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On stretching time

Someone told me yesterday that David Eagleman gets a thousand e-mails a day. Little wonder, then, that an hour after his talk on time perception at the Society for Neuroscience meeting in Washington DC, the cognitive neuroscientist from Baylor College of Medicine in Houston, Texas, was still taking questions from a three-deep throng of interested researchers.

His whirlwind talk started with a personal memory: as a child he slipped and fell off the roof of a house still under construction. Although he hit the ground after less than a second, the journey seemed to take much longer; it was as if time had slowed down.

Eagleman’s lab famously tested this effect a few years ago using volunteers who fell 31 metres before landing on a net. Just as with Eagleman’s childhood tumble, time did seem to slow down for the volunteers: they perceived their own falls as lasting 36% longer than the falls they watched.

It doesn’t take a potentially fatal free fall to stretch time, though. In his talk, Eagleman flashed a series of identical images of a shoe on the screen and then an image of an alarm clock. Although the pictures appeared for the same amount of time, the clock seemed to appear on the screen for longer than the shoes — 13% longer to be precise, based on laboratory experiments on a number of volunteers.

Other scientists had explained this trick — the oddball effect — in terms of attention. The alarm clock, because it was new, prompted volunteers to pay closer attention and therefore it seemed to appear for a longer duration. If that explanation were true, Eagleman reasoned, then photos that are more arousing — such as images of guns or tarantulas — would seem to last even longer. That didn’t turn out to be the case. Guns and spiders stretched time just as much as clocks and shoes.

Instead, Eagleman hypothesizes that the oddball effect, subjective duration and time perception in general can be explained by a fundamental property of the brain called adaptation (or repetition suppression), in which neurons become less responsive to something after repeated exposure. Scientists see this in patch-clamped neurons, live animals implanted with electrodes that measure neural spikes, as well as in humans undergoing functional neuroimaging. With repeated exposure to something, a neuron or population of neurons finds more energy-efficient ways of encoding that information.

To tie this property into time perception, Eagleman and his team trawled through the psychology literature looking for examples of subjective duration. Brighter things feel as though they last longer than dim objects; bigger things warp time more than small objects; the number 8 lasts longer than the number 3, and so on. “Anything that induces a larger neural response seems to last longer,” he says.

In their own experiments, he and his team found that people who see a series of faces while in a functional magnetic resonance imaging, or fMRI, scanner report that new faces seem to be presented for longer than a face they’ve just seen. A brain area that is sensitive to faces, called the fusiform gyrus, is also more active when volunteers view the new faces.

Eagleman concedes that these observations are just correlations. But with a subjective conscious experience such as time perception, correlation may be the only way to accrue evidence for his model.

He already thinks it offers an explanation for his childhood tumble: “During the fall your brain is on fire trying to figure out what the heck is going,” he says. “When you look back on a duration like that you say, ‘Wow, that lasted forever.’ ”

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    Mike Maxwell said:

    Any way this sort of thing could be related to the fact that the Moon looks larger near the horizon than overhead?

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