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Guest post: Thinking in time

9 Feb, 2011

Do we 'decode' changes in the clouds?
Where does our sense of time come from? Recent research suggests external factors play a key part in how our brain perceives the passage of time, writes Misha Ahrens.

Our sense of time passing is important, and typically assumed to originate from timekeeping circuitry within the brain. But a dedicated ‘brain clock’ has not yet been found. We know that our sense of time can be distorted by external events – for example, if people watch a visual scene lasting one second, but showing fast motion, it can appear to last 1.2 seconds.

Our research asked where our sense of time comes from, and whether it partly originates from the outside world. When we judge quantitative aspects of the world, we combine information from multiple senses. Watching a person’s lips, for instance, helps in understanding the verbal content of the sounds they produce. We wondered if our sense of time works in a similar way. Does watching the outside world help shape our sense of duration?

To answer this question, my PhD supervisor Maneesh Sahani and I first showed that, theoretically, there is information about the passage of time in visual scenes we typically observe. Take, for example, the movement of clouds. Larger changes encourage us to believe in longer durations. Because we have an unconscious model for the way clouds change over time and how fast they change shape, on average, we can ‘decode’ the changing of clouds in any particular instance to tell us how fast time might be passing.

Two key experiments suggested that, indeed, part of our time perception is anchored in the outside world. In one experiment, participants watched small circles of light appear on a screen twice in a row. They were then asked to say which appearance lasted longer. When the circles were accompanied by a mottled pattern programmed to change randomly, but at a regular rate, participants’ judgments about the circles were more accurate. This suggests that they used the rate of change in the patterns to help judge the passing of time.

In the second experiment, we asked participants to judge how long a brief presentation of similar mottled patterns lasted, but we varied the rates at which those patterns changed. When the patterns changed faster, participants judged the appearance of the patterns to have lasted longer – again showing that sensory change shapes our sense of time.

People have certain expectations of how fast the world (particularly visual scenes) changes. We argue that, through these expectations, people actually use visual perceptions of the world as a type of clock. Watching a clock or hourglass clearly improves one’s accuracy of how quickly time passes, but we show, theoretically, that it is possible to also derive information about time from, say, watching clouds change shape. Comparing the changes we see to the ‘average’ rate we expect helps us judge how much time has passed, and refines our internal timekeeping. Similarly, it appears that watching a stimulus, even if it is fairly random, can improve one’s accuracy at judging time intervals.

The fact that we can bias people’s perception of time does not fit with the theory of a rigid internal brain clock. Instead, time perception is rooted not only within the brain, but also in our experience of the outside world.

  • Ahrens MB, & Sahani M (2011). Observers exploit stochastic models of sensory change to help judge the passage of time. Current biology : CB, 21 (3), 200-6 PMID: 21256018

  • Misha Ahrens

    Misha Ahrens is a Sir Henry Wellcome Postdoctoral Fellow at the University of Cambridge and conducted this study with Dr Maneesh Sahani of the Gatsby Unit, UCL.

    Image credit: Mark, Vicki, Ellaura and Mason on Flickr

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