Non-invasive characterisation of neurovascular coupling in the human brain using simultaneous DC-magnetoencephalography and time-resolved near-infrared spectroscopy

Functional brain imaging methods map neuronal activations indirectly through the accompanying neurovascular response. However, awareness is increasing that the link between neuronal and vascular task-related responses is all but a simple linear transform. Recently, we have shown that in a technically complementary approach neurovascular coupling can be analyzed non-invasively in the human brain in principle by simultaneous DC-magnetoencephalography (DC-MEG with brain-to-sensor modulation) and near-infrared spectroscopy (NIRS). In extension to first nonselective NIRS and DC-MEG recordings we demonstrate here that time-resolved multichannel NIRS in combination with DC-MEG permits to analyze neurovascular coupling in the human cortex. Simultaneously DC-MEG signals and time-resolved multichannel NIRS at three wavelengths (687 nm, 803 nm, 826 nm) allowing for depth-resolved analysis of absorption changes were recorded over the left primary motor cortex hand area in healthy subjects during finger movements of the right hand (30 s followed by 30 s rest; n=25 periods). DC-fields and NIRS parameters (deoxy-Hb and oxy-Hb) followed closely the motor task cycles revealing statistically significant differences between periods of finger movements and rest. In subjects with good signal-to-noise ratio the variance analysis of photon time of flight for a 2-layer model (d=10mm) demonstrated changes originating from the deeper layer, i.e., the cortex. Notably, whereas MEG signals reached 50% of the maximum level between 0.3s and 3s after finger movement onset the oxy-Hb signal reached this level around 1.5s later. This dual approach provides a new opportunity to analyse non-invasively the „hemodynamic inverse problem“ which refers to the challenge of making valid and precise estimates of underlying cortical neuronal activity from measured hemodynamic resposes, e.g., in NIRS and fMRI.