Abstract
The medial posterior parietal cortex (PPC) is involved in the complex processes of visuomotor integration. Its connections to the dorsal premotor cortex, which in turn is connected to the primary motor cortex (M1), complete the fronto-parietal network that supports important cognitive functions in the planning and execution of goal-oriented movements. In this study, we wanted to investigate the time-course of the functional connectivity at rest between the medial PPC and the M1 using dual-site transcranial magnetic stimulation in healthy humans. We stimulated the left M1 using a suprathreshold test stimulus to elicit motor-evoked potentials in the hand, and a subthreshold conditioning stimulus was applied over the left medial PPC at different inter-stimulus intervals (ISIs). The conditioning stimulus affected the M1 excitability depending on the ISI, with inhibition at longer ISIs (12 and 15 ms). We suggest that these modulations may reflect the activation of different parieto-frontal pathways, with long latency inhibitions likely recruiting polisynaptic pathways, presumably through anterolateral PPC.
Highlights
Parieto-frontal networks are actively involved in monitoring arm movements such as reaching and grasping in monkeys [1,2,3,4,5,6,7] and in humans [8,9,10,11,12,13,14,15,16]
inter-stimulus intervals (ISIs) longer than 10 ms have never been tested for medial posterior parietal cortex (PPC)–M1 interactions; the novelty of the present study is to provide new evidence about functional interactions involving medial PPC–M1 with longer latency, which can offer novel insights into clinical conditions associated with altered connectivity patterns
We suggest that the inhibition we found at long ISIs could be the result of the activation of polysynaptic routes between hV6A and the frontal cortex
Summary
Parieto-frontal networks are actively involved in monitoring arm movements such as reaching and grasping in monkeys [1,2,3,4,5,6,7] and in humans [8,9,10,11,12,13,14,15,16]. Given the knowledge of monkey V6A connections with the occipital, parietal, mesial and frontal cortices [23,24,25], area V6A may integrate sensory and motor-related input to estimate the limb state during arm movement, and participates in the exchange of information with the frontal cortex, with the dorsal premotor cortex (PMd), which is in turn connected to the primary motor cortex (M1), and this is necessary to perform accurate interactions with objects in the peripersonal space [2,26,27]. Given the fundamental importance of such parieto-frontal connections in orchestrating our movements, it appears to be crucial to investigate the time course of interactions between hV6A and M1 in healthy humans. This knowledge represents a fundamental step for a deeper understanding of possible dysfunctions of such connections in pathological conditions. It has been reported that latency of the mutual information exchange is altered in neurological conditions such as Parkinson’s disease and mirror dystonia [31,32]
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