Anticipating excitation

Functional magnetic resonance imaging (fMRI) has revealed a reason why the excitement of unwrapping presents dwindles as our brains get older and more jaded. According to a new study, a biochemical pathway is responsible for mellowing our expectations.

As most parents know, there is nothing more exciting to children than the anticipation of present giving and the unwrapping of those presents. Sometimes, even the unwrapping is more exciting than the gift within and many a parent spends a disappointing morning looking forlornly at expensive toys, while junior plays with the cardboard box in which it came.

Now, Karen Faith Berman and colleagues at the National Institute of Mental Health, Clinical Brain Disorders Branch, Bethesda, Maryland, have investigated how that sense of giddy expectation changes as we get older and have uncovered a biochemical explanation as to why older adults tend not to be so excited by the receiving of surprise gifts or the anticipation of other kinds of reward as younger people.

Berman and her colleagues are well known for their use of a variety of neuroimaging techniques in their investigations of the neurobiology of neuropsychiatric disorders, including schizophrenia, and genetic disorders, such as Williams Syndrome, that affect cognition and brain function.

The researchers knew that the brain chemical dopamine is thought to be involved in the brain's reward processing circuitry. Indeed, it is described metaphorically as the brain's currency of pleasure. It is also understood that the brain's dopamine system declines naturally as a part of normal aging. But until now, the consequences of this decline have remained elusive.

"Significant losses over a normal lifespan have been reported for dopamine receptors and transporters, but very little is known about the neurofunctional consequences of this age-related dopaminergic decline," the researchers explain.

In an experiment designed by Berman and Jean-Claude Dreher now at the Reward and Decision Making Group, Center of Cognitive Neuroscience, of the Centre National de la Recherche Scientifique (CNRS) in Bron, France, the team, which includes Andreas Meyer-Lindenberg (now director of the Central Institute of Mental Health, Mannheim and University of Heidelberg, Germany) and NIMH Special Expert Philip Kohn, used fMRI to observe which parts of the brain are stimulated by rewards. It is not only rewards themselves but simply the promise of rewards that present differently it seems to younger and older adults.

The researchers found, perhaps not surprisingly, that fMRI scans show that activation of dopamine-triggered brain regions differs between older and younger adults. They measured the effects on the brain of anticipation and the receiving of a reward through a video slot machine, rather than the receiving of a wrapped gift, the team explains. They found the effect was stronger in younger subjects with an average age of 25 years, compared to their older counterparts, with an average age of 65 years.

Intriguingly, the researchers point out that Positron Emission Tomography (PET) scans revealed no difference in the production of dopamine in the brains of either the young or old subjects. However, they did find a link between the biochemical synthesis of dopamine (as measured by PET using 6-[¹⁸F]fluoroDOPA) and the reward-related brain activity in people that they had observed using fMRI. This, the team explains, reveals some of the changes in the brain's regulatory circuitry that accompanies normal aging.

"We directly demonstrate a link between midbrain dopamine synthesis and reward-related prefrontal activity in humans, show that healthy aging induces functional alterations in the reward system, and identify an age-related change in the direction of the relationship (from a positive to a negative correlation) between midbrain dopamine synthesis and prefrontal activity," the researchers say.

The results suggest that there is a dopaminergic tuning mechanism that takes place in the aging brain. This leads to changes in the cortical reward processing during otherwise healthy aging. The researchers, however, concede that there are limitations to their experiment, and another explanation may be possible. This will require detailed follow-up work to provide additional evidence for their conclusions. Normal aging, they explain, can also affect the cerebrovascular system, which means that the neurovascular effects observed in their fMRI scans may be different because of simple physical differences between older and younger subjects, rather than actual changes in neural activity alluded to in their work.

That could explain why some adults do still enjoy a sense of excited anticipation. One aspect of this work that remains a puzzle though, is how grandparents manage to feign such great excitement so well when opening presents with the children. Perhaps there is more to the mellowing that Berman and colleagues discuss than meets the eye.

Reference:

J.-C. Dreher, A. Meyer-Lindenberg, P. Kohn, K. F. Berman (2008). Age-related changes in midbrain dopaminergic regulation of the human reward system Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0802127105

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