Stimulus frequency dependence of blood oxygenation level-dependent functional magnetic resonance imaging signals in the somatosensory cortex of rats.

Understanding the mechanism of coupling between neuronal events and hemodynamic responses is important in non-invasive functional imaging of the brain. The stimulus frequency dependence of hemodynamic responses has been studied using a rat somatosensory cortex model; most results for short stimulus durations reveal peak frequencies at which the hemodynamic response is maximized. However, such peak frequencies have not been observed in studies using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signals with long stimulus durations. To clarify whether the stimulus frequency dependence of BOLD signals depends on the stimulus duration, we measured BOLD signals at 7 T with short- and long-stimulus durations for stimulating rat forepaw at 1-10 Hz using spin-echo echo-planar imaging to enhance changes in activation focus. For both these durations, BOLD signals were significantly higher at stimulus frequencies of 3 or 5 Hz in agreement with the results of previous studies using optical techniques. Our results show that stimulus duration has little influence on the stimulus frequency dependence of BOLD signals in the rat somatosensory model. The discrepant results of most previous fMRI studies using gradient-echo sequence may be ascribed to the difference of imaging to enhance activation focus or draining vein.