Serious and Sillier

Life is a roller coaster. One moment we are up, the next we are down. When we are on top of the world we can be sure we are heading for a big dive. But at least when we hit the bottom the only way to go is up.

Those swinging feelings

We all go through swings in life. Perhaps we find the right job after years of floundering. Or we find the right partner. For a while we feel great. Unfortunately this good feeling doesn't last forever. The new job becomes routine. The honeymoon period comes to an end.*1 Soon everything becomes mundane again. Euphoria switches back to normality.

Our physical pains and pleasures swing up and down in the same way. For me chocolate eclairs are usually delicious. But on occasion they can be absolutely revolting. To be more precise: the first eclair I eat is always very tasty. After about two or three of them, though, the next eclair isn't tasty at all. If I'm not yet disgusted by them, I certainly will be by the time I've eaten the fourth or fifth.

What is curious about this is that even though I'm always eating the same type of eclair, the amount of pleasure I get from each eclair is different. The same stimulus is giving me different intensities of pain or pleasure. This flexibility seems to be true of many, if not all, of our feelings. Most things we do for fun are the most enjoyable if we do them only occasionally and become less enjoyable or unpleasant if we do them too much. *2

Why are feelings flexible like this? The reason is to tailor our wants to meet our needs. If the hundredth eclair tasted as good as the first, I would be heading towards gastric disaster. And if we were always on a high, we wouldn't be motivated to do anything. So our pains and pleasures have to rise and fall only by as much as is necessary to make us react appropriately to the situations we find ourselves in.

A bit of biochemistry

Where does this flexibility come from? To answer this we have to know what the rises and falls are made of. We also have to know a bit about biochemistry and a chemical law called Le Chatelier's principle.

Biochemical systems generally operate through a series of small changes, each of which is reversible. The reversibility of these changes is important: it allows pathways to be easily regulated so that, for instance, organisms produce only the amount of a biochemical that is needed.

It is reasonable, therefore, to suppose that our feelings are also regulated through reversible processes. In previous posts I have speculated that rises and falls in dopamine in the brain may be responsible for pleasurable and painful feelings respectively. Which reversible processes, then, could be responsible for rises and falls in dopamine levels?

Let us imagine that these rises and falls occur in specific synapses. The synapse is a space surrounded by cell membranes. This space, however, is not completely insulated from the outside. Biochemicals such as dopamine can pass through the membranes at certain points, via "gates" which regulate how much of the biochemical comes in and out. We can express the diffusion of dopamine between the synapse and the outside as follows:

[Dopamine] _i  " [Dopamine] _e

Where:  [Dopamine] means "concentration of dopamine"

i means inside the synapse,

e means outside the synapse,

and " means a reversible process.

In this case the reversible process is the movement of dopamine through the cell membranes: dopamine can travel both ways, in and out of the synapse. If there hasn't been any change for a while, the concentrations of dopamine inside and outside of the synapse reach a balance. This means the concentrations remain steady because the flow of dopamine in equals the flow out. At this balanced point the process is said to be at dynamic equilibrium.

Other reversible processes inside the synapse itself may be responsible for the concentration of dopamine reaching dynamic equilibrium. Catalysed by enzymes in the synaptic fluid, dopamine may be converted into another substance; and in the same reaction, this other substance may be converted back into dopamine. We can express this conversion as follows:

[Dopamine] " [Pre-dopamine]

Where [Pre-dopamine] represents the concentration of a compound which is similar to dopamine but which doesn't have its neurological properties.

Returning to the point

What happens if there is a influx of dopamine into the synapse? Because the concentration has risen rapidly, more dopamine will flow out of the synapses than will flow in. In other words, dopamine will diffuse out, bringing the concentration of dopamine back down. The influx will also trigger a net conversion of dopamine to pre-dopamine. Both these processes - diffusion and conversion - will eventually lead to the concentration of dopamine returning to a point of equilibrium.*3

In Attraction I imagined how a system of rising and falling dopamine might work in practice. I had myself walking past a bakery shop and smelling a pleasant aroma. Messages were sent from my nose to dopaminergic neurons in my brain. This triggered a release of dopamine into synapses. After this sudden rise I wrote levels of dopamine would begin to fall. I suggested a graph of dopamine concentration over time would look something like this:

"

Of course, the reason why dopamine levels start to fall after the influx is because of the processes of diffusion and conversion described above. The equilibrium has been upset and, as we expect from Le Chatelier's principle, begins to restore itself. The concentration of dopamine gradually returns to a position of equilibrium, which in the graph above I have called the basal concentration.

It's more than attraction

I proposed in that post that this rise in dopamine followed by a fall is responsible for our feelings of attraction. The rising part of the curve corresponds to the pleasure I feel on smelling the aroma; the falling part of the curve represents the pain of wanting - the feeling which motivates me to go into the bakery and buy something. The same sequence of events would happen if I saw chocolate eclairs in the shop window, or indeed whenever I came across any stimulus which I find attractive.

It is my guess, though, that this sequence is responsible not only for feelings of attraction but also for each and every one of our feelings of pleasure.*4 Suppose I next go into the bakery, buy and begin to eat an eclair. Sensory receptors on my tongue and in my nose send messages to dopaminergic neurones in my brain which again release dopamine into synapses. This means I feel pleasure - as I usually do after taking my first bite of an eclair.

Next we can expect that the levels of dopamine begin to fall. My consciousness interprets this fall as the pain of wanting. That makes me take another bite, which again causes another rise and fall in my dopamine levels. That again makes me take another bite...and I am likely to continue eating as long as I experience these waves of pain and pleasure.

Why we stop eating

Why don't I carry on eating eclairs forever? What makes me stop eating long before my stomach ruptures? The answer lies with the dynamics of the dopamine equilibrium. We can see in the graph above that it takes a relatively long time before dopamine returns to its basal concentration. The fall takes longer than the initial rise - in fact, so long that it ends at some time off the graph.

What would happen if I took my second bite before the dopamine level returned to the basal concentration? A graph showing the change in dopamine concentration for the first and second bite would look something like this:

"

Where 1 is the time of the first bite and 2 is the time of the second bite. We can see that the concentration of dopamine rises after the second bite, but this rise is not as steep as the first. The time from the second bite to the second peak is also shorter than for the first.

Why are there these differences in the two peaks? The reason has to do with the dynamics of reversible processes. The higher the concentration of dopamine is, the faster the rate of reaction is back towards the basal concentration. We could imagine that this happens because of a kind of "pressure" which brings the processes back to equilibrium - further the dopamine level is away from the basal concentration, the greater this "pressure" is.*5

In practice this means that the dopamine produced after the second bite is mopped up quicker that it was after the first bite. A greater "pressure" pulls the concentration back down again, which not only makes the rise briefer but makes it more gradual too.

Rises in dopamine are interpreted by the consciousness as pleasure, and, I believe, less steep rises are interpreted as less intense pleasure. The pleasure from the second bite, therefore, is slightly less intense and doesn't last as long. Already my eclair is beginning to be less tasty.

The eclair still gives me enough pleasure though to make me want to finish it. But what happens when I eat two eclairs in quick succession and start on my third? My dopamine levels already have been pushed very high which means that with each subsequent bite the rises become flatter and shorter. By the time I begin my fourth eclair the rises are negligible and probably aren't detected by the consciousness at all. At this point the eclairs have stopped being tasty.

Disgust

The fact that the eclairs become less tasty usually is enough to persuade me to stop eating well before my fourth eclair - but sometimes, unfortunately, I get carried away and keep on eating until I feel disgusted. Where does this disgust come from? Partly, I think, it comes from the same equilibrium which made the eclairs tasty in the first place. When my dopamine levels are high from eating to many eclairs, the equilibrium force pulling the concentration down again is strong. That means sharp falls in dopamine, or, in other words, pain.

Why, though, isn't this pain felt as a want to eat more eclairs, as it was when I ate my first one? The reason, I guess, is because there wasn't a significant rise before the fall. Somehow the brain only interprets a pain as a want "for more" when that pain is linked to a specific pleasure. Simply put - if the eclair doesn't taste good, I won't want to eat it any more.

How honeymoons end

Can high levels of dopamine also explain why honeymood periods finish in big crashes? Probably to some extent - but that's not the whole picture. The pains and pleasure of eclairs are quite short-lived. Eclairs become tasty again after a day or two following a binge. In contrast, honeymoon periods can last for years but their euphoric spirit can rarely be repeated. What's the reason for these differences?

We have to remember that a honeymoon isn't only one experience: it is a host of experiences all put together. Each new sensation pushes the dopamine levels up, giving us pleasure. But we don't feel the downs too much, because as soon as the pleasure of one sensation begins to turn to pain, we switch our attention to another new sensation which is still pushing the dopamine levels up. Consciousness jumps from one pleasure to another, and can go on like this for a long time - but not, of course, for ever.

There is one more factor which makes honeymoons come to an end: boredom. New sensations are exciting but old ones are boring. This occurs because of the process of learning - that is, new information, experienced consciously, is increasingly fixed into unconscious concepts. This is a topic I hope to examine more fully in another post.

It's not that simple

In comparison to the complexity of honeymoons, we could say that eating eclairs is a simple event. Actually, though, it isn't a single experience and it's not that simple either. When I eat eclairs different kinds of receptors in my eyes, on my tongue, in my nose and in my stomach are all stimulated. For each kind of stimulus dopamine levels will be sent up, or possibly down. My consciousness will track each of these ups and downs, and will decide, on the basis of the steepness of the rises and falls, whether I will take another bite or not.

It could happen, for example, that even though my stomach is already sending pain signals, the sharp rises I get from sugar will mean that I will concentrate on those instead and will carry on eating. Similarly disgust, unlike in my description above, is unlikely to be caused by the falling of dopamine from one type of stimulus, but because of a combination of different stimuli: let's say, the sugar and the cream stop being tasty and I begin to feel discomfort in my stomach.

Regardless of whether experiences are simple or complex, the pains and pleasures they give are probably all regulated in a relatively straightforward way by one dynamic equilibrium involving dopamine. In this post I have focussed on experiences which are mainly pleasurable. In the next post I will concentrate on the dynamics of painful experiences.

*1. In this post I use the word honeymoon to mean any euphoric or joyful phase, not only the time immediately following a wedding.

*2. This means the roller coaster isn't a perfect metaphor for feelings: roller coasters move on fixed rails and so their movements aren't flexible.

*3. In these two processes I am applying Le Chatelier's principle. This states that processes which are at dynamic equilibrium, and which then are subjected to a change, in time return to a new equilibrium by counteracting the change.

*4. On reflection I realise I need to qualify this sentence: ".....every one of our feelings of pleasure caused by pleasurable stimuli." We can feel pleasure from painful stimuli too and this occurs through a different (even if similar) mechanism. I discuss this mechanism in The dynamics of pain.

*5. What really happens is that as there is more dopamine at high concentrations, dopamine molecules are either more likely to react or more likely to diffuse out, and so the rates of these processes in these particular directions are faster.