This post first appeared on pygaze.org, in November 2015.
Threatening elements (e.g. a spider) in your surroundings tend to grasp your attention more strongly than non-threatening things (e.g. a puppy). Some scientists believe that your brain is wired to notice threatening stimuli quicker, via a special sub-cortical route. In a new experiment, we show that task-irrelevant threatening stimuli are prioritised over, but not processed any quicker than non-threatening stimuli. In essence, you will spend more attentional resources on a spider than on a puppy, but you won’t be any quicker to find out it’s RIGHT NEXT TO YOU – AAAAAH!!
(In internet-lingo, TL;DR is an abbreviation of ‘Too Long; Didn’t Read’. Some people don’t like reading long text, and require a super-short summary.)
Scary stuff stands out: things that are threatening to you seem to have a special saliency. You notice them more often than other things, and you might find it hard to focus on anything else. Imagine you just saw the film Eight Legged Freaks, and now you are faced with a house spider in your bedroom: When you walked in, you immediately noticed that freak, and now you can’t keep your eyes of it. This is what we psychologists tend to call an attentional bias, in this case towards spiders, but you will also have it to other threatening (real or imagined) things.
A very prominent idea in the current scientific literature, is that of the direct route of threatening visual information. The logic is a bit like this: Normally, visual information is processed mostly in the occipital lobe at the back of your head, which receives information from your eyes through the thalamus (specifically the lateral geniculate nucleus of the thalamus, or LGN for friends). However, another neuronal path from the eyes leads through the superior colliculus, a super-interesting area that is thought to be heavily involved in directing your attention and your eye movements. A crucial point here, is that the superior colliculus also connects (via the pulvinar) to the amygdala, an area that is often associated with the processing of fear. (Note: The amygdala has also been associated with nearly every other emotion, and a handful of other things such as your sexual or political orientation, but let’s forget about that for now.) So, goes the reasoning, if the eyes are more or less directly connected to the amygdala, that’s a much more direct way for visual information to travel to the ‘fear centre‘. Remember: the other way is from the eyes, to the back of your head, then along your skull (via the ventral and dorsal routes), via some more detours, and eventually some of the processed visual information will reach the amygdala. The route via the superior colliculus has been labelled the direct route, and the route through the visual system in the occipital lobe has been named the indirect route.
At this point, you might be wondering why we have this elaborate visual system at all, if information could just bypass it and still be processed. Well, you see, we need it to know what it exactly is that we are looking at. The direct route is thought to be a quick-and-dirty way of making a little bit of sense of your environment (“a threat might be present”), whereas the visual system will actually tell you what precisely is going on (“there is a spider on your pillow”). The visual system really is the only place where accurate vision is constructed (that’s why they call it the visual system!). The idea some people base on this, is that there might not quite be a ‘direct route’ of visual processing at all. They think that visual information is processed in the occipital lobe, and that subcortical areas (such as the amygdala) can guide what information is prioritised.
Top-down versus bottom-up
A crucial prediction of the ‘indirect’ and ‘direct’ pathway idea, is that threatening visual information is processed earlier. In contrast, the alternative hypothesis (without the ‘direct’ route) would still predict a prioritisation of threatening visual information, but not a quicker one.
A number of studies have reported quicker processing of threatening information, but the stimuli used in those studies has always been relevant to the task. That is, people always had to attend to and process the threatening stimulus, and then do something with it (move their eyes to or from it, or judge it in one way or the other). This is a problem, because it stimulates people to adopt strategies that prioritise the task-relevant (and incidentally threatening) stimuli. A mental strategy that prioritises information is what we call top-down attention: You consciously guide your attention to the stimuli that you want to focus on.
The opposite, which is crucial to the current scientific ideas on attentional biases to threat, is bottom-up attention. This involves an automatic process: Whenever something threatening appears, it automatically grasps your attention.
So, where does that leave us? Well, we set out to test if task-irrelevant and threatening information is processed quicker than task-irrelevant non-threatening information. In addition, we set out to find out how this information is prioritised.
Crucially, when I write about ‘us’ and ‘we’, I mean Manon Mulckhuyse and myself. And by that I mean Manon did most of the work, and I could happily tag along.
Eye movements and threat
As attention researchers, we love eye movements. Seriously, eye movements are absolutely fantastic. They are quick, they are very closely tied to your attention. In general, where your attention goes is where your eyes go. And they are incredibly quick: You can move your eyes to a suddenly appearing stimulus in 150 ms, whereas a hand movement would be at least twice as long (and usually about four times, or even slower). So, in order to test the quick attentional effects of threatening stimuli, we really want to look at people’s eye movements.
In this experiment, we ask people to look at a circle of simple, grey circles. Each of them has a little plus in it. At some point, almost the pluses suddenly disappear, but one remains visible. The idea is that participants move their eyes to that plus as quick as possible. Crucially, sometimes another circle (green or red) appears when the pluses disappear. People get distracted by this extra circle, and sometimes accidentally make an eye movement towards that instead of towards the plus.
So far so good, but where is the threatening information? Good question! Before the experiment, we made people look at red and green circles. For some people, the red circles were accompanied by a loud and aversive noise, while the green circle was safe. For other people, the green circle was associated with the nasty sound, and the red circle was safe.
By now you might realise our trick: the red and green distracting circles that appeared during the task were still associated with the aversive sounds. (Throughout the experiment, we occasionally played the loud noise, just to reinforce the association between the coloured circle and the aversive sound.) The coloured circle that was associated with the sound is what we call the CS+, and the other is what we call the CS-.
What we tested, was A) Whether the aversive circle grasped attention more than the non-aversive circle, B) Whether participants were slower to disengage from aversive circle than from the non-aversive circle, and C) Whether people were quicker to look at the coloured circle that was associated with the aversive sound, .
More eye movements towards the threat
First of all, it’s important to check whether participants actually felt a bit threatened by the loud noise. This was the case: people indicated that they felt more anxious during the experiment. In addition, they were very good at learning the association between one colour and the aversive noise. They were also more threatened by the coloured circle that was paired with the loud noise than by the coloured circle that was safe. (This was all tested using response scales and statistics; see the paper for the precise methods.)
Now that we know our experimental manipulation worked, let’s get on with the results! First of, the amount of eye movements people made towards the distractors. When none was present, people first looked at the target (the plus that did not disappear) in 72.6% of all trials. When a safe distractor was present, people first looked at the target in only 53.0% of the trials; and when the threatening distractor was present, it was only 47.8%. These differences were statistically significant, so people looked at the target less when a distractor was present, and this was worse when that distractor was threatening.
When a distractor was not threatening, people first looked at it in 12.5% of the trials. When the distractor was threatening, people first looked at it in 20.6% of the trials. That difference was statistically significant too, so we can conclude that people looked at the threatening distractor more often than at the non-threatening distractor.
Slower disengagement from the threat
We now know that the threatening distractor captures your attention more than a non-threatening distractor. But once it does grasp your attention, does it do so for longer? In other words: does it take more time to disengage your attention from a threat?
In order to answer this question, we looked at the average time it took participants to make an eye movement to the target in the presence of each distractor. The idea here is that you can only make a correct eye movement to the target, once your attention is at the target. If your attention was captured by a distractor, you need to disengage it first, and then turn to the target instead. So when it takes you longer to make an eye movement to the target, your attention lingered at the distractor for longer.
When no distractor was present, eye movements to the target were quite quick: 257 milliseconds on average. They were about as quick when a non-threatening distractor was present: 263 milliseconds. However, when a threatening distractor was present, participants were a bit slower to make an eye movement to the target: 271 milliseconds. The difference between no distractor and a safe distractor were not significant. The difference between no distractor or a safe distractor, and a threatening distractor were significant. This means we can conclude that the threatening distractor grasped participants’ attention for longer: it was harder to disengage from.
Quicker distraction by danger?
The final thing we wanted to know, is whether a threatening distractor is also quicker to grasp your attention. You might remember that this is a crucial prediction of the two-path model: the ‘direct’ route for threatening visual information is supposed to make you quicker to respond to threatening stimuli than to other visual information.
When people fail to make an eye movement to the target, but instead go for the distractor, we say their eyes were captured by the distractor. The two-routes theory would predict that eye movements that were captured by the threatening distractor would be quicker, than those captured by the non-threatening distractor. In fact, they are not. We divided all eye movements into three bins: really quick, quick, and slightly less quick. The quickest eye movements to the threatening distractor started about 170 milliseconds after its onset, and those to the non-threatening distractor 173 milliseconds. The medium-quick eye movements started 189 milliseconds after the onset of the threatening distractor, and 188 milliseconds after the non-threatening distractor. For the slightly-less-quick eye movements, this was 226 milliseconds for the threatening, and 218 for the non-threatening. You can clearly see how small the differences are, and how they are not even in the right direction. It will not come as a surprise that these differences were not significant.
So, there is no difference in how quick threatening and non-threatening stimuli capture your attention. This suggests that your brain is not processing threatening information is any quicker than non-threatening information.
Threatening stimuli are prioritised by our visual system. They are more potent than non-threatening stimuli in distracting us from an unrelated task. They also engage our attention for longer. However, threatening distractors do not grasp our attention any quicker than any other distractor. This suggests visual information on threats is not processed through a special, ‘direct’ route in the brain.
- Mulckhuyse, M., & Dalmaijer, E.S. (in press). Distracted by danger: Temporal and spatial dynamics of visual selection in the presence of threat. Cognitive, Affective, and Behavioral Neuroscience. doi:10.3758/s13415-015-0391-2