请教一个关于global signal regression的问题

Submitted by gongbiyan on Fri, 04/01/2011 - 18:14

在Kevin Murphy（The impact of global signal regression on resting state correlations: Are anti-correlated networks introduced?）一文中：global signal regression is used as a regressor in a general linear model (GLM) to remove the associated variance.This technique assumes
that fMRI experiments are concerned with local changes in neuronal activity and that global signals represent uninteresting sources of noise. However, this assumption is only accurate when the global signal and experimental conditions are orthogonal to each other, that is, uncorrelated.
我对fMRI experiments are concerned with local changes in neuronal activity 和the global signal and experimental conditions are orthogonal to each other, that is, uncorrelated.不太理解，fMRI experiments具体指哪些情况？experimental conditions 具体代表什么？the global signal and experimental conditions 怎样才能是不相关的，即orthogonal to each other？
我是个新手，真诚感谢！

Submitted by dongzy08 on Sat, 04/02/2011 - 09:02

fMRI experiments are concerned with local changes in neuronal activity 这句话是说fMRI研究关注的是特定的脑区，而不是把整个人脑看成一个整体。比如说研究杏仁核和前额叶的功能连接，杏仁核和前额叶就是特定的脑区，它们的活动变化就是指local changes。但是很少的研究是是把整个人脑看成一个整体来研究的，比如做人脑和心脏的功能连接。fMRI experiments指的是对人脑的fMRI研究。

the global signal and experimental conditions are orthogonal to each other, that is, uncorrelated.这是一个假设。作者想说如果这个假设成立，才可以去除全脑信号协变量。但实际上这个假设几乎是不可能成立的，所以去除全脑协变量也是不对的。这是作者想表达的意思。
experimental conditions指的是所有的实验环境，比如实验任务，机械噪声，生理信号等等。不可能不相关。