No abstract

Fronto-basal ganglia pathways play a crucial role in voluntary action control, including the ability to inhibit motor responses. Response inhibition might be mediated via a fast hyperdirect pathway connecting the right inferior frontal gyrus (rIFG) and the presupplementary motor area (preSMA) with the subthalamic nucleus or, alternatively, via the indirect pathway between the cortex and caudate. To test the relative contribution of these two pathways to inhibitory action control, we applied an innovative quantification method for effective brain connectivity. Functional magnetic resonance imaging data were collected from 20 human participants performing a Simon interference task with an occasional stop signal. A single right-lateralized model involving both the hyperdirect and indirect pathways best explained the pattern of brain activation on stop trials. Notably, the overall connection strength of this combined model was highest on successfully inhibited trials. Inspection of the relationship between behavior and connection values revealed that fast inhibitors showed increased connectivity between rIFG and right caudate (rCaudate), whereas slow inhibitors were associated with increased connectivity between preSMA and rCaudate. In compliance, connection strengths from the rIFG and preSMA into the rCaudate were correlated negatively. If participants failed to stop, the magnitude of experienced interference (Simon effect), but not stopping latency, was predictive for the hyperdirect-indirect model connections. Together, the present results suggest that both the hyperdirect and indirect pathways act together to implement response inhibition, whereas the relationship between performance control and the fronto-basal ganglia connections points toward a top-down mechanism that underlies voluntary action control.

The ability to suppress one's impulses and actions constitutes a fundamental mechanism of cognitive control, thought to be subserved by the right inferior frontal cortex (rIFC). The neural bases of more selective inhibitory control when selecting between two actions have thus far remained articulated with less precision. Selective inhibition can be explored in detail by extracting parameters from response time (RT) distributions as derived from performance in the Simon task. Individual differences in RT distribution parameters not only can be used to probe the efficiency and temporal dynamics of selective response inhibition, but also allow a more detailed analysis of functional neuroimaging data. Such model-based analyses, which capitalize on individual differences, have demonstrated that selective response inhibition is subserved by the rIFC. The aim of the present study was to specify the relationship between model parameters of response inhibition and their functional and structural underpinnings in the brain. Functional magnetic resonance imaging (fMRI) data were obtained from healthy participants while performing a Simon task in which irrelevant information can activate incorrect responses that should be selectively inhibited in favor of selecting the correct response. In addition, structural data on the density of coherency of white matter tracts were obtained using diffusion tensor imaging (DTI). The analyses aimed at quantifying the extent to which RT distribution measures of response inhibition are associated with individual differences in both rIFC function and structure. The results revealed a strong correlation between the model parameters and both fMRI and DTI characteristics of the rIFC. In general, our results reveal that individual differences in inhibition are accompanied by differences in both brain function and structure.