Near-infrared spectroscopy and electroencephalography during spreading depression

Spreading depression (SD) is an ultra-slow (frequency < 0.05Hz) brain electrical activity caused by the high concentration of extracellular potassium ions (K+) and plays an essential role in the pathophysiology of epilepsy. Due to filter effects of the scalp and in the presence of other non-neuronal slow shift potentials (e.g., electrode low-frequency shifts and motion artifact), the recording of an SD signal with the non-invasive method can be a difficult task. Near-infrared spectroscopy (NIRS) is wavelength-dependent absorption spectroscopy. Light absorption is a function of the molecular properties of substances within the light path. Hemodynamic variations accompany the propagation of the SD. Thus, using near-infra-red spectroscopy beside EEG can provide an additional biomarker to distinguish SD from non-neuronal EEG slow shifts. This study used NIRS/EEG, a dual-modal NIRS and ultra-low frequency (0.01Hz to 80Hz) EEG device to record signal from five Wistar rats (anesthetized). One NIRS source, NIRS detector, and EEG electrode were positioned above the somatosensory neocortex on the depilated skin. The EEG reference electrode was close to the rat's nasion. The distance between source and detector was 8mm. KCL solution (3 mole/L, 10 mu l) was injected into the rat neocortex to generate the SD like-wave, and NIRS/EEG device performed the simultaneous recording. The increase of HHb (deoxyhemoglobin) accompanied by the slow shift of EEG was detected during SD. The rise of THb (Total hemoglobin) was also detected during the induced SD.