You're driving by your local fast-food joint. The aroma of meaty burgers tickles your nostrils. Wow, that smells good! I'm hungry! you realize consciously at some point-probably several seconds after you've made an illegal turn from the left lane and whipped up to the drive-thru window.
"Would you like fries with that?"
You get a mental image of succulent, golden brown potatoes drenched in ketchup. "Sure. Why not? Sounds too good to resist."
You might regret that impulse later when trying to button your waistband, but there's no escaping the daily drives of appetite, hunger, and eating behavior. They are hardwired into your brain. You experience a desire for food several times daily, but the motivation isn't always the same. The aroma of grilled meat may trigger your appetite, but so can the sight of a sugar doughnut, the bell of the ice cream truck, or simply the clock on the wall that says it's lunchtime. Hunger arises internally, too, well before you become aware of tummy pangs. The decreasing concentration of glucose in the blood stimulates appetite. The lower your blood glucose level falls, the hungrier you feel . . . and if you haven't eaten for six or eight hours, you're likely to feel ravenous.
Sure, the taste of food begins with nerve impulses in your mouth, but it's your brain that perceives (and perhaps craves) sweet and salty. Your food-wanting and food-liking and food-seeking behaviors arise in your brain, too, and maybe you've noticed that your brain can have a mind of its own when it comes to food. Do sights and smells trigger your food-seeking behavior? Do you change your eating pattern when you're stressed or lonely? Do you eat when you're not hungry or eat more than you know is good for you? Do you crave some foods and not others?
The answers to all those questions lie in more fundamental questions about how the brain is organized. For 150 years, neuroscientists have likened the organization of the brain to the command structure of a military organization, say the U. S. Army. The higher, more recently evolved brain centers of the neocortex act as the generals--so the theory goes--taking charge and issuing directives, while the lower, more primitive brain centers work as privates do, performing routine tasks and taking orders from higher up in the chain of command. It all sounds efficient and orderly, but as any weight watcher knows, the top-down command model of the brain doesn't explain much when it comes to appetite control. Internal signals and external cues trigger undesired eating behavior in even the most determined dieter. Willpower is notoriously unreliable, and neuroscientists are probing the reasons why. In the process, they are constructing new, perhaps more accurate, models of brain organization.
This week in the Proceedings of the National Academy of Sciences, Richard Thompson and Larry Swanson (shown below) of USC suggest that the brain is more like the Internet than the Army. Instead of a top-down chain of command, the brain may be organized in a system of interconnecting circuits in which--along one path or another--everything is connected to everything else and no central command-and-control center operates. The brain's structural connectivity may depend on a network of circuits, meaning that virtually every part of the brain can influence every other part.
The Internet metaphor helps to explain the system of interconnecting circuits Swanson has found in studying the feeding and appetite regions of the brain over the last dozen years. The nucleus accumbens (NA), little known or studied until about ten years ago, is an important part of that circuit. It influences how good a food tastes. Its actions explain, in part, why food tastes better when you're hungry than when you're full.
In this newly published experiment, Thompson and Swanson injected tracer chemicals into a tiny region of the NA in rat brains. The region is renowned as a "hedonic hot spot"; its action stimulates feeding behavior, perhaps because it enhances the pleasure of sweet tastes. The tracers allowed the researchers to follow the routes that nerve impulses travel through various brain structures. The tracings showed that signals travel among the NA and other regions of the brain known to control appetite, stress, and depression.
[For the anatomists among you (see diagram), the tracers revealed a neural circuit of four nodes of a closed loop: a tiny region of the nucleus accumbens (ACBdm); the anterior lateral hypothalamic area (LHAa); the anterior paratenial and paraventricular nuclei (PTa/PVTa); and the cortical infralimbic area (ILA). Some of the pathways were excitatory: they triggered impulses in another part of the brain. Others were inhibitory: they stopped impulses from arising. Some of the outputs innervated brain regions that control metabolism and feeding behavior.]
Around and around in a series of loops, messages travel in all directions. Thus, the "circuit diagram" for appetite isn't the straight-line organizational chart of a military organization; it's a feedback network best compared to computer networks such as the Internet. Appetite isn't the only brain function that may work in an Internet-like fashion. "A lot of influences on behavior are not top-down," Swanson says. "Attention, arousal, the sleep-wake cycle--your behavioral state--those kinds of things aren't under voluntary control either."
The tracing technique may be used to construct a wiring diagram of the entire nervous system, Swanson says. That notion is the impetus for several newly funded projects that will allow neuroscientists to do for the brain what geneticists did for the genome: the goal is to map the "connectome," which means all the interconnecting circuitry of the brain. Swanson is already working to map the connectome of the mouse. Eventually, he and other neuroscientists hope to trace the pathways through which any one of the brain's 500 distinct parts can communicate with all the others. "If you get enough links in the chain, you get back to everywhere," Swanson says, "and once you get into that complicated network, you can knock out different parts of it and it still works--maybe not perfectly, but there are alternative ways to get around."
Thompson and Swanson hope their work will stimulate new mathematical approaches and new ways of thinking about how the brain is organized. "The hierarchical way is the way most people think about it," Swanson says, "[but] when you have loops and loops, the actual definitions of feedback and feed-forward and top and bottom get very fuzzy, and maybe those aren't the best to describe what's happening."
Admittedly, knowledge of the brain's appetite circuitry isn't going to make resisting that juicy burger any easier, but the better we understand our brains, the better equipped we become to fight the battle of the bulge. And thinking about new models of brain circuitry may get your mind off food--for a little while anyway.
For More Information:
Part Three, "Taste," in Brain Sense provides a full discussion (and some entertaining stories) about hedonic hot spots, food cravings and aversions, and experimentation on the nucleus accumbens.
The new research paper is Richard H. Thompson and Larry W. Swanson, "Hypothesis-driven Structural Connectivity Analysis Supports Network over Hierarchical Model of Brain Architecture," Proceedings of the National Academy of Sciences, August 9, 2010.