近年来,有关意识的生物和物质基础的理论层出不穷。好的理论能够指导实证研究,解释实验数据,开发新型实验技术,以及扩展我们控制感兴趣的实验现象的能力。然而,在意识领域,目前的理论如何相互关联或者能否用实验鉴别,尚不清楚。为了阐明这一复杂的领域,我们回顾了四类重要的意识理论(Theories of Consciousness,TOC)路径:高阶理论(higher-order theories,HOT)、全局工作空间理论(global workspace theories,GWT)、整合信息论(integrated information theory,IIT)和复馈/预测处理理论(re-entry and predictive processing theories)。通过指明它们试图解释的意识问题、它们的神经生物学承诺、以及它们引证的实验证据,刻画了这四类路径的主要特征。我们考虑了一些重要的实验争论如何可能区分不同理论,并列举了三个意识理论需要发展的方向,使它们能够被神经科学实验所检验。有理由相信,意识理论的迭代发展、验证和比较将导向更深刻的理解。
一、引言
在意识科学复苏的最初几十年,研究者们侧重于寻找“意识的神经相关物” (neural correlates of consciousness, NCC)。严格地说,NCC是足以构成某种意识状态的最小神经事件集;在实践中,寻找NCC包括寻找与意识最密切相关的大脑状态和过程[1-3]。专注于寻找NCC的工作是有用的,因为 NCC“理论中立”,为具有不同理论和哲学背景的研究人员提供了共同的语言和方法。然而,NCC框架的局限性变得越来越明显,比如难以将NCC从意识的神经先决条件和附带影响中区分出来[4-7]。为了应对这些限制,人们越来越关注意识理论(theories of consciousness, ToC)的发展。在ToC的帮助下,我们能超越基于NCC的方法,转而去探索具有解释性洞见的意识模型。事实上,意识科学的主要目标就是得到被实验验证的ToC[8,9]。NCC的研究方法强调意识与大脑活动之间的相关性,而ToC则侧重于鉴别意识特征与神经机理之间的解释性联系[10]。话虽如此,理论家们经常采用不同的话语体系来解释神经活动和意识之间的联系。一部分人认为,令人满意的ToC应该缩小“解释鸿沟”(方框 1),使神经活动和意识之间的关系,像水的化学结构与其总体特性之间的关系一样,清晰明了[11]。另一部分人认为,“解释鸿沟”不一定能完全消除,但仍然希望有一个框架可以解释意识的某些方面,减少或消除有关意识的生理基础的神秘感[12,13]。还有一部分人认为, “解释鸿沟”具有误导性,不应该被意识科学认真对待[14,15]。
ToC之间“鸡同鸭讲”的主要原因之一是它们通常具有不同的解释目标。因此,我们首先考虑,一个详尽的ToC需要解释哪些内容。即使这个问题也存在争议,理论家们在ToC应该解释什么样的意识现象上经常无法达成一致。意识的核心问题是“第一人称体验”或“主观知觉”。虽然,除了循环定义以外,无法对这些术语进行定义,但意识研究的对象可以用一些直观的特征来进行区分和限定。比如使用“有某种感觉”(something it is like)来描述某个生物体具有意识[20],使用“感觉起来怎么样”(what it is like)来区分两种不同的意识状态。一个详尽的ToC不仅需要解释为什么一些生物体或系统有意识而另一些没有意识,还需要解释为什么不同的过程导致不同的意识状态。意识状态可以分为两类:全局状态和局部状态。全局状态描述生物体的整体主观特征,并与觉醒水平的变化、行为反应的变化有关。常见的全局状态包括清醒、做梦、镇静、最低意识状态(或许还包括迷幻状态)。这些全局状态有时被称为“意识水平”,但我们更喜欢“全局状态”这个术语,因为它留下了这样一种可能性,即这些状态无法在单一维度上被完整地排序,而最适合在多维空间内被描述[21]。局部状态——通常被称为“意识的内容”或具有“感质(qualia)”的状态——的特征被“这种状态感觉起来怎么样”所刻画。头痛的局部状态与闻咖啡的局部状态不同,因为头痛的感觉与闻咖啡的感觉不同。局部状态的描述可以具有多层级的颗粒度,从低级的感知特征(例如颜色)到单个物体,再到完整的多模态场景感知。一些重要的局部状态构成了自我体验的基础,自我体验包括情绪、情感、意志、拥有身体的感觉、明晰的自传体记忆等[13, 22-24]。虽然神经生物学理论倾向于关注具有感觉和感知内容的局部状态,但意识还包括具有认知和命题内容的局部状态,例如解决填字游戏时出现的想法。重要的是,在特定的时刻,一个生物体具有的众多局部状态并非简单而独立地出现,相反,它们作为单一的意识场景的组成部分,被绑定在一起[25, 26]。第二个区别是意识的现象属性和功能属性之间的区别。现象属性是指意识的主观体验特征,也就是说“感觉起来怎么样”。功能属性关注的是,对具有意识并因此获得好处的生物体而言,在它的认知经济性方面,心智状态承担了什么角色。(这里的“功能”包括目的论功能——由进化形成的功能——和部署性功能——一个过程在它所属的更大系统的运作过程中所起的作用;见参考文献 27)例如,有意识地看到咖啡杯可以实现一系列功能,包括灵活地使用杯子(可能是从杯子里喝水,或者把它扔到房间的另一头),对上述事件进行情景记忆,以及提供有关上述体验的口头报告。虽然我们区分了意识的现象属性和功能属性,但并不是说它们相互独立(它们很可能不是独立的),只因为它们为 ToC 提供了不同的解释目标。正如我们将看到的,一些 ToC 关注意识的现象特征,另一些则关注意识的功能特征,还有一些试图同时解释意识的功能特征和现象特征。第三个区别是ToC可能试图回答的,有关局部状态(“内容”)的两种问题。一方面,人们可能会问为什么生物体处于某个局部状态(而不是另一个局部状态)。另一方面,人们可能会问,为什么一个特定的局部状态具有它所具有的主观体验特征(而不是其它类型的特征)。这种区别可以用双目竞争实验来解释。在双目竞争实验中,被试的每只眼睛都接收到不同的刺激(例如,右眼呈现房子,左眼呈现人脸),被试的视觉体验在左眼刺激和右眼刺激之间交替[28]。在特定时刻,意识内容只包含房子,而不包含人脸,此时,我们可以问,为什么在这一刻对应于“房子”的心理状态是有意识的(而“人脸”的心理状态无意识),我们也可以问为什么房子的视觉体验具有它们所具有的独特的体验特征,而不是具有看到一张脸,听到铃铛响或感到疼痛时的体验特征。值得注意的是,可能有一些内容是不可能有意识的(例如,初级感知或调节系统中的低级处理过程),而其它内容只能是有意识的(例如,全局整合的感知场景)。因此,除了解释为什么某些心理内容在某些情景中是有意识的,而在其它情景中不是,另一个挑战是解释,为什么某些内容永远不会是有意识的,而某些内容只能作为有意识的存在。大多数ToC并没有试图同时解决我们刚刚阐明的所有意识问题,它们试图解释意识的某些方面的问题,它们或许将这种努力看做迈向详尽完整的意识理论的一步。虽然,只研究意识的某一方面,这种局限本身并不构成对某一ToC的批驳和反对,但这确实意味着意识理论之间的相互比较变得没有那么简单而直接了。如果不同理论针对意识的不同方面(例如,一种理论专注于意识的现象特征,另一种理论关注其功能特征),那它们乍一看可能不像“对手”。本文回顾的ToC分为四类:高阶理论(HOT),全局工作空间理论(GWT),整合信息论(IIT)以及复馈/预测处理理论。虽然某些对意识的解释跨越了多个类别,而另一些则无法合理地归入这些类别中的任何一个(表格 1),但这种四分类方法提供了一个视角,有助于理解意识科学的研究现状(方框2;关于其它对理论进行分组的方式,见[29])。在下文中,我们将介绍每个类别的关键元素,描述一些值得注意的类别内差异,并指明最密切相关的意识问题。然后,我们根据一些经典的实验争论,来说明这些ToC如何相互关联,并提出了几个建议,我们认为这些建议将有助于推动意识的理论发展和实验研究之间的良性循环。
意识的神经相关物(Neural correlates of consciousness)共同足以构成某种意识状态的最小神经事件集解释鸿沟直觉(Explanatory gap intuitions)认为不可能用物理的、机械的术语给予意识完全令人满意的解释的直觉。对抗式合作(Adversarial collaborations)一类研究项目。其中,不同理论的支持者共同设计一个实验来区分他们喜欢的理论,而且,对实验结果如何更有利于哪一种理论,他们需要事先达成一致。全局状态(Global states)与生物体的整体意识状态有关,通常与唤醒和行为反应有关,并与意识的“水平”有关。局部状态(Local states)与特定的有意识的心智状态有关,例如有意识的知觉、情感或思想。局部状态也经常被称为有意识的内容。双目竞争(Binocular rivalry)向两只眼睛分别呈现不同的图像,有意识的感知将在两个图像之间交替,这种现象叫双目竞争。现象特征(Phenomenal character)局部状态的主观体验特质,比如看到红色时感受到“红色的感觉”或牙痛时感受到疼痛——有时也称为感受性质(qualia)。元表征(Meta-representation)以另一种心智表征为目标的心智表征。无报告范式(No-report paradigms)被试不提供主观(口头、行为)报告的行为实验。Φ指的是一个系统所规定的,不可由它的部件所规定的信息量所化约的信息量。Φ有许多变体,每一种都有着不同的计算方法,做出不同的假设。后部热区(Posterior hot zone)靠近大脑皮层后部的一系列脑区,包括顶叶、颞叶和枕叶区,以及楔前区等区域。复合体(Complex)在整合信息论(IIT)中,支撑着最大的不可化约的整合信息的一个物理系统的一个子集。意识的统一性(Unity of consciousness)指的是下列事实:一个智能体在某一时期的各种体验,似乎总是作为单个复杂体验的组成部分而发生。认知访问(Cognitive access)指的是一种功能属性,即一种心理状态可以接触到一系列认知过程,通常包括语言和/或行为报告。内感受预测(Interoceptive predictions)指的是对源自身体内部的感觉信号的原因的预测(内感受指的“从内部”对身体的感知)。计算(神经)现象学(Computational (neuro) phenomenology)指的是使用计算模型从(神经)机制的角度来解释意识状态的现象学特征。测量问题(The measurement problem)指的是确认某一心智状态是否是有意识的的问题,或确认某一有机体或系统,是否是,或者是否有能力是有意识的的问题。类大脑器官(Cerebral organoids)实验室生长的神经结构,它能自我组织成具有细胞和网络特征的系统,类似于发育中的人脑。
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