“WHEN a man sits with a pretty girl for an hour,” said Albert Einstein, “it seems like a minute. But let him sit on a hot stove for a minute, and it’s longer than any hour.” Einstein was describing one of the most profound implications of his Theory of General Relativity – that the perception of time is subjective. This is something we all know from experience: time flies when we are enjoying ourselves, but seems to drag on when we are doing something tedious.
The subjective experience of time can also be manipulated experimentally. Visual stimuli which appear to be approaching are perceived to be longer in duration than when viewed as static or moving away. Similarly, participants presented with a stream of otherwise identical stimuli, but including one oddball, or “deviant”, stimulus, tend to perceive the deviant stimulus as lasting longer than the others. The underlying neural mechanisms of this are unknown, but now the first neuroimaging study of this phenomenon implicates the involvement of brain structures which are thought to be required for cognitive control and subjective awareness.
The apparent prolonged duration of a looming or deviant stimulus is referred to as the time dilation illusion, and three possible, but not mutually exclusive, explanations for why it might occur have been put forward. First, the stimulus might be perceived as lasting longer because it has unusual properties which require an increased amount of attention to be devoted to it. Alternatively, the perceived duration of the stimulus might reflect the amount of energy expended in generating its neural representation (that is, duration is a function of coding efficiency). Finally, the effect might be due to the intrinsic dynamic properties of the stimulus, such that the brain estimates time based on the number of changes in an event.
Of particular relevance to the third hypothesis is the observation that looming stimuli are associated with a distorted subjective perception of time, such that their duration is perceived to be longer than it actually is. Marc Wittmann and his colleagues exploited this in their new study. They recruited 20 participants and scanned their brains as they viewing a stream of five visual events. All five stimuli were static and of an identical duration, except for the fourth. This ‘deviant’ target consisted of an expanding or shrinking disc – which mimicked an object moving toward, or away from, the participants, respectively – and whose duration was systematically varied for each trial. The participants were required to judge the duration of the deviant stimulus in comparison to the other four, by answering the question, “Is the target longer or shorter than the other events?”
The time dilation effect was only observed in trials which included looming deviant targets. The looming stimulus, which had a duration 409 milliseconds (ms, thousandths of a second), was reported by the participants to be equivalent to the four static stimuli, which lasted 490 ms. The opposite effect was observed in trials including the receding stimulus. The shrinking discs, which actually lasted 511 ms, were also reported to being of equivalent duration to the static stimuli. The extent of this latter effect was, however, lesser than the time dilation effect – whereas looming stimuli were perceived to last 81 ms (or almost one tenth of a second) longer than they actually were, the duration of the receding stimuli was ‘compressed’ by just 21 ms (just over one fiftieth of a second).
Thus, the participants consistently overestimated the duration of the looming stimulus and underestimated that of the deviant receding stimulus. Analysis of the fMRI data revealed that both types of deviant stimulus produced similar brain activation patterns. When they compared the brain’s response to looming versus static stimuli with its response to receding versus static stimuli, the researchers observed increased activation in a network of areas including the left insula and surrounding areas, the anterior cingulate gyrus, the right middle frontal cortex and the left and right superior frontal regions. Additionally, receding stimuli alone activated the right insula and the entire cingulate gyrus in both hemispheres.
The looming and receding stimuli therefore led to asymmetrical activation in the so-called “core control network“, which is critical for selecting, switching between and attending to prominent features in the environment. Differences between the two activation patterns further revealed the neural correlates of the time dilation effect. The looming stimulus was found to produce stronger activation of the left middle and superior frontal cortex, including the cingulate gyrus, and the posterior cingulate and pre-cuneus. Some of these structures, which are located near the brain’s midline, have been associated with a “default network” which is engaged when the brain is at rest, but whose activity is suspended during any goal-directed action. They are also associated with the processing of self-referential information.
The researchers observed a strong time dilation effect in the looming, but not the receding, condition. They suggest that it is the occurance of a looming stimulus, and not an oddball or deviant stimulus per se, that causes an overestimation of time, but do not rule out that attentional and emotional factors might also be involved. They speculate that the expanding discs – which mimic an approaching object – evoke self-referential processes which act to signal the presence of a potential threat. In such a situation, an illusion of time dilation could facilitate an effective escape. People often report that time seems to slow down during dangerous events, such as a car accident or a robbery, but it remains to be seen whether looming visual stimuli are interpreted as threatening under experimental conditions.
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Wittmann, M., et al (2010). The neural substrates of subjective time dilation Front. Hum. Neurosci. DOI: 10.3389/neuro.09.002.2010.
Eagleman, D. & Pariyadath, V. (2009). Is subjective duration a signature of coding efficiency? Phil. Trans. R. Soc. B. 364: 1841-1851. [PDF]
van Wassenhove V., et al. (2008) Distortions of Subjective Time Perception Within and Across Senses. PLoS ONE 3: e1437. DOI: 10.1371/journal.pone.0001437.
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