On some days, time flies by, while on others it seems to drag on. Worn out brain cells may explain why the endless summers of childhood turn into whole years hurtling past as we get older, according to Japanese and US scientists. 

They scanned the brains of people watching a grey circle for set lengths of time, 30 times in a row. They were then shown another image.

The researchers say that when people viewed the new image for the same length of time they’d been watching the grey circle, the activity of brain cells in an area of the brain called the supramarginal gyrus (SMG) dropped.

These cells fire in response to a specific length of time, and the researchers say the repeated exposure to a stimulus of a fixed duration wears them down. Since other brain cells continue firing normally, the wear and tear can skew our perception of time, they conclude.

A new study from JNeurosci reveals why: time-sensitive neurons get worn out and skew our perceptions of time.

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.

Hayashi and Ivry measured brain activity with fMRI as human participants engaged in a time comparison task. Healthy adult participants viewed a visual adaptor (a grey circle) for a set length of time, 30 times in a row. After this adaptation period, they were shown a test stimulus and indicated its duration. 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. The extent of skewed time perception correlated with how much the activity in the SMG decreased — greater fatigue led to greater time distortion.

Download the full-text PDF: Duration-Selectivity in Right Parietal Cortex Reflects the Subjective Experience of Time

From: Society for Neuroscience