Proactive inhibitory control of movement assessed by event-related fMRI

Many neuronal processes play a role in the overall performance of inhibition tasks, often making it difficult to associate particular behavioral results to specific processes and structures. Indeed, in classical Go/NoGo, Stop or subliminal masked-prime tasks, inhibition is usually triggered at the same time as the sensorimotor processes involved in movement selection and conflict monitoring. To account for motor inhibition, many conflicting candidate structures, which depend on specific task requirements, have been proposed. In the present paper, first we used a simple reaction (RT) time task and, second, we took advantage of the fact that volitional inhibition is usually implemented before any stimulus occurs when subjects are aware that a warning signal will be presented before a target. This proactive inhibition would be intended to prevent anticipated responses and would be lifted as soon as the warning signal has been identified. In other words, we postulate that the same event does not trigger both inhibition and target processing, and that, indeed, these mechanisms can be separated in time. Event-related fMRI revealed that the medial prefrontal cortex and the inferior parietal cortex may be responsible for proactive inhibition, and that the primary motor cortex, the supplementary motor cortex and the putamen are the likely targeted sites of this inhibition. We conclude that executive control in these tasks may consist of switching from controlled inhibition (suppression of the neuronal processes underlying movement initiation) to automatic sensorimotor processing. The possible contribution of the medial prefrontal cortex to the tonic inhibition state adds new perspectives to possible meanings of a "default mode of brain function".