为啥我们一到下午就想偷懒 | Journal of Neuroscience 论文推荐

又是奖赏系统的锅

作者 David Disalvo

日期 August 22 2017

翻译 梁珩

审校 阿金 谭坤

图片来源：Shutterstock

我们大多数人对于在下午两点左右没动力干活已经司空见惯了。不管这称之为‘午餐眩晕’或‘午间低潮’，这种来自大脑压倒性的困顿感使得你只是盯着显示屏走神儿，并开始想着要不下午去一趟星爸爸？

一项新研究从另一角度给予了解释：一切都关乎“奖赏”。需要特别说明的是，大脑专注于寻找奖赏：奖赏能带来动力，让人们 在午休后能继续工作。

首先我们需要知道，这里的“奖赏”指的是啥。你可以说我们早起，去上班，完成每日任务等做各种事的原因都是为了奖赏——那些使得我们努力实现、争取、学会、成就以及改变的一切。大脑生来就会几乎把所有时间放在关注各种奖赏上。但正如这项研究所示，这种对奖赏的追求的程度在一天内会有起伏。

研究人员利用 fMRI（功能性核磁共振影像技术）扫描了一小组志愿者在三个时间点（早上10点，下午2点，晚上7点）的大脑活动，并专注于他们脑部“奖赏机制”的活动。他们发现脑部一个称之为‘左侧豆状核（left putamen）’的区域，在上午和晚上的活动最频繁，在午后则最为安静。左侧豆状核区在奖赏处理和奖赏预期中发挥重要作用，并且这个研究也揭示了它的脑频在早上和晚上早些时段会达到峰值，但在下午2点左右处于低谷。

研究者认为脑部的变化在于大脑对什么时候能预期“奖赏”存在“预测误差（prediction error）”。奖励预期值的低潮在上午和晚上，所以需求奖赏的动力会更高（也就说当你认为得到什么的概率比较低时候，你会愿意花额外的精力来得到它）。预期值在中午达到峰值，因此这时候就失去了努力的动力。预测误差阶段有着重要意义——因为我们的期待值和实际会获得奖赏并不一致，所以我们才想在这段时间内学到或完成更多的事情（这只是我们大脑里许多矛盾之一呀）。

虽然这个解释是基于推测，但这种感觉你应该早就习以为常了。吃个丰盛的午餐来加点血糖，或者早上连灌上三杯咖啡后中午的空虚无力，都会让你懂得为什么最好不要中午去完成需要做的大项目了。

论文基本信息：

【题目】Time of daydifferences in neural reward functioning in healthy young men

【作者】Jamie E. M.Byrne，MatthewE. Hughes，GregMurray，et al

【期刊】Journal of  Neuroscience

【日期】21 August 2017

【DOI】https://doi.org/10.1523/JNEUROSCI.0918-17.2017

【地址】http://www.jneurosci.org/content/early/2017/08/21/JNEUROSCI.0918-17.2017    

【摘要】

Reward function appears to bemodulated by the circadian system, but little is known about the neural basisof this interaction. Previous research suggests that the neural reward responsemay be different in the afternoon; however the direction of this effect iscontentious. Reward response may follow the diurnal rhythm in self-reportedpositive affect, peaking in the early afternoon. An alternative is that dailyreward response represents a type of prediction error, with neural rewardactivation relatively high at times of day when rewards are unexpected (i.e.,early and late in the day). The present study measured neural reward activationin the context of a validated reward task at 10.00h, 14.00h, and 19.00h inhealthy human males. A region of interest blood oxygen level dependent (BOLD)functional magnetic resonance imaging (fMRI) protocol was used to investigatethe diurnal waveform of activation in reward-related brain regions. Multi-levelmodelling found, as expected, a highly significant quadratic time-of-day effectfocusing on the left putamen (p <.001). Consistent with the‘prediction error’ hypothesis, activation was significantly higher at 10.00hand 19.00h compared to 14.00h. It is provisionally concluded that the putamenmay be particularly important in endogenous priming of reward motivation atdifferent times of day, with the pattern of activation consistent withcircadian-modulated reward expectancies in neural pathways; viz., greateractivation to reward stimuli at unexpected times of day. This study encouragesfurther research into circadian modulation of reward, and underscores themethodological importance of accounting for time of day in fMRI protocols.

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