Steliana Yanakieva | 17 FEB 2021
Every day we experience the world through our senses – we see colours, hear sounds, taste, and smell food, and feel the sun or the rain on our skin. But how do we sense time? It is certain that we do experience a sense of time, both consciously, for example when we look at a clock, and subconsciously (e.g. in the order in which we do things), and that our sense of time is highly integrated with our other senses. However, even if we lost our ability to see, smell or hear, we would still have a sense of time passing.
Sensing time (time perception) seems to be a product of evolution. As far as we know, humans are the only species consciously aware of the passage of time and our own mortality. Despite how it might seem, we do not perceive time itself, but rather we perceive changes in events occurring in time. Hence, unlike our other senses, time perception does not have a dedicated sensory system. Instead, time is a construction of the brain that enables us to perceive a unified sensory picture of the world, which underlies our conscious experience. This idiosyncratic sixth sense is fundamental to our understanding of sequential events, allowing us to perceive our lives as an uninterrupted stream of events.
We are only truly aware of a few seconds of time at any one moment, a phenomenon termed “specious present” by E.R. Clayand and later elaborated by William James (James, 1890). For example, whilst we can plan for events that have not yet occurred, we are incapable of perceiving durations in the future. In fact, durations of events (intervals) can only be perceived after they have ended, so technically the “specious present” moment you are aware of has already happened. David Eagleman (2009) explains this in his famous essay “Brain Time”. He argues that different types of information are not only processed by distinct neural pathways, but also at different speeds. In order to perceive a continuous unified picture, our brain has to overcome this difference by waiting for the slowest sensory information to arrive before making us aware of what is happening ‘now’. This delay of around 100 milliseconds allows us to watch TV unaware of the fact that our brain processes auditory stimuli faster than visual stimuli. So, if you have ever experienced the frustration of unsynchronised TV audio and video, there is a delay of over 100 milliseconds, that your brain is programmed to pay attention to.
In a cognitive sense, attention is a mental process that allows you to selectively attend to information relative to completing a task. Hence, if you are in a boring class, thinking about how long you have got to the end of it – you will be more aware of the passage of time and therefore overestimate its duration (e.g. time appears to pass slower). On the other hand, time will appear to pass much faster when you are having fun. This goes to show that intact perception of small intervals of times is essential to our day-to-day functioning.
Mechanisms of Time Perception
Durations in the milliseconds to seconds ranges, in psychology, are referred to as interval timing. Impairments have been observed in psychiatric disorders marked by disruptions of consciousness, such as schizophrenia (Allman & Meck, 2012), dissociative disorders (Simeon, et al., 2007; Spiegel et al., 2013), Parkinson’s disease (te Woerd et al., 2014; Gulberti et al., 2015) and Huntington’s disease (Beste et al., 2007). Specifically, impairments in time perception are associated with symptoms such as tremor and hallucinations. Therefore, understanding the neural mechanisms of interval timing would allow scientists to develop new therapies for these symptoms underlined by timing deficits. Over the years, there have been several theories about the neural mechanism of time perception (Gibbon, 1977; Matell & Meck, 2004), and even though scientists cannot agree on a unified model of interval timing, one thing we know for sure is that time perception is a multifaceted process, dependant on other cognitive process, particularly attention.
Both schizophrenia and Parkinson’s disease are associated with aberrant dopamine concentrations in the brain (Brisch et al., 2014; Davie, 2008), which, interestingly, have been linked to the speed of our “internal-clock” (Cheng et al., 2007). Excessive dopamine, as seen in schizophrenia, appears to lead to overestimation of time intervals whilst dopamine depletion, as seen in Parkinson’s disease, appears to lead to its underestimation (Hass & Durstewitz, 2016; Meck, 1996). We can see these effects without relying on neurological conditions because stimulant drugs, such as caffeine, cocaine and amphetamines, increase brain dopamine levels and can lead to overestimating time intervals, while depressant drugs, such as ketamine, have the opposite effect, likely through the effects such psychoactive substances have on attention.
One way of understanding this phenomenon is that psychoactive drugs will either excite or inhibit the firing of dopaminergic neurons in the brain. Whilst stimulants increase the rate of neuronal firing, allowing the brain to register more events within a given time interval and leading to the perception of time speeding up, inhibitory drugs decrease the firing rate of neurons, resulting in a slowing down perceived time. However, since such drugs also impact attention, it is difficult to disentangle whether the observed effects on timings are due to the dopaminergic manipulation, per se, or if they are caused indirectly due to increased/decreased attention to time.
A promising solution to this problem appears to be microdosing of hallucinogens, such as LSD, which appear to alter interval time
perception without marked disturbances to attention, concentration, and memory (Yanakieva, et al., 2018).
In a cognitive sense, attention is a mental process that allows you to selectively attend to information relative to completing a task.
Overall, our perception of time is one of the most fascinating sensory experiences. Despite research literature on timing dating back 150 years, how our brains process time is still a mystery. Can understanding the neural basis of time perception unravel the hard problem of consciousness? Can it explain the altered states of consciousness observed in schizophrenia and the dissociative disorders? Are animals aware of passage of time and does this make them conscious? These are just a few of the questions that remain to be answered. However, each scientific experiment raises more questions than answers, all highly intriguing and deserving of attention.
Editors: Matt Higgs and Uroosa Chughtai
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