SUMMARY – when we turn our heads quickly, our vision is shut down in a series of ‘saccades’… this causes ‘saccadic masking’… drivers at junctions turn their heads quickly left and right, and generate saccades… if the motorcycle falls behind a saccade, the driver can appear to look right at the bike and yet will not see it…
Although the ‘looked but failed to see’ collision between a motorcycle and another vehicles is frequently blamed on a lack of visual conspicuity, we also have to consider how the brain processes the visual data. A key point to accept is that our eyes and the brain are not the equivalent of the lens and the camera. I’ve already discussed the limitation of the eye’s foveal vision, but there are two other issues we need to understand:
- saccadic masking
- motion camouflage
We previously saw that only in the fovea, a tiny part the retina, do we have clear, colour, focused vision, and that to examine an object of interest in detail requires a shift of our line-of-sight to a new position. Once the attractant is in our line-of-sight, it comes in to our awareness, and we can examine it in detail with the fovea’s high-resolution vision. And if it’s moving, we can track it by moving our eyes using what are called ‘smooth pursuits’. Smooth pursuits are very common in our everyday visual behaviour.
But there’s a problem. When we move our eyes to scan a scene, the background would move rapidly through this zone of vision. This would cause disorientation and dizziness. So our eyes don’t move smoothly across the background, as we commonly think. Instead they move very rapidly from one selected object – a fixation – to another. These movements are called saccades. This mechanism is very effective in processing complex scenes, but there is a drawback. The brain ceases to process retinal images between saccades. This is known as saccadic masking or saccadic suppression.
So smooth movement as we track a moving object is quite distinct from the two other events:
- the fixations that occur when we examine at a point of interest
- the saccades that occur when we move our eyes quickly from one fixation to another
Once again, this phenomenon has been known about for a very long time.
Back in the 1880s, French ophthalmologist Émile Javal observed how the eyes moved when reading and discovered that it involves a succession of rapid but discontinuous individual movements. He that coined the term ‘saccades’, after the French word for ‘jerk’.
Saccadic masking is exploited by dancers during fast turns – the dancer ‘spots’ by keeping head and eyes still as long as possible by focusing on a fixed location, then turning the head faster than the body. Spotting enhances the dancer’s control and prevents dizziness.
Now, ask yourself: “what does a driver do at a junction?” The answer is that he / she looks right and left, momentarily fixating in each direction before moving head and eyes rapidly from one scene to the other. Just like the dancer, to prevent blurring of vision and disorientation, saccadic masking shuts down the visual processing system as the eyes move. As a result, it is only during the stationary fixations that an image is processed. In consequence, all drivers (and motorcyclists too) are left with gaps in visual perception whenever we scan both ways at a junction. Because the brain synthesizes the missing visual data to give the impression of continuous vision during the scan, drivers – just like dancers – are unaware of the visual shut-down and believe they have completed a full scan of the entire zone whereas the true visual feed is more akin to a series of snapshots.
There’s an excellent video on saccadic masking by the Alliance of British Drivers – you can find it here on YouTube. Whilst you’re watching that, notice just how the A pillar blocks the view too – I snipped the screenshot below from the video.
Drivers (and riders) typically look for around 0.4 seconds in each direction before turning their head, so how much data do we lose? Saccadic masking shuts off the visual flow for around 0.03 to 0.04 seconds before the rapid head turn, and for around 0.1 seconds after the initiation of the saccade (Roessger et al 2015).
So, even an attentive driver, looking in both directions to check for oncoming traffic, may fail to see an approaching motorcycle if it falls within a saccade. This isn’t ‘carelessness’ or ‘failing to look properly’, it’s a fundamental limitation – and illusion – of the human visual system. Watch the driver’s head – when the driver’s head is moving both ways, there’s a very real risk that our motorcycle will fall within a saccade. Only if we can clearly see that the driver’s head movements are tracking our motorcycle can we be reasonably sure we’ve been spotted, and that the driver’s visual system is using ‘smooth pursuit’ to follow our path.
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Roessger, L., Lenne, M., G., Underwood, G. (2015) “Increasing motorcycle conspicuity – design and assessment of interventions to enhance rider safety” Routledge Press
Thursday 13 June – added information about ‘smooth pursuit’, minor edit for clarity
Wednesday 1 May 2019 – minor edit for clarity, added screenshot and link to ABD saccade video
Friday 23 November 2018 – minor edit for clarity and to fix typos
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Photo credit Paul Townsend https://www.flickr.com/photos/brizzlebornandbred/20001313491