A temporal illusion is a distortion in the perception of time that occurs for various reasons, such as due to different kinds of stress. In such cases, a person may momentarily perceive time as slowing down, stopping, speeding up, or even running backwards, as the timing and temporal order of events are misperceived. When we say that time slows down, what we actually mean is that our internal clock speeds up, which gives the impression that time in the rest of the world slows down.

Kappa Effect

The kappa effect is a form of temporal illusion which can be verified by experiment. It refers to occasions when the temporal duration between a sequence of consecutive stimuli is thought to be relatively longer or shorter than its actual elapsed time, as a result of the spatial separation between consecutive stimuli.

As an example, if three light sources are flashed successively in the dark with equal time intervals between each of the flashes, the temporal interval between the two light sources that are closer together tends to be perceived to be shorter than that between the two light sources that are further away from each other (even though the time intervals are actually equal). As another example, if two consecutive journeys take an equal amount of time, the journey that covers more distance may appear to the individual perceiver as taking longer than the journey covering less distance, even though they actually take an equal amount of time.

Several theories have been put forward to explain the kappa effect, mainly based on the brain’s prior expectations about stimulus velocity or speed. It seems that the brain is wired to expect temporal intervals that would produce constant velocity (i.e. uniform motion). There is a related spatial illusion called the tau effect, where the spatial separation between stimuli is constant and the temporal separation is varied (as might indeed be expected if the explanation of the effects concerns expected velocity).

Chronostasis

Chronostasis, also known as the stopped clock illusion, is where the first impression following the introduction of a new event or task demand to the brain appears to be extended in time. The most commonly encountered example is when the second hand of an analog clock appears to freeze in place for a short period of time after a person initially looks at it. A similar illusion can also be found within the auditory system.

It appears to occur as a result of a disconnect in the communication between visual sensation and perception, specifically triggered by quick eye movements (technically known as saccades) which can disrupt this flow of information, although research is still ongoing and the complete mechanism is still not well understood.

To some extent, though, this is just an exaggerated example of a more commonly experienced phenomenon (see the Oddball Effect below). Psychological testing has shown that reactions to exposure to ANY new auditory or visual stimuli take longer than reactions to known or repeated stimuli, suggesting an internal slowing down of time perception, perhaps to accommodate increased brain arousal, activity and attention.

Oddball Effect

The so-called “oddball effect” occurs when the brain experiences something unusual or out of the normal run of events. In this case, the brain pays special attention and spends more time processing the event, recording as much information as possible on the novel circumstances, which can lead to a feeling that time has slowed down.

This is particularly apparent when a person perceives an unusual or stressful sensory stimulus, particularly one that appears to be a potential threat or a possible mate. For example, time seems to slow down when a person skydives or bungee jumps, or when a person suddenly and unexpectedly senses the presence of a potential predator or mate. Similar effects occur when emotions of fear are artificially generated by watching a horror film. It is thought that changes in the neurotransmitter norepinephrine (adrenaline) is responsible for this slowing down our internal clock.

There may be an evolutionarily advantage to such a response, in that it may enhance our ability to make quick and good decisions in moments that may be of critical importance to our survival (or procreation), where one has a short space of time in which to decide whether to attack or run away (fight or flight). To a bystander watching a life-threatening situation such as an accident, time is moving at a normal speed, but the individual in the accident may perceive time as having slowed down, allowing them “more” time to think and act during these potentially life-threatening events. David Eagleman has carried out experiments that show quite clearly that, in high adrenaline situations, the brain is actually capable of processing information significantly faster than under normal circumstances.

However, it is still not completely clear whether the effect is actually a function of a real increase in time resolution during the event, or just an illusion of memory of emotionally important or salient events. It looks increasing probable that it may only be a retrospective assessment that brings a person to feel that time ran in slow motion during a life-threatening or stressful event. More specifically, the brain does its best to record everything possible about a stressful, threatening or exciting situation in memory, in case they may be useful in future situations. The more memories that are accumulated about the situation, the more data there is to subsequently “leaf through”, giving the impression that more time must have passed.

Emotional States

Positive or negative emotions can affect the subjective perception of time. “Time flies” is a common proverbial expression, since its first appearance in Vergil’s Georgics in 29BCE as “tempus fugit” (literally “time flees”). It is particularly commonly encountered in the phrase ”time flies when you are having fun”, which refers to the way that time seems to pass very quickly, and without notice, when one is otherwise occupied with something enjoyable. The key here seems to be partly that one is not attending to, or consciously thinking about, the passing time, but also that the underlying emotion is one of happiness and enjoyment, particularly enjoyment with a motivation or a goal. A tedious or unpleasant task, on the other hand, appears to take a disproportionately long time to complete.

Even the perception of another person’s emotions can be enough to slightly change our sense of time. The neurological mechanism here may be quite different, though, resting on a process called embodied cognition (an internal process that mimics or simulates another’s emotional state) and mirror neurons (a neuron that fires both when a person acts and when they observe the same action performed by another).

Unexpectedly and perhaps counter-intuitively, studies have shown that sensory deprivation (such as in experimental isolation chambers) tends to compress the experience of time, so that minutes, hours and days seem to pass about twice as fast as usual. Time spent under these unpleasant and undeniably tedious conditions paradoxically feels shorter than normal time, and some other effect seems to be at work here.

Ageing

The subjective perception of the passing of time tends to speed up with increasing age in humans. Older people often complain that the years (and even the days) pass much more quickly than they used to. This same effect also causes older people to underestimate given intervals of time. For example, one study showed how estimates of a 3 minute period among a group of 19-24 year olds yielded an average of 3 minutes 3 seconds, while the estimates of a group of 60-80 year olds averaged 3 minutes 40 seconds.

Various explanations for this common experience have been put forward, including: the fact that younger people are still living through new and interesting (rather than repeated and routine) experiences, requiring more neural resources and brain power, and are less subject to the neural adaptation experienced by older people; the fact that a single day (or an hour) represents a much larger proportion of the lives of young people as compared to older people; the general slowing down of most organic processes in the bodies of older people; the lower dopamine levels in the ageing brain; etc. It may even be that our internal biological clocks slow down in some way as we age.

Very young children appear to have little or no conception of the passing of time, which is thought to be due to the continuing maturation of the prefrontal cortex and hippocampus in young children.

Psychoactive Drugs

The brain’s judgement of time can be affected and impaired by various psychoactive drugs. This probably occurs because of the way such drugs affect level of activity in the brain of neurotransmitters such as dopamine and norepinephrine. They either excite or inhibit the firing of neurons in the brain, with a greater firing rate allowing the brain to register the occurrence of more events within a given interval (so that time seems to slows down), and a decreased firing rate reducing the brain’s capacity to distinguish events occurring within a given interval (time speeds up).

For example, stimulants (or “uppers”), which are typically intended to enhance alertness, wakefulness and locomotion, tend to lead to an overestimation of time intervals as time seems to slow down for the user. Caffeine, nicotine, amphetamines, cocaine, etc, are examples of stimulants. On the other hand depressants (or “downers”), which typically decrease arousal and mental/physical function, have the opposite effect, as time seems to speed up and time intervals may be underestimated. Alcohol, cannabis, heroin and other opioids are all depressants.

Neuroscientist Warren Meck has carried out experiments with trained rats to demonstrate how drugs affect the rats’ internal clocks and their estimation of time periods. Rats on cocaine perceived a 12-second period as being around 8 seconds, while rats on marijuana estimated about 16 seconds for the same period.

Experiments have even been carried out to ascertain whether it is possible to use drugs to “speed up” our mental processes relative to a duration of physical time, thus making us more mentally productive and effectively allowing us to learn more per minute. However, none of these experiments have yielded any success yet.

Meditation

Given its emphasis on moment-to-moment awareness, it seems logical that mindfulness meditation (which purports to improve attention, working memory capacity, and reading comprehension, among other things) would alter time perception to some extent, and experiments have indeed shown that it may lead to a relative overestimation of time durations.

Body Temperature

Some studies have shown that time perception may speed up as body temperature rises, and slow down as body temperature lowers. The neural and physiological mechanisms for such an effect are unclear.

Clinical disorders

Medical conditions that result in, or are caused by, abnormal dopamine levels in the brain (e.g. Parkinson’s disease, schizophrenia, attention deficit hyperactivity disorder, etc) may be linked to noticeable impairments in time perception. For example, in time estimation tasks, children with ADHD feel that time passes very slowly for them. Some Parkinson’s patients find it difficult to clap to a regular beat, despite their own perception of having completed the task quite effectively. Schizophrenic patients may stop perceiving time as a flow of causally linked events, and there is often a delay in time perception in schizophrenic patients compared to normal subjects.

The brain’s internal clock, which is typically used to time durations in the seconds-to-minutes range, has been shown to be specifically linked to dopamine function in the basal ganglia region of the brain, so it is perhaps no surprise that dopamine abnormalities might affect time perception in this interval range.

Some autistic savants have an incredibly developed and accurate sense of the passage of time, and may be able to tell the exact time to the minute at any point in the day or night, or to state exactly how much time has passed, without looking at (or even being able to read) a clock.

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