Resting-state or “intrinsic connectivity” fMRI measures spontaneous low frequency (<0·08-0·1 Hz) fluctuations in the blood oxygen level dependent (BOLD) signal while subjects lie quietly in the scanner and perform no specific task.24 The BOLD signal reflects changes in the ratio between oxy- and deoxy-haemoglobin following neuronal activity, therefore resting fMRI provides an indirect marker of neuronal function on a time scale of seconds. Functional connectivity is defined by temporal correlations (over minutes of data acquisition) of the BOLD signal between spatially distinct regions.24
