The world at your feet — the cue to our desires

David H Zald, professor of psychology and psychiatry at Vander bilt University, Nash ville, and a fellow of the Center for Integrative and Cognitive Neuro science, studies the neural basis of emotional processing. Using latest imaging techniques he has explored how smell and taste is processed in the brain, and also the reward mechanism with the neuro chemical dopamine at its centre.Prof Zald is also one of a handful of people who has mastered the Chapman Stick, a ten-stringed instrument played without strumming or bowing. Here, he tells Subodh Varma how the brain strives to seek reward, and how specialist chemicals bring this about.
Dopamine is supposed to induce "rewardseeking" behaviour. What does that mean? Is 'reward' just food and sex? What about listening to somebody playing the Chapman Stick?
Reward seeking refers to any situation in which an individual takes actions aimed at obtaining a reward — regardless of the type of reward. This can include both searching for a reward and performing actions directly related to obtaining the reward. Rewards in this context can be natural rewards (such as food, drink, sex or pleasant sensory experiences), social rewards (such as positive comments from a parent or friend), the effects of drugs of abuse, and monetary rewards that allow ac cess to other rewards cess to other rewards -and for some people listening to music played on the Chapman Stick.
Dopamine plays a couple of roles here. First, the cells that make dopamine respond to cues that signal potential rewards, and appear to help motivate the animal to search for the reward, or work to obtain the reward.They also fire when a reward that is not fully predicted is received. Firing upon getting a reward does not appear to relate to pleasure itself, but provides a learning signal to help the individual better predict rewards in the future.
Does having delicious food or inhaling a pleasurable smell raise dopamine levels? Why do various smells have different effects on the 'mood'?
If the sensory input is experienced as rewarding or is a cue for a potential reward, it will transiently increase dopamine, but note that these increases may be shortlived. In many cases the dopamine increase may last only for a few seconds.There are situations when an individual is working for a goal that there is a longerlasting increase in dopamine, but these longer-lasting increases are not in response to the reward itself.
The issue of smell and mood is an interesting one, but is not specific to dopamine in most cases, and likely reflects the fact that the olfactory system is intimately connected with areas of the brain that are involved in mood. This includes projections to an area called the amygdala, which is heavily involved in emotional arousal, and the orbitofrontal cortex at the base of the frontal lobe, which is involved in mood. The one place where dopamine is likely to be involved is when the olfactory cue creates a desire for a specific reward — like the smell of freshly baked naan, causing a desire to eat.
In somebody suffering from depression, will neurochemicals associated with various kinds of behaviour be deficient? How do these neurochemicals interact with each other?
It would be wrong to think that depression reflects a neurochemical deficiency. Neither serotonin nor dopamine deficits alone are likely to be responsible for depression.That said, some alteration in dopamine functioning may be related to deficits in reward-seeking behaviour in depression, and in some cases drug therapy that includes increasing dopamine functions may be helpful. My colleagues and I have speculated that in cases where the depressed person has strong deficits in motivation, treat ment to improve the dopamine system may be particularly useful.
It is notable in this regard that one empirically validated form of psychotherapy for depression, called behavioural activation therapy , primarily focuses on getting the individual to do activities that can bring about rewards, despite their low motivation.
Dopamine deficit is associated with Parkinson's disease. Are all key neuro chemicals associated with some physiological disorder?
The issue here is complicated in two ways. First, because neuro transmitters such as dopamine effect many processes and may have different behavioural effects in different areas of the brain, there is generally not going to be a one-to one mapping between the neurochemical and just one disease, but rather dysfunction of the neurochemical can impact multiple diseases and their expression.
Treating Parkinson's disease with L-DOPA is the best example of being able to easily rectify a deficit in the production of a neurotransmitter. However, similar approaches may not work for other neurotransmitter-related pathologies due to the need to have high specificity (for instance, only targeting a certain neurotransmitter receptor), or due to the need to target just one brain region.
For most neurological and psychiatric disorders, the neurochemical situation only tells part of the story , and in such cases, pharmacology alone is unlikely to be curative.