Missing a bus is so much more frustrating if you're late by a fraction of second, than by half an hour, even though the outcome is the same in both cases. It's most frustrating of all if you first run all the way to the bus, and then only just miss it by a whisker. In the scientific jargon these two factors are "proximity", and "effort expended" and being thwarted in the context of both of these is enough to unleash your inner HULK.

Now, for the first time, neuroscientists have looked at what's happening in your brain when this kind of extreme frustration fills you with rage (or minor annoyance, at least). They think they may have found the HULK brain circuit - OK, those are my words not theirs, but that's basically what they're saying.

To research this topic, Rongjun Yu and his colleagues must have had great fun devising an incredibly frustrating task for their participants. I was hoping they might have introduced an itch in the middle of their participants' backs, or taunted them with chocolates tied to a piece of string - SNAP, "no you can't have it!"

Unfortunately it was a lot drier than that. Participants had to click the correct key, within a limited time window, according to whether arrows on a screen pointed left or right. Succeed at this enough times and they would win two pounds (about $3.30). A progress bar on screen showed them how many more trials they had to complete to win their reward (the most was four). Participants had several attempts to win two pounds, but the whole thing was actually fixed so that a lot of times they were thwarted at various stages of proximity to the reward, and after expending various degrees of effort.

The set-up worked, in the sense that both proximity and expended effort led participants to report feeling more frustrated, and to show more frustration. The display of frustration was measured by how hard participants pressed a key to confirm whether they'd won the money or failed. After getting really close, but missing out, or after completing more trials and then missing out, the participants pressed the confirmation key harder. There's no mention of any participants hurling their response pads onto the ground, which is disappointing.

Next Yu and his team repeated the whole exercise with more participants and scanned their brains at the same time. Again, proximity and effort expended both increased the frustration caused by failing to win. Both these factors led to increased activity in the same circuit of brain structures, including the left amygdala, left midbrain PAG (periaqueductal gray), left anterior insula, and dorsal anterior cingulate cortex.

What does all that mean? Well, the researchers point out that this spread of activity is very similar to a "rage circuit" that's been identified in rats. According to Rongjun Yu and his team, the evolutionary purpose of this brain response following frustration might be to "transfer unfulfilled motivation into subsequent behavioral vigour to overcome goal-blocking obstacles." In plain English, being thwarted in tantalizing fashion gives you a jolt of energy so you try harder to get what you want.

But here's why this is possibly the most frustrating brain scanning study ever - not only because it's about frustration, but also because we haven't learned much. The network of brain regions activated by frustration was relatively large and varied, and includes structures previously associated with a range of emotional functions. It would have been pretty amazing if these structures hadn't shown some kind of response when participants were frustrated. The only really surprising result was that both greater "proximity" and "effort expended" led to the exact same activation patterns (although it remains possible that more detailed/powerful analysis in the future will uncover differences). I was left wondering how much the results would have differed if the researchers had looked at rising anger, rather than rising frustration - what exactly is the distinction?

The authors admit in their discussion of the study that many questions remain unanswered about the psychology of frustration, including: why do proximity and effort expended lead us to feel more angry and explosive? Yu and his team make a few suggestions, including counterfactual thinking (i.e. getting so close probably encourages us to imagine "what if" we had succeeded), and there's also probably a progressive build up of "anticipated regret" - fear of failing - the closer we get to a reward. Those are sound suggestions, but it's a frustrating shame the brain scanning doesn't give us any extra insight. HULK says do more psychology research next time, only then scan brain.

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