Given my own over-indulgence this Easter, I needed little excuse to hunt around in the neuroscience literature for some possibly-beyond-consious-control neural mechanism that means my unabashed chocolate scoffing is entirely not my fault (of course, that raises the whole issue of consciousness and the neural basis of free will, but that’s another story for another blog post…).
It’s an issue that isn’t entirely out of my remit, so this is also a great opportunity to intoduce a few key ideas related to my own research that will probably form the bulk of some future blogposts. I spend most of my time studying a group of structures bang in the middle of the brain known as the ‘basal ganglia’ (not, as my sister calls them, the basal danglies). They’re pretty old, evolutionarily speaking, and are heavily implicated in lots of different functions.
A couple of important functions here are believed to be learning associations between stimuli and reward (think Pavlov’s dog learning to associate a bell with food), and in representing predictions of the ‘reward value’ of an event or stimulus. A quick caveat here: in the neurosciences, ‘reward’ is a term that can be bandied about a bit carelessly without being properly defined. This can lead to a lot of talking across purposes and confusing poor PhD students in meetings (true story). So, just to be clear, by reward I don’t just mean something that is intrinsically pleasurable, though this is included in the term. I regard reward as something that tends to ‘reinforce’ the behaviour that brought it about; it encourages us to do again whatever caused the reward**. So, ‘reward value’ may be thought of as the degree to which an event or stimulus is either pleasurable or reinforcing.
It has been demonstrated widely that (expected) reward value may be encoded in a region of the basal ganglia known as the ventral striatum (though trying to figure out whether it’s pronounced stree-ah-tum, stree-ay-tum or stry-ay-tum has robbed me of a disproportionate amount of good procrastination time). This part of the brain is also implicated in influencing the actions we take based on motivational information. Projecting to this area is a region known as orbitofrontal cortex. This is a ‘new’ part of the brain in evolutionary terms, and sits just above the eyeballs. It too appears to represent information relating to reward value, and has also been implicated in high level functions like suppressing instinctive responses and urges. People who have damage to this region often show behavioural dysfunction such as impulsitivity and compulsiveness.***
So – where does chocolate come into all this? I can only speak for myself of course, but I certainly find chocolate rewarding. Spectacularly rewarding. So rewarding that I’ve had to stop bringing spare change to work since the arrival of ‘Claudia’, our beautiful new departmental vending machine, for fear of ending up the size of a house before the year is out (it’s the galaxy caramels that really do a lot of damage). To study the – rewarding, amongst other – effects of the sight and taste of chocolate on neural activity, Edmund Rolls and Ciara McCabe of Oxford University**** performed an fMRI study examining neural responses to chocolate in chocolate cravers and non-cravers. The results were pretty interesting.
First of all, cravers showed more brain activity than non-cravers at the sight and the taste of chocolate in the orbitofrontal cortex (which I have mentioned above). Even more interesting, sight and taste combined produced an effect even greater than the sum of the effects of sight and taste alone, and this was also yet more pronounced in cravers. Cravers also showed a high correlation between brain activity here and how pleasant they said they found chocolate. This shows that the higher the ‘pleasantness’ rating, the stronger the activity was. Again, this was the case more for cravers than non cravers.
What is really interesting though, is that brain regions involved directly in the representation of taste, most specifically the anterior insula, did not show greater activation in cravers. Neither was activity here correlated with pleasantness ratings. Also, while ventral striatum showed greater activation in cravers at the sight of chocolate, there was no difference here between cravers and non cravers for the taste of chocolate.
This might all seem like an awful lot of interactions, but let’s look at it simply: brain regions involved in representing reward value, and those involved in suppressing instinctive behaviour, were generally more active in cravers, particularly when anticipating – rather than consuming – chocolate. Regions involved in taste showed no difference. Cravers also seemed more aware of their own future responses to receiving chocolate. The upshot is that it probably isn’t the actual sensory experience of eating chocolate that influences cravings, but that the anticipation of a pleasurable experience is greater for cravers, and the subsequent reward is represented to a greater degree. It may be that cravers have a stronger learned association between the notion of chocolate (including the sight of it) and the expectation of reward – this would explain the greater drive for cravers to eat chocolate.
So next time you find yourself craving a dairy milk, don’t be too hard on yourself. Clearly, you’re just a brilliant learner, and that association between chocolate and reward is incredibly strongly represented in your highly efficient brain. At least, that’s what I like to tell myself…
* Disclaimer bumpf: I nabbed this picture from wikipedia. I don’t own it… I think I’m allowed to use it.
**Those in the know will be surprised that I haven’t mentioned the ‘phasic dopamine’ signal here. It is certainly relevant and may well act as a ‘do-it-again’ signal. The reason I haven’t gotten into it here is because the exact nature of the signal is still hotly debated and to attempt to outline it in a paragraph or two would be to traverse a minefield/labyrinth/dreary conference hall of empassioned academics. However, watch this space, and it may well form the bulk of an extended future blog post.
***One famous case is that of Phineas Gage whose frontal lobes were severely damaged when a tamping iron impaled his skull.
****The super interested can find the original paper here