TY - JOUR T1 - The hemodynamic initial-dip consists of both volumetric and oxymetric changes correlated to localized spiking activity JF - bioRxiv DO - 10.1101/259895 SP - 259895 AU - Ali Danish Zaidi AU - Niels Birbaumer AU - Eberhard Fetz AU - Nikos Logothetis AU - Ranganatha Sitaram Y1 - 2018/01/01 UR - http://biorxiv.org/content/early/2018/02/22/259895.abstract N2 - The “initial-dip” is a transient decrease frequently observed in functional neuroimaging signals, immediately after stimulus onset, and is believed to originate from a rise in deoxy-hemoglobin (HbR) caused by local neural activity. It has been shown to be more spatially specific than the hemodynamic response, and is believed to represent focal neuronal activity. However, despite being observed in various neuroimaging modalities (such as fMRI, fNIRS, etc), its origins are disputed and its neuronal correlates unknown. Here, we show that the initial-dip is dominated by a decrease in total-hemoglobin (HbT). We also find a biphasic response in HbR, with an early decrease and later rebound. However, HbT decreases were always large enough to counter spiking-induced increases in HbR. Moreover, the HbT-dip and HbR-rebound were strongly coupled to highly localized spiking activity. Our results suggest that the HbT-dip helps prevent accumulation of spiking-induced HbR-concentration in capillaries by flushing out HbT, probably by active venule dilation.BOLDblood-oxygen level dependent signalCPVsCapillary and Peri-capillary Vessels (precapillary-arterioles, capillaries and post-capillary venules)fMRIfunctional Magnetic Resonance ImagingfNIRSfunctional Near Infra-Red SpectroscopyHbOconcentration of oxy-hemoglobinHbRconcentration of deoxy-hemoglobinHbTconcentration of total hemoglobinLFPlocal field potentialRWilcoxon’s one-tailed rank-sum testŠWilcoxon’s two-tailed signed rank testT1Tetrode 1 (0.55mm from emitter)T2Tetrode 2 (1.75mm from emitter)T3Tetrode 2 (2.95mm from emitter) ER -