Hack18.When Time Stands Still

Hack 18. When Time Stands Still

Our sense of time lends a seamless coherence to our conscious experience of the world. We are able to effortlessly distinguish between the past, present, and future. Yet, subtle illusions show that our mental clock can make mistakes.

You only have to enjoy the synchrony achieved by your local orchestra to realize that humans must be remarkably skilled at judging short intervals of time. However, our mental clock does make mistakes. These anomalies tend to occur when the brain is attempting to compensate for gaps or ambiguities in available sensory information.

Such gaps can be caused by self-generated movement. For example, our knowledge about how long an object has been in its current position is compromised by the suppression of visual information [Hack #17] that occurs when we move our eyes toward that objectwe can have no idea what that object was actually doing for the time our eyes were in motion. This uncertainty of position, and the subsequent guess the brain makes, can be felt in action by saccading the eyes toward a moving object.

2.7.1. In Action

Sometimes you'll glance at a clock and the second hand appears to hang, remaining stationary for longer than it ought to. For what seems like a very long moment, you think the clock may have stopped. Normally you keep looking to check and see that shortly afterward the second hand starts to move again as normalunless, that is, it truly has stopped.

This phenomenon has been dubbed the stopped clock illusion. You can demonstrate it to yourself by getting a silently moving clock and placing it off to one side. It doesn't need to be an analog clock with a traditional second hand; it can be a digital clock or watch, just so long as it shows seconds. Position the clock so that you aren't looking at it at first but can bring the second hand or digits into view just by moving your eyes. Now, flick your eyes over to the clock (i.e., make a saccade [Hack #15] ). The movement needs to be as quick as possible, much as might happen if your attention had been grabbed by a sudden sound or thought [Hack #37] ; a slow, deliberate movement won't cut it. Try it a few times and you should experience the "stopped clock" effect on some attempts at least.

Whether or not this works depends on exactly when your eyes fall on the clock. If your eyes land on the clock just when the second hand is on the cusp of moving (or second digits are about to change), you're less likely to see the illusion. On the other hand, if your eyes land the instant after the second hand has moved, you're much more likely to experience the effect.

2.7.2. How It Works

When our gaze falls on an object, it seems our brain makes certain assumptions about how long that object has been where it is. It probably does this to compensate for the suppression of our vision that occurs when we move our eyes [Hack #17] . This suppression means vision can avoid the difficult job of deciphering the inevitable and persistent motion blur that accompanies each of the hundred thousand rapid saccadic eye movements that we make daily. So when our gaze falls on an object, the brain assumes that object has been where it is for at least as long as it took us to lay eyes on it. Our brain antedates the time the object has been where it is. When we glance at stationary objects like a lamp or table, we don't notice this antedating process. But when we look at a clock's second hand or digits, knowing as we do that they ought not be in one place for long, this discord triggers the illusion.

This explanation was supported and quantified in an experiment by Keilan Yarrow and colleagues at University College, London and Oxford University.1 They asked people to glance at a number counter. The participants' eye movements triggered the counter, which then began counting upward from 1 to 4. Each of the numerals 2, 3, and 4 was displayed for 1 second, but the initial numeral 1 was displayed for a range of different intervals, from 400 ms to 1600 ms, starting the moment subjects moved their eyes toward the counter. The participants were asked to state whether the time they saw the numeral 1 was longer or shorter than the time they saw the subsequent numerals. Consistent with the stopped clock illusion, the participants consistently overestimated how long they thought they had seen the number 1. And crucially, the larger the initial eye movement made to the counter, the more participants tended to overestimate the duration for which the initial number 1 was visible. This supports the saccadic suppression hypothesis, because larger saccades are inevitably associated with a longer period of visual suppression. And if it is true that the brain assumes a newly focused-on target has been where it is for at least as long as it took to make the orienting saccade, then it makes sense that longer saccades led to greater overestimation. Moreover, the stopped clock illusion was found to occur only when people made eye movements to the counter, not when the counter jumped into a position before their eyesagain consistent with the saccadic suppression explanation.

You'll experience an effect similar to the stopped clock illusion when you first pick up a telephone handset and get an intermittent tone (pause, beeeep, pause, beeeep, repeat). You might find that the initial silence appears to hang for longer than it ought to. The phone can appear dead and, consequently, the illusion has been dubbed the dead phone illusion.

The clock explanation, however, cannot account for the dead phone illusion since it doesn't depend on saccadic eye movement.² And it can't account, either, for another recent observation that people tend to overestimate how long they have been holding a newly grasped object,³ which seems like a similar effect: the initial encounter appears to last longer.

One suggestion for the dead phone illusion is that shifting our attention to a new auditory focus creates an increase in arousal, or mental interest. Because previous research has shown that increased arousalwhen we're stressed, for instancespeeds up our sense of time, this could lead us to overestimate the duration of a newly attended-to sound. Of course, this doesn't fit with the observation mentioned before, that the stopped clock illusion fails to occur when the clock or counter moves in front of our eyessurely that would lead to increased arousal just as much as glancing at a clock or picking up a telephone.

So, a unifying explanation for "when time stands still" remains elusive. What is clear is that most of the time our brain is extraordinarily successful at providing us with a coherent sense of what happened when.

2.7.3. End Notes

1. Yarrow, K., Haggard, P., Heal, R., Brown, P., & Rothwell, J. C. (2001). Illusory perceptions of space and time preserve cross-saccadic perceptual continuity. Nature, 414(6861), 302-305.

2. Hodinott-Hill, I., Thilo, K. V., Cowey, A., & Walsh, V. (2002). Auditory chronostasis: Hanging on the telephone. Current Biology, 12, 1779-1781.

3. Yarrow, K., & Rothwell, J. C. (2003). Manual chronostasis: Tactile perception precedes physical contact. Current Biology, 12(13), 1134-1139.

Christian Jarrett