A Short Route for Reach Planning between Human V6A and the Motor Cortex

Abstract

In the macaque monkey, area V6A, located in the medial posterior parietal cortex (mPPC), contains cells that encode the spatial position of a reaching target. It has been suggested that during reach planning this information is sent to the frontal cortex along a parieto-frontal pathway that connects V6A-premotor cortex-M1. A similar parieto-frontal network may also exist in the human brain and we aimed here to study the timing of this functional connection during planning of a reaching movement toward different spatial positions. We probed the functional connectivity between human area V6A (hV6A) and the primary motor cortex (M1) using dual-site, paired pulse transcranial magnetic stimulation with a short (4ms) and a longer (10ms) inter-stimulus interval while healthy participants (18 men and 18 women) planned a visually-guided or a memory-guided reaching movement toward positions located at different depths and directions. We found that, when the stimulation over hV6A is sent 4ms before the stimulation over M1, hV6A inhibits motor evoked potentials during planning of either rightward or leftward reaching movements. No modulations were found when the stimulation over hV6A was sent 10ms before the stimulation over M1, suggesting that only short medial parieto-frontal routes are active during reach planning. Moreover, the short route of hV6A-premotor cortex-M1 is active during reach planning irrespectively of the nature (visual or memory) of the reaching target. These results agree with previous neuroimaging studies and provide the first demonstration of the flow of inhibitory signals between hV6A and M1.Significance Statement:All our dexterous movements depend on the correct functioning of network of brain areas. Knowing the functional timing of these networks is useful to gain a deeper understanding of how the brain works to enable accurate arm movements. In this paper, we probed the parieto-frontal network and demonstrated that it takes 4ms for the medial posterior parietal cortex to send inhibitory signals to the frontal cortex during reach planning. This fast flow of information seems not to be dependent on the availability of visual information regarding the reaching target. This study opens the way for future studies to test how this timing could be impaired in different neurological disorders.

Visual sensitivity at the service of action control in posterior parietal cortex

“…Indeed, the error signal may be extremely useful to premotor areas, that have a direct input from V6A ( Matelli et al, 1998 ; Galletti et al, 2004 ; Gamberini et al, 2009 ; 2021 ) when orchestrating successful motor control. These neuronal properties representing differences in the degree to which cells are influenced by feedback control versus feedforward planning of reaching and reach-to-grasp find a parallel in the human brain with recent transcranial magnetic interference with V6A activity during reaching and grasping ( Breveglieri et al, 2021 ; Breveglieri et al, 2022 ; Breveglieri et al, 2023 ), showing a causal involvement of human V6A in state estimation for reaching and grasping.…”