Imagine the typical office birthday party.

It's after lunch, so everybody is full. Then, in comes a luscious chocolate confection. The sight, the smell -- even the sound of the word 'cake!' -- stimulate the reward-and-pleasure circuits of the brain, activating memory centers and salivary glands as well.

Those reactions quickly drown out the subtle signals from the stomach that are saying, in effect, 'Still digesting down here. Don't send more!' Social cues add pressure and permission to indulge. Soon, everybody is having a slice -- or two.

Scholars have understood the different motives for eating as far back as Socrates, who counseled, 'Thou shouldst eat to live, not live to eat.' But nowadays, scientists are using sophisticated brain-imaging technology to understand how the lure of delicious food can overwhelm the body's built-in mechanism to regulatehunger and fullness, what's called 'hedonic' versus 'homeostatic' eating.

One thing is clear: Obese people react much more hedonistically to sweet, fat-laden food in the pleasure and reward circuits of the brain than healthy-weight people do. Simply seeing pictures of tempting food can light up the pleasure-seeking areas of obese peoples' brains.

Two conferences this week on obesity are each examining aspects of how appetite works in the brain and why some people ignore their built-in fullness signals. Scientists hope that breakthroughs will lead to ways to retrain people's thinking about food or weight-loss drugs that can target certain brain areas.

In a study presented this week at the International Conference on Obesity in Stockholm, researchers from Columbia University in New York showed pictures of cake, pies, french fries and other high-calorie foods to 10 obese women and 10 non-obese women and monitored their brain reactions on fMRI scans. In the obese women, the images triggered a strong response in the ventral tegmental area (VTA), a tiny spot in the midbrain where dopamine, the 'desire chemical,' is released. The images also activated the ventral pallidum, a part of the brain involved in planning to do something rewarding.

'When obese people see high-calorie foods, a widespreadnetwork of brain areas involved in reward, attention, emotion, memory and motor planning is activated, and all the areas talk to each other, making it hard for them to resist,' says lead investigator Susan Carnell, a research psychiatrist at the New York Obesity Research Center at Columbia University.

Similar brain reactions occurred in the obese subjects even when researchers merely said the words 'chocolate brownie' -- but not when they saw or heard about lower calorie foods such as cabbage and zucchini. Reactions were far less pronounced in the non-obese subjects.

More such studies are being presented in Pittsburgh this week at the annual meeting of the Society for the Study of Ingestive Behavior. In one, neuroscientists from Yale University's John B. Pierce Laboratory had 13 overweight and 13 normal-weight subjects smell and taste chocolate or strawberry milkshakes and observed their brains with fMRI scans.

The overweight subjects had strong reactions to the food in the amygdala -- the emotional center of the brain -- whether they were hungry or not. The healthy-weight subjects showed an amygdala response only when they were hungry.

'If you are of normal weight, your homeostatic mechanisms are functioning and controlling this region of the brain,' says lead investigator Dana Small. 'But in the overweight group, there is some sort of dysfunction in the homeostatic signal so that even though they weren't hungry, they were vulnerable to these external eating cues.'

Studies have found that a diet of sweet, high-fat foods can indeed blunt the body's built-in fullness signals. Most of them emanate from the digestive tract, which releases chemical messengers including cholecystokinin, glucagon-like peptide and peptide YY when the stomach and intestines are full. Those signals travel up to the brain stem and then the hypothalamus, telling the body to stop eating.

Obesity also throws off the action of leptin, a hormone secreted by fat tissue that tells the hypothalamus how much energy the body has stored. Leptin should act as a brake against overeating, and it does in normal-weight people. But most obese people have an overabundance of leptin, and somehow their brains are ignoring the signal.

All these findings beg the question, which came first? Does obesity disrupt the action of leptin, or does a malfunction in leptin signaling make people obese?

Similarly, are some people obese because their brains overreact to tempting food, or do their brains react that way because something else is driving them to overeat? Researchers at Yale and elsewhere are turning to such questions next. 'It's possible that these changes reflect how the brain has adapted to eating patterns in obese people, and that could create a viciouscircle, putting them at risk for even more disordered eating,' says Dr. Small.

Melinda Beck

• reward [ri´wɔ:d] n.&v.报答；报酬；奖赏 (初中英语单词)
• stomach [´stʌmək] n.胃；胃口，食欲 (初中英语单词)
• pressure [´preʃə] n.压榨 vt.对...施压力 (初中英语单词)
• permission [pə´miʃən] n.允许；同意；许可 (初中英语单词)
• delicious [di´liʃəs] a.美味的，可口的 (初中英语单词)
• hunger [´hʌŋgə] n.饥饿；渴望 (初中英语单词)
• appetite [´æpitait] n.欲望；食欲 (初中英语单词)
• international [,intə´næʃənəl] a.国际的，世界的 (初中英语单词)
• conference [´kɔnfərəns] n.讨论(会)；会谈 (初中英语单词)
• columbia [kə´lʌmbiə] n.哥伦比亚 (初中英语单词)
• emotion [i´məuʃən] n.感情；情绪；激动 (初中英语单词)
• research [ri´sə:tʃ] n.&vi.调查；探究；研究 (初中英语单词)
• cabbage [´kæbidʒ] n.卷心菜；洋白菜 (初中英语单词)
• pittsburgh [´pitsbə:g] n.匹兹堡 (初中英语单词)
• annual [´ænjuəl] a.每年的 n.年刊 (初中英语单词)
• pierce [piəs] v.刺穿；突破；洞察 (初中英语单词)
• emotional [i´məuʃənəl] a.易动感情的；情感的 (初中英语单词)
• normal [´nɔ:məl] a.正规的 n.正常状态 (初中英语单词)
• chemical [´kemikəl] a.化学的 n.化学制品 (初中英语单词)
• energy [´enədʒi] n.活力，精力；能力 (初中英语单词)
• elsewhere [,elsweə] ad.在别处；向别处 (初中英语单词)
• reflect [ri´flekt] v.反射；反响；表达 (初中英语单词)
• circle [´sə:kəl] n.圆圈 v.环绕；盘旋 (初中英语单词)
• typical [´tipikəl] a.典型的；象征的 (高中英语单词)
• stimulate [´stimjuleit] v.刺激；服兴奋剂 (高中英语单词)
• saying [´seiŋ, ´sei-iŋ] n.言语；言论；格言 (高中英语单词)
• indulge [in´dʌldʒ] v.(使)沉迷；沉溺；放任 (高中英语单词)
• overwhelm [,əuvə´welm] vt.压倒；淹没；打翻 (高中英语单词)
• regulate [´regjuleit] vt.管理；调整；规定 (高中英语单词)
• seeing [si:iŋ] see的现在分词 n.视觉 (高中英语单词)
• ignore [ig´nɔ:] vt.忽视，不理，不顾 (高中英语单词)
• response [ri´spɔns] n.回答；响应 (高中英语单词)
• pronounced [prə´naunst] a.发出音的；显著的 (高中英语单词)
• behavior [bi´heiviə] n.举止，行为 (高中英语单词)
• laboratory [lə´bɔrətəri] n.实验室；研究室(所) (高中英语单词)
• strawberry [´strɔ:bəri] n.草莓 (高中英语单词)
• external [ik´stə:nəl] a.外部的；外面的 (高中英语单词)
• tissue [´tiʃu:, -sju:] n.织物，薄绢，纸 (高中英语单词)
• mechanism [´mekənizəm] n.机械装置；机制 (英语四级单词)
• fullness [´fulnis] n.满；充实；彻底 (英语四级单词)
• tempting [´temptiŋ] a.引诱人的，吸引人的 (英语四级单词)
• widespread [´waidspred] a.广布的；普遍的 (英语四级单词)
• network [´netwə:k] n.网状物 vt.联播 (英语四级单词)
• investigator [in´vestigeitə] n.调查；审查者 (英语四级单词)
• vicious [´viʃəs] a.不道德的；刻毒的 (英语四级单词)
• luscious [´lʌʃəs] a.味道甘美的 (英语六级单词)
• socrates [´sɔkrəti:z] n.苏格拉底 (英语六级单词)
• calorie [´kæləri] n.卡(热量单位) (英语六级单词)
• digestive [di´dʒestiv] a.消化的，易消化的 (英语六级单词)