请教一个关于global signal regression的问题

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！