A correlation study of averaged and single trial MEG signals: The average describes multiple histories each in a different set of single trials
A correlation study of averaged and single trial MEG signals: The average describes multiple histories each in a different set of single trials
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DOI：
10.1007/bf01186914
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发表时间：
1996-06-01
期刊：
影响因子：
2.7
通讯作者：
Ioannides, AA
中科院分区：
文献类型：
--
作者：
Liu, LC;Ioannides, AA
我们对脑电活动和行为之间联系的理解是基于间接测量。正电子发射断层扫描（PET）和功能性磁共振成像（fMRI）依赖于血流动力学过程，比表征正常和病理脑功能的过程慢两到三个数量级。另一方面，电活动的直接侵入性测量产生的观点过于局部，无法显示维持意识和认知的大规模一致性。在另一个极端，像脑电图（EEG）和单通道或少通道脑电图（MEG）这样的电活动的粗略测量直到最近还依赖于从许多重复的生理无关刺激的平均值中提取的简单点模型。引入超过30个通道的多通道探针（Hamalainen et al. 1993）和使用分布式源分析（loannides et al. 1990a）首次为研究单个试验的响应提供了可能性。在这项工作中，我们直接解决了从平均信号分析中提取的事件描述的代表性如何的问题。我们使用最简单的例子：皮层对一个简单的1khz音调的反应，重点关注刺激开始后100毫秒左右的早期和一般承认的“自动”反应。为了避免受试者间的差异，我们研究了单个受试者左、右皮质区域对单侧和对侧刺激的反应；为了测试再现性，我们使用了睁眼和闭眼条件。由于从每个试验中提取完整的三维描述所涉及的计算需求太大，我们已经用特殊技术补充了分布式源分析，这使我们能够扫描每一个单独的试验，并识别出与平均信号相似的每个皮质激活。因此，我们能够明确地表明，由平均信号分析得出的事件顺序不能代表个别试验中发生的情况。这个序列是由发生在不同试验中的事件组成的，反映了可能存在许多平行路线，每条路线都从耳朵的输入到最终的“计算”。
Our understanding of the link between electrical events in the brain and behaviour is based on indirect measures. Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI) rely on haemodynamic processes which are slower by two to three orders of magnitude than the processes characterizing normal and pathological brain function. Direct invasive measurements of the electrical activity on the other hand produce too local a view which fails to show the large scale coherence which sustains awareness and cognition. On the opposite extreme, gross measures of the electrical activity like Electroencephalography (EEG) and single or few channel Magetoencephalography (MEG) had until recently to rely on simplistic point like models extracted from the averages of many repetitions of physiologically irrelevant stimuli. The introduction of multichannel probes with over 30 channels (Hamalainen et al. 1993), and the use of distributed source analysis (loannides et al. 1990a) opened up for the first time the possibility to study the response of single trials. In this work we address directly the question how representative is the description of events extracted from the analysis of the average signal. We use the simplest possible example: the cortical response to a simple 1 kHz tone, focusing on the early and by general admission ''automatic'' response around 100 ms after stimulus onset. To avoid the confounding inter-subject variability we have studied the responses over the left and right cortical areas to ipsi- and contralateral stimulation in a single subject; for testing reproducibility, we have used both the eyes open and eyes closed conditions. Since the computational demands involved in extracting a full three dimensional description from each trial are too great,we have complemented the distributed source analysis with special techniques, which allow us to scan through each and every single trial and identify each cortical activation similar to the ones picked out in the average signal. We are thus able to show conclusively that the sequence of events suggested by the analysis of the average signal is not representative of what is happening in individual trials. The sequence is made up of events which occurred in different trials reflecting probably the existence of many parallel routes each of which leads from the input at the ear to a final ''computation''.