Are you going to work on time this morning? In addition to thanking the great god of transportation, you have to take time to thank yourself for your brain. The exceptionally accurate biological clock inside the brain allows us to perceive the passage of time, a skill that is especially important for achieving many everyday functions. In the absence of the ability to track the passage of time, we may continue to consume it in the bathroom in the morning. If there is no sense of uneasiness, we are likely to drive too long and miss a fixed high-speed exit.
But how does the brain generate this precisely adjustable mental clock? Neuroscientists believe that we have different nervous systems to deal with different types of time, such as maintaining a rhythm of day and night, controlling the timing of fine body movements, and sensing the passage of time. Until recently, most neuroscientists still believed that the latter kind of time processing—that is, the time that would warn you about breakfast delays—was supported by the same brain system. However, new research suggests that the model of a single neural clock may be too simple. Recently, neuroscientists at the University of California, Irvine, published a new study in the Journal of Neuroscience, which shows that the brain actually has another way to perceive the passage of time. The author also suggests that this second set of circadian clocks not only works in parallel with the clock of the main nerve but may even compete with it.
Past research has shown that a region of the brain called the striatum is the core of our central circadian clock, which integrates time information with the surrounding cerebral cortex. For example, when people pay attention to how long the time has passed, the striatum is activated; however, for patients with Parkinson's syndrome, they lack the concept of time. Parkinson's disease is a neurodegenerative disease that interrupts the input to the striatum in the patient's brain.
For the perception of time-lapse, the brain is required not only to measure time, but also to maintain a dynamic memory of time-lapse. Scientists have long discovered that a region of the brain called the hippocampus is very important for empirical memory. Now, they believe that the region is equally important in the passage of memory. A study of animal brain bioelectricity has shown that neurons in the hippocampus respond to specific moments, but tracking events do not necessarily involve the hippocampus. It is worth noting that people with damaged hippocampus can still accurately remember the passage of short-term time, but the long interval of memory can cause difficulties. These findings suggest that the hippocampus is important for responding to certain time information, but not for all information. If so, what is this time coding used for, and why is it so selective?
The researchers tried to unlock the mystery by training the rats to distinguish between different time intervals. They use the reward mechanism to train rats to choose different odors, allowing rats to show their perceived passage of time. Prior to some trials, the scientists also injected rats with an agent that temporarily inhibits hippocampal activity. In this way, they can test the necessity of normal working hippocampus to distinguish between different time intervals.
As in the control group, rats in which the hippocampus does not work can be distinguished by a larger time interval (for example, 3 minutes and 12 minutes), but they are helpless for similar time intervals (such as 8 and 12 minutes). This suggests that the hippocampus is important for distinguishing between similar time intervals, but it is not necessary for very different intervals. Surprisingly, this model is only established over a longer time span; rats whose hippocampus does not work can not only distinguish between shorter-scale intervals (such as 1 to 1.5 minutes) but can distinguish better.
Therefore, when the hippocampus responds to the passage of time, it also plays a special role. It is especially capable of distinguishing similar time intervals at large scales - about a few minutes. When you know that you are bathed for 10 minutes instead of 15 minutes, you should thank your hippocampus. But when you perceive 1 and 1.5 minutes, or 20 minutes and 1-hour difference, the other brain areas take over the internal timer.
Although it is a bit strange that the hippocampus has such a highly specialized feature, it is consistent with what we know in other ways. The hippocampus is best known for its ability to partition overlapping objects or cross-experience, a process called pattern separation. This study shows that its model distinguishes multiple levels of experience and detects subtle differences in objects, places, and time intervals.
For things that happen in seconds, the hippocampus may not be noticed, but we can indeed track these very fast moments. Considering that the striatum can track time in seconds, the authors believe that the hippocampus and striatum may compete with each other. For example, when the hippocampus rests, the striatum may work more efficiently than usual. Although I don't recommend that you damage your hippocampus (so you will incur more serious problems), doing so will theoretically enhance your ability to track very short intervals.
So if you go to work on time tomorrow, you have to thank not only one, but multiple clocks inside your brain, and a healthy hippocampus.