Another fundamental question concerning the interpretation of BOLD-fMRI data lies in the neurophysiological origin of BOLD signals. There have been debates regarding whether the BOLD signal arises from neuronal inputs or outputs (i.e. synaptic or spiking activities, respectively). Early studies [36, 37] suggest the dependence of the BOLD responses upon neuronal spiking activity. This conclusion was drawn from the observations that the peak magnitudes of the BOLD signals measured in human V5 [36] and V1 [37] were linearly correlated with the averaged spiking rates invasively recorded in analogous areas in primates. More recently, Mukamel et al. also found a close correlation between the BOLD signals measured from healthy subjects and the neuronal spiking activity recorded from epilepsy patients when both groups underwent identical stimuli [38]. However, questions may be raised, since these studies compared the BOLD and neuronal spiking signals acquired from different species or subjects.
