Human Brain
The way we sense and perceive time is essentially a construction of the brain

Time is not directly perceived, and so time perception is essentially a construction of the brain, which can therefore be manipulated and distorted in various ways (see the section on Temporal Illusions). Biopsychology, also sometimes known as behavioural neuroscience or psychobiology, studies the way the brain (at the level of nerves, neurotransmitters, brain circuitry and basic biological processes) does that.

Although another person’s perception of time obviously cannot be directly experienced or understood, there are techniques within psychology and neuroscience that can allow us to objectively study the phenomenon.

Internal Clock

The actual mechanism by which the brain perceives and processes the concept of time is complex and not fully understood. The judgement and perception of time is known to involve different part of the brain in a highly distributed system, and the cerebral cortex, cerebellum and basal ganglia are all involved to some extent. However, experiments on rats that have had their cortexes completely removed show that they can still successfully estimate a time interval of about 40 seconds, suggesting that time estimation may actually be a more low-level or sub-cortical process.

Neurotransmitters such as dopamine and norepinephrine (adrenaline) are integrally involved in our perception of time, although the exact mechanism is still not well understood. Some neuropharmacological research indicates that the human brain possesses some kind of “internal clock” (distinct from the biological or circadian clock), that is typically used to time durations in the seconds-to-minutes range. This timing mechanism appears to be specifically linked to dopamine function in the basal ganglia region of the brain, and norepinephrine also serves to slow down our internal clock (as do some drugs – see the section on Temporal Illusions).

Neuroscientist Warren Meck has carried out experiments showing how specific neurons near the base of the brain become active when a person is asked to estimate a duration of time. Neurochemicals are released by these cells that trigger other cells in the frontal cortex, which is what allows us to judge the passing of time. Meck also believes that the brain may have several different clocks working together but independently, and that the brain selects a “winner” from these different possible timings depending on the context.

In experiments with rats in conditions of sensory deprivation, psychologist Howard Eichenbaum discovered that certain neurons in the hippocampus region of the brain (an area important in memory function among other things) seem to fire in sequence almost like the ticking of a clock. For example, some cells fired when the rat first enters the sensory deprivation area, some in the next second, some in the third second, some in the fourth, etc. Over extended periods, some cells drop out of the “ticking”, some fire at different times, and some that were not firing earlier begin to fire. Eichenbaum has called these neurons “time cells”, similar to the “place cells” which are also found in the hippocampus (i.e. some cells seem to respond mostly to distance or location, while some respond mostly to time).

Delays in Time Perception

Although thought and perception appear to take no time at all, they are nevertheless constrained by the speed of neurological processes (e.g. the time for signals to leap across synapses, for action potentials to move along the axons of neurons, etc). The brain processes different types of sensory information (e.g. auditory, tactile, visual, etc) at different speeds using different neural architectures. But it appears to be able to overcome these speed disparities in order to achieve a temporally unified representation of the outside world, through a process sometimes referred to as temporal binding. As an example, if touch our nose and our toes at the same time, the signal from our distant toes must take longer to arrive at the brain than the signal from our nearby nose, but we perceive them as occurring simultaneously.

The brain also uses this process, also known as integration, to integrate our sense of time into a seamless and fluid experience. This works in a similar way to the way in which the brain makes our sense perceptions of the outside world into a complete and unitary picture, glossing over any discontinuities and inconsistencies (e.g. the way we perceive a smoothly-moving movie, rather than a series of discrete and separate frames, and the way we can usually piece together meaning from a partially heard sentence).

Neuroscientists have found that our brain actually waits about 80 milliseconds for all the relevant input to come in before we experience a “now”, rather like a time delay in broadcasting “live” television or radio. So, if the discrepancy in time between different inputs is less than about one-tenth of a second, the brain is able to process the different sensory input together. If two images are flashed in fast enough succession, therefore, we are not able to tell which came first and which second. To use a real-world example, so long as television audio and video signals are synchronized to within one-tenth of a second, viewers’ brains are able to automatically re-synchronize the signals; any more of a delay and a mis-synchronization becomes noticeable.

Time Expectations

There is an increasing body of research suggesting that the brain operates on some kind of an expected order and speed of events, and alterations to these expectations may lead to illusions like the kappa effect (see the section on Temporal Illusions). One study has shown how, when a video game player becomes used to a slight delay in computer mouse reaction time, and that delay is then removed, they may even experience a reversal in temporal perception judgement, feeling as though the effect on the screen happened just before they commanded it.

Other studies have shown that, when a pair of tactile stimuli are delivered to each hand in rapid succession, and the subject then crosses their arms across the body’s midline, they may experience the order of the stimuli as reversed. Interestingly, this reversing effect was not observed among congenitally blind subjects (as opposed to late-onset blind subjects), suggesting that the brain has a whole set of tactile/visual/spatial associations as regards time perception, which it develops during childhood.

Tests have shown that a person under hypnosis can judge time more accurately than the same person in a normal waking state. Unconscious time perception may therefore actually be more accurate than conscious time perception, possibly due to the lack of trained or conditioned responses and expectations that are present in the conscious state.

Mental Chronometry

The speed of neuron firing in the brain is also of interest to psychologists and neuroscientists for other reasons.

Mental chronometry is a technique used in experimental and cognitive psychology to assess how fast an individual can execute certain mental operations. This involves measuring a person’s reaction time, i.e. the elapsed time between the presentation of a sensory stimulus and their subsequent behavioural response, typically the pressing of a button or sometimes an eye movement or vocal response. This can then be used as a measure of cognitive processing speed and efficiency, from which an assessment of the person’s general intelligence or IQ can be made. Mental chronometry techniques are also used in other areas of cognitive and behavioural neuroscience and psychophysiology.

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