Unknown desires, the pallidum, and Parkinson's
By David Dobbs
This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
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Welcome to
Mind Matters
where top researchers in neuroscience, psychology, and psychiatry explain and discuss the findings and theories driving their fields. Readers can join them. We hope you will.
This week we take stock of
Unknown Desires
You may grab for gold even when you don't know it's there.
Image copyright iStockPhoto
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Introduction
by David Dobbs
Editor, Mind Matters"Thy strength shall be according to the measure of thy desire," asserts an Arab proverb. This wisdom now has an brain-imaging study to back it up. In the study ("
How the Brain Translates Money into Force: A Neuroimaging Study of Subliminal Motivation," published this April in
Science, a team of researchers at the
University College London Centre for Neuroimaging combined subliminal messages and the lure of money to reveal that people will work harder (and display more hand strength) for a larger reward even when they're not consciously aware of the award's size. This finding doesn't simply physicalize the obvious. As University of Toronto medical student and Rhodes Scholar
Navindra Persuad describes below, the paper also refines our view of neural circuits of reward, motivation, and action that may be important in afflictions ranging from drug addiction to Parkinson's disease.
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Why are you working so hard?
Navindra Persaud
University of Oxford, Oxford, UKPeople generally work harder when the stakes are higher. For this reason employees are offered commissions or performance-based bonuses and the playoffs are more exciting than regular season games. But do we need conscious awareness of what the stakes are in order to adjust our level of effort? This
study of subliminal motivation by Mathias Pessiglione and colleagues suggests not. Sometimes we work harder without consciously knowing the stakes. Pessiglione and colleagues gave people the opportunity to earn money by squeezing a hand grip many times. There was a performance-based incentive: people earned more if they squeezed harder. But the value of each squeeze varied. Sometimes up to one British pound could be earned by squeezing hard. Other times you could earn no more than a penny regardless of how forcefully the person clutched the grip. The maximum value of each squeeze was presented before each round as a picture of either a pound coin or a penny flashed on a screen. Not surprisingly, when people saw that a pound was at stake they really worked their muscles; they did not squeeze so hard when they saw that even maximum effort would earn them only a penny. The researchers then added a wrinkle that let them test subconscious motivation: On some trials they flashed the pound or penny image for only 1/50th of a second -- a glimpse so brief that the subjects were not conscious of seeing the image. Strikingly, despite being unable to consciously determine which coin was shown, people still squeezed harder after the pound coin than after the penny. This reaction to a motivation never consciously perceived is the equivalent of a salesperson working harder to close a larger deal than a smaller one despite not knowing at a conscious level which will bring the larger commission. What was going on in these people's brains? The researchers identified one part of the brain, the
pallidum, that was always more active when the pound coin was shown, regardless of whether it was consciously seen. The pallidum thus seems to represent both conscious and subconscious motivation.
Desire and decision Although the effects of subconscious motivation in this study were very small (people only squeezed slightly harder for the subconsciously "seen" pound coin), the principle supports the notion that subconscious motivations influence our everyday behavior. Deciding how much time or effort to invest in an endeavor is often as important as deciding what to do. For example, gauging the relative amount of effort you put into each of your upcoming exams may be as important as determining that you'll study at all. This example also raises the question of whether there is a difference between subconscious motivation and subconscious decision-making. Is there a distinction between saying that the people in the experiment were subconsciously motivated to squeeze hard (when the pound coin was shown) and saying that they subconsciously decided to squeeze hard? This question may sound like splitting hairs. But the novelty and significance of this study relies on such a subtle distinction because there are already a large number of papers about subconscious decision-making. Brain activation data may help disentangle decisions and motivations. The pallidum was thought to represent motivation in this task because it was more active for the pound than for the penny. But the brain must convert the image of the pound coin or penny into a motivation before it is used. Are the brain mechanisms involved in this conversion the same as those used in other types of subconscious decision-making? If so, the distinction between subconscious decision-making and subconscious motivation would start to blur.
Motivation in health and disease These findings may ultimately have clinical implications, because an excess or lack of motivation can lead to ill health. For example, drug addicts continue to abuse substances even when they know at a conscious level that their addiction is destructive. Might their most desperate drug-seeking behavior share a common neural mechanism with the benign squeezes of the participants in this study? Intriguingly,
a number of studies have implicated the pallidum in the formation and maintenance of addictions. Perhaps addiction is driven partly by a motivation that is strong but unrecognized -- and therefore harder to counter. On the other hand, people with depression suffer a reduced level of motivation. Could they be subconsciously seeing pennies where others see pounds? Activation in the pallidum has been associated with the pleasure and desire most people feel regarding things such as chocolate. Perhaps such subliminal motivation is missing or inhibited in depression. This study may also provide a new framework for understanding the difficulty that Parkinson's disease patients have in initiating movements. Some people with
Parkinson's disease have difficulty starting to walk on their own accord and yet can walk when prompted to do so by, for example, cues painted on the floor. Severely affected patients who can hardly walk across the room have been known to walk briskly outdoors in response to a fire alarm. Walking may require both a conscious drive and a subconscious motivation that is amiss in Parkinson's, perhaps because of abnormalities in the pallidum and its associated neural circuitry. Fire alarms and other cues may trigger (or bypass) this dysfunctional subconscious motivation system. The pallidum's consistent activation in this study suggests it may serve as a useful index of subconscious motivation. Measuring the average effect that a prospective treatment of, for example, Parkinson's disease has on pallidal activation in a subliminal motivation task (like the one used in this study) may offer some insight into the treatment's function. Measuring the effect of an existing treatment in a given patient may help to predict which treatment best suits the individual. The prospect of such practical applications, although still distant, justifies the excitement that this study has generated.
Navindra Persaud is a graduate student at the University of Oxford and a medical student at the University of Toronto. One of his papers on measuring consciousness was reviewed in Mind Matters by Christof Koch and Kerstin Preuschoff.