1 引言
神经生理学的研究表明多感觉通道信息的整合方式是人进行感知觉加工的主要方式(Falchier, Clavagnier, Barone, & Kennedy, 2002)。在日常生活中, 我们能够通过多个感觉通道从自然环境中感知信息, 来自不同感觉通道的信息会导致多个感觉通道之间产生交互作用, 这种交互作用既可以表现为通道间的信息整合, 也可以表现为通道间的信息竞争(Chen & Zhou, 2013; Driver & Noesselt, 2008; Koelewijn, Bronkhorst, & Theeuwes, 2010; Spence, 2011; Talsma, 2015)。根据统计最优理论, 在加工多感觉通道信息时, 大脑可以根据每种感觉信息的可靠性来决定其在多感觉整合过程中所占的比例, 进而导致在某些情况下, 一个感觉通道的信息得到优先加工而处于主导性的地位, 即出现感觉主导效应(Chen & Zhou, 2013; Koelewijn et al., 2010; Spence, 2011; Zhou, Jiang, He, & Chen, 2010)。
声音诱发闪光错觉(SiFI)是一种典型的听觉占主导性的多感觉整合现象, 即当一个视觉闪光伴随着两个听觉声音呈现时, 一个视觉闪光会被错误知觉为两个, 被称为裂变错觉(Shams, Kamitani, & Shimojo, 2000, 2002); 当两个视觉闪光伴随着一个听觉声音同时呈现时, 两个视觉闪光会被错误知觉为一个, 也被称为融合错觉(Andersen, Tiippana, Sams, 2004)。自声音诱发闪光错觉现象被发现后, 许多研究者开始探讨影响声音诱发闪光错觉的因素。一些研究考察了刺激的大小、形状、对比度以及闪光与声音的相对位置等自下而上的因素, 研究结果发现改变刺激大小、形状和对比度等属性虽然能影响闪光错觉的错觉量大小, 但声音诱发闪光错觉效应依然稳定存在(Shams et al., 2000; Shams, Ma, & Beierholm, 2005)。也有一些研究考察了自上而下的因素对声音诱发闪光错觉的影响, 研究结果发现虽然自上而下的因素也能够影响闪光错觉的错觉量, 但声音诱发闪光错觉的现象也依然稳定存在(Mishra, Martinez, & Hillyard, 2009; 于薇, 王爱君, 张明, 2017; 张明 等, 2018)。关于声音诱发闪光错觉的神经机制, 研究者采用功能性磁共振成像(fMRI)、经颅直流电刺激(tDCS)、经颅磁刺激技术(TMS)和脑磁图(MEG)等技术针对大脑在加工声音诱发闪光错觉范式中的视听觉信息时, 哪些脑区参与加工进行了研究。研究表明, 视觉皮层(Maccora et al., 2019; Watkins, Shams, Josephs, & Rees, 2007; Watkins, Shams, Tanaka, Haynes, & Rees, 2006; Zhang & Chen, 2006)、颞上沟和上丘(Watkins et al., 2006, 2007)、前额叶皮层和内侧枕叶皮层(Bolognini, Rossetti, Casati, Mancini & Vallar, 2011; Jiang & Han, 2007)、角回等顶叶网络(Kamke, Vieth, Cottrell, & Mattingley, 2012)等广泛区域均参与加工了声音诱发闪光错觉。
已有研究发现, 声音诱发闪光错觉效应存在个体差异, 即在同样的测量条件下, 不同个体的声音诱发闪光错觉的错觉量存在差异。Mishra, Martinez, Sejnowski和Hillyard (2007)的研究发现, 声音诱发闪光错觉现象在个体间的差异较大, 闪光错觉发生的比例范围在个体间是3%~86% (Mishra et al., 2007)。从大脑结构层面来看, de Haas, Kanai, Jalkanen和Rees (2012)的研究表明, 大脑灰质体积的差异造成了个体对声音诱发闪光错觉敏感性的差异(de Haas et al., 2012)。也有研究表明, 个体α频率(individual alpha frequency, IAF)的峰值和闪光错觉出现的时间窗大小之间存在正相关(Cecere, Rees, Romei, 2015)。除此之外, 从个体在年龄层面的差异角度看, 一些研究也发现相对于年轻人, 老年人更容易产生声音诱发闪光错觉, 且视听刺激整合的时间窗口更大(Chan, Connolly & Setti, 2018; Diederich, Colonius, & Schomburg, 2008; Hirst, Setti, Kenny, & Newell, 2019; Setti, Burke, Kenny, & Newell, 2011)。DeLoss, Pierce和Andersen (2013)使用声音诱发闪光错觉范式考察声音诱发闪光错觉是否存在年龄差异。结果发现, 与年轻人相比, 老年人抑制无关跨通道信息能力降低, 整合信息能力更强, 即表现出更多的错觉现象(DeLoss et al., 2013)。McGovern, Roudaia, Stapleton, McGinnity和Newell (2014)的研究进一步发现, 老年人即使在刺激的时间间隔(SOA)较长的情况下也会产生裂变错觉。但是对于融合错觉, 老年人和年轻人之间的错觉量并不存在显著差异(McGovern et al., 2014)。
尽管目前现有的研究发现老年人与年轻人的视听整合能力存在差异, 进而导致声音诱发闪光错觉的错觉量存在差异。但对老年人声音诱发闪光错觉产生机制的研究较少。本研究采用静息态功能磁共振技术考察老年人声音诱发闪光错觉与大脑静息态自发活动区域的关系, 以此揭示相对于年轻人而言, 老年人声音诱发闪光错觉量更大的机制。静息态功能性磁共振成像技术(resting-state functional magnetic resonance imaging, RS-fMRI)是基于血氧水平依赖(blood oxygenation level-dependent, BOLD)信号的变化来获取一定时间内大脑功能活动水平的技术, 已广泛用于测量大脑的功能以及功能连接。RS-fMRI由于它独特的优势得到研究者越来越多的关注, 在神经科学的研究中得到了广泛的应用(Biswal, Yetkin, Haughton, & Hyde, 1995)。由于老年人认知能力和运动能力的下降, 任务态功能磁共振成像技术受实验范式、实验任务和被试特点等因素的影响对于老年人有效地完成任务可能存在一定的困难, 因此RS-fMRI在研究老年人这一相关问题上具有优势。静息态功能磁共振成像中, 自发低频波动振幅(ALFF)是反映自发神经元活动程度的一种广泛使用的指标(Zang et al., 2007), 被广泛用作测量大脑活动的自发低频波动。而且有研究发现, ALFF能够预测任务诱发神经活动的大小, 即ALFF值越高, 任务诱发的神经活动越强(Mennes et al., 2011)。此外, ALFF的研究也已经被应用于老年人和疾病患者的研究之中, 包括老年人注意缺陷多动障碍(Zang et al., 2007)和早期阿尔茨海默病(He et al., 2007)。静息态功能磁共振研究不受实验任务和实验范式的制约和影响, 能够可靠地测量到大脑内在的自发活动(Zuo et al., 2010), 而ALFF作为静息态研究常用的指标, 通过计算全脑每一体素低频振荡的幅度大小, 来表示每个体素的BOLD信号强度, 从能量代谢的角度反映静息状态下神经元自发活动水平的高低(Zang et al., 2007)。鉴于静息态功能磁共振成像技术在探讨老年人认知功能方面具有一定的优越性, 加之目前声音诱发闪光错觉的机制研究均采用任务态功能像以及结构像, 鲜有研究采用静息态功能磁共振成像技术。因此, 本研究采用静息态功能磁共振成像技术的ALFF指标考察老年人声音诱发闪光错觉与大脑静息态自发活动区域的关系, 并且通过与年轻人对比揭示从发展的角度看, 老化对于声音诱发闪光错觉的影响及其机制。
2 方法
2.1 被试
采用G*Power 3.1.9.2对实验中的样本量进行了计算。采用2 (被试群体: 老年人vs.年轻人) × 2 (错觉类型: F1B2 vs. F2B1)的重复测量方差分析作为统计检验, 设置参数effect size ƒ为0.25, I类错误的概率α err prob为0.05, 检验效能Power (1 - β err prob)为0.80, 计算得到样本量为34 (老年人和年轻人各17名)。本研究招募了51名老年女性被试, 但数据处理过程中将频动超过了2 mm或者转动超过了2°的被试排除, 因此剩余50名被试纳入统计分析, 年龄介于50~69岁(M = 61.78, SD = 4.17)。为了与老年人的结果进行对比, 本研究招募了50名年轻被试群体, 年龄介于17~27岁(18男, 32女; M = 20.86, SD = 2.17)。所有被试视力正常或者矫正视力正常, 没有任何精神病史。之前均未参加过类似实验, 实验后被试获得一定的报酬。所有被试在实验前填写“磁共振(MRI)扫描被试检查单”, 符合磁共振检查要求的被试在正式实验前将被详细告知实验的过程, 并签署知情同意书。实验程序经某大学伦理委员会审核通过, 在研究过程中严格按照预定的实验程序实施。
2.2 实验仪器和材料
行为实验的所有刺激呈现在View Sonic P220f VS10284型号的显示器上, 屏幕分辨率为1024 × 768像素, 刷新率为60 Hz。实验中所有视觉刺激呈现在黑色的背景上由Presentation软件编程(Neurobehavioral Systems Inc), 其中视觉闪光刺激为呈现在中央注视点下方5°视角处的白色圆盘(半径视角为2°), 呈现的时间为17 ms。之所以将视觉闪光刺激呈现在中央注视点下方5°视角处是因为在听觉声音刺激伴随下, 视觉闪光刺激位于外周视野时的错觉效应最大(Shams et al., 2002)。实验中的听觉声音刺激通过头戴式铁三角耳机(ATH-WS99)呈现。听觉声音刺激的响度为75 dB, 频率为3.5 kHz, 呈现时间为7 ms。
2.3 实验设计和流程
被试在正式实验之前先进行练习实验, 主要目的是为了熟悉实验任务以及排除掉不能分辨清楚视觉闪光和听觉声音个数的被试。正式实验为2(被试群体: 老年人vs.年轻人) × 2 (视觉闪光刺激: 1 vs. 2) × 3 (听觉声音刺激: 0 vs. 1 vs. 2)的混合设计。所有视觉闪光刺激和听觉声音刺激构成了6种实验条件, 即F1、F2、F1B1、F1B2、F2B1和F2B2。为了便于论述, 这些试次类型均使用英文缩写来表示, 如“F1B1”表示呈现一个视觉闪光刺激时伴随着一个听觉声音刺激的试次; “F1B2”表示呈现一个视觉闪光刺激时伴随着两个听觉声音刺激的试次。两组被试均完成6种实验条件, 每种实验条件下80个试次, 一共480个试次(每个block有120个试次, 共4个block, 每个block间被试可进行休息)。6种实验条件下的刺激在实验中随机呈现, 试次间的时间间隔以100 ms为步长从400 ms到700 ms随机。在F1B1、F1B2、F2B1和F2B2条件下, 第一个听觉声音刺激与第一个视觉闪光刺激的开始时间相同, 听觉声音刺激呈现7 ms, 视觉闪光刺激呈现17 ms。F2、F2B1和F2B2条件下, 两个视觉闪光刺激的时间间隔为66 ms; F1B2和F2B2条件下, 两个听觉声音刺激的时间间隔为76 ms (刺激呈现示意如图1所示)。实验要求被试判断视觉闪光刺激的个数, 而忽略听觉声音刺激。
图1
2.4 静息态功能性磁共振数据采集与分析
2.4.1 数据采集
磁共振图像采集于MRI磁共振室的飞利浦3.0T磁共振扫描仪, 所有被试的功能静息态像采用梯度回波平面序列(gradient echoplanar imagine, EPI)获得, 扫描参数为: TR = 2000 ms, TE = 30 ms, 视野(FOV) = 220 mm × 220 mm, 翻转角 = 90°, 矩阵 = 64 × 64, 层数 = 36层, 层厚 = 4 mm, 扫描时间400 s。所有被试的3D-T1图像采用快速梯度回波序列(magnetization-prepared gradient echo, MPRAGE)。扫描视野FOV = 240 mm × 240 mm × 150 mm, 扫描矩阵matrix size: 256×256×256, TR/TE = 8.5/3.4 ms, 射频翻转角(FA) = 12°。采集静息态图像期间, 被试者平躺于扫描仪内, 头部调整至舒适位置后给予适当固定, 要求被试保持头部与身体不动, 自然清醒放松状态, 尽可能不作任何动作和意向性思维。
2.4.2 数据预处理
采用SPM8 (http://www.fil.ion.ucl.ac.uk/spm)和中国科学院心理研究所严超赣研究员团队开发的DPABI软件(Yan, Wang, Zuo, & Zang, 2016) (DPABI: Data Processing & Analysis for Resting-State Brain Imaging)搭载在Matlab2013b平台上进行分析处理。将所有被试的DICOM图像转化为NIFTI图像后, 预处理过程包括: 删除前10个时间点、头动矫正(将频动超过了2 mm或者转动超过了2°的被试进行排除)、时间校正(参考层为每个全脑扫描过程中位于中间时间点的那一层)和去除协变量(包括6个头动参数和白质信号、脑脊液信号)。之后采用DARTEL空间标准化, 将分割获得的功能图像参数以蒙特利尔标准脑为模板进行标准化, 体素大小为3 mm × 3 mm × 3 mm。对标准化后的图像使用4 mm全宽半最大值(FWHM)滤波器进行空间平滑处理。然后去除线性趋势, 对图像进行时间带通滤波(0.01~0.08 Hz), 减少低频漂移和高频噪声(Biswal et al., 1995; Pan et al., 2014)。
2.4.3 ALFF指标计算
ALFF计算结果通过静息态fMRI数据处理软件DPABI计算。ALFF反映了大脑区域自发波动的强度。ALFF分析提供了一种在低频范围(0.01~0.08 Hz)有效评估voxel-wise BOLD信号动态特性的方法。ALFF为低频BOLD信号范围内振幅的总和, 因此与低频振荡(LFOs)的整体强度有关。将预处理的图像时间序列经由快速傅里叶变换到频域, 得到其功率谱。然后计算功率谱上每个频率的平方根, 最后对每个体素在频率为0.01~0.08 Hz频段的范围内计算均值就得到每个体素在0.01~0.08 Hz频段的ALFF值(Zang et al., 2007; Zuo & Xing, 2014)
2.4.4 ALFF与错觉量的相关分析
采用DPABI软件(Yan et al., 2016)以年龄作为协变量以排除个体年龄差异对结果的影响(Pan et al., 2016; Wang et al., 2016; Lei et al., 2017; Li et al., 2014; 罗扬眉, 李宝林, 刘杰, 毕重增, 黄希庭, 2015), 对平均ALFF值与行为实验错觉量结果进行偏相关分析(Partial correlation analysis)。为了控制I型错误进行蒙特卡罗模拟(Monte carlo simulations) (包括: 个体体素p = 0.05, 1000次双尾模拟, 4 mm FWHM过滤, AlphaSim校正, 矫正后p < 0.05)。
3 结果
3.1 行为数据
将两组被试的正确率进行2(被试群体: 老年人vs.年轻人) × 2 (视觉闪光刺激: 1 vs. 2) × 3 (听觉声音刺激: 0 vs. 1 vs. 2)的重复测量方差分析(见图2)。结果发现, 被试群体的主效应显著, F(1, 98) = 11.91, p = 0.001, η2 = 0.20; 视觉闪光刺激的主效应不显著, F(1, 98) = 1.43, p = 0.238; 听觉声音刺激的主效应显著, F(2, 97) = 118.70, p < 0.001, η2 = 0.83; 被试群体和视觉闪光刺激的交互效应显著, F(1, 98) = 11.91, p = 0.019, η2 = 0.11; 被试群体和听觉声音刺激的交互效应不显著, F(2, 97) = 1.49, p = 0.236; 视觉闪光刺激和听觉声音刺激的交互效应显著, F(2, 97) = 219.40, p < 0.001, η2 = 0.90; 被试群体、视觉闪光刺激和听觉声音刺激三者的交互效应显著, F(2, 97) = 8.08, p = 0.001, η2 = 0.25。
图2
为了进一步了解视觉闪光刺激和听觉声音刺激在不同被试群体中的交互作用, 将老年人和年轻人行为反应的正确率分别进行2(视觉闪光刺激: 1 vs. 2) × 3(听觉声音刺激: 0 vs. 1 vs. 2)的重复测量方差分析。结果发现, 在老年人群体中, 视觉闪光刺激的主效应不显著, F < 1; 听觉声音刺激的主效应显著, F(2, 48) = 64.93, p < 0.001, η2 = 0.73; 两者交互作用显著, F(2, 48) = 126.20, p < 0.001, η2 = 0.84; 为了进一步了解视觉闪光刺激和听觉声音刺激的交互关系, 对听觉声音条件影响下的视觉闪光条件进行简单效应分析: 当没有听觉声音刺激时, 所知觉到的视觉闪光个数正确率之间存在显著差异, t(49) = 5.35, p < 0.001, Cohen’s d = 0.96, 95% CI = [0.07, 0.16]; 当听觉声音刺激为1个时, 所知觉到的视觉闪光个数正确率之间存在显著差异, t(49) = 11.01, p < 0.001, Cohen’s d = 2.26, 95% CI = [0.39, 0.56]; 当听觉声音刺激为2个时, 所知觉到的视觉闪光个数正确率之间存在显著差异, t(49) = 12.51, p < 0.001, Cohen’s d = 2.70, 95% CI = [0.46, 0.64]。说明了对于老年被试来说, 一个视觉闪光的正确率比两个视觉闪光的正确率高。F1B1与F2B1之间存在显著差异, 说明声音诱发了融合错觉, 即当呈现1个听觉声音时, 错误地将2个视觉闪光知觉为1个。F2B2与F1B2之间存在显著差异, 这说明声音诱发了裂变错觉, 即当呈现2个听觉声音时, 错误地将1个视觉闪光知觉为2个。除此之外, 对F1B2和F2B1进行配对样本t检验之后, 结果表明, 裂变错觉和融合错觉在错觉量上并没有显著差异, t(49) = 1.36, p = 0.18, Cohen’s d = 0.24 (本研究同时也进行了贝叶斯检验, 结果与t检验的结果一致)。依据Cohen提出对值效应量大小的判断标准(Cohen, 1988), d = 0.2, d = 0.5和d = 0.8分别对应于小、中、大的效应量(温忠麟, 范息涛, 叶宝娟, 陈宇帅, 2016)。因此, 简单效应分析结果表明, 研究存在显著的裂变错觉和融合错觉。
在年轻人群体中, 视觉闪光刺激的主效应显著, F(1, 49) = 6.42, p = 0.02, η2 = 0.12; 听觉声音刺激的主效应显著, F(2, 48) = 69.97, p < 0.001, η2 = 0.75; 两者交互作用显著, F(2, 48) = 85.69, p < 0.001, η2 = 0.78。为了进一步了解视觉闪光刺激和听觉声音刺激的交互关系, 我们对听觉声音条件影响下的视觉闪光条件进行简单效应分析: 当没有听觉声音刺激时, 所知觉到的视觉闪光个数正确率之间没有显著差异, t(49) = 1.29, p = 0.20; 当听觉声音刺激为1个时, 所知觉到的视觉闪光个数正确率之间存在显著差异, t(49) = 6.04, p < 0.001, Cohen’s d = 1.24, 95% CI = [0.16, 0.32]; 当听觉声音刺激为2个时, 所知觉到的视觉闪光个数正确率之间存在显著差异, t(49) = 11.08, p < 0.001, Cohen’s d = 2.21, 95% CI = [-0.52, -0.36]。F1B1与F2B1之间存在显著差异, 这说明声音诱发了融合错觉, 即当呈现1个听觉声音时, 错误地将2个视觉闪光知觉为1个。F2B2与F1B2之间存在显著差异, 这说明声音诱发了裂变错觉, 即当呈现2个听觉声音时, 错误地将1个视觉闪光知觉为2个。除此之外, 对F1B2和F2B1进行配对样本t检验, 结果表明, 裂变错觉的错觉量显大于著融合错觉的错觉量, t(49) = 3.44, p = 0.001, Cohen’s d = 0.63, 95% CI = [-0.27, -0.07] (本研究同时也进行了贝叶斯检验, 结果与t检验的结果一致)。
进一步2(被试群体: 老年人vs.年轻人) × 2 (错觉量: 裂变错觉vs.融合错觉)的重复测量方差分析。结果发现, 被试群体的主效应显著, F(1, 98) = 13.72, p = 0.001, η2 = 0.22; 错觉量的主效应显著, F(1, 98) = 10.95, p = 0.002, η2 = 0.18; 被试群体和错觉量的交互作用不显著, F(1, 98) = 1.98, p = 0.17。对老年人和年轻人的裂变错觉和融合错觉分别进行独立样本t检验之后, 结果发现, 对于裂变错觉, 老年人的错觉量显著大于年轻人, t(98) = 2.13, p = 0.036, Cohen’s d = 0.43, 95% CI = [-0.23, -0.01]; 对于融合错觉, 老年人的错觉量也显著大于年轻人, t(98) = 3.86, p < 0.001, Cohen’s d = 0.78, 95% CI = [-0.32, -0.10]。
3.2 ALFF与裂变错觉
对于老年人, F1B2条件下, 当cluster size > 127 voxels, 个体voxel cluster p < 0.05时, 校正后的显著性水平为p < 0.05; 将与F1B2错觉量相关的脑区在DPABI-VIEW (Yan et al., 2016) ch2bet提供的模板上进行显示, 所有显著相关均以MNI坐标表示。图3上(A)结果显示, F1B2错觉量和内侧前额叶皮层(medial prefrontal cortex, MPFC)区域呈显著正相关, 表现为内侧前额叶皮层活动越强烈, 被试的裂变错觉量越大(peak voxel at x = -27, y = 51, z = 0, cluster size k = 304 voxels, rpeak = 0.56, rcluster = 0.55, p < 0.001), 见图3上(B)。对于年轻人, F1B2条件下, 当cluster size > 117 voxels, 个体voxel cluster p < 0.05时, 校正后的显著性水平为p < 0.05; 将与F1B2错觉量相关的脑区在DPABI-VIEW (Yan et al., 2016) ch2bet提供的模板上进行显示, 所有显著相关均以MNI坐标表示。图3下(A)结果显示, F1B2 错觉量和右侧枕上回(superior occipital gyrus, SOG)和左侧舌回(Lingual)呈显著负相关, 表现为两个区域的活动越弱, 被试的裂变错觉量越小, 以右侧枕上回为例(peak voxel at x = 24, y = -87, z = 9, cluster size k = 237 voxels, rpeak = -0.50, rcluster = -0.45, p = 0.001), 见图3下(B)。
图3
图3
上(A) 老年人F1B2条件下ALFF与错觉量呈现显著相关的脑区; (B) 散点图显示错觉量与mean ALFF在Prefrontal_Med_L上呈现显著相关(r = 0.55, p < 0.001)。下(A) 年轻人F1B2条件下ALFF与错觉量呈现显著相关的脑区; (B) 散点图显示错觉量与mean ALFF在Occipital_Sup_R上呈现显著相关(r = -0.45, p = 0.001)。(Color bars represent R-values, L = Left, R = Right. MNI = Montreal Neurological Institute.)
3.3 ALFF与融合错觉
对于老年人, F2B1条件下, 当cluster size > 132 voxels, 个体voxel cluster p < 0.05时, 校正后的显著性水平为p < 0.05。将与F2B1错觉量相关的脑区在DPABI-VIEW (Yan et al., 2016) ch2bet提供的模板上进行显示, 所有显著相关均以MNI坐标表示。图4上(A)结果显示, F2B1错觉量和左外侧额下回(inferior frontal gyrus, IFG)、右侧额中回(middle frontal gyrus, MFG)、左侧颞下回(inferior temporal gyrus, ITG)呈显著正相关, 与左侧枕上回(superior occipital gyrus, SOG)、左侧舌回(Lingual)、右侧小脑区(Cerebelum)呈显著负相关。表现为左外侧额下回、右侧额中回和颞下回的活动越强烈, 被试的融合错觉的错觉量越大, 而左侧枕上回和舌回的活动越弱, 被试的融合错觉量越大。以左外侧额下回为例(peak voxel at x = -54, y = 24, z = 33, cluster size k = 343 voxels, r peak = 0.50, rcluster = 0.40, p = 0.004 < 0.05), 见图4上(B)。对于年轻人, F2B1条件下, 当cluster size > 116 voxels, 个体voxel cluster p < 0.05时, 校正后的显著性水平为p < 0.05。将与F2B1错觉量相关的脑区分别在DPABI- VIEW (Yan et al., 2016) ch2bet提供的模板上进行显示, 所有显著相关均以MNI坐标表示。图4下(A)结果显示, F2B1错觉量和左侧额上回(superior frontal gyrus, SFG)区域呈显著正相关。表现为在外侧额上回区域活动越强烈, 被试的融合错觉量越大(peak voxel at x = -21, y = 36, z = 39, cluster size k = 118 voxels, rpeak = 0.57, rcluster = 0.55, p < 0.001), 见图4下(B)。
图4
图4
上(A) 老年人F2B1条件下ALFF与错觉量呈现显著相关的脑区; (B) 散点图显示错觉量与mean ALFF在Frontal_Inf_Oper_L上呈现显著相关(r = 0.40, p = 0.004)。下(A) 年轻人F2B1条件下ALFF与错觉量呈现显著相关的脑区; (B) 散点图显示错觉量与mean ALFF在Frontal_Sup_L上呈现显著相关(r = 0.55, p < 0.001)。(Color bars represent R-values, L = Left, R = Right. MNI = Montreal Neurological Institute.)
4 讨论
本研究采用静息态功能磁共振成像的ALFF指标考察老年人声音诱发闪光错觉与大脑静息态自发活动区域的关系, 并且通过与年轻人进行对比来揭示从发展的角度看, 老年人视听整合能力对于声音诱发闪光错觉的影响及其机制。行为结果发现, 无论是老年人还是年轻人都存在声音诱发闪光错觉这种听觉主导效应, 即被试既可以产生裂变错觉, 也可以融合错觉。研究发现, 无论是裂变错觉还是融合错觉, 老年人的错觉量均显著大于年轻人的错觉量。静息态功能磁共振ALFF值与行为结果的相关分析发现, 对于裂变错觉, 老年人的内侧前额叶皮层活动越强烈, 其裂变错觉量越大。年轻人右侧枕上回和左侧舌回的活动越强烈, 其裂变错觉量越小。对于融合错觉, 老年人左外侧额下回、右侧额中回和颞下回的活动越强烈, 其融合错觉的错觉量越大, 而左侧枕上回和舌回的活动越强烈, 被试的融合错觉量越弱。年轻被试外侧额上回区域活动越强烈, 被试的融合错觉量越大。
以往研究采用年轻人作为被试均发现, 存在声音诱发闪光错觉效应, 且裂变错觉比融合错觉的效应更大(Shams et al., 2000; Wozny, Beierholm, & Shams, 2008; 于薇 等, 2017)。本研究的结果发现, 相比于年轻人, 老年人的裂变错觉和融合错觉效应均更大。之所以表现多感觉错觉效应增大的原因在于老年人多感觉整合能力可能比年轻人更强。以往关于多感觉整合年龄效应的研究表明老年人的多感觉整合能力强于年轻人(Mahoney, Li, Ohpark, Verghese, & Holtzer, 2011; DeLoss et al., 2013), 例如DeLoss等人(2013)的研究发现, 老年人的视听整合能力优于年轻人, Peiffer, Mozolic, Hugenschmidt和Laurienti (2007)的研究也发现了同样的结果(Peiffer et al., 2007)。关于老年人多感觉整合效应增强的原因Laurienti, Burdette, Maldjian和Wallace (2006)认为可能是由于补偿机制。补偿机制是指多感觉通道信息的加工能力较强, 以补偿单一感觉通道信息加工能力的不足(Laurienti et al., 2006)。以往研究表明, 当单一感觉通道信息不可靠时, 人脑可以通过整合多感觉通道的信息来增强对外界的感知, 即当知觉单一感觉通道信息的加工能力下降时, 多感觉通道信息的加工对个体的知觉过程影响更大。对于老年人来说, 当视觉通道信息减弱时, 听觉通道信息可以作为补偿提供更多关于环境特性的信息(Klatzky, Marston, Giudice, Golledge, & Loomis, 2006)。以往声音诱发闪光错觉的研究也发现, 相对于年轻人, 老年人更容易产生错觉效应(Mcgovern et al., 2014; Setti et al., 2011 )。此外, 对具有一定认知功能受损(Chan et al., 2015)以及平衡能力受损(Merriman, Whyatt, Setti, Craig, & Newell, 2015; Stapleton, Setti, Doheny, Kenny, & Newell, 2014)的老年人来说, 对错觉的敏感性更强, 即更容易产生声音诱发闪光错觉。因此, 对声音诱发闪光错觉的敏感性可能是衡量多感觉通道信息加工能力的指标, 即对错觉越敏感, 表明多感觉整合的能力越强。特别是对于老年人来说, 对错觉量的增加可能是由于其对于单一视觉通道信息的加工能力受损, 而更加依赖于听觉通道的信息导致(Chan et al., 2018)。
关于老年人多感觉整合效应增强的原因也有可能是由于选择性注意下降而导致的。以往的研究发现, 老年人很难抑制与任务无关的信息(Schmitz, Cheng, & de Rosa, 2010), 由于无法抑制听觉信息, 老年人在声音诱发闪光错觉上可能和年轻人有所差异(DeLoss et al., 2013)。因此, 老年人抑制无关听觉刺激的缺陷可能是其错觉效应增大的原因(Mcgovern et al., 2014)。同样地, Stapleton等人(2014)通过对比易跌倒老年人和健康老年人的声音诱发闪光错觉发现, 易跌倒老人的错觉量更大。研究表明, 健康老年人可能不需要像易跌倒的老年人那样需要分配注意资源来保持身体平衡, 因此他们可能有更多的注意资源来分配给当前的感知觉加工任务(Stapleton et al., 2014)。
以往关于声音诱发闪光错觉的研究发现, 裂变错觉和融合错觉存在着不同的神经机制。例如, 研究表明裂变错觉的产生可以使大脑中的初级视觉皮层的活动增加, 也表现出上丘、右侧额顶注意网络和颞上回的活动增加(de Haas et al., 2012; Watkins et al., 2006); 而对于融合错觉的产生, 大脑中的初级视觉皮层的活动减少, 同时也表现出右侧颞上回的活动增加(Mishra et al., 2009; Watkins et al., 2007)。此外, 也有研究发现, M通路(Magno Pathway)和P通路(Parvo Pathway)可以诱发裂变错觉, 而融合错觉只能在P通路中诱发(Kaposvari, Bognar, Csibri, Utassy, & Sary, 2014)。本研究的结果发现, 对于裂变错觉, 老年人和年轻人的错觉量与不同的脑区存在显著的相关。老年人之所以存在较大裂变错觉的错觉量主要体现在默认网络的内侧前额叶皮层, 表现为内侧前额叶皮层活动越强烈, 裂变错觉的错觉量越大。结果表明, 首先老年人在低注意需求的情况下裂变错觉效应更大(李雨, 舒华, 2014; Raichle et al., 2001), 因此可以说明, 相对于年轻人群体, 老年人群体在完成视觉闪光判断的过程中由于其在认知层面存在老化, 特别是视觉注意的加工能力存在不足, 因此表现出裂变错觉效应更大。其次也表明了老年人存在跨通道的促进效应, 导致视听整合的能力增强, 因此内侧前额叶区域的活动增强可能是因为补偿作用(Diaconescu, Hasher, & McIntosh, 2013; Freiherr, Lundstrom, Habel, & Reetz, 2013)。而年轻人的错觉效应则集中在枕叶相关的脑区, 这也与以往的研究较为一致, 即裂变错觉是与大脑枕叶的视觉加工区域有关(de Haas et al., 2012; Mishra et al., 2007; Shams et al., 2001, 2005; Watkins et al., 2006, 2007)。
对于融合错觉, 老年人和年轻人的错觉量也与不同的脑区存在显著相关。研究发现, 老年人之所以存在较大融合错觉主要表现为左外侧额下回、右侧额中回和颞下回的活动越强烈, 融合错觉的错觉量越大, 这以往的研究结果发现颞中回和额叶区域自上而下的影响反映了被试个体倾向于融合错觉较为一致(Keil, Nadia, Hartmann, & Weisz, 2013; Watkins et al., 2007)。从本研究中额中回与额下回的自发活动与老年人融合错觉存在显著正相关来看, 前额叶皮层是执行功能区域, 而执行功能是所有复杂的高级认知过程的集合, 是对低水平、自动化的基本认知加工的自上而下的心理调控, 用来解决任务之间的干扰, 灵活协调多个认知子系统活动的复杂认知过程的总和(Gilbert & Burgess, 2008)。在Baddeley和Mc Dowell对患者的研究中发现, 前额叶受损伤患者的双任务执行功能受到影响, 研究表明了执行功能与前额叶皮层有显著相关(Baddeley, Della Sala, Papagano, & Spinnler, 1997; Mcdowell, Whyte, & D’ Espisito, 1997)。此外, 以往的研究发现, 无论采用何种范式, 只要存在任务干扰, 那么额叶皮层就会发挥其执行功能作用(谭金凤, 伍姗姗, 徐雷, 王丽君, 陈安涛, 2013; Szameitat, Schubert, Müller, & Von Cramon, 2002)。因此, 本研究中老年人在进行按键任务时, 由于会受到听觉声音刺激的干扰, 为了确保尽可能地判断正确, 老年人需要动用执行功能相关的脑区来完成任务。除此之外, 本研究还发现左侧枕上回和舌回的活动越弱, 错觉量越大。这与以往的研究发现初级视觉皮层的活动越弱, 融合错觉效应越强相一致(Watkins et al., 2007)。舌回在功能上对应的是布鲁德曼17和18区, 有着处理视觉信息的功能(Kawachi, 2017), 老年人错觉量的增大可能的原因之一是由于老年人视觉敏感性下降, 导致其不能对视觉闪光刺激进行很好地加工。此外, Chan等人(2018)的研究也发现, 对于老年人来说, 对错觉量的增加可能是由于其对于单一视觉通道信息的加工能力受损, 而更加依赖于听觉通道的信息导致(Chan et al., 2018)。对于年轻人来说仅仅则表现在外侧额上回区域活动越强烈, 错觉效应越大。融合错觉在不同被试群体间的差异一方面体现在自上而下的认知加工过程中老年人群体与年轻人群体的差异导致出现了不同脑区的差异, 另一方面也是因为相对于裂变错觉, 融合错觉效应更为不稳定(Andersen et al., 2004; Innes-Brown & Crewther, 2009; Shams et al., 2000), 且更为容易受到被试在视觉加工层面上的个体差异影响(Mishra et al., 2009; Shams & Kim, 2010), 因此在不同的脑区呈现出了相关。
然而, 本研究也存在一定的局限性。首先, 本研究的老年人群体的样本均为女性群体, 虽然目前关于声音诱发闪光错觉这类听觉主导效应的研究均未发现性别差异, 如Setti等人(2011)的研究发现不同性别的群体在声音诱发闪光错觉实验中表现相似, 但是将结论推广到整个老年人群体时仍需谨慎, 未来研究可以进一步探究老年人的性别差异对于声音诱发闪光错觉的影响。其次, 虽然与声音诱发闪光错觉相对应的其他自发脑活动差异没有被观察到, 或者在多次比较校正后没有显现出来, 但仍需在其他被试群体中进行验证, 以证明研究结果的可靠性。最后, 通过本研究仅能得到的结论是声音诱发闪光错觉这种听觉主导效应与自发脑活动之间存在相关关系, 但并不能揭示两者的因果关系。因此, 未来研究应结合静息态磁共振成像中的其他指标, 以及采用功能连通性的分析方法进行进一步的探究。
5 结论
老年人群体的声音诱发闪光错觉量大于年轻人群体, 且老年人和年轻人的错觉量与不同脑区的静息态低频振幅存在相关。
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