Cortico-cortical connections that reach the primary motor cortex (M1) are thought to transmit crucial information relevant to build the final motor output required to perform a selected motor plan. These connections originate from key areas of the parietal and frontal lobes, such as the posterior parietal cortex (PPC), the supplementary motor area (SMA) and the ventral (PMv) and dorsal premotor cortex (PMd). Multifocal transcranial magnetic stimulation (TMS) has been recently investigated as a powerful method able to track within millisecond time scale direct information on the causal functional connectivity of these non-primary motor areas with the M1 that would explain how their activity may modulate the spatial pattern of output from primary motor areas preceding execution of a movement. A conditioning stimulus (CS) is first used to activate putative pathways to the motor cortex from, for example, the PPC or the PMd, while a second, test stimulus (TS), delivered over the M1 a few milliseconds later probes any changes in excitability that are produced by the CS. When tested at rest, the activation of these cortico-cortical projections may induce both a transient facilitation and a inhibition in the M1 ipsilateral or contralateral to the site of conditioning. However these interactions are not fixed, but may change critically during a certain motor task, giving important information on how the strength of the connection changes over time and during a specific task, and providing crucial information on the causal effects that a specific cortical region exerts over the M1. Moreover, it is possible to combine these measurements of functional connectivity together with diffusion tensor magnetic resonance imaging (DTI) to obtain insight into the white matter pathways that mediate these interactions. Here, we review studies describing the functional role of parallel cortico-cortical connections among the PMd, PMv, PPC and M1 in specific motor tasks such as action selection, action reprogramming, action observation and goal- directed reaching and grasping movements, showing that the functional interplay between these areas is not fixed, but is promptly activated depending on the behavioural state.
|Title of host publication||Cortical Connectivity: Brain Stimulation for Assessing and Modulating Cortical Connectivity and Function|
|Publisher||Springer-Verlag Berlin Heidelberg|
|Number of pages||20|
|ISBN (Print)||9783642327674, 3642327664, 9783642327667|
|Publication status||Published - Jul 1 2012|
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