What does fear do to our vision?

Consider the following scenario. A policeman is on patrol, maybe he's quite new to working in the field. He sees a suspicious young man and decides to follow him.

He turns the corner and sees that the man has drawn a gun from his pocket. In a snap second – almost too fast to think twice – he takes out his own gun and shoots the man dead.

Only the man didn't have a gun at all, it was a mobile phone.

Sadly, it's a familiar story. An incident exactly like it occurred only last week (January 2016) and a quick trawl though more newspaper reports shows how commonly it occurs.

When people make snap decisions in situations like this, they are often under intense momentary stress. This can provoke a host of automatic mental and physical effects that some psychologists refer to as "freezing behaviour". We usually think of this kind of reaction as occurring in animals – a mouse paralysed with fear or a deer trapped motionless in the headlights (resulting in much road kill).

In other words, it's the moment before an animal decides what to do to do, whether to "fight or flee". This is believed to be an innate response to a predator, to avoid being seen or heard. Research has shown that an animal's heart rate actually decreases when in this state.

Although we hear about it less in humans, our physiological response can be similar. For instance one 2005 study found that in response to pictures of mutilated bodies, participants' physical movements reduced and their heart rates slowed. The same effect was found in a 2010 study in response to pictures of threatening faces.

However, there's still much more we need to learn about the effects of the human freezing response – for example, what effect does it have on visual perception, and could any effects help explain some of the tragic instances when police have mistaken phones and other harmless objects for guns? A new study published in the Journal of Experimental Psychology aimed to find out.

Maria Lojowska of Radboud University and colleagues in the Netherlands tested 34 participants between the ages of 18 and 30. To create a situation that elicited freezing behaviour, the researchers occasionally gave their participants a mild electric shock, which was always preceded by a red dot. Participants were told they were taking part in a visual perception task and were fully informed about the nature of the shocks before the experiment started.

It was not the shock itself that made the participants show "freezing behaviour" (as measured by their heart rate), rather it was the anticipation of the shock. When participants saw a green dot (which did not presage a shock), they relaxed, but when they saw a red dot they felt more scared, regardless of whether a shock was actually given or not.

The participants' task was to judge as accurately as possible the orientation of the lines inside small squares, which appeared on a computer screen on the left or right of their visual fields. The squares either had several lines (high detail) or few lines (indicating low detail), as you can see below. Crucially, the researchers found that the participants' visual performance was affected by whether or not they were stressed and showing physiological signs of freezing. When they were afraid and stressed, their performance at judging the squares with high detail was impaired but their ability to judge the squares with coarse visual details actually improved.

The researchers said that previous research in animals had suggested that the freezing response leads to an overall improvement in vision, but their new findings suggest a more nuanced situation – it seems that when we're afraid, we perceive some aspects of the world more clearly, but at the cost of ignoring much of the detail.

Intuitively, it makes sense that an animal or human only sees the most basic detail of a potentially threatening object. It would take too much time to take in all the detail of a scene. Our brain has a clever way of quickly reconstructing what every object is likely to be using its memory of similar events and situations, rather than analysing each new thing afresh, in depth. It is these shortcuts that can result in errors and visual illusions.

Despite these potential flaws in our visual perception, it's important for us to be able to perceive things quickly. If you are walking in a desert and glimpse a shape that could be a snake (but is more likely a stick), it's better to show caution and stop than assume it's a stick and walk right into danger.

Now that we better understand how our visual perception changes when we feel fear, Maria Lojowska and her team plan to discover exactly what's going on in the brain when this happens. Meanwhile, the researchers hope their findings might help inform training programmes to improve a person's performance when they are in a stressful environment. Many police forces in the US already train their officers to overcome their implicit bias towards race and sex. It would be helpful to add the limits of our visual perception to the list.

About the author

Post written by Melissa Hogenboom (@melissasuzanneh) for the BPS Research Digest. Melissa is BBC Earth's feature writer.

Further reading

Lojowska, M., Gladwin, T., Hermans, E., & Roelofs, K. (2015). Freezing promotes perception of coarse visual features. Journal of Experimental Psychology: General, 144 (6), 1080-1088 DOI: 10.1037/xge0000117