Temporal illusions

6 minute read

Your nose is closer to your brain than your knee. Obvious.

You can use your hands to touch both your nose and foot. Obvious. [1]

When you do touch both your nose and foot at the same time, you feel the touches at the same time. Obvious, and now you’re wondering where this is going. But let’s pause here and think about this. How is it that you feel the touch on your nose, which is closer to your brain, at the same time that you feel the touch on your foot, which is further away? The non-intuitive conclusion is that our brain has a slight window of time where it processes signals and then interprets what we feel. Brain, time, and memory are linked. Perception then, is a temporal illusion.

I had the privilege of hearing David Eagleman talk at the Long Now anniversary event [2] a while back where he talked about this and other temporal illusions. I recently realised he wrote a post about his research too. The post is worth reading in its entirety, but below are the parts I found most interesting:

We all know about optical illusions, in which things appear different from how they really are; less well known is the world of temporal illusions. When you begin to look for temporal illusions, they appear everywhere. In the movie theater, you perceive a series of static images as a smoothly flowing scene. Or perhaps you’ve noticed when glancing at a clock that the second hand sometimes appears to take longer than normal to move to its next position—as though the clock were momentarily frozen.

Obvious in hindsight, but movies and animation are such an easy way to realise how simply our brains can be tricked regarding perception of time. Nothing is ‘actually’ moving but we see it that way.

go look in a mirror. Now move your eyes back and forth, so that you’re looking at your left eye, then at your right eye, then at your left eye again. When your eyes shift from one position to the other, they take time to move and land on the other location. But here’s the kicker: you never see your eyes move. What is happening to the time gaps during which your eyes are moving?

Another simple experiment to convince you if you haven’t already been persuaded.

This is what we find in the lab, but might something different happen during real-life events, as in the common anecdotal report that time “slows down” during brief, dangerous events such as car accidents and robberies?

We’ve either experienced ourselves or seen the trope before where events seem to take longer to occur when we get an adrenaline rush. Do we actually see these events over an extended duration, or is this just a consequence of how our brain processes events and creates a narrative after? David and his grad student Chess crafted an ingenious experiment to tease these effects apart.

His essay goes into more detail about the experimental design, and essentially what they had participants do was look at a digital display that changed rapidly while participants were undergoing a scary event. If we did see time slow down, we should be able to identify what was shown on the display accurately. Think of how you might be able to see a bee flapping its wings when you slow down a video to quarter speed or less. If we actually did see things in slow-motion, we should also be able to see the displayed image. If we don’t actually see time slow down, we would not be able to read the display accurately.

The result? Participants weren’t able to read the numbers in free fall any better than in the laboratory. This was not because they closed their eyes or didn’t pay attention (we monitored for that) but because they could not, after all, see time in slow motion (or in “bullet time,” like Neo in The Matrix). Nonetheless, their perception of the elapsed duration itself was greatly affected.

We feel like a longer time has passed than actually did, but unfortunately don’t actually experience slow-mo.

The answer is that time and memory are tightly linked. […] So in a dire situation, your brain may lay down memories in a way that makes them “stick” better. Upon replay, the higher density of data would make the event appear to last longer.

The time dilation effect is only something we notice on recalling the event, not an actual physical reality. I want to emphasise their point on how time, memory, and the brain are all inter-related. This is counterintuitive but the most plausible explanation so far.

A partially related point is how time seems to speed up as we get older. There’s a new paper out [3] that theorises this is because our nerves grow longer and degrade as we age. This causes images to be processed slower and sense fewer new images. We feel that time passes more quickly when we sense less new images, hence this common experience as we age.

But there is a deeper challenge the brain must tackle, without which feature-binding would rarely be possible. This is the problem of temporalbinding: the assignment of the correct timing of events in the world. The challenge is that different stimulus features move through different processing streams and are processed at different speeds. The brain must account for speed disparities between and within its various sensory channels if it is to determine the timing relationships of features in the world.

We’ve known that our different sensory inputs occur at different speeds, e.g. audio travels faster for our brains compared to visual [4]. This implies that our brain must somehow be stitching all of this together to form a coherent story.

So if the visual brain wants to get events correct timewise, it may have only one choice: wait for the slowest information to arrive. To accomplish this, it must wait about a tenth of a second. In the early days of television broadcasting, engineers worried about the problem of keeping audio and video signals synchronized. Then they accidentally discovered that they had around a hundred milliseconds of slop: As long as the signals arrived within this window, viewers’ brains would automatically resynchronize the signals; outside that tenth-of a-second window, it suddenly looked like a badly dubbed movie.

Put another way, we lag. And our brains have a brief period of time in which they’ll consider all inputs to be occuring simultaneously. Tangentially, the big concern with Google’s Stadia game streaming project seems to be latency concerns. [5]

This raises a question: what is the use of perception, especially since it lags behind reality, is retrospectively attributed, and is generally outstripped by automatic (unconscious) systems? The most likely answer is that perceptions are representations of information that cognitive systems can work with later. Thus it is important for the brain to take sufficient time to settle on its best interpretation of what just happened rather than stick with its initial, rapid interpretation.

This reminds me a bit of Kahneman’s system 1 and system 2 thinking. Seems our brains are trying to construct a narrative that makes sense for us after the event happened, that may help us for future encounters.

I don’t know if this has any relevance to how we live our lives; just thought it was something fascinating to learn about. We often take many things for granted but being surprised by something counterintuitive like this is fun.


  1. Heck or even use the same hand if you’re so inclined.
  2. A fascinating foundation by the way and I strongly encourage people to check it out. Plenty of people talk about thinking long term but don’t act on it. How many organisations think in 10,000 year time frames and want to build something that can last that long?
  3. Full paper here
  4. Why sprinters react faster to the sound rather than sight of a gun
  5. I’m unsure if this is the right way to think about it, but should I assume that anything <100ms latency would work? Or does it have to be half of that to account for the time it takes for me to see the output and then press a key action, <50ms ? Hoping someone can clarify this for me