执行缺陷与半脑
《决策大脑》执行缺陷与半脑,页面无弹窗的全文阅读!
额叶损伤往往导致两种症状：持续症和场依存行为。在大多数严重的案例中，这两种症状会表现为附和行为。我将在本书的第8章详细介绍这两种症状。在这里我只是指出，持续症指的是一种无法从一种活动完全切换至另一种活动的病理现象，表现为之前活动的某些元素可能会延续至正在进行的活动，对后者造成干扰。可以将持续症理解为缺乏认知灵活性。相反，场依存行为指的是因缺乏抵抗环境中偶然出现的干扰而无法专注于眼前的任务。可以将场依存行为理解为心理操作缺乏稳定性。在一些极端案例中，这种缺陷可能会表现为动作模仿（对他人行为的身体模仿）或言语模仿（对他人话语的语言模仿）。
临床神经心理学文献通常将持续症和场依存行为视为截然不同的两种症状，每一种都是由额叶介导的执行控制的不同方面的故障导致的。此外，上述两种不同的症状往往被用来说明执行功能的两个截然不同的方面：一是通过内部表征指导行为的能力，二是根据不断变动的偶然事件切换认知模式的能力。执行控制的上述两个方面有时候被称作“稳定性”和“可塑性”，二者在正常认知中保持着某种动态平衡。在额叶受损后，这种平衡关系可能会被打破，导致持续症（心理操作过于稳定）或场依存行为（心理操作的可塑性过高）。
由额叶介导的认知稳定性和认知可塑性的机制在认知和计算神经科学领域仍然是一个需要进行大量研究和辩论的问题，由此产生了“双稳态”这一概念，“双稳态”指的是有机体以一种稳健、稳定的方式维持一系列心理活动，然后相对快速地切换到另一系列心理活动（同样以一种稳健、稳定的方式将其维持下去）的能力。研究人员提出了若干机制来解释双稳态。其中之一涉及多巴胺调节的双重特性。我们已经知道，存在多种多巴胺受体，它们似乎是以不同的方式运作的。有观点认为，D1受体对低浓度的多巴胺做出反应，其激活能使有机体“稳健”地维持当前的认知机制。相反，D2受体仅对高浓度的多巴胺做出反应，其激活能使有机体的认知机制产生不稳定的“迅速更新”。如果这是真的，那么D2受体的偏侧化（偏向右半脑）就非常耐人寻味。这可能是认知新奇性与右半脑之间联系紧密的部分原因吧。虽然持续症和场依存行为表面上有着巨大差异，甚至是完全对立的，但它们的内在机制之间有何种差异还远远没有定论。兰德尔·欧·莱利和宗像裕子在“认知神经科学的计算探索”一文中提出了一种不同于临床观测给我们的直觉的重要见解。该见解的核心是，持续症和场依存行为反映的不是额叶内两种不同机制的崩溃。事实上，它们被视为由额叶介导的对后部神经网络的某种单一调节的失败（更准确地说，是没能发挥作用），后部神经网络位于左右半脑后部，由两种不同的方式被组织。换句话说，额叶症状学的双重性质不仅在于额叶本身，还在于额叶对之起作用的后神经网络的组织方式的多样性。如果后部神经网络是由一系列子网络组成的，各个子网络内部有着紧密的联系，但各个子网络之间的联系不够紧密，那么额叶介导的失败就会导致激活模式被“卡在”子网络当中，无法传递出去，因而导致持续症。另一方面，如果后部神经网络是相对同质化的，那么额叶介导的失败将导致激活模式过度扩散，从而产生场依存行为。
对这两种神经架构的描述（将其分为独特的子网络和相对同质化的子网络）听起来和本章提到的两个半脑的组织方式颇为类似：左半脑由相对独特的子网络组成，而右半脑由相对同质化的子网络组成。接下来我们得到了一个有趣的推论。如果欧·莱利和宗像裕子的假说与此处对半脑差异的描述（以及之前在《智慧大脑》中的描述）都是正确的，那么左额叶损伤主要导致持续症，右额叶损伤主要导致场依存行为。我们有可以检验该预测的数据吗？答案是肯定的。不管怎样，我的导师亚历山大·卢里亚都避开了心理测量学的标准测验。相反，他选择了一系列临床试验，一些是他自己设计的，另一些是基于上一代了不起的欧洲神经科医生的成果改编而成的。卢里亚根据手头的诊断问题以一种即兴的、灵活的方式进行试验。大部分程序的设计初衷都是为了引发阳性症状而非阴性症状，也就是疾病所引起的表现，而不是正常行为的一部分，不是行为的减退。几乎毫无例外的是，阳性症状由于自身的独特性能够提供比阴性症状更多的信息，因而，在有经验的研究人员看来，卢里亚的临床试验是一种异常强大的诊断工具。如今，这种神经心理学诊断方法有时会被夸张地冠以“过程导向型”“量化”“假设检验”等名称，用来强调其不同的方面，并与“固定组”的方法相区别。卢里亚的临床团队中的所有成员（包括我在内）都以一种类似学徒的方式从卢里亚独特的苏格拉底式的灵活诊断中获得了很大启发。在卢里亚的临床试验中，有一些用来获取包括持续症和模仿行为在内的“额叶”症状（或者说迹象）的独创性程序。
多年以后，已经身在美国的我决定让这些程序更加正规化，将卢里亚的独创性和对标准化的普遍预期结合起来。这一努力的成果就是我与我之前的学生兼同事鲍勃·比尔德、朱迪·耶格和肯·波德尔合著的《执行控制组》（The Executive Control Battery）一书。它由4个简单的子测验组成，其中两个被用来引发各种类型的书写持续症和运动持续症，另外两个被用来引发各种类型的模仿反应。除了用于神经健康的正常个体外，我们还将执行控制组实验用于各种临床案例，其中就有单侧额叶损伤患者。
研究结果颇具戏剧性。左额叶损伤导致的持续症的严重程度几乎是右额叶损伤的两倍。相反，右额叶损伤导致的模仿行为的严重程度是左额叶损伤的两倍。上述发现证实了我多年前进行的一项非正式观察，当时我还在卢里亚在莫斯科的布尔登科神经外科研究所工作，我发现持续症和场依存行为通常是由不同侧的单侧额叶损伤导致的。假设欧·莱利和宗像裕子的假说是正确的，这正是根据本章前面提到的两个半脑神经网络的组织方式的差异所做的预测。虽然上述发现无法证明左右半脑神经网络的组织方式非常不同，即一个由各不相同的子网络组成，另一个显得更同质化和平滑，但二者显然与该模型相容。类似地，如果我们这里所说的两个半脑神经网络的组织方式存在差异的假说是正确的，那么执行控制组实验的发现对欧·莱利和宗像裕子的假说提供了有力支撑，即持续症和场依存行为背后有一种单一机制，只是该机制因为两个半脑神经网络的组织方式存在差异而表现出了不同。
不过，这并不是全部。有关持续症和模仿行为与左右额叶损伤的双重分离只存在男性身上。对女性来说，这种双重分离并不明显，左右额叶损伤的影响非常相似。顺着我们的逻辑进行推理，这是不是意味着女性的两个半脑的神经组织方式比男性的更相似？我们将在第7章更深入地探讨这个问题。
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