How do we judge the speed of human movements?

The speed at which people move - their gestures, walking pace, and so on – conveys useful social information about the meaning and intent behind their actions, as well as clues about their emotional state and temperament. A rapid flick of an eyebrow, for instance, is a common form of greeting while a slow rise and fall can indicate surprise or fear. Walking speed is generally slower when someone is feeling sad rather than happy.

In order for us to use information about action speed in these ways, we need to be able to estimate speed reliably. In this project we asked two questions: How do we perceive the speed of human movements? Does the brain have special neurones that respond according to human movement speed?

In a newly published research article, we attempt to answer these questions by studying how observers make judgements about movement speed in video clips. We report evidence that speed judgments are not fixed and stable, but rely on an internal standard or norm for movement speed that can be altered by relatively short periods of prior experience. The participants in our experiments viewed short video clips of walking or running human figures (recorded on the local high street or at the London Marathon). The playback rate of the clips varied between slow-motion and fast-forward, and participants were asked to judge whether the movement speed of the figures appeared too slow or too fast. This should be an easy task, but we found that participants showed consistent biases in their judgements.

This  clip shows a slow-motion video similar to those we used in the research:

After viewing slow-motion movements for a short period, normal-speed playback appeared to be too fast, and had to be slowed down in order to appear normal. The opposite effect occurred after viewing fast-forward movement for a while. So our judgements of speed are unconsciously influenced by previously viewed speeds. This ‘adaptation’ effect has some interesting implications.

In the silent movie era, cine cameras and projectors were hand-cranked, so playback rate in the movie theatre was very variable (typically between 16 and 24 cine frames per second, depending on how quickly the projectionist cranked the projector, often influenced by programming considerations). Our results indicate that the variation in playback rate was accepted by moviegoers because they quickly adapted to whatever playback rate was used; after a short while the actors appeared to be moving normally, whatever the playback rate.

In a modern context, after you have driven along a motorway at 70mph for a while, you may have had the experience that upon leaving the motorway it is easy to misjudge slow speeds, so your exit speed may be too fast. Our results indicate that this may occur because, perceptually, 70mph became ‘normal’ speed after spending some time on the motorway, causing a speed of 30mph on the slip road to appear slower than it usually does while driving in a city. The neurones in your brain had acclimatised to fast motorway speeds.

Slow-motion video replays from security cameras are increasingly used in criminal prosecutions, to help juries decide whether the crime was premeditated. A recent study in the USA found that after participants had viewed slow-motion replays of criminal acts they were more likely to decide that the defendant had acted in a premeditated way. Intentionality judgements are affected by perceived speed of movement (quick actions tend to be seen as off-the-cuff, without thought). So changes in perceived speed caused by the adaptation effect we report may well be a source of bias influencing legal decisions based on video evidence.

Next season the Football Association will review video footage of fouls in football matches in order to decide whether a player intended to deceive the referee by feigning injury or pretending to have been fouled. If these reviews involve repeated viewing of slow-motion replays, they may well be biased due to the adaptation effect we reported. Offending players may appear to move naturally but have more time, because the total duration of the slow-motion video is longer.

Further reading

Caruso, E. M., Burns, Z. C., Converse, B. A. (2016). Slow motion increases perceived intent. Proceedings of the National Academy of Sciences, 113, 9250–9255.

Mather, G., Sharman, R.J., Parsons, T. (2017) Visual adaptation alters the apparent speed of real-world actions. Scientific Reports, 7, 6738.

Morewedge, C. K., Preston, J., Wegner, D. M. (2007). Timescale bias in the attribution of mind. Journal of Personality and Social Psychology, 93(1), 1-11.

Why did the blade in Hull's Victoria Square look so strange?

These images appear to be unconvincing digital superimpositions of a large cylindrical object on photographs of Victoria Square in Hull. 

I visited the square and saw the object for myself; the photographs are real and not re-touched, the object really was installed in the square. So why does it seem to be superimposed in these photographs? Perhaps it is because the object is so large and smooth compared to the buildings and other objects in the scene that it does not seem to fit. This may be part of the explanation, but I think there is also something else going on which relates to the unconscious assumptions that our brain makes when we try to make sense of visual scenes.

Although the object in the photographs appears to be shaped like a cylinder, it is actually a relatively flat, 75-metre long blade from a wind turbine. At its lower edge as seen in the photographs the blade has a rounded, convex profile, but the upper edge is much sharper and turns over slightly toward the viewer to create a slightly concave shape near the top. As a result, the top third of the blade is in the shadow of the daylight overhead and therefore relatively dark, as is the bottom third (which faces the ground). The middle section of the blade appears light because it faces slightly upward. This kind of shading profile is normally seen on cylindrical-shaped objects that are lit from the side, so we tend to perceive the shape of the blade in this way. 

Of course the rest of the street scene is lit by natural daylight from above, not from the side.  So the apparent inconsistency in lighting between the blade (side-lit) and the rest of the scene (top-lit) makes it appear completely out of place. It creates the false impression that the blade was not present in the scene; that the photograph is a composite of two different scenes. Many more images have appeared on the web  and in the print media. Whenever they show the same side of the blade as in the photographs above, there is a tendency to misinterpret its shape and so interpret the lighting as inconsistent. The illusion could even be experienced while viewing the blade at the site, at least if one eye was closed to remove shape cues based on stereo vision.

The blade was  made by turbine manufacturer Siemens and placed placed in Victoria Square as an art installation to mark the start of Hull’s year as the UK City of Culture 2017. As Hull City of Culture (https://www.hull2017.co.uk/whatson/events/blade/) states:

"Conceived by artist Nayan Kulkarni, Blade has been created for Look Up, a programme of temporary artworks created for the city’s public spaces and places.

It uses one of the first B75 rotor blades made in Hull and changes its status to that of a readymade artwork.  At 75 metres it is the world’s largest, handmade fibreglass component – cast as a single element."

The Blade was removed from Victoria Square in March at the end of its exhibition period, and Siemens has yet to decide where its permanent home will be.

A more detailed discussion of this interpretation of the Blade is now published in the journal i-Perception.