In contrast, more studies have demonstrated that BOLD-fMRI reflects synaptic activity rather than spiking activity [39–42]. In a very influential and technically demanding experiment, Logothetis and his colleagues recorded the spiking activity (i.e. single-unit activity (SUA) and multi-unit activity (MUA)) and the synaptic activity (i.e. local field potential (LFP)) simultaneously with the BOLD-fMRI signals from monkeys [39]. Through linear transformation systems, the LFP was found to yield better estimates of BOLD responses than the MUA. Hence, they conclude that BOLD signals primarily reflect synaptic activities [39, 43, 44]. Following this study, similar findings were also reported in several independent studies [40–42].
