It is essential to elucidate the relationship between blood oxygenation level-dependent (BOLD) signals and neuronal activity for the interpretation of the functional magnetic resonance imaging (fMRI) signals; this relationship has been quantitatively investigated by animal studies measuring evoked potentials as indices of neuronal activity. Although most human fMRI studies employ the event-related task design, in which the stimulus duration is short, few studies have investigated the relationship between BOLD signals and evoked potentials at short stimulus durations. The present study investigated this relationship in the somatosensory cortex of anesthetized rats by using electrical forepaw stimulation at a short duration of 4 s and comprehensively analyzed it at different frequencies (1-10 Hz) and currents (0.5-2.0 mA). Somatosensory evoked potential (SEP) responses were measured at the scalp using silver ball electrodes. The sum of the peak-to-peak amplitude (ÎŁSEP) and average SEP (avg. SEP) responses were calculated. BOLD signals were obtained using a spin-echo echo-planar imaging sequence at 7 T. The relationship between the avg.SEP and BOLD signals varied with frequency, whereas that between ÎŁSEP and BOLD signals showed a significant correlation at varying frequencies and currents. In particular, the relationship between ÎŁSEP and ÎŁBOLD, which is the sum of the BOLD signals obtained at each time point reflecting the area under the BOLD response curves, mostly converged, irrespective of the frequency. Our results suggest that ÎŁBOLD obtained using a spin-echo sequence reflects the neural activity as quantified by ÎŁSEP, which was determined at different frequencies and currents.
Section: Regulatory SystemsKeywords: BOLD; fMRI; Forepaw; Neurovascular coupling; SEP Abbreviations: BOLD, blood oxygenation level-dependent; CBF, cerebral blood flow; CMRO 2 , cerebral metabolic rate of oxygen consumption; EPI, echo planar imagining; TE, echo time; fMRI, functional magnetic resonance imaging; LDF, laser Doppler flowmetry; SEP, somatosensory evoked potential; T 2 , transverse relaxation time -2 -
IntroductionFunctional magnetic resonance imaging (fMRI) based on blood oxygenation level-dependent (BOLD) effect has been widely used in neuroscience and psychology to study brain function. fMRI reflects cerebral hemodynamic changes, and its signals have been interpreted by assuming a neuronal hemodynamic response (Friston et al., 1995) based on the 19 th century hypothesis of Roy and Sherrington that neuronal activity induces an increase in the regional cerebral blood flow (CBF) (Roy and Sherrington, 1890). Thus, fMRI is an indirect method for assessing neuronal activity. Therefore, it is essential to elucidate the relationship between BOLD signals and neuronal activity for interpreting the fMRI signals; this relationship has been investigated mainly by animal studies measuring evoked potentials as indices of the neuronal activity in the somatosensory cortex (Brinker et al., 1999;Van Camp et al., 2006;Goloshevsk...