LEC activity in the first few trials indeed fit this model: specifically, firing rate was an accurate predictor of trial versus intertrial periods. But after the initial trials, LEC activity decreased across the board, and the neurons appeared to stop coding for time altogether.
The researchers hypothesize that this peculiar result suggests that LEC neurons are not metric time coders but instead “experience coders”. In their words, “temporal information in the LEC arises simply because the animal’s moment-to-moment experience constantly changes, and time can be extracted from this changing flow of experience” (60). In the continuous alternation (turning) task, LEC neurons initially fired due to the task’s novel nature, but once the animals learned what was going on, and their behavior became automatic, LEC activity decreased.
Upon first glance, this portrait of LEC neurons seems to be continuous with most people’s notions of episodic time. Rapidly-changing situations with high levels of “experience” to code for may feel slower than familiar, everyday tasks. On the other hand, next time you’re stuck in a meeting and can’t stop looking at the clock, see if you can’t find some repetitive task to do: things might go by a little quicker … just make sure your boss doesn’t notice!
References:
Tsao et al. (2018) Tsao A, Sugar J, Lu L, Wang C, Knierim JJ, Moser M-B, Moser EI. Integrating time from experience in the lateral entorhinal cortex. Nature. 2018; 561:57–62.
You can follow me on Twitter here. I tweet about psychedelics, neuroscience, and more.
