‘Pleasure is fundamental to us,’ says Morten Kringelbach who holds a dual appointment as senior research fellow at the Department of Psychiatry at Oxford University and professor at Aarhus University, Denmark. He should know – he’s been studying the basis of pleasure and how the emotion is generated by the brain. ‘It’s what gets us up in the morning, or guides us in choosing whether we want coffee or tea.’
‘It’s no accident,’ he points out, ‘that food and sex are our primary sources of pleasure. They are critical for our survival, so having dedicated pleasure networks in the brain that tend to make us seek them out makes absolute sense.’
Being around other people is also pleasurable, he says. Social interactions are part of what sustains us as humans, and pleasure plays a key role in this. ‘It’s one reason why food is more enjoyable with other people, for example.’
On top of the basic pleasure-inducing elements of food, sex, and other people which have helped our survival, there are the higher-order sources of enjoyment such as music, art and beauty, or monetary rewards. But these are processed in the brain in similar ways. ‘Brain scanning experiments have shown that music hits the same parts as food or sex,’ says Professor Kringelbach.
His work can also tell us about when this impulse goes wrong. This is very important in depression, which can be associated with less engagement with other people and a lack of normal feelings of pleasure in situations where it might be expected. But, to understand that, the layers of systems and brain processes involved in experiencing pleasure, both conscious and unconscious, must be stripped back.
One set of experiments by Kringelbach and colleagues used functional magnetic resonance imaging [fMRI] and neatly identified the regions of the human brain involved in experiencing pleasure.
Hungry volunteers were placed in the MRI scanner and were asked to rate their enjoyment of small aliquots of tomato juice or chocolate milk on a subjective pleasantness scale from -2 to +2. Both foods were on average rated around +1.5 by the participants. They were then taken out of the scanner and fed as much as they wanted of either tomato juice or chocolate milk. The now-full volunteers went back into the scanner and the experiment of feeding them both liquid foods was repeated exactly as before. Except that the participants now strongly disliked the food that they had just been fed and rated their subjective experience as negative at -1.5. In contrast, the other food was still experienced as very positive (on average +1.5 or better).
In this way the changes in brain activity in both parts of the experiment could be compared, separating those parts of the brain involved purely in taste and sensory perceptions from those higher systems involved in the subsequent subjective experience of pleasure. The result was that the subjective experience of pleasure appeared to be linked to activity in a particular region of the brain just behind the eyebrows (see figure below). ‘Activity in the mid-anterior orbitofrontal cortex correlates very strongly with how much we like something,’ says Professor Kringelbach.
Other results have similarly found that the rush caused by drugs, music and other pleasures are all linked to the same network of pleasure centres of which the orbitofrontal cortex appears to be the most important in humans. Of course, it was Dutch researchers that confirming the same was true of sex, finding that the difference between real and fake orgasms in women is experienced in the orbitofrontal cortex.
Work elsewhere has also given an insight into the chemical basis of pleasure. US research from Kringelbach’s close collaborator Professor Kent Berridge has shown that pleasure can be studied in other animals such as rats who lick their lips in proportion to the amount of sugar in their water, giving a behavioural measure of pleasure.
Dopamine, a chemical involved in neural processes in the brain, was for many years thought to be the brain’s pleasure chemical. But rodents with more dopamine in their brains didn’t lick their lips any more than normal. Instead they went to the sugar water more quickly. So there seems to be a difference in the brain processes of ‘liking’ (licking lips) and ‘wanting’ (going to get the food reward). This is of interest in understanding addiction, which is associated with a lot of ‘wanting’ or at least an imbalance between liking and wanting. Conversely, opioid drugs such as morphine (which are used to reduce pain), are found to increase ‘liking’ processes.
‘The question is can we use the picture we are building up of pleasure, its causes and its purpose, to be able to restore it. Can we find out what is wrong in treatment-resistant disorders including depression?’ asks Professor Kringelbach. ‘Will it be possible to change the baseline of pleasure in people, possibly even from early childhood?’
To answer these questions, Professor Kringelbach is using both MRI and a technique called magnetoencephalography [MEG], which allows much greater time resolution, to begin to see more of what neurons are actually doing rather than just look at the much slower changes in oxygenation of blood flow in the brain measured with MRI. In this way, it may be possible to monitor real changes in the brain that may result from cognitive behaviour therapies, he suggests. His transnational research group, Hedonia: TrygFonden Research Group [http://kringelbach.org] is using these techniques in collaboration with Professor Alan Stein at Department of Psychiatry to understand the pleasures of the parent-infant relationship. They have recently discovered a potential neural signature for parental instinct in the adult brain, which could potentially help with post-natal depression.
He is also working closely with Professor Tipu Aziz of Oxford University to understand the mechanisms of deep brain stimulation [DBS], which can help in otherwise treatment-resistant cases. DBS, described by Kringelbach as a ‘pacemaker for the brain’, involves implanting electrodes very carefully in specific areas of the brain and applying an electric stimulus at the right frequency to override or correct aberrant brain impulses. It has been used to great effect in a number of patients with movement disorders, such as Parkinson’s and in patients with very severe chronic pain, where nothing else has been successful. Many of these patients are suffering from an acute lack of pleasure in their lives and they describe the pain relief from DBS as deeply pleasurable.
‘DBS is a radical solution and it’s not for everyone,’ says Professor Kringelbach. ‘However, there are people in which this surgery have led to a remarkable change in their quality of life.’