Brain scanning

No hiding place

Oct 28th 2004 | SAN DIEGO From The Economist print edition

Studies using functional brain-imaging take on sophisticated topics

FEW recent innovations have transformed a field of research as much as functional magnetic-resonance imaging (fMRI). The technique has revolutionised the study of the human brain. By making visible the invisible (the activity of different bits of the living brain on a second-by-second basis), it has revolutionised the study of that organ. But what started out as a medical instrument is now used routinely to probe complex questions about behaviour and motivation. That was the lesson of two studies presented to a meeting of the Society for Neuroscience, held in San Diego earlier this week.

In one of the studies, Jonathan Cohen, of Princeton University, and his colleagues tried to explain an anomaly that has been nagging economists for decades. If humans were fully rational (at least, rational in the way that economists define the word), they would attach the same monetary value to a week’s delay in receiving a payment, regardless of when that week began. So, if someone is offered $10 at the beginning of any given week, or $11 at the end of it, he should make the same choice, whether that week starts now or a year from now. But that turns out not to be how most people judge it. In most cases, they will take the $10 today but the $11 in a year and a week.

Dr Cohen reasoned that this inconsistency might reflect the influence of different neural systems in the brain. To test this, he recruited 14 students, the traditional workhorses in such studies. While lying in his brain scanner, the students were offered the choice of receiving an Amazon.com gift certificate worth somewhere between $5 and $40 immediately, or getting one worth 1% to 50% more in a couple of weeks’ time.

When a participant chose the earlier reward, there was an increase in the activity of his limbic system. This is a region of the brain that is involved in emotion. In contrast, when the choice was to delay gratification in exchange for a bigger reward, brain activity was concentrated in the “thinking” regions, such as the prefrontal cortex. The inconsistency therefore seems to be the result of different sorts of calculation happening in the two cases.

Of course, that does not answer the ultimate question of why evolution has equipped the brain this way. Dr Cohen speculates that it may have something to do with survival when the arrival of resources is scarce and unpredictable, rather than the subject of contracts and an efficient banking system. But it does shine a new light on issues such as drug addiction and procrastination, which are both situations where the temptation of immediate reward can lead to choices that might ultimately be detrimental.

While Dr Cohen’s group wrestles with how people make choices, Klaus Mathiak, of the University of Tübingen, in Germany, and his colleagues, are using fMRI to study the effects which certain sorts of choice have on brain activity. Specifically, the team is looking at what goes on in the heads of dedicated video-games players during violent “social interactions” within a game.

Dr Mathiak enlisted 13 gamers who played video games for, on average, 20 hours a week. While the gamers stalked and shot the enemy from the relative discomfort of a scanner’s interior, the researchers recorded events in their brains.

As a player approached a violent encounter, part of his brain called the anterior cingulate cortex became active. This area is associated with aggression in less fictional scenarios, and also with the subsequent suppression of more positive emotions, such as empathy. Dr Mathiak noted that the responses in his gamers were thus strikingly similar to the neural correlates of real aggression. As he puts it, “Contrary to what the industry says, it appears to be more than just a game.”

近来很少有创新像功能性磁共振成像这样，可以改变一个领域的研究。这项技术革新了人脑的研究。通过使无形的变有形(对有生命的大脑不同部分进行逐秒记录的活动)，这一技术使对器官所作的研究有了翻天覆地的变化。不过作为医学仪器开始应用，如今是在例行检查方面，以探究有关行为和动机的复杂问题。就是本周早些时候于圣迭戈召开的Society for Neuroscience会议上提交的那两项研究。

在其中一项研究中，Princeton University的Jonathan Cohen和他的同事们试图解释一个挑剔了经济学家们几十年的怪人。如果人们都是完全理性的(至少是经济学家们定义的那样)，那么对于延迟一周支付应赋予相同的货币价值，无论那一周什么时候开始。所以，对于在某一周开始时得到10美元、或者在这一周结束时得到11美元，一个人应当做出相同的选择，不管这一周是现在还是一年以后。但是那并不能证明人们究竟如何判断的。大多数情况下，人们会选择今天拿10美元而不是一年零一周之后的11美元。

Dr Cohen解释道这一矛盾可能反映了大脑中不同中枢神经系统的影响。为了验证这一解释，他招募了14名学生，一般在这样的研究中都是由学生来充当牛马。在处于大脑监测仪的监测之下时，学生们被要求在是现在得到价值5到40美元的亚马逊礼券还是几周以后得到多出1%到50%的礼券之间做出选择。

当一个参与者选择了先期报酬时，他的大脑边缘系统活动增强了。这是大脑中与情绪有关的一个区域。相反，选择以延期支付换取更多的报酬时，大脑活动集中在了“思考”区域，如前额大脑皮层。所以矛盾看起来是两种情况下不同类型的考虑的结果。

当然，那并没能回答为什么进化会以这种方式装备大脑这一根本问题。Dr Cohen推测可能是在资源奇缺且不可预料时有什么与生存相关的因素，而不是合同和有效的银行系统这些原因。但是这却使解决像毒瘾和拖延这样的问题有了一丝光亮，这两者都是眼前回报的诱惑会导致最终作出有害选择的情形。

就在Dr Cohen的团队不断斟酌人们怎么做出选择时，德国Tübingen大学的Klaus Mathiak和同事们正使用功能性磁共振成像来研究特定类型的选择对于大脑活动产生的影响。他们尤其注意观察社会交际暴力游戏中专注于视频游戏的玩家们的大脑。

Dr Mathiak招募了13个平均每周玩20小时视频游戏的玩家。当玩家们追踪并向敌人射击时，研究人员从内置扫描仪中记录下了他们大脑相对处于不舒服的状态。

当一个玩家开始一场暴力遭遇战时，他大脑中称为前扣带脑皮质的部分开始活跃。这一区域在虚构情节较少时与进攻相关联，还在后来感同身受一样的更积极的情绪出现时与抑制有关。Dr Mathiak指出玩家们的反应和真正的进攻相关联的中枢惊人地相似。正如他所指出的，“与业内同行所说的相反，这看来似乎远不止于仅仅是一场游戏。”