Civilizations throughout history have constructed ways of timekeeping. Whether that be the astrological Mayan calendar, Celtic use of seasonal weather change to observe festivals, or modern day New Year’s Eve, humans around the globe and across history measure our lives with the construct of time. While these holidays help keep track of time on a cultural scale, our brains also do it automatically. Time estimation on large scales, facilitated by our use of calendars, gives us the ability to plan ahead and maintain a routine, but our innate circadian rhythm governs natural sleep-to-waking schedules as well as changes in body temperature and hormone levels. Without a doubt we time things, not only as a society through constructed uniform calendars, but also naturally as individuals. Of course we know how calendars work, but how do we explain our internal understanding of time?
Reactions to external stimuli are processed on the second-to-minute scale, motor functions such as balance and object manipulation rely on millisecond time estimates, and humans are adept at spotting timing and rhythm infrequencies within beats [1]. With minimal effort, you can probably estimate how long you’ve slept, how long an activity lasted, or the number of seconds it takes you to slam the brakes to avoid rear-ending another car. However, biological timekeeping is not always consistent and can be distorted. For example you’ve probably lost track of the number of times you have stared at a clock, with the seconds ticking by like a dripping tap or sat in an exam where 45 minutes feels like five. This distortion occurs even on a larger scale. In lockdown, for example, whole seasons passed in days that went by painfully slowly. The minutes themselves have not suddenly altered their steady march, but rather the way you experience them most likely has.
Timekeeping can be approached in multiple ways. The fields of experimental psychology, psychophysics, and neuroscience all having painstakingly investigated internal timekeeping and how it can become distorted. However, the exact neural details are yet to be discovered [2]. What we do know is that internal mechanisms provide a way for our brain to discriminate between different stimuli within seconds or minutes [1]. Our ability to keep time automatically allows the brain to fulfill a variety of behavioral functions and is helpful for a number of activities such as sports and music. Studies to investigate timekeeping and its nuances have been carried out on performance athletes and musicians alike [3]. If one thing is clear, it is that timekeeping relates to a myriad of functions and can affect much of our life.
In an effort to understand how we keep track of time, some researchers have turned to memory. These studies have shed light on where exactly in the brain timekeeping happens, with research focusing on the hippocampus—a structure in the brain highly involved in forming memories. The long-term perception of time within our brain is associated with locations and certain events and is known as episodic memory. In one study, researchers discovered that the hippocampus is responsible for placing events retrospectively on a mental timeline [4]. This discovery was achieved by using rats and fluorescent imaging to see what areas of the brain were active as the rats were presented with olfactory stimuli in successive trials and tasked with determining how much time had elapsed between trials. Without the temporal context provided by the hippocampus, it would be difficult for us (and the rats) to have an understanding of the events that make up our mental timelines [4].
On a more short-term scale, interval timing tasks are used in neuroscience to investigate our brain’s aptitude for timing stimuli at the levels of seconds and minutes. Interval timing allows us to predict events and adjust our behavior on the basis of passing time. When learning a task, we make predictions about how to act based on timing between relevant stimuli. In order for our brain to make these predictions, however, internal signals must exist [5]. The exact neural pathways that trigger these signals are not concrete, although it is clear that interval timing differs hugely from 24 hour timekeeping. Interval timing also operates with significantly more flexibility, allowing humans to time a range of stimuli on varying timescales, rather than the set 24 hours that circadian rhythms allows us [2]. Without interval timing, our lives would certainly be boring, as it provides us with skills that allow us to enjoy everyday activities such as playing sports or music [3].
Such is the importance of interval timing that the quest to find the most accurate internal clock model for short-term interval timescale has been ongoing for some time, with some success but with much scope for further expansion. Two such models are Scalar Expectancy Theory (SET) and Striatal Beat Frequency (SBF). Although these models differ in their mechanisms, both heavily involve the same neurotransmitter to transmit information between neurons: dopamine.
Developed from a reference frame of behavioral psychology, which looks to connect our brains and behaviour, the SET model offers an insight into cognitive timekeeping. In simple terms, the SET model looks at interval timing through the metaphor of a pacemaker [6]. This theoretical pacemaker emits pulses steadily and regularly. When an external stimulus triggers a switch within the pacemaker, the brain measures the number of pulses between the beginning and end of the stimulus. In order for this switch to be triggered, our attention must be commanded by a stimulus, like the sound of a car door closing, or the screech of a brake. Upon the appearance of the stimulus, dopamine, which controls the opening as well as the closing of the switch, is immediately released in our brain. This allows us to later determine the time that elapsed over the course of the stimulus [7].
While the pacemaker metaphor of the SET model is accessible, there are numerous models of other internal clock mechanisms which are more neurobiologically involved. These models are based on the theory that interval timing is controlled by oscillations within the brain [8]. Oscillations refer to synchronized rhythmic brain activity in the brain and spinal cord [9]. For example, the Striatal Beat Frequency (SBF) model illustrates interval timekeeping as oscillations between the cortex (the outermost layer of the brain) and the striatum (located in the basal ganglia at the base of the brain). It is unlikely that only one internal structure governs timekeeping, and the SBF model accounts for that by illustrating timekeeping across several dynamic areas. Dopamine bursts are still incorporated to signal the onset of a stimulus to the brain [7].
It is noteworthy that the SET model has been applied in the field of behavioural psychology, with the concept of a pacemaker proving helpful [7]. The SET has been successfully applied to ecological studies concerning foraging that investigated the way in which the SET model can provide a theoretical basis for decision making and internal timing. In the ecological study, starlings were trained to find food within a specified area and, upon finding hidden food, tasked with the decision to either expend the necessary time to retrieve it or to move on in search of more easily accessible food. Researchers used the SET model as a psychological reference model to account for the decision making the starlings did based on their internal perception of time. [10]. Biologically and in terms of anatomical details, the SBT model offers a more accurate account for how the brain processes time than the SET model.
Dopamine is a key player in both models for internal timekeeping. It is also sometimes colloquially associated with the feeling of happiness, due to its role in the reward and punishment system in our brain [11]. But the effects of dopamine go far beyond affect. Many factors can regulate dopamine, as change in our senses, emotions, and neurological disorders are all circumstances that can alter dopamine release. One study on the midbrain, a structure towards the base of the brain connected to the spinal cord, has shown how integral dopamine is in judgement of elapsed time [12]. In this study, mice were trained to perform tasks that required temporal discrimination. Dopamine levels were then measured and compared with the time taken for tasks to be completed. Activity within midbrain neurons were found to relate to variability in the speed of internal timekeeping in the mice [12].
Dopamine. Interval timing. As outlined, these are intricately linked within neural structures. If we have some understanding of how we measure time internally, we can pivot to see how that can change depending on the situation we are in.
Even when distracted by your emotion, your brain is still keeping time, and it is these emotions that skew your perception of time. Confrontation, anger, and threatening situations cause an overestimation of duration of events in humans and other animals, resulting from an increase of the speed of the interval timing clock. To account for this, the internal clock models come in handy. This perceived increase in speed can be represented by a higher number of dopamine pulses as per the SET model, or alternatively more oscillations via the SBF model. Attention levels of subjects tended to increase when presented with fear-inducing stimuli, accounting for increased internal clock speed—recall that attention is the trigger factor for the pacemaker’s switch in the SET model [10, 13]. Thus, when considering the duration of an event, it is important to consider the emotions that were at play over the course of the event, as these can distort your perception of time.
Experimentally, it is difficult to examine how our environment influences our perception of time, largely because of the emotions at play. Controlled laboratory environments are not ideal for reproducing real life situations, especially those that involve emotions because they are extremely subjective. Some scientists disagree with the SBT/SET theories because they require a specific set of neural mechanisms devoted to an internal clock. These scientists believe that no one set of neural mechanisms is devoted to internal timekeeping, making looking for an accurate internal clock model a redundant search [7]. The field of neuroscience is extensive and challenges have risen when studying time perception for various reasons, including the difficulties in designing experiments that accurately mimic real world timing tasks, as well as the highly variable nature of dopamine levels per person. Your reaction as a pedestrian to a busy highway involves many dynamic factors, which makes it hard to recreate this real life simulation in a controlled laboratory environment.
Outside of the lab collective, and even unprecedented events can occur that affect society’s emotions as a whole. Wars, economic recessions, pandemics, and terrorist attacks are useful and pertinent examples of this [14]. In the context of trauma research, the current COVID-19 pandemic has societal implications not only for public health but also for personal timelines [15]. The feeling of timelessness in the context of trauma exposure is known as “discontinuity” [16] and is applicable to the wide-spread trauma experienced throughout communities during the pandemic [15]. Discontinuity has accumulated as a direct consequence of the pandemic, as well as the domino effect induced by it. Economic uncertainty and strained social interactions are two examples of the secondary outcomes of the pandemic. The pandemic amplifies both [15].
The overnight dissolution of a normal routine has disordered the concrete psychological idea we once had of time and set events, as a relaxed Sunday night ritual became interchangeable with your Friday night party plans. A simple Google search will show the growth in interest in studying anxiety levels and resilience across population samples throughout the pandemic, with the COVID-19 Stress and Health Study being conducted by King’s College London, the University of Nottingham, and the University of Auckland one such example [17]. Accordingly, anxiety has an effect on perception of time, contextualized psychologically by the SET model, as does depression [18, 19]. It is not uncommon to hear of time becoming surreal or “slowing down” in times of grief or in patients with depression [7]. Hypotheses for widespread warping of time during national or regional lockdowns can be drawn from this, with time seeming longer as a result of increased levels of anxiety or depression. The lack of structure and general uncertainty surrounding how long the pandemic could last is a primary factor in this warping [18]. Although a lack of everyday structure was experienced, long term timescale indicators were also diminished as event after event was cancelled and the once solid start dates for academic years, conferences, and the like became irrelevant. Individual experiences during the pandemic have differed, the various disruptions caused conditions that can cause psychological confusion and contribute to the discontinuity of time [15].
The ways in which time is measured in the brain is not a one-size fits all situation, and we clearly have different mechanisms depending on the timescale our brain needs to operate in. An activity such as driving comes down to the millisecond: an event like a global pandemic lockdown, not so much. There is still much to be discovered about how exactly we process time, with no single pathway in our brains is governing it all. [20] Challenges within the field of neuroscience include finding a model of time processing that is interdisciplinary and can accurately connect the perception of time to motor processes and memory. Furthermore, there is much scope to apply the fields of genetics, pharmacology, and behavior to the neurobiology of time processing in real world situations [20]. Their advancement will also hopefully allow us to make sense of the distortion of the past year.
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