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“Sorry mate, I didn’t see you”: perceptual errors and inattentional blindness

“Sorry mate, I didn’t see you.” It’s a common refrain heard after many a road-traffic collision. So common, in fact, that if you say “SMIDSY” to a UK motorcyclist, they’ll most likely wince and offer a story of how they or a colleague came to grief. Perhaps you’ve had SMIDSY said to you, or even had to utter those words yourself?

SMIDSY describes the all-too-common type of motorbike accident when a car pulls out at an intersection. The driver’s sure that the road is clear, but discovers too late that something is coming. Even if you haven’t been involved in such an incident, you can probably recall some occasion on which you were driving and had a near miss with a car or bike you’d swear wasn’t there a moment ago.

It turns out that these sorts of events might be more complicated than first appear. It’s quite possible for you to look right at the other vehicle, but for your brain to fail to process the information associated with it. These sorts of situational awareness failures may in fact result from a well-described, but not well-known, psychological phenomenon called “inattentional blindness”.

Most people believe their senses work a bit like a video camera. You direct attention towards an object and your brain automatically and reliably records. Although this is our day-to-day experience, perception is in reality a much more active process, with a number of filters operating between information arriving, and you becoming consciously aware of it.

A potentially limitless amount of information exists in the environment around you, but little of it is relevant from moment to moment. Rather than ‘clutter up’ consciousness with a surfeit of useless information, the subconscious monitors these unnecessary items and only ‘alerts’ the consciousness when something relevant occurs.

Under normal circumstances your brain is fairly efficient at subconsciously monitoring events around you. Imagine holding a conversation in a noisy restaurant: you are probably only consciously aware of the conversation you are directly involved in (your primary task), but if your name is mentioned elsewhere, you will turn around to find out why. Your brain has been subconsciously monitoring that stream of conversation, and when something personally relevant occurs (your name is a very powerful trigger, carrying a high degree of ‘cognitive saliency’) you can devote your attention to it.

Problems occur when you have to concentrate harder on a primary task. The more cognitive demands placed on you, the narrower your focus becomes. It’s surprising how big an event you might miss: the classic demonstration of this effect is known as the “Invisible Gorilla” and was devised by Harvard psychologists Dan Simons and Christopher Chabris in 1999. Observers were asked to follow two teams of basketball players, counting the passes made by one of the teams. Caught up in the counting task, 50% of the participants failed to notice as a collaborator, dressed in a gorilla costume, marched between the players and stopped to beat her chest before marching out again. Making the primary task more difficult, by asking the observers to count bounce- and aerial-passes separately, caused the noticing rate to fall to 33%. Most observers were inattentionally blind to the gorilla and many expressed shock when shown their error, some even accusing the experimenters of showing two different videos. Although these perceptual errors are an innate and universal feature of human cognitive architecture, it’s a common finding that insight into their effects is very poor. Almost everyone significantly overestimates their ability to notice the unexpected.

Increasing workload has been well described as a risk for this form of perceptual error. Interestingly, people with professional basketball experience are much more likely to notice the gorilla in the Simons video, but athletes from other disciplines perform much as the general public does. Whilst expertise is certainly protective to a degree (although does not eliminate the risk altogether), it does seem to be very task-specific.

How does inattentional blindness affect medical practice? The short answer is that no-one really knows. High-profile disasters such as the case of Elaine Bromiley make vivid reminders of the devastating consequences of medical errors, however, it is well recognized that daily errors occur in every institution around the world. In the UK, errors account for 2.5% of the national health budget annually.

Loss of situational awareness is thought to be the leading cause of error in time-critical situations, and there can be no doubt that clinicians labor under mentally taxing circumstances. Of course, doctors are well trained. Training brings expertise, and surely expertise protects against perceptual error? Possibly, but perhaps not to the extent that you might expect.

A few studies have looked into inattentional blindness in medical personnel, mainly by showing people items such as radiographs with a gorilla superimposed. These experiments showed large numbers of even experienced staff miss the anomaly. Our group in Oxford took this a little further, creating a recording of an adult resuscitation scenario into which we inserted a series of events, designed to test for the presence of different types of perceptual error. We showed this video to a more than 140 people and demonstrated that overall, more than seven people in ten missed events that would contribute to poor patient outcome (were they to be missed in ‘real life’). As one might expect, experts in the group (all experienced, accredited instructors of adult resuscitation) did perform better. In their case around six in ten missed it…

So does this prove that inattentional blindess is a problem for us, as experienced clinicians? Not yet, but it does raise some questions about how reliably individuals can maintain situational awareness, and offers some insight into the mechanisms by which even highly trained personnel might make mistakes. By research, using tools such as high-fidelity simulation, we can start to investigate how frequently perceptual errors actually do contribute to loss of situational awareness, who is most vulnerable to these effects, and most importantly, how can we mitigate them.

Heading image: Optics: page to a partwork on science, with pictures of optical phenomena. Coloured lithograph by J. Emslie, 1850. CC BY 4.0 via Wellcome Images.

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