Padoa-Schioppa Camillo
Department of Anatomy and Neurobiology, Washington University in St Louis, St Louis, MO 63110, USA.
J Neurosci. 2009 Nov 4;29(44):14004-14. doi: 10.1523/JNEUROSCI.3751-09.2009.
While making economic choices, individuals assign subjective values to the available options. Values computed in different behavioral conditions, however, can vary substantially. The same person might choose some times between goods worth a few dollars, and other times between goods worth thousands of dollars, or more. How does the brain system that computes values -- the "valuation system" -- handle this large variability? Here we show that the representation of value in the orbitofrontal cortex (OFC), an area implicated in value assignment during economic choice, adapts to the behavioral condition of choice and, more specifically, to the range of values available in any given condition. In the experiments, monkeys chose between different juices and their choice patterns provided a measure of subjective value. Value ranges were varied from session to session and, in each session, OFC neurons encoded values in a linear way. Across the population, the neuronal sensitivity (defined as the change in neuronal activity elicited by the increase in one value unit) was inversely proportional to the value range. Conversely, the neuronal activity range did not depend on the value range. This phenomenon of range adaptation complements that of menu invariance observed in a previous study. Indeed, the activity of each neuron adapts to the range values it encodes but does not depend on other available goods. Our results thus suggest that the representation of value in the OFC is at one time instantiative of preference transitivity (menu invariance) and computationally efficient (range adaptation).
在做出经济选择时，个体为可用选项赋予主观价值。然而，在不同行为条件下计算出的价值可能会有很大差异。同一个人有时可能会在价值几美元的商品之间进行选择，而有时则会在价值数千美元或更多的商品之间进行选择。大脑中计算价值的系统——“估值系统”——是如何处理这种巨大差异的呢？在这里，我们表明眶额皮质（OFC）中的价值表征，这一在经济选择过程中与价值赋予相关的区域，会适应选择的行为条件，更具体地说，会适应任何给定条件下可用的价值范围。在实验中，猴子在不同果汁之间进行选择，它们的选择模式提供了主观价值的一种度量。价值范围在不同实验时段有所变化，并且在每个实验时段，OFC神经元以线性方式编码价值。在总体上，神经元敏感性（定义为一个价值单位增加所引发的神经元活动变化）与价值范围成反比。相反，神经元活动范围并不依赖于价值范围。这种范围适应现象补充了先前一项研究中观察到的菜单不变性现象。实际上，每个神经元的活动会适应它所编码的价值范围，但并不依赖于其他可用商品。因此，我们的结果表明，OFC中的价值表征一方面体现了偏好传递性（菜单不变性），另一方面在计算上是高效的（范围适应）。
