加工需求驱动下词汇阅读神经通路的动态协作机制
2024-01-09党敏蔡文琦陈发坤王小娟杨剑峰
党敏 蔡文琦 陈发坤 王小娟 杨剑峰
摘 要 建构统一的认知和神经生理模型是词汇阅读认知神经科学研究面临的核心问题。阅读的认知理论模型一致认为阅读是语音和语义加工通路分工协作的结果, 认知神经科学研究也表明词汇阅读是背侧和腹侧神经通路动态协作的结果。为了系统地阐述阅读网络的这种动态协作机制, 结合神经功能和生理基础两个层面, 从以下三个方面对最新研究进展进行系统疏理:首先, 指出潜在的加工需求是背/腹侧神经通路动态协作的实质; 然后进一步阐明潜在加工需求驱动了不同正字法深度下背/腹侧神经通路的分工合作模式; 最后, 深入剖析了潜在加工如何通过语言经验塑造了神经通路间的动态协作。从而揭示出阅读神经通路动态协作的实质可能是特定任务下加工需求驱动的结果, 这种动态协作可能成为跨语言普遍的词汇阅读理论模型。
关键词 词汇阅读, 模型, 通路, 动态协作
分类号 B842
1 引言
随着认知神经科学的迅速发展, 词汇阅读的脑机制研究取得了丰富的成果。基于对词汇阅读加工成分(如字形、语音和语义)相关功能脑区的识别, 研究者尝试建立与认知理论模型相统一的神經生理模型(Smith et al., 2021)。词汇阅读的两大认知理论模型(双通路理论, Coltheart et al., 2001; 联结主义三角模型, Seidenberg, 2011)虽然在模型结构、算法实现和内在加工机制上都存在差异, 但它们一致认为词汇阅读需要依赖于语音、语义两条加工通路的相互协作。相应地, 研究者指出, 阅读的神经生理模型存在背侧和腹侧两条分工不同的神经通路。背侧通路主要是经由视觉分析之后通达左侧颞顶皮层和额下回岛盖部; 而腹侧通路是经由腹侧枕颞皮层通达颞中回和额下回眶额部及三角部(Carreiras et al., 2014; Taylor et al., 2013)。基于白质纤维束的研究也为这两条神经通路提供了生理解剖基础(Saur et al., 2008; Wandell & Le, 2017)。
对词汇阅读中背/腹侧神经通路的功能探讨一度成为研究者广泛关注和争论的问题。近年来, 基于神经网络思想的研究发现阅读的神经生理模型并不能简单地分离为不同功能的神经通路。一方面, 研究者发现即使是加工与视觉词汇相似的刺激材料, 都需要广泛的阅读相关脑区的参与激活, 不存在刺激类型特异或加工成分特异的阅读相关脑区, 词汇阅读是由相关脑区组成的神经网络活动的结果(Wang et al., 2011)。另一方面, 大量研究发现的词汇阅读的脑机制是在特定实验条件下背侧和腹侧通路动态协作的结果(Dickens et al., 2019; Hoffman et al., 2015; Oliver et al., 2017)。
从而, 词汇阅读的背侧和腹侧神经通路的动态协作可能在认知和神经层面达成统一, 成为普遍的阅读理论模型。本文基于神经网络的最新取向从以下方面对此展开论述:首先, 阐明背/腹侧通路在刺激属性和任务需求驱动下表现出动态协作的实质; 其次, 揭示不同正字法深度下背/腹侧通路分工合作模式的跨语言差异; 最后, 深入剖析这两条神经通路的动态协作是如何在语言经验塑造作用下发展起来的。从而指明阅读网络内神经通路间的动态协作可能成为跨语言普遍的阅读神经机制。
2 阅读背/腹侧神经通路的分工协作
长期以来, 研究者通过对比不同的词汇类型或者对比不同阅读任务的加工区别, 揭示出阅读涉及的字形、语音和语义加工相关的功能脑区。越来越多的研究表明, 阅读相关脑区的激活具有动态变化的特点。这些脑区的激活不仅受到自下而上刺激属性的影响(Guo et al., 2022b; Taylor et al., 2019), 还受到了自上而下任务需求的调节(Ludersdorfer et al., 2019; Mattheiss et al., 2018)。此外, 有研究结果显示阅读相关脑区的激活同时受到了刺激属性和任务需求交互作用的调节(Pattamadilok et al., 2017; Yang et al., 2012)。下面详细分析阅读神经网络是如何在刺激和任务的调节下, 通过脑区间的动态协作来完成词汇阅读过程的。
2.1 刺激类型特异的阅读神经通路
基于功能定位的思想, 研究者提出阅读加工的双通路模型(Dual Route Cascaded Model, DRC), 并认为不同的词汇类型依赖于不同的加工通路(Coltheart et al., 2001)。一条是存储着形素?音素转换规则的亚词典通路, 主要负责形?音规则的低频词和可发音假词的阅读; 还有一条是通过查字典的方式提取相应语音信息的词典通路, 主要负责形?音不规则的低频例外词的阅读, 高频真词也通过词典通路完成阅读。
通过不同类型词汇阅读的脑机制对比, 研究者尝试揭示词汇阅读的大脑神经通路。真词和假词阅读的对比研究发现, 假词更多地激活了背侧顶叶皮层, 如左侧顶下小叶和缘上回并延伸至中央前回, 而真词更多地激活了左侧颞中回以及额下回的腹侧脑区(Dickens et al., 2019; Woolnough et al., 2022)。采用参数相关的方法(Protopapas et al., 2016)以及元分析结果(Taylor et al., 2013)都得到了类似的发现。对比规则词和例外词阅读的研究也发现, 两种词汇阅读对背侧和腹侧神经通路具有不同程度的依赖(Price, 2012; Taylor et al., 2014)。例如, 有研究表明, 规则词更多激活了背侧通路的左侧顶下小叶与中央前回, 而例外词阅读更多激活了腹侧通路的左侧颞下回, 颞中回和颞上回(Cummine et al., 2013)。
基于有效连接分析的fMRI研究发现, 不同类型的词汇阅读不仅存在功能脑区的激活差异, 还存在着脑区间的连接差异。例如, Levy等人(2009)通过建构结构方程模型, 发现假词使左侧枕颞交接区到顶叶皮层的背侧通路脑区间的有效连接增强, 而真词使左侧枕颞交接区到额下回(BA 45)的腹侧通路脑区间有效连接增强。建构词汇阅读的动态因果模型也得到了类似的结果。例如, 有研究发现假词使左侧缘上回与额下回岛盖部等背侧脑区的功能连接增强(Barton? et al., 2023; Junker et al., 2023), 而真词则使左侧梭状回前部与颞中回、腹侧额下回等脑区的功能连接增强(Mechelli et al., 2005; Woollams et al., 2018)。此外, 对比规则词与例外词的研究发现, 规则词的阅读更多依赖从腹侧枕颞皮层到背侧通路(左侧中央前回)的连接, 而例外词的阅读则更加依赖从腹侧枕颞皮层到腹侧通路(左侧颞前叶)的连接(Hoffman et al., 2015)。
2.2阅读神经通路间的动态协作
基于神经网络的思想, 研究者提出了阅读的联结主义模型, 认为阅读需要字形、语音和语义系统之间的相互作用(又称三角模型, 详见综述: Seidenberg, 2011; 楊剑峰 等, 2018)。该理论指出, 词汇阅读需要语音和语义加工的共同作用, 在神经生理层面则体现为阅读神经网络内不同脑区以及神经通路动态协作的结果。从而, 研究者认为不存在对特定词汇类型敏感的阅读功能脑区或神经通路(Wang et al., 2011), 阅读不同类型词汇或跨语言的阅读机制, 则是不同认知加工/神经通路间动态协作的结果。
阅读不同类型的词汇需要背侧、腹侧神经通路的分工协作, 具体表现为词汇阅读中语音与语义加工脑区分工合作的模式差异。随着词汇中涉及的语音信息增多, 语音加工脑区(左侧额下回)的激活增强; 随着词汇中语义信息的增多, 对应的语义加工脑区(左侧颞中回和角回)激活增强(Frost et al., 2005)。参数相关的fMRI研究也发现, 随着词汇拼读一致性降低, 负责语义到语音编码的左侧颞中回和颞下沟表现出激活增强; 而随着词汇频率和语义可表象性的增加, 负责字形到语义编码的双侧角回和左侧楔前叶表现出激活增强(Graves et al., 2010)。Boukrina和Graves(2013)则使用有效连接的搜索算法探讨了随语义信息的变化, 语音和语义脑区之间连接模式的变化趋势。结果发现, 加工语义高表象词汇时阅读网络会表现出语义加工脑区(颞下沟)促进语音加工脑区(颞上回后部)的激活优势; 与之相反, 加工语义低表象词汇时表现为语音加工脑区促进语义加工脑区的激活优势。除了对真词的考察, Wang等人(2016)对不同假字类型诱发的脑激活进行考察, 同样发现了阅读网络中语音(左侧额下回和脑岛)和语义加工脑区(左侧颞中回后部和角回)的分工协作。
2.3潜在加工需求可能是阅读神经通路动态协作的实质
为了阐明阅读加工成分对应的大脑神经机制, 研究者普遍的做法是对比不同阅读任务的脑机制差异。研究表明, 相同的任务需求下, 即使是不同的语言体系也会表现出类似的神经激活(Krafnick et al., 2016; Rueckl et al., 2015), 从而通过任务对比就可以识别阅读相关的加工成分。比如, 正字法任务更多地激活与字形加工相关的梭状回(Guo & Burgund, 2010; Welcome & Joanisse, 2012); 语音任务更多地激活负责形?音转换加工的左侧颞顶皮层(D?bska et al., 2019; Qu et al., 2022); 而语义加工任务则需要与语义表征和计算相关的颞中回后部和腹侧额下回等脑区的参与(Hodgson et al., 2021; Zhang et al., 2019)。
任务对比差异不仅体现在个别脑区的激活差异, 还体现在脑区间的功能连接差异。例如, 研究者要求被试完成三种不同类型的阅读任务:出声阅读、动嘴默读与不动嘴默读。结果发现, 虽然这三种任务都表现出从左侧额下回到顶下小叶以及颞上回的功能连接, 但随着任务对发音动作需求的减弱, 阅读相关脑区之间不仅表现出功能连接数量的增多, 而且还伴随着从运动皮层(左侧辅助运动区)到阅读相关脑区功能连接数量的减少(Wan et al., 2018)。另一项研究通过听觉和阅读任务的比较发现, 虽然这两种任务都引起左侧枕颞沟中部脑区与左侧额顶控制网络(额下联合区)的功能连接, 但与此同时, 听觉比阅读任务表现出左侧枕颞沟中部与背侧注意网络(顶下沟后部)更强的功能连接(Qin et al., 2021)。这说明, 任务需求差异反映了潜在的认知加工不同, 并体现在了脑区间的连接模式上。
研究者通过操纵语音和语义任务来探讨阅读的背侧和腹侧神经通路。相关研究发现, 语音任务使背侧通路相关脑区之间的功能连接得到增强。例如, 在押韵判断任务下, 随着语音加工需求的增加, 从左侧顶下小叶到左侧额下回的功能连接显著增强(Zhu et al., 2016)。相应地, 语义任务使腹侧通路相关脑区的功能连接得到增强。例如, Jackson等(2016)的研究发现被试在完成语义判断任务时激活了核心的语义网络, 且随着加工需求的增加, 左侧背外侧颞前叶与左侧角回、额下回、前额叶皮层中部以及枕叶的功能连接也随之增强。Zhang等(2019)不仅在语义判断任务中发现左侧额下回和颞中回之间较强的功能连接, 还发现这些脑区的功能连接越强, 被试的行为表现越好。同时考察语音和语义任务的研究发现, 同音判断任务使中央前回与背外侧额叶皮层(BA 9/46)和背侧额下回的功能连接增强, 而语义判断任务使左侧颞中回与背外侧额叶皮层(BA 46)和腹侧额下回之间的功能连接增强(Liu et al., 2022)。根据以往研究, 阅读网络会在语音或语义加工需求的调节下, 表现出背侧和腹侧神经通路动态的分工协作。
潜在的认知加工可能是阅读神经通路动态协作的根本原因。表现为阅读相关脑区同时受到刺激类型和任务需求交互作用的影响(Pattamadilok et al., 2017; Yang et al., 2012)。对汉字阅读fMRI研究的元分析结果也显示, 刺激对比和任务对比共享了相同的阅读脑网络, 并且, 刺激对比发现的脑区激活都可以被任务对比所解释(Zhao et al., 2017)。因此, 潜在的认知加工可能决定了阅读相关脑区的参与激活。比如, 认为存在汉字特异激活的左侧额中回(Tan et al., 2005), 在加工法语词汇时同样得到了激活(Feng et al., 2020)。可见, 跨语言普遍性以及语言特异性的问题归根结底可能是任务需求的问题。
3 阅读背/腹侧通路分工协作跨语言差异的实质
词汇阅读是否具有跨语言普遍的认知和神经机制, 是研究者长期关注的问题。跨语言比较的研究认为正字法深度是影响词汇阅读网络的一个重要因素(Paulesu et al., 2000)。通过对深层和浅层正字法条件下发展性阅读障碍者与正常读者的对比, Richlan (2014)也指出, 正字法深度差异是预测发展性阅读障碍行为表现与脑损伤的一个重要参考。
基于神经网络的思想, 越来越多的研究者认为跨语言的词汇阅读具有普遍的认知加工机制(见综述: Smith et al., 2021), 并激活了普遍的大脑神经网络(Nakamura et al., 2012; Rueckl et al., 2015)。而前人发现的跨语言特异脑区, 如左侧颞顶皮层或额中回的激活只是特定实验室条件下发现的结果(Murphy et al., 2019; Wang et al., 2015)。因此, 跨语言差异主要是受语料属性及其对应加工策略的影响, 从而在神经层面表现出阅读功能脑区间的协作模式差异。下面从阅读脑机制的跨语言比较和二语脑机制研究两个层面对此进行详细阐述。
3.1 阅读神经通路的跨语言比较
早期的跨语言比较研究发现, 不同正字法深度下的词汇阅读需要依赖不同的功能脑区激活。对比不同正字法深度的词汇阅读, 研究者发现形?音对应规则(透明文字)的意大利语要更多激活左侧颞顶皮层, 而形?音对应相对不规则(不透明文字)的英语词汇阅读需要更多激活腹侧枕颞皮层和左侧额叶脑区(Paulesu et al., 2000)。Tan等人(2005)对比汉语和英语词汇阅读的元分析发现, 相较于不透明的汉语, 形音对应规则的英语阅读更多激活了负责形?音转换加工的左侧颞顶皮层后部, 而汉字阅读更多激活了负责寻址语音加工的左侧额中回。跨语言比较的结果表明, 阅读形?音对应相对规则的透明文字, 需要更多负责形?音转换加工的颞顶皮层参与, 而阅读不透明的文字需要更多腹侧通路, 如腹侧枕颞区和额叶的参与。
阅读的跨语言差异还表现为阅读脑区间的连接模式差异。例如, 对比日语汉字(Kanji)和平假名(Hiragana)在词汇判断任务中脑区间有效连接的差异。结果发现, 不透明的日语汉字使左侧视觉皮层到腹侧枕颞皮层的双向连接增强, 而透明的平假名使背侧通路, 从左侧视觉皮层到缘上回及布洛卡区之间的双向连接增强(Duncan et al., 2014)。采用多变量模式分析技术, Li等人(2022)将阅读任务的神经激活模式与汉字的语音特征相关联, 发现腹侧通路的相关脑区, 如左侧额下回、颞下回、颞中回和双侧梭状回与汉字的语音信息具有显著相关。而拼音文字的相关研究则发现背侧通路脑区, 如左侧颞上回(Wang et al., 2023)和缘上回(Graves et al., 2023)对英语语音信息的激活模式更敏感。
而且, 脑区激活的跨语言差异具有神经解剖的生理基础。采用弥散张量成像技术(Diffusion Tensor Imaging, DTI), 研究者发现不透明的乌尔都语使腹侧白质纤维束, 连接枕叶与眶额皮层的下额枕束的部分各向异性值(Fractional Anisotropy, FA的大小反映纤维束的髓鞘化程度)显著大于透明正字法的印地语, 表明腹侧通路对不透明语言的加工更加有利(Kumar & Padakanaya, 2019)。对比汉语与英语被试的大脑结构连接, 研究者发现透明正字法的英语使背侧白质纤维束, 连接顶下小叶与额叶皮层的上纵束的FA值显著高于汉语, 表明背侧通路对透明语言的加工更重要(Zhang et al., 2014)。而汉语被试则表现出对腹侧白质纤维束的独特依赖。例如, 来自汉语阅读障碍儿童的DTI研究发现, 除了表现出与字母语言相似的背侧白质纤维束損伤外, 汉语阅读障碍儿童还表现出腹侧白质纤维束, 连接视觉皮层到视觉词形区(Visual Word Form Area, VWFA)下纵束的损伤(Su, Zhao, et al., 2018)。最新的综述研究也表明, 连接词汇阅读相关脑区的背侧弓状束和腹侧下额枕束与下纵束分别构成了汉字阅读的背侧与腹侧结构通路(Guo et al., 2022a)。
跨语言的脑机制差异, 主要还是潜在加工需求的差异。在相同的任务下, 即使是不同的语言体系也会表现出类似的神经激活。例如, 一项4种语言的对比研究发现, 成人被试在完成相同的语义分类任务时, 西班牙语、英语、希伯来语和汉语读者脑机制的语言变异性有限。即4种语言共同诱发了普遍性的阅读脑网络, 包括与语音和语义加工相关的双侧额下回、颞中回到颞上回, 左侧顶下小叶以及皮层下的双侧脑岛、壳核与丘脑(Rueckl et al., 2015)。儿童研究也得到了类似的发现。如, 采用字词识别任务分别对汉语和法语儿童进行考察, 发现这两种语言在左侧梭状回、颞上回、中央前回和额中回具有普遍性激活(Feng et al., 2020)。
随着阅读单位由词汇向更加自然的语料(如段落、语篇)过渡, 阅读者更注重对大单元语义和语法进行整合加工, 词汇等小单元的字形、语音和语义通达等细节分析则会更加自动化, 从而跨语言的差异变小(Wang et al., 2015)。Dehghani等人(2017)考察了母语为英语、汉语以及波斯语三类被试对篇章语义的神经表征解码。结果发现, 三种语言的篇章解码表现出相似的神经激活模式, 这些脑区包括后内侧皮质, 内侧前额叶以及外侧顶叶皮层。最近, Malik-Moraleda等(2022)对12个语系中45种语言诱发的激活模式进行考察并发现, 额?颞?顶语言网络的左偏侧化以及关键脑区的语言功能具有跨语言的普遍性。
因此, 不同正字法深度导致的阅读脑机制差异主要表现为对背侧和腹侧神经通路的不同依赖程度。形?音对应规则的透明正字法语言更多激活了背侧神经通路, 而依赖语音词典或语义提取的不透明正字法语言更多地激活了腹侧神经通路。
3.2双语研究的证据
探討双语者在两种语言下的词汇阅读脑机制, 为阅读背侧/腹侧神经通路的分工合作提供丰富的证据。
首先, 双语者在阅读两种不同正字法深度的词汇时, 会根据输入语言选择性地依赖背侧或腹侧神经通路。相对而言, 形?音对应透明的文字在阅读时需要更多使用形?音转换规则, 从而更依赖背侧通路中左侧枕颞皮层和颞上回的激活, 而形?音对应不透明的文字在阅读时更多依赖于腹侧通路, 如腹侧枕颞皮层的激活(Cao et al., 2017; Das et al., 2011; Jamal et al., 2012)。来自印地语?英语双语儿童的研究也支持了背/腹侧通路的跨语言差异(Cherodath & Singh, 2015)。此外, 汉?英双语者的研究得到了类似的发现。即阅读汉字更依赖于腹侧通路的梭状回和颞中回后部脑区, 而阅读英语更依赖于背侧通路的左侧颞顶皮层(Sun et al., 2011; Tan et al., 2003)。对汉?英双语者的神经因果研究发现, 经颅直流电刺激(tDCS)抑制双语者的腹侧通路脑区只选择性地干扰了汉字阅读, 而抑制双语者的背侧脑区则对英语和汉字阅读都产生了干扰。该研究从神经因果的角度表明双语者在阅读不同语言词汇时会选择性地依赖阅读的神经通路(Bhattacharjee et al., 2020)。
其次, 阅读相关脑区的功能连接分析也发现, 双语者加工不同语言的词汇时, 脑区间的功能连接会根据输入语料的不同表现出动态变化的连接模式。对比母语相同的两类双语者, 西班牙?巴斯克双语者在加工正字法透明度较高的巴斯克语时会诱发腹侧枕颞区与背侧通路, 如左侧颞顶皮层和额下回岛盖部更强的功能连接; 而西班牙?英语双语者在加工正字法相对不透明的英语时, 双语者的腹侧枕颞区与腹侧通路, 如额下回三角部有更强的功能连接(Oliver et al., 2017)。英语?威尔士语双语者的研究发现在语义分类任务中, 正字法不透明的英语比威尔士语在左侧视觉词形区后部与腹侧通路脑区, 如颞下回、颞中回以及梭状回具有更强的功能连接(Tainturier et al., 2019)。此外, 汉?英双语者的研究也发现, 被试在阅读汉字时表现出腹侧通路梭状回前部与颞前叶和额下回更强的连接, 而阅读英语时则表现出背侧通路脑区颞顶皮层与额下回更强的连接(Dong et al., 2020)。
最后, 语言间正字法透明度的相似性(正字法距离)会影响到阅读脑区的动态激活。比如, 研究者以韩?汉?英三语者为被试, 使用韵律判断任务来检验三种语言的激活脑区。通过计算这些语言的激活相似程度, 结果发现, 相比于韩语和汉语之间较低的正字法透明度相似性, 韩语和英语之间较为相似的正字法透明度诱发了大脑更高的激活相似性(Kim et al., 2016)。另外一项维吾尔?汉?英三语者的研究成果也表明, 正字法透明度的相似性影响语言间跨语言模式的相似性。与汉语相比, 维吾尔语和英语词汇阅读在语音加工的大脑区域, 如左侧颞上回后部、缘上回、角回和中央前回表现出更大的跨语言模式相似性。研究者认为这可能是由于维吾尔语和英语词汇阅读都更加需要从正字法到语音映射的背侧通路的参与(Dong et al., 2021)。Shen和Tufo(2022)探讨了语言间正字法距离对阅读相关脑区静息态连接的影响。结果发现, 语言间的正字法透明度越不相似(正字法距离越大), 左侧缘上回和右侧缘上回、额中回、额下回和脑岛之间, 梭状回与左侧楔前叶之间的功能连接越强。可见, 语言间正字法透明度的相似性也会影响到母语和二语的大脑激活, 进而对阅读的背侧和腹侧通路产生影响。
4 阅读经验对背侧和腹侧通路动态协作的塑造作用
成人阅读的认知和神经机制是长期阅读经验积累的结果。基于相同的阅读计算机制, 不同正字法深度的书写系统具有不同的输入语料特性, 阅读者根据输入语料的特性发展出与之相适
阅读经验能够促进VWFA与阅读相关的语音和语义加工脑区的功能连接增强。对比儿童与成人的静息态功能连接, 研究发现成人比儿童在VWFA与左侧缘上回以及VWFA与左侧额下回的功能连接更强(Li et al., 2017)。任务态功能连接的研究得到了类似的发现。如, 在语音判断任务中, 正常成人比儿童表现出从VWFA到左侧顶下小叶以及从VWFA到左侧额中回更强的有效连接(Siok et al., 2020)。这一现象也在不同群体的儿童脑激活上有所体现。比如, 最新的研究发现, 随着儿童阅读技能的提高, 有阅读经验的儿童比初学儿童表现出从VWFA到阅读相关脑区(左侧顶下小叶、中央前回与额下回)有效连接的增强(di Pietro et al., 2023)。而与正常发展儿童相比, 阅读障碍儿童往往表现出VWFA与额下回等阅读相关脑区的功能连接异常。研究者发现无论是在阅读任务(Morken et al., 2017; Wang et al., 2020)還是静息状态下(Koyama et al., 2013; Schurz et al., 2015), 阅读障碍儿童的VWFA与左侧化阅读相关脑区之间往往表现出减弱的功能连接。
阅读经验还塑造了功能脑区皮层下白质连接的神经基础。对儿童阅读习得的大脑结构发育进行考察发现, 随着阅读技能的获得, 学龄儿童(9~10岁)比学前儿童(5~6岁)表现出VWFA和左侧化阅读网络, 如左侧顶下小叶、颞中回、辅助运动皮层与中央前回更多的结构连接(Simon et al., 2013)。来自追踪研究的证据也显示, 学前儿童(5.5~6.5岁)在接受一年的词汇学习后, VWFA和左侧颞顶皮层的结构连接表现出径向扩散率(radial diffusivity, RD)的显著降低, 表明词汇学习可以促进白质纤维束髓鞘化的成熟(Moulton et al., 2019)。阅读强化训练也能在一定程度上增强VWFA和其它阅读脑区间的结构连接。例如, 在一项纵向干预研究中, 实验组(7~12岁)儿童接受为期8周的语音和正字法强化训练, 而控制组参加正常的学校学习。结果发现, 随着干预时间和阅读技能的增长, 实验组儿童弓状束和下纵束的FA值快速增加, 表明与控制组相比, 实验组儿童这两条神经纤维的髓鞘化程度更高(Huber et al., 2018)。更多的研究发现, 经VWFA连接额?颞?枕叶的下额枕束与学前儿童(5~6岁)的字母和语音意识相关, 而与早期学龄儿童(7~8岁)的正字法加工有更强的相关, 表明阅读经验影响了下额枕束在阅读中的功能(Vanderauwera et al., 2018)。
4.2阅读经验增强了形?音加工区与其它阅读功能脑区的连接
研究显示左侧颞顶皮层(Temporal Parietal Cortex, TPC)随着儿童语音加工能力的成熟, 词汇阅读过程中会诱发左侧TPC更强的激活(Cao et al., 2015; Moulton et al., 2019)。而存在词汇阅读障碍的群体往往表现出左侧TPC脑区的激活不足(Braid & Richlan, 2022; Richlan & Wimmer, 2011)。
阅读经验改变了左侧TPC与阅读网络中其它脑区之间的功能连接, 也成为拼音文字系统阅读能力发展的重要生理指标。采用纵向追踪的研究发现, 儿童语音加工能力与左侧TPC到梭状回和额下回脑区连接强度的变化存在显著相关(Yu et al., 2018)。与儿童初学者相比, 有一定阅读经验的儿童从梭状回到TPC的有效连接更强; 而阅读障碍儿童比正常组儿童表现出从TPC到梭状回更强的有效连接(di Pietro et al., 2023)。汉语发展性阅读障碍研究也发现, 阅读障碍儿童存在梭状回与TPC的连接异常(Cao et al., 2008)。
相应地, 阅读经验增强了TPC与其它阅读功能脑区的神经生理连接。TPC脑区主要通过背侧弓状束将腹侧枕颞皮层与额下回相连, 在词汇阅读的语音加工中起着重要作用(Lerma-Usabiaga et al., 2018)。随着阅读能力的获得, 连接左侧TPC与腹侧枕颞皮层的弓状束后部的FA值变大, 反映了阅读经验对背侧语音通路的塑造作用(de Schotten et al., 2014)。研究发现, 弓状束的FA值与儿童的语音意识(Zuk et al., 2021)以及语音编码能力(Cross et al., 2023)显著相关。而且, 儿童(7~12岁)的阅读能力与弓状束的发育模式显著相关。具体的, 阅读能力高于平均水平的儿童随着年龄增长, 左侧弓状束的FA值逐渐变大; 而阅读能力低于平均水平的儿童, 左侧弓状束的FA值随时间推移逐渐下降(Yeatman et al., 2012)。阅读障碍儿童的研究也发现, 与正常组儿童相比, 这些儿童往往表现出左侧弓状束更小的FA值(van der Auwera et al., 2021; Su, de Schotten, et al., 2018)。
4.3阅读经验塑造了背、腹侧通路的分工合作
正常阅读者的背侧通路和腹侧通路具有不同的发展轨迹。在词汇学习初期, 儿童主要依靠形素到音素的转化规则进行发音, 背侧通路在阅读中发挥着重要作用。随着年龄和阅读能力的增长, 儿童越来越依赖腹侧通路并能够快速地识别整词, 而不需要进行形?音转换的加工(Pugh et al., 2001)。一项对儿童和成人脑激活对比的元分析研究也发现, 儿童的词汇阅读更依赖于形?音转换加工的背侧脑区, 随着阅读能力的增长, 成人读者表现出对腹侧通路的依赖增强(Martin et al., 2015)。
随着儿童阅读经验的增长, 对两条神经通路依赖性的动态变化反映了阅读能力的发展。例如, Younger等人(2017)发现高、低阅读能力组儿童(8~14岁)表现出阅读神经通路不同的发展趋势。其中, 高阅读能力组随时间表现出背侧通路由强变弱以及腹侧通路的稳定增强; 而低阅读能力组的背侧通路随时间几乎没有连接差异, 且腹侧通路随时间表现出逐渐减弱的趋势。Caffarra等人(2021)综述前人的研究指出, 随着词汇习得以及阅读熟练度的提高, 正常儿童的腹侧枕颞皮层后部及其通过垂直枕束与顶叶皮层相连(背侧通路)的结构连接强度逐渐减弱, 而腹侧枕颞皮层前部及其通过弓状束与颞叶与额叶相连(腹侧通路)的结构连接逐渐增加; 然而, 对于阅读障碍儿童, 这一转变过程可能会延迟或受损。
而且, 随着儿童年龄和阅读经验的增长, 阅读脑网络的背侧语音和腹侧语义脑区之间相互作用逐渐增强。比如, 儿童在两个不同年龄段(T1, T2)完成语义判断任务, 结果发现与T1 (M= 10.1岁)的脑激活相比, 儿童在T2 (M= 12.0岁)时表现出左侧颞中回后部对弱关联词汇更强的激活及其与顶下小叶更强的功能连接, 研究者认为这一现象主要反映了语音加工对早期语义表征的促进(Lee et al., 2016)。
综上所述, 语言经验对VWFA和TPC两个阅读重要脑区及其与其它脑区的连接机制具有重要的塑造作用。也正是随着语言经验的不断丰富和发展, 阅读背/腹侧神经通路的分工协作机制也逐渐形成, 从而表现出在特定刺激和任务条件下阅读神经网络的激活差异。
5 总结与展望
本文涉及的重要理论问题是认知和神经生理统一的阅读理论模型。研究者基于不同的认知模型尝试建立认知加工与大脑功能的对应关系。基于视觉加工的背/腹侧神经通路, 研究者提出了词汇阅读的背/腹侧神经通路(Pugh et al., 2000); 基于双通路理论, 研究者对比不同类型词汇阅读的脑区激活差异, 在脑机制上提出了双通路神经生理模型(Jobard et al., 2003); 随后更多的研究者尝试在不同的加工任务下识别词汇阅读的神经通路(Levy et al., 2009; Richardson et al., 2011)。而在联结主义阅读理论取向下, 研究者在识别词汇阅读的语音/语义加工通路的同时, 还考察了加工通路在刺激属性和加工任务双重驱动下的动态协作机制(Carreiras et al., 2014; Price, 2012)。如图1所示, 研究者尝试建立认知与神经统一的理论模型, 但是还有很多问题亟待解决, 如语义加工涉及了众多脑区的参与, 未来还需要厘清语义加工神经通路的具体机制。
在神经网络的研究取向下, 探讨词汇阅读中神经通路间的动态协作机制是研究者关注的热点和焦点问题, 目前的研究可以得出以下结论:1) 特定实验条件下发现的阅读脑区激活由潛在的加工需求决定, 并表现出阅读背/腹侧神经通路的动态协作; 2) 背/腹侧神经通路的动态协作是跨语言普遍的阅读脑机制; 3) 阅读经验对背/腹神经通路动态协作机制的形成起着至关重要的作用。
本文尝试将阅读的认知和神经模型统一为潜在加工需求下阅读神经通路的动态协作机制, 但是, 对一些基本问题还需要进一步厘清:首先, 需要深入阐明潜在加工需求的实质。这里的潜在加工需求是指特定实验任务下涉及的具体加工, 不同的任务需求对阅读中的正字法、语音和语义加工具有不同的依赖。而特定任务下不同的阅读加工成分是否具有实质相同的加工需求, 如加工负荷(Taylor et al., 2013), 或者在多大程度上不同的阅读加工成分能够统一为基本的认知加工, 这些问题都还需要深入细致地探讨。
其次, 需要阐明动态性的具体体现。一方面, 本文提出的动态性体现在受到实验任务和实验材料(词汇属性、语言特性)的影响时, 背/腹侧阅读通路表现出脑区激活或功能连接在空间拓扑上的变化。另一方面, 从阅读经验对背侧和腹侧通路动态协作的塑造作用, 尝试从发展的角度论述阅读通路协作模式的动态性变化。具体表现为随着儿童学习经验的积累, 脑区的参与程度(激活强度和白质密度等)、以及体现在阅读的背侧和腹侧通路的连接效率呈现出不同的趋势。但是, 在“时间”维度上, 文中关于儿童阅读发展的大尺度研究与其略有相关, 对于更加精细的加工进程层面, 本文没有提及。如果能从加工进程的“时间”维度阐明阅读神经通路间的动态协作, 无疑将有利于揭示阅读网络动态性的时空特征。
同时, 未来的研究还需要对词汇阅读的神经网络展开深入探讨:1) 阅读相关脑区的功能认识还存在争论, 很大程度上可能是特定刺激或任务对比导致的潜在加工需求不同并导致了特定的实验结果。这就需要透过实验任务或刺激条件的表象, 从潜在加工本质的角度来考察阅读相关脑区的功能。2) 从神经网络的角度探讨阅读的脑机制, 不仅需要考察脑区/神经通路间的动态协作机制, 还需要考察特定脑区作为跨认知功能网络的一部分, 在不同认知加工需求下表现出来的动态激活特性。同时, 词汇阅读与视觉客体加工神经网络的普遍性与特异性问题还需要深入探讨。阅读神经网络在多大程度上是语言特异性或领域一般性的, 是未来研究需要解决的重要理论问题。3) 当前词汇阅读的认知和神经生理模型还不统一, 未来研究需要进一步构建并完善词汇阅读的理论模型。
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Dynamic collaboration of reading neural pathways driven by the processing demands
DANG Min, CAI Wenqi, CHEN Fakun, WANG Xiaojuan, YANG Jianfeng
(School of Psychology, Shaanxi Normal University, Xian 710062, China)
Abstract: Constructing unified cognitive and neurophysiological models is the central problem in the cognitive neuroscience of word reading. The cognitive models agree that reading is the collaborative outcome of phonological and semantic processing pathways, and studies of cognitive neuroscience have also shown that reading results from a dynamic collaboration between dorsal and ventral neural pathways. In order to systematically elaborate this dynamic collaboration mechanism of the reading network, the latest research progress is systematically disentangled from the following three aspects by combining the two levels of neural function and physiological basis. Firstly, it points out that the underlying processing demand is the essence of the dynamic collaboration between dorsal and ventral neural pathways. Secondly, it further elucidates how underlying processing demand drives the division of labor and collaborative patterns between the dorsal and ventral neural pathways under different levels of orthographic depths. Finally, it profoundly analyzes how latent processing shapes the dynamic collaboration between neural pathways through the language experience. In conclusion, the essence of the division and collaboration between neural pathways might be driven by processing demand under the specific task. It might become a universal cross-language word reading model.
Keywords: word reading, model, pathway, dynamics of collaboration