Abstract
Functional magnetic resonance imaging (fMRI) detects hemodynamic responses with high spatial and temporal resolution. Capabilities of correlating psychophysical parameters with fMRI allow a better understanding of the neuronal processes. The objective of this study is to examine how fMRI BOLD response parameters vary with behavioral outcome measures (e.g. reaction time). We utilized a lexical decision task with two character-frequency levels and pseudocharaters. In the left inferior frontal lobe, the activation volumes were smaller to high-frequency characters than to low-frequency characters and pseudocharacters (587±261 vs. 3280±822 mm3, 587±261 vs. 2025±548 mm3 ), which is correlated well with the reaction times (417±9 vs. 631±22 ms, 417±9 vs. 658±15 ms ). The linear correlation between the measured reaction times and FWHM (full width half maximum) of the fMRI responses was also observed in the left inferior frontal lobe (p＜0.001). There was no significant correlation between reaction time and onset time of the hemodynamic response. These findings imply left inferior frontal lobe is active during the entire reaction time when executing the lexical decision task. Conversely, the FWHM of fMRI responses in the left primary motor cortex (M1) related to pressing a button for responding at the end of task is not correlated to reaction time. Our results are also coherent with respect to a functional model of lexical decision task. Thus, the different patterns of neuronal activity during the cognitive process could be detected and distinguished through the fMRI hemodynamic responses. In this study, we have demonstrated that the time-resolved fMRI technique has the capacity to exploit the cognitive events by tracing in the temporal responses of fMRI, even if the reaction time is at the time scale of hundred milliseconds.