Efficiency of go/no-go task performance implemented in the left hemisphere.

It is well known that the efficiency of response inhibition differs from person to person, but the neural mechanism that implements the efficiency is less understood. In the present fMRI study, we devised an index to evaluate the efficiency of response inhibition in the go/no-go task, and investigated the neural correlates of the efficiency of response inhibition. The human subjects who perform the go/no-go task with a shorter reaction time in go trials (Go-RT) and with a higher percentage of correct no-go trials (Nogo-PC) are thought to have the ability to conduct response inhibition more efficiently. To quantify the efficiency, we defined an efficiency index as the difference in the Nogo-PC between each subject and an ordinarily efficient subject, under the same Go-RT. An across-subject correlation analysis revealed that the brain activity in multiple regions in the left frontal and parietal cortex positively correlated with the efficiency index. Moreover, a test of hemispheric asymmetry with regard to the across-subject correlation revealed left-hemispheric dominance. The significant correlation in the left frontal and parietal regions complements the results of previous studies that used the stop-signal reaction time (SSRT), a well known index to evaluate the efficiency of response inhibition used in the stop-signal task. Our results also indicate that, although it is well known that the neural substrates for response inhibition common in a subject group exist dominantly in the right hemisphere, the neural substrates for efficiency exist dominantly in the left hemisphere.