Prefrontal-Premotor Pathways and Motor Output in Well-Recovered Stroke Patients

Abstract

Structural brain imaging has continuously furthered our knowledge how different pathways of the human motor system contribute to residual motor output in stroke patients. Tract-related microstructure of pathways between primary and premotor areas has been found to critically influence motor output. The motor network is not restricted in connectivity to motor and premotor areas but these brain regions are densely interconnected with prefrontal regions such as the dorsolateral (DLPFC) and ventrolateral (VLPFC) prefrontal cortex. So far, the available data about the topography of such direct pathways and their microstructural properties in humans are sparse. To what extent prefrontal-premotor connections might also relate to residual motor outcome after stroke is still an open question. The present study was designed to address this issue of structural connectivity of prefrontal-premotor pathways in 26 healthy, older participants (66 ± 10 years old, 15 male) and 30 well-recovered chronic stroke patients (64 ± 10 years old, 21 males). Probabilistic tractography was used to reconstruct direct fiber tracts between DLPFC and VLPFC and three premotor areas (dorsal and ventral premotor cortex and the supplementary motor area). Direct connections between DLPFC/VLPFC and the primary motor cortex were also tested. Tract-related microstructure was estimated for each specific tract by means of fractional anisotropy and alternative diffusion metrics. These measures were compared between the groups and related to residual motor outcome in the stroke patients. Direct prefrontal-premotor trajectories were successfully traceable in both groups. Similar in gross anatomic topography, stroke patients presented only marginal microstructural alterations of these tracts, predominantly of the affected hemisphere. However, there was no clear evidence for a significant association between tract-related microstructure of prefrontal-premotor connections and residual motor functions in the present group of well-recovered stroke patients. Direct prefrontal-motor connections between DLPFC/VLPFC and the primary motor cortex could not be reconstructed in the present healthy participants and stroke patients.

The Superior Frontal Longitudinal Tract: Connection between the Dorsal Premotor and the Dorsolateral Prefrontal Cortices

“…Based on this study's findings and previously published research, the SLFT represents a 285 connection between the DLPFC and the PMd cortex. The BA4/6-caudal subcomponent seems to 286 exist genuinely in humans at a much lower rate than BA6-rostral/8, which might explain why other 287 similar studies in humans could not confirm its existence (Schulz et al, 2019). Still, we do not have a…”

The superior frontal longitudinal tract: a connection between the dorsal premotor and the dorsolateral prefrontal cortices

“…Based on this study's findings and previously published research, the SFLT represents a connection between the DLPFC and PMd cortex. The BA4/6-caudal subcomponent seems to exist in humans at a much lower rate than BA6-rostral/8, which might explain why other similar studies in humans could not confirm its existence 4 ; however, such an interpretation should be made with caution since no direct connection between the caudal PMd (PMd proper) and DLPFC was detected in animal studies 1,15 . The available data concerning a human direct structural pathway between the rostral PMd and DLPFC are still limited, apart from a few functional studies involving healthy participants 3 , well-recovered chronic stroke patients 4 and postmortem examinations 6 .…”

“…The current work extensively studied the anatomy of the SFLT using GQI tractography with a limited white fibre dissection, which focused on the anatomical terminations’ subcomponents and their asymmetry; this aspect, in particular, received little attention in previous reports 4 – 6 . In all the samples studied, the SFLT was found to be a frontal intralobar tract in distinction from the superficial U-fibres of the sFLS, which differs from some previous reports 5 , 6 .…”

“…The brain’s frontal lobe structural connectivity, namely the connection between the dorsal premotor area (PMd) and the dorsolateral prefrontal cortex (DLPFC) is not well understood due to lack of sufficient data unfolding its details in humans. Several animal tracing, functional MRI, and probabilistic tractography analysis reports support the assumption of its existence 1 – 4 . An early study of primate brain tracing concluded that several premotor areas are interconnected with the prefrontal cortex, and the major site for interactions is the ventral part of the premotor area.…”

“…They were able to reconstruct probable trajectories connecting the DLPFC with the premotor (dorsal and ventral) and supplementary motor area (SMA) in both groups. Also, the DLPFC-premotor trajectories were restricted only to premotor regions with higher connectivity for SMA than for PMd; however, they could not reconstruct a direct prefrontal-motor connection between the DLPFC and primary motor cortex 4 .…”