Time flies when you’re having fun, as the old saying goes, and now scientists have worked out why – along with why time crawls when you’re bored.
It turns out that our brain cells become ‘worn out’ from certain tasks, skewing our sense of time.
Neurons in the supramarginal gyrus (SMG), which process touch and speech, become fatigued when exposed to the same thing again and again, over a long period.
This affects people’s sense of time, researchers discovered in tests where volunteers performed tasks on a computer screen.
Corresponding author Dr Masamichi Hayashi, a psychologist at University of California, Berkeley, said: "These results indicate duration-tuned neural populations in the right of the SMG reflect the subjective experience of time."
The findings, published in the journal JNeurosci, were based on 18 men and women aged 18 to 27 who underwent brain scans as they performed tasks on a computer screen.
They first viewed a 'visual adaptor' – a grey circle – for a set length of time 30 times in a row.
After this period, they were shown a test stimulus and indicated its duration.
Hayashi said: "If the adaptor duration was long, the participants underestimated time; if the adaptor duration was short, they overestimated time."
Activity in the SMG decreased when the adaptor and test stimulus were similar in length, indicating neuron fatigue.
Hayashi said: "The extent of skewed time perception correlated with how much the activity in the SMG decreased – greater fatigue led to greater time distortion.
"Neurons in the supramarginal gyrus (SMG) fire in response to a specific length of time.
"If repeatedly exposed to a stimulus of a fixed duration, the neurons fatigue. Since other neurons continue firing normally, our subjective perception of time becomes skewed."
The supramarginal gyrus is a portion of the parietal lobe near the centre of the brain.
Hayashi said: “To investigate the neural basis of subjective time, we conducted an fMRI (functional magnetic resonance imaging) study, using an adaptation procedure that allowed us to manipulate perceived duration while holding physical duration constant.
"Regions within the occipital cortex and right parietal lobe showed duration tuning that was modulated when the test stimuli were similar in duration to the adaptor.
"Moreover, the magnitude of the distortion in perceived duration was correlated with the degree of duration tuning modulation in the parietal region.
"These results provide strong physiological evidence that the population coding of time in the right parietal cortex reflects our subjective experience of time."