Inhibition of an ongoing reaction tendency for adaptation to changing environments is a major function of the human prefrontal cortex. This function has been investigated frequently using the go/no-go task and set-shifting tasks such as the Wisconsin Card Sorting Test (WCST). Studies in humans and monkeys suggest the involvement of the dorsolateral prefrontal cortex in the two task paradigms. However, it remains unknown where in the dorsolateral prefrontal cortex this function is localized, whether a common inhibitory mechanism is used in these task paradigms and how this inhibitory function acts on two different targets, i.e. the go response in the go/no-go task and the cognitive set in the WCST. In the go/no-go task of this study, subjects were instructed to either respond (go trial) or not respond (no-go trial), depending on the cue stimulus presented. The signals of functional MRI (fMRI) related to the inhibitory function should be transient by nature. Thus, we used the temporal resolution of fMRI (event-related fMRI) by which transient signals in go and no-go trials can be analysed separately and compared with each other. We found a focus that showed transient no-go dominant activity in the posterior part of the inferior frontal sulcus in the right hemisphere. This was true irrespective of whether the subjects used their right or left hands. These results suggest that the transient activation in the right inferior prefrontal area is related to the neural mechanism underlying the response inhibition function. Furthermore, this area was found to be overlapped spatially with the area that was activated transiently during cognitive set shifting in the WCST. The transient signals in the go/no-go task peaked 5 s after the transient expression of the inhibitory function, and the transient signals in the WCST peaked 7s after the transient expression, reflecting different durations of neuronal activity in the two inhibitory task paradigms. These results imply that the right inferior prefrontal area is commonly involved in the inhibition of different targets, i.e. the go response during performance of the go/no-go task and the cognitive set during performance of the WCST.

Inhibition of inappropriate responses is an essential executive function needed for adaptation to changing environments. In stop-signal tasks, which are often used to investigate response inhibition, subjects make "go" responses while they prepare to stop at a suddenly given "stop" signal. However, the preparatory processes ongoing before response inhibition have rarely been investigated, and it remains unclear how the preparation contributes to response inhibition. In the present study, a stop-signal task was designed so that the extent of the preparation could be estimated using behavioral and neuroimaging measures. Specifically, in addition to the conventional go trials where preparation to stop was required ("uncertain-go" trials), another type of go trial was introduced where a stop-signal was never given and such preparation was unnecessary ("certain-go" trials). An index reflecting the "preparation cost" was then calculated by subtracting the reaction times in the certain-go trials from those in the uncertain-go trials. It was revealed that the stop signal reaction time, a common index used to evaluate the efficiency of response inhibition, decreased as the preparation cost increased, indicating greater preparation supports more efficient inhibition. In addition, imaging data showed that response inhibition recruited frontoparietal regions (the contrast "stop vs uncertain-go") and that preparation recruited most of the inhibition-related frontoparietal regions (the contrast "uncertain-go vs certain-go"). It was also revealed that the inhibition-related activity declined as the preparation cost increased. These behavioral and imaging results suggest preparation makes a complementary contribution to response inhibition during performance of a stop-signal task.

The contribution of the right inferior frontal cortex to response inhibition has been demonstrated by previous studies of neuropsychology, electrophysiology, and neuroimaging. The inferior frontal cortex is also known to be activated during processing of infrequent stimuli such as stimulus-driven attention. Response inhibition has most often been investigated using the go/no-go task, and the no-go trials are usually given infrequently to enhance prepotent response tendency. Thus, it has not been clarified whether the inferior frontal activation during the go/no-go task is associated with response inhibition or processing of infrequent stimuli. In the present functional magnetic resonance imaging study, we employed not only frequent-go trials but also infrequent-go trials that were presented as infrequently as the no-go trials. The imaging results demonstrated that the posterior inferior frontal gyrus (pIFG) was activated during response inhibition as revealed by the no-go vs. infrequent-go trials, whereas the inferior frontal junction (IFJ) region was activated primarily during processing of infrequent stimuli as revealed by the infrequent-go versus frequent-go trials. These results indicate that the pIFG and IFJ within the inferior frontal cortex are spatially close but are associated with different cognitive control processes in the go/no-go paradigm.

The Wisconsin Card Sorting Test, which probes the ability to shift attention from one category of stimulus attributes to another (shifting cognitive sets), is the most common paradigm used to detect human frontal lobe pathology. However, the exact relationship of this card test to prefrontal function and the precise anatomical localization of the cognitive shifts involved are controversial. By isolating shift-related signals using the temporal resolution of functional magnetic resonance imaging, we reproducibly found transient activation of the posterior part of the bilateral inferior frontal sulci. This activation was larger as the number of dimensions (relevant stimulus attributes that had to be recognized) were increased. These results suggest that the inferior frontal areas play an essential role in the flexible shifting of cognitive sets.

We investigated the response inhibition function of the prefrontal cortex associated with the go/no-go task using functional magnetic resonance imaging in five human subjects. The go/no-go task consisted of go and no-go trials given randomly with roughly equal probability. In go trials a green square was presented and the subjects had to respond by promptly pushing a button using their right or left thumbs, but in no-go trials a red square was presented and subjects were instructed not to respond. When brain activity in no-go trials is dominant over that in go trials in areas in the prefrontal cortex, this no-go dominant brain activity would reflect the neural processes for inhibiting inherent response tendency. We used a new strategy of image data analysis by which transient brain activity in go or no-go trials can be analysed separately, and looked for the prefrontal areas in which the brain activity in no-go trials is dominant over that in go trials. We found the no-go dominant foci in the posterior part of the right inferior frontal sulcus reproducibly among the subjects. This was true whether the right or left hand was used. These results suggest that this region in the prefrontal cortex is related to the neural mechanisms underlying the response inhibition function.

The resting-state human brain networks underlie fundamental cognitive functions and consist of complex interactions among brain regions. However, the level of complexity of the resting-state networks has not been quantified, which has prevented comprehensive descriptions of the brain activity as an integrative system. Here, we address this issue by demonstrating that a pairwise maximum entropy model, which takes into account region-specific activity rates and pairwise interactions, can be robustly and accurately fitted to resting-state human brain activities obtained by functional magnetic resonance imaging. Furthermore, to validate the approximation of the resting-state networks by the pairwise maximum entropy model, we show that the functional interactions estimated by the pairwise maximum entropy model reflect anatomical connexions more accurately than the conventional functional connectivity method. These findings indicate that a relatively simple statistical model not only captures the structure of the resting-state networks but also provides a possible method to derive physiological information about various large-scale brain networks.

Functional brain organization of macaque monkeys and humans was directly compared by functional magnetic resonance imaging. Subjects of both species performed a modified Wisconsin Card Sorting Test that required behavioral flexibility in the form of cognitive set shifting. Equivalent visual stimuli and task sequence were used for the two species. We found transient activation related to cognitive set shifting in focal regions of prefrontal cortex in both monkeys and humans. These functional homologs were located in cytoarchitectonically equivalent regions in the posterior part of ventrolateral prefrontal cortex. This comparative imaging provides insights into the evolution of cognition in primates.

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