Researchers have come up with convincing models for how time is measured by memory

PHYSICAL EVENTS proceed on an objective timeline, biological cycles are regulated by internal clocks [pacemakers] but humans are aware of the passage of time and our perception of time duration can vary a lot depending on circumstances. This is explained by cognitive psychologist Dan Zakay in current edition of The Psychologist. Please stick with this article to the end, when you will be rewarded with bonus information of inestimable value!

When you recall the duration of a past experience you must rely on your memory of the event – “retrospective timing”. The main psychological model that explains retrospective timing is the “contextual change model”. You estimate the duration of the event by recalling the data stored in your memory of the event. The more data stored, the longer the estimation of the duration of the event.

However, different amounts of information can be stored in memory during identical clock-time intervals, depending on several factors, eg the intensity of the information processing in which one is engaged. The higher the intensity, the longer the duration seems to be. In a classic experiment, participants were asked to memorise either a simple [a circle] or complex figure . Although the clock-time allocated to each task was identical, participants later estimated the duration of memorising the complex shape to be significantly longer than for the simple shape.

Other factors that lengthen retrospective timing estimations are the amount of contextual changes that occur during the interval, or the level of interval segmentation into sub-intervals. These are interpreted by the contextual change model simply as adding to the amount of information stored in memory.

The other main way of estimating time duration is “prospective timing”. Consider the task of memorising the simple versus the complex figure again, but this time you are told in advance that after completing the task you have to estimate its duration. You will now use prospective timing and the result will be the opposite to that of by retrospective timing – you will estimate the duration of memorising the complex figure as shorter than memorising the simple figure. Retrospective and prospective timings are based on different cognitive processes, with the former based on attentional processes, not memory processes. The “attentional gate model” explains prospective timing.

Biological cycles are measured by an internal clock that emits steady signals. The signals emitted over a given interval are counted by something called an “accumulator”. The counts can be stored in memory by an animal and used to repeat certain durations by counting signals until they match the count stored in the memory. No awareness of the passage of time is necessary.

Humans however are aware of the passage of time and are easily influenced by attentional demands over a target interval. Humans have an “attentional gate” through which the signals from the clock must pass in order to reach the accumulator. If the individual decides that the passage of time is important , then the attentional gate is opened wide and signal accumulation is maximised. If the passage of time is unimportant then the gate is narrowed and fewer signals are accumulated. Assuming that the estimate of time duration depends on the count registered by the accumulator, it is easy to see that the same objective time duration, eg 15 minutes, will seem longer when waiting for interview that while relaxing. And, memorising the complex figure requires more attentional resources than memorising the circle, leading to a narrower gate and a lower signal count.

And now, a little homework. Using the attentional gate model of prospective timing, explain why “a watched pot never boils”, why earthquakes feel longer than they are, and why the “return trip” always feels shorter.

Finally, here is a “guaranteed” way to lengthen your life. Childhood holidays seem to last forever, but as you grow older time seems to accelerate. “Time” is related to how much information you are taking in – information stretches time. A child’s day from 9am to 3.30pm is like a 20-hour day for an adult. Children experience many new things every day and time passes slowly, but as people get older they have fewer new experiences and time is less stretched by information. So, you can “lengthen” your life by minimising routine and making sure your life is full of new active experiences – travel to new places, take on new interests, and spend more time living in the present – see Making Time by Steve Taylor .

William Reville is an emeritus professor of biochemistry and public awareness of science officer at UCC. See http://understandingscience.ucc.ie/