Comprehensive correlation between neuronal activity and spin-echo blood oxygenation level-dependent signals in the rat somatosensory cortex evoked by short electrical stimulations at various frequencies and currents

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 (ΣSEP) 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 ΣSEP and BOLD signals showed a significant correlation at varying frequencies and currents. In particular, the relationship between ΣSEP and ΣBOLD, 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 ΣBOLD obtained using a spin-echo sequence reflects the neural activity as quantified by ΣSEP, which was determined at different frequencies and currents.