The human body does not have time sensing receptors; thus, there is no association
between time and sensory-specific systems[1]. We have receptors under the skin for sensing touch, temperature, pain, pressure,
and vibration. In addition, we have proprioceptors in muscles and tendons, which continuously
generate action potentials to the central nervous system with information about the
length and tension of muscles; this signaling allows us to make accurate movements.
We also have highly specialized sensory systems, such as vision, hearing, olfaction,
and gustation, which provide us with environmental information. Although we are not
endowed with time receptors[2]
,
[3], our brains have an inherent ability to process time through the functions of neuronal
mechanisms and neurotransmitters.
We carry watches to know the hour precisely, but we do not need similar devices to
determine whether it is cold or whether a sound is loud or soft. Similarly, our perception
of time refers to subjective experience and depends on an internal clock. The subjective
duration of events may be based on brain units that accumulate over time[4]
,
[5]
,
[6]
,
[7]. It is commonly said that “time passes faster” in older people[8], which may be due to central dopamine-related mechanisms and memory[9]
,
[10]. This study aimed to estimate the passage of time in different age groups, to test
the truth of this saying.
METHOD
This study included 233 healthy subjects, including 129 women. Three groups were formed
according to age: Group 1 comprised 86 subjects, aged 15-29 years; the mean age was
22.4 years. Group 2 comprised 76 subjects, aged 30-49 years; the mean age was 38.9
years. Group 3 comprised 71 subjects, aged 50-89 years; the mean age was 59.7 years.
Each of the participants was asked to count mentally with eyes closed the passage
of 120 s (2 min). When the participant finished, one of the examiners marked down
the actual elapsed time; thus, the mental time or “brain clock” was compared with
the actual time interval according to the clock. For instance, if a subject count
mentally 120 s (“brain clock”) in just 90 s (“real clock”), he/she time perception
is faster. For them, time goes faster. All subjects were conscious, fully oriented
with no past history of neurological disease or memory complaints. In all cases, the
examiners checked with them before the test if they had an understanding of time and
the meaning of 1 second. The use of drugs, in general, was not a criterion for exclusion,
because it is difficult to find older individuals that do not take medications. The
use of anxiolytics, in small doses, and antidepressants, was accepted, but these individuals
represented less than 10% of the study cohort. Individuals were also stratified by
education level.
All data were entered in a worksheet and tabulated for the 3 age groups. Statistical
analyses for multiple comparisons between groups were performed with the ANOVA-Bonferroni
test. The study was approved by the local ethics committee, and all subjects signed
informed consent forms.
RESULTS
Within each group, the times elapsed during mental counting followed a normal (Gaussian)
distribution. Mean elapsed times were: G1, 114.9 ± 35 s; G2, 96.0 ± 34.3 s; and G3,
86.6 ± 34.9 s. Multiple comparisons with the ANOVA-Bonferroni test showed that G3
times were significantly different from G1 times (p < 0.001; [Figure]).
Figure Mental perceptions of the passage of 120 s in different age groups. Group 1: aged
15-29 years; Group 2: aged 30-49 years; Group 3: aged 50-89 years. Values indicate
the mean (upper line) and median (lower line) of each group; error bars indicate standard
deviations.
DISCUSSION
Our data clearly confirmed the assertion that “time passes faster for older individuals”.
We found that the mental perception of the passage of 120 s among older people (>
50 years) was significantly reduced compared to the perception in younger people (<
30 years). The difference between these age groups was 24.6% (28.3 s) for mean times
and 28.4% (32.5 s) for median times. These data indicated that the perception of time
passage was accelerated in aging. This phenomenon may be related to two mechanisms:
(1) a lack of new experiences[11] and (2) a reduction in dopamine neurotransmission[9]
,
[10]
,
[12].
New experiences
The time it takes to learn something new (“the first time”) is always subjectively
prolonged, such as a first sexual relationship, a first job, a first trip without
parents, or the first experience of living away from home[13]. Novelty has a strong impact on memory. Generally, we can vividly remember the beginning
of each new experience that occurred between ages 15 and 25 years. This may partly
explain why a random day can seem longer for a child than an adult[14]. Most external and internal experiences are new to children, and most experiences
are repetitive for adults.
Dopamine and neuropathways
The ability of our internal clock to estimate the passage of time depends on both
attention and memory[9]. For example, estimating the time necessary to perform a given task requires both
concentration and memorization of a sequence of information. Concentration depends
on the function of dopamine neurotransmission in the basal ganglia. Memory depends
on acetylcholine neurotransmission in brain areas related to memory in the hippocampal-prefrontal
cortex axis[2].
The neurotransmitter dopamine was associated with variations in time at the level
of seconds and minutes[6]. Experimental studies have shown that rats can estimate time intervals up to 40
s, despite the removal of their entire cerebral cortex. Those findings indicated that
time estimation must be a subcortical process[6].
In humans, time intervals are generally overestimated with the use of stimulants and
underestimated with the use of depressants and anesthetics. Patients with Parkinson
disease, schizophrenia, or attention-deficit hyperactivity disorder, which are all
associated with abnormalities in brain dopamine levels, typically exhibit abnormalities
in time perception[15]
,
[16]. Stimulants that increase the function of dopamine in the brain have the effect
of accelerating the perception of time. Conversely, antipsychotic drugs, or emotions
of sadness and depression, block dopamine receptors and have the opposite effect[17].
The consciousness of time develops during childhood, when we form the capacities for
both attention and short-term memory. The integration of these capacities depend on
the slow maturation of the hippocampus and prefrontal cortex[18]. For example, when we become accustomed to a certain time period, e.g., a class
of 30 min, that time depends on memory (and the neurotransmitter, acetylcholine).
Indeed, it does not take long for us to notice that a red traffic light is defective
after it fails to turn green, because the memory of that time is stored[9]. On the other hand, when we are at a party and someone asks how long we have been
there, we imagine a value in the absence of memory, and variations can be extensive
(dependent on dopamine levels).
Imagined time also depends on behavioral mechanisms. The perception of time during
a stressful or fearful situation seems much longer than the actual time that passes[4]
,
[19]
,
[20]. Terror movie scenes are imagined as lasting 2-fold, and sometimes 3-fold longer
than the actual time that passes. This was demonstrated in a classic study, where
people with arachnophobia were invited to look for spiders for 45 s – an activity
that elicited intense fear; those individuals greatly overestimated the time elapsed[13]. Similarly, when someone becomes angry with the lengthy service in call centers,
the time is substantially overestimated, frequently by two-fold.
Why does time seem to accelerate as we age?
The apparent rapidity of time is a major cognitive illusion[8]. When we are reminded of school holidays, or for example, when we learn to swim
or fly a kite, the memory seems endless. But that is an illusion. Adulthood does not
hold the constant, never-ending discovery of new things that is inherent in childhood.
Also, the higher the demand required of a cognitive task, the greater the perceived
duration. Thus, repeated stimuli appear briefer than new stimuli of equal duration.
As a result, it is possible that learning new things can slow the internal sense of
time. Moreover, among healthy individuals, aging is typically connected to reduced
interest in learning new things that require great effort; thus, a reluctance to learn
may contribute to an accelerated internal sense of timing.
In conclusion, we found a significant reduction in the mental perception of the passage
of 120 s for older people compared to younger people. The reasons for this phenomenon
may be explained by differences in dopaminergic and/or cholinergic pathways or the
integration between these two pathways. Future studies should investigate these possibilities.
