The neurotransmitter dopamine is one we all seem to have heard about, claimed to be central to love, gambling, reward, addiction, etc. A recent article by Howe et al. shows a bit more nuance than previously assumed in what dopamine levels are signaling. They ramp up during navigation towards an expected reward. I thought I would pass on a clip from Niv’s summary of their findings, followed by the Howe et al. abstract
a, Dopaminergic neurons in the midbrain project to all brain areas, most prominently to the striatum (black arrows). These cells fire at a constant rate of 3–5 spikes per second, with occasional phasic bursts or pauses on the occurrence of positive reward prediction errors (discovering that the local supermarket now supplies your favourite coffee beans, b) or negative reward prediction errors (sipping your coffee and finding that the milk has gone sour, c), respectively. The background (tonic) level of dopamine fluctuates slowly, possibly tracking the average rate of rewards (not shown). d, By measuring dopamine concentrations in the striatum of rats navigating mazes, Howe et al.1 reveal a third mode of dopaminergic signalling: when a prolonged series of actions must be completed to obtain a reward (for instance, all the steps it takes to make a cup of coffee), dopamine concentration ramps up gradually, at each point in time signalling the predicted distance from the goal.
Predictions about future rewarding events have a powerful influence on behaviour. The phasic spike activity of dopamine-containing neurons, and corresponding dopamine transients in the striatum, are thought to underlie these predictions, encoding positive and negative reward prediction errors. However, many behaviours are directed towards distant goals, for which transient signals may fail to provide sustained drive. Here we report an extended mode of reward-predictive dopamine signalling in the striatum that emerged as rats moved towards distant goals. These dopamine signals, which were detected with fast-scan cyclic voltammetry (FSCV), gradually increased or—in rare instances—decreased as the animals navigated mazes to reach remote rewards, rather than having phasic or steady tonic profiles. These dopamine increases (ramps) scaled flexibly with both the distance and size of the rewards. During learning, these dopamine signals showed spatial preferences for goals in different locations and readily changed in magnitude to reflect changing values of the distant rewards. Such prolonged dopamine signalling could provide sustained motivational drive, a control mechanism that may be important for normal behaviour and that can be impaired in a range of neurologic and neuropsychiatric disorders.
If you have ever wondered why people’s arms and legs twitch suddenly as they are drifting off to sleep, our resident psychologist Tom Stafford has the answer.
As we give up our bodies to sleep, sudden twitches escape our brains, causing our arms and legs to jerk. Some people are startled by them, others are embarrassed. Me, I am fascinated by these twitches, known as hypnic jerks. Nobody knows for sure what causes them, but to me they represent the side effects of a hidden battle for control in the brain that happens each night on the cusp between wakefulness and dreams.
Normally we are paralysed while we sleep. Even during the most vivid dreams our muscles stay relaxed and still, showing little sign of our internal excitement. Events in the outside world usually get ignored: not that I’d recommend doing this but experiments have shown that even if you sleep with your eyes taped open and someone flashes a light at you it is unlikely that it will affect your dreams.
But the door between the dreamer and the outside world is not completely closed. Two kinds of movements escape the dreaming brain, and they each have a different story to tell.
The most common movements we make while asleep are rapid eye-movements. When we dream, our eyes move according to what we are dreaming about. If, for example, we dream we are watching a game of tennis our eyes will move from left to right with each volley. These movements generated in the dream world escape from normal sleep paralysis and leak into the real world. Seeing a sleeping persons’ eyes move is the strongest sign that they are dreaming.
Hypnic jerks aren’t like this. They are most common in children, when our dreams are most simple and they do not reflect what is happening in the dream world - if you dream of riding a bike you do not move your legs in circles. Instead, hypnic jerks seem to be a sign that the motor system can still exert some control over the body as sleep paralysis begins to take over. Rather than having a single “sleep-wake” switch in the brain for controlling our sleep (i.e. ON at night, OFF during the day), we have two opposing systems balanced against each other that go through a daily dance, where each has to wrest control from the other.
Deep in the brain, below the cortex (the most evolved part of the human brain) lies one of them: a network of nerve cells called the reticular activating system. This is nestled among the parts of the brain that govern basic physiological processes, such as breathing. When the reticular activating system is in full force we feel alert and restless - that is, we are awake.
Opposing this system is the ventrolateral preoptic nucleus: ‘ventrolateral’ means it is on the underside and towards the edge in the brain, ‘preoptic’ means it is just before the point where the nerves from the eyes cross. We call it the VLPO. The VLPO drives sleepiness, and its location near the optic nerve is presumably so that it can collect information about the beginning and end of daylight hours, and so influence our sleep cycles.
As the mind gives in to its normal task of interpreting the external world, and starts to generate its own entertainment, the struggle between the reticular activating system and VLPO tilts in favour of the latter. Sleep paralysis sets in. What happens next is not fully clear, but it seems that part of the story is that the struggle for control of the motor system is not quite over yet. Few battles are won completely in a single moment. As sleep paralysis sets in remaining daytime energy kindles and bursts out in seemingly random movements. In other words, hypnic jerks are the last gasps of normal daytime motor control.
Some people report that hypnic jerks happen as they dream they are falling or tripping up. This is an example of the rare phenomenon known as dream incorporation, where something external, such as an alarm clock, is built into your dreams. When this does happen, it illustrates our mind’s amazing capacity to generate plausible stories. In dreams, the planning and foresight areas of the brain are suppressed, allowing the mind to react creatively to wherever it wanders - much like a jazz improviser responds to fellow musicians to inspire what they play.
As hypnic jerks escape during the struggle between wake and sleep, the mind is undergoing its own transition. In the waking world we must make sense of external events. In dreams the mind tries to make sense of its own activity, resulting in dreams. Whilst a veil is drawn over most of the external world as we fall asleep, hypnic jerks are obviously close enough to home - being movements of our own bodies - to attract the attention of sleeping consciousness. Along with the hallucinated night-time world they get incorporated into our dreams.
So there is a pleasing symmetry between the two kinds of movements we make when asleep. Rapid eye movements are the traces of dreams that can be seen in the waking world. Hypnic jerks seem to be the traces of waking life that intrude on the dream world.
Why do some people prefer adventure and the company of others, while others favour being alone? It’s all to do with how your brain processes rewards.
Will you spend Saturday night in a crowded bar, or curled up with a good book? Is your ideal holiday adventure sports with a large group of mates and, or anywhere more sedate destination with a few good friends? Maybe your answers to these questions are clear – you’d love one option and hate another – or maybe you find yourself somewhere between the two extremes. Whatever your answers, the origin of your feelings may lie in how your brain responds to rewards.
We all exist somewhere on the spectrum between extroverts and introverts, and different circumstances can make us feel more one way or the other. Extraverts, a term popularised by psychologist Carl Jung at the beginning of the 20th Century, seem to dominate our world, either because they really are more common, or because they just make most of the noise. (The original spelling of “extravert” is now rarely used generally, but is still used in psychology.) This is so much the case that some have even written guides on how to care for introverts, and nurture their special talents.
A fundamental question remains – what makes an extrovert? Why are we all different in this respect, and what do extraverts have in common that makes them like they are? Now, with brain scans that can record activity from deep within the brain, and with genetic profiling that reveals the code behind the constructions of the chemical signalling system used by the brain, we can put some answers to these decades-old questions.
In the 1960s, psychologist Hans Eysenck made the influential proposal that extroverts were defined by having a chronically lower level of arousal. Arousal, in the physiological sense, is the extent to which our bodies and minds are alert and ready to respond to stimulation. This varies for us all throughout the day (for example, as I move from asleep to awake, usually via few cups of coffee) and in different circumstances (for example, cycling through the rush-hour keeps you on your toes, heightening arousal, whereas a particularly warm lecture theatre tends to lower your arousal). Eysenck’s theory was that extroverts have just a slightly lower basic rate of arousal. The effect is that they need to work a little harder to get themselves up to the level others find normal and pleasant without doing anything. Hence the need for company, seeking out novel experiences and risks. Conversely, highly introverted individuals find themselves overstimulated by things others might find merely pleasantly exciting or engaging. Hence they seek out quiet conversations about important topics, solitary pursuits and predictable environments.
More recently, this theory has been refined, linking extroversion to the function of dopamine, a chemical that plays an intimate role in the brain circuits which control reward, learning and responses to novelty. Could extroverts differ in how active their dopamine systems are? This would provide a neat explanation for the kinds of behaviours extroverts display, while connecting it to an aspect of brain function that we know quite a lot about for other reasons.
Researchers lead by Michael Cohen, now of the University of Amsterdam, were able to test these ideas in a paper published in 2005. They asked participants to perform a gambling task while in the brain scanner. Before they went in the scanner each participant filled out a personality profile and contributed a mouth swab for genetic analysis. Analysis of the imaging data showed how the brain activity differed between extroverted volunteers and introverted ones. When the gambles they took paid off, the more extroverted group showed a stronger response in two crucial brain regions: the amygdala and the nucleus accumbens. The amygdala is known for processing emotional stimuli, and the nucleus accumbens is a key part of the brain’s reward circuitry and part of the dopamine system. The results confirm the theory – extroverts process surprising rewards differently.
When Cohen’s group looked at the genetic profiles of the participants, they found another difference in reward-related brain activity. Those volunteers who had a gene known to increase the responsiveness of the dopamine system also showed increased activity when they won a gamble.
So here we see part of the puzzle of why we’re all different in this way. Extrovert’s brains respond more strongly when gambles pay off. Obviously they are going to enjoy adventure sports more, or social adventures like meeting new people more. Part of this difference is genetic, resulting from the way our genes shape and develop our brains. Other results confirm that dopamine function is key to this – so, for example, genes that control dopamine function predict personality differences in how much people enjoy the unfamiliar and actively seek out novelty. Other results show how extroverts learn differently, in keeping with a heighted sensitivity to rewards due to their reactive dopamine systems.
Our preferences are shaped by the way our brains respond to the world. Maybe this little bit of biological psychology can help us all, whether introverts or extroverts, by allowing us to appreciate how and why others might like different things from us.