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事件相关电位在测谎研究中得到了广泛的关注并取得了较好的成果[1-4],但某些与说谎相关的自发脑电的变化一度被忽略。一个内在或外在的刺激事件不仅会产生事件相关电位(event-related potential, ERP),同时也会引起自发脑电的变化,表现为事件相关同步ERS或去同步(event-related desynchronization, ERD)现象[5]。已有研究显示EEG特定频率成分的ERS现象与大脑的记忆、注意、决策等认知功能相关[6-8],而说谎是一个涉及知觉、记忆、情感、冲突心理、决策和反应等复杂的心理过程[2, 9-11]。因此,本文主要对测谎实验中的EEG信号在delta、theta频带的ERS/ERD现象进行探索性研究,以期从EEG信号的特定频率成分中获得一些可以反映说谎心理加工过程的指标,更多地揭示“说谎”与“说真话”时脑电信号的差异。
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ERS/ERD反应了控制自发EEG的特定频率成分的主神经元和中间神经元的局部交互作用的活动变化[5]。当外部的刺激事件发生时,自发脑电的事件相关现象通常以特定频率成分的能量增加或减少的形式出现,前者称之为事件相关同步,后者被称为事件相关去同步。因此,对于自发脑电信号的ERS/ERD的研究必须在特定的频率范围内,同时具有一定时段的基线数据作为参考。
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ERD/ERS的一个基本特征是自发EEG的特定频带能量相对于参考时间段相同频带能量的百分比。文献[5]给出了ERD/ERS的精确定义,并最早提出了一种经典的能量法来量化ERD/ERS的时程。后来相关学者在之前的经典能量量化方法的基础上,区分了脑电活动的相位锁定成分和非相位锁定成分并考虑了相位锁定的ERP对量化ERS/ERD时程的影响,提出了一种改进的量化ERS/ERD时程的能量方法,称作Intertrial variance方法[12-13]。作为本文研究的基础,本文简单介绍该方法量化ERS/ERD的基本过程。用X(i, j)代表N次实验中第i次实验的第j个时刻的脑电信号样本值,则量化ERS/ERD的步骤如下。
1) 对脑电信号进行特定频带滤波,得到滤波后各次实验的脑电时间序列${X_f}(i,j)$,本文采样小波包分解和重构方法对EEG信号进行滤波,主要提取delta(0.5~4 Hz)和theta(4~8 Hz)频带信号;
2) 计算每次实验的滤波后脑电信号各采样点的幅值平方,得到脑电能量序列${X_f}{(i,j)^2}$;
3) 计算$N$次实验所有脑电能量序列的叠加平均,得到特定频段内脑电能量的时程变化,各个时刻点j的平均脑电能量为:
$${\bar X_f}(j) = \frac{1}{N}\sum\limits_{i = 1}^N {{X_f}{{(i,j)}^2}} $$ (1) 4) 计算各次实验中的脑电能量序列与所有实验的脑电能量平均值之差的平方,得到各次实验的能量方差:
$$A(j) = \frac{1}{{N - 1}}\sum\limits_{i = 1}^N {{{\{ {X_f}(i,j) - {{\bar X}_f}(j)\} }^2}} $$ (2) 5) 选择刺激信息出现前大脑放松状态的一段时间内的平均特定频带脑电能量作为参考,记做$R$,则第$j$点的ERS可计算为:
$${\rm{ERS}}(j) = \frac{{A(j) - R}}{R} \times 100\% $$ (3) 式中,$R$为参考时间段的特定频带脑电的平均能量;
6) 对ERS(j)序列在时间上进行平滑处理,本文对计算得到的ERD数据在125 ms的时窗内求平均值,然后以25 ms时移在下一个时窗内求平均值,最后得到平滑后的ERD/ERS的时程。
通过上述6个步骤,可得到某一种大脑认知状态下的脑电信号能量的百分比变化,从而量化了ERD/ERS的时程。很明显,ERD对应于负的百分比值,ERS对应于正的百分比值,为了描述方便,在后续研究中统一用ERS表示。
本文选择测谎实验中刺激出现前1 000 ms的自发脑电作为参考时间段数据,并应用小波包分解重构方法对自发脑电进行相应频带的滤波,对刺激出现后的0~1 000 ms的脑电数据计算其ERS/ERD的时程变化。
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本文应用小波包分解重构方法对EEG信号的delta(0.5~4 Hz)和theta(4~8 Hz)节律进行滤波,选择Daubechies系列的db4作为小波基函数对截取的脑电信号数据段进行小波包分解。实验中采集的脑电信号的频率范围为0.01~70 Hz,经小波去噪后脑电信号在0.5~30 Hz频率范围。可以对delta(0.5~4 Hz)和theta(4~8 Hz)频带的脑电信号进行滤波。对于采样率为1 000 Hz,且经过小波去噪预处理的脑电信号,采样8级小波包分解和重构可获得delta(0.5~4 Hz)和theta(4~8 Hz)节律成分。并根据文献[13]提出的Intertrial variance方法分别对两个节律的ERS时程进行量化。
The Study of Event Related Synchronization Based on EEG during Mentality Facticity
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摘要: 为揭示"说谎"与"说真话"时脑电信号的差异,该文对测谎实验中自发脑电信号的事件相关同步(ERS)进行了深入研究。分别对脑电的delta、theta节律的事件相关同步进行了详细分析,发现delta和theta节律的ERS可以反映说谎相关的心理状态,其中delta节律的ERS与P300高度相关,而theta节律的ERS效应与经典的P300效应尽管在时间上有一定的重叠,但具有不同的头皮分布特性。通过对比Fz、Cz和Pz 3个电极的主效应,发现theta同步的最大主效应位于前额中线的Fz电极,而P300的最大主效应在枕区中线的Pz电极。这说明theta节律的ERS与P300成分可能从不同角度反映了说谎心理的认知加工过程。Abstract: In order to reveal the electroencephalogram (EEG) differences between truth and lying, the event related synchronization (ERS) is deeply studied for spontaneous EEG during the lying detection experiment. The ERS corresponding to delta and theta rhythm of EEG is analyzed in detail. The results indicate that ERS corresponding to delta and theta rhythm can reflect the deception-related psychological states. The delta synchronization is strongly related with P300. The theta synchronization effect overlaps with the ERPs effect, but ERPs and the synchronization effects have different scalp distribution character. The results show that the theta synchronization effect is strongest in Fz electrode and the ERPs effect is maximal on Pz electrode. This evidence implies that ERS corresponding to theta rhythm and P300 might reflect the deception-related cognitive psychological process from different aspects.
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Key words:
- EEG /
- event related synchronization(ERS) /
- lie detection /
- mentality facticity
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