I guess we all pretty much know that our brains don’t seem to capture everything that goes on around us – thankfully we can filter out a lot of unnecessary information (no, I don’t want to know what that funny noise outside is right now!) so that we can focus on what is important. When it comes to pain, the limited capacity of our brains to process information has been used to explain why there are times that, despite a large amount of nociceptive input, we can remain pretty much oblivious to it. Clearly it would be wonderful if we could harness this and use it as a form of analgesia.
The place to start, I guess, is with recognition that one function of our brain is to keep us focused on important goal-directed activity. Because of this function, some information that is available both from inside the body and from outside, needs to be prioritised. ‘Top down’ processing models suggest that our brains have a process of intentionally choosing information that is relevant for whatever goals we are aiming for. In this model, the sensitivity of neural responses is modulated up or down, depending on the goal, and it’s been postulated that prefrontal and parietal areas are likely to influence these mechanisms.
‘Bottom up’ models suggest that there is a process of unintentional ‘capture’ of attention by the stimulus itself – if a fire alarm goes off without warning, we’ll usually be alerted to it without having to stop and think too hard! The term used to describe the characteristic of this ‘alerting’ feature is ‘salience’, or the degree to which that specific stimulus contrasts with surrounding stimuli. For something to be salient, it needs to be novel or new, not often present, or directly relevant to our goals perhaps from previous experience. It’s thought that ‘pre-attentional’ systems are involved in detecting salient or novel stimuli – two separate systems have been found for the auditory system for example. These ‘bottom up’ systems can act as a distraction from ‘top down’ goals because the attention can be switched to unimportant features of the environment or body and reduce performance.
It’s thought that ‘bottom up’ attention, while unintentional, is influenced by ‘top down’ processes because the top down processes establish the amount of attention focused on a task (attentional load), so that when load is high, irrelevant information is not processed as much. Top down processes also establish the ‘attentional set’ or the type of information that is prioritised as being relevant to the goal – if a stimulus that is irrelevant matches one aspect of these features, it’s likely to capture attention.
Pain is known to capture attention ‘bottom up’ when its new or novel, intense, or is threatening. This means that vague and commonplace pains like epigastric pain, can often be ignored. Because most of us will have had heartburn, and know that it usually passes without too much of a problem, we don’t usually pay too much attention to it. The situation would change, however, if we’ve had an MI – because that pain now represents another whole new threat.
The parts of the brain known to be associated with attentional capture by pain are the brain areas underlying the P2 responses of laser-evoked potentials. The midcingulate cortex (MCC) is one of the main generators of this P2 signal, and it’s known that this part of the brain is associated with novelty detection, directing attention toward a specific stimulus, and where our behaviour needs to be regulated. The MCC can be seen as a part of the brain that is needed to coordinate a speedy motor response because of its close relationship with premotor areas.
Enough about bottom up attention – what about top down?
Well, there are many studies showing that bottom up attention to nociception can be influenced – the amplitude of P2 potentials is decreased when people are engaged in demanding activities, responses in the MCC, paracentral and operculo-insular areas are decreased when people are doing absorbing tasks, and responses in the primary and secondary somatosensory cortices are influenced by how demanding the task is (attentional load) all supporting the idea that top down attention affects nociceptive processing by biases somatosensory brain activity. This is why, when we’re engaged in an activity that is interesting and requires a bit of brain effort, our perception of pain is reduced.
Unfortunately, top down attention can also direct us towards painful events – especially when the nociceptive stimulus is being delivered to a part of the body that is also engaged in an activity (similar attentional set), and can be influential in individuals who expect or catastrophise about pain, where it has been shown that there is greater activity in operculo-insular and MCC areas. It can work both ways.
Prefrontal and parietal areas are known to be involved with top down attention to or from nocicpetive and nonnociceptive stimuli. Exactly what their job is is unclear, but some experiments suggest that the dorsolateral prefrontal cortex is important for maintaining priorities relevant to goals, by increasing the involvement of executive functions to the job of processing relevant information, thus avoiding the interference from irrelevant stimuli. Maybe the dorsolateral prefrontal cortex (DLPFC) and the intraparietal sulcus(IPS) are able to maintain both attentional load and attentional set, to make sure we’re able to focus on the goal we had in mind, rather than being distracted by pain.
So what does this mean for us?
Well, the authors of the paper I’ve been paraphrasing, suggest that the bottom up factors act as a signal to salient events, and this influences the operculo-insular areas, and by the MCC – this triggers attentional bias towards nociceptive input. Both attentional load and attentional set seem to modulate how much these salient events disrupt our attention away from the task in hand. Remember that attentional load is the cognitive ‘grab’ of whatever we are focusing on, so it also allocates resources or effort to maintain that focus. The DLPFC keeps this focus by directing executive functions to the most important task. Attentional set guides that ‘what is important’ or matching qualities that the person is attending to in order to achieve the goal, and all stimuli that meet at least one of those qualities will capture attention. This is thought to be associated with activity of the IPS which detects similarities of various stimuli and focuses toward or away from responses to features in the environment. So there is a sort of balance or dynamic between the bottom up and top down influences on attention.
This model is pretty important – most experiments in pain have been with acute pain in controlled settings where there is some degree of expectation that something will happen. This is not a lot like real life where pain is often unexpected and happens in a busy and demanding context. For people with chronic pain, it’s already been found that cognitive deficits occur and while there could be some truth in the current explanation of ‘anxiety’ or ‘hypervigilance to pain’, the neurological explanation may well be more along the lines of this top down and bottom up allocation of resource. It’s especially useful to consider this model when looking at executive functions and how these, and self regulation, can be impaired in people with chronic pain – maybe it’s because it’s more difficult for people with chronic pain to exert down-regulation or to switch attention from one thing to another – or even to inhibit irrelevant signals in order to focus on important aspects.
The authors also suggest that attention management may become a more important part of a cognitive behavioural approach than it has in the past – mindfulness occurs to me as a particularly useful way to develop effective executive control.
Legrain V, Damme SV, Eccleston C, Davis KD, Seminowicz DA, & Crombez G (2009). A neurocognitive model of attention to pain: behavioral and neuroimaging evidence. Pain, 144 (3), 230-2 PMID: 19376654