Limited Capacity for Ipsilateral Secondary Motor Areas to Support Hand Function Post-Stroke

Abstract

Key pointsr Ipsilateral-projecting corticobulbar pathways, originating primarily from secondary motor areas, innervate the proximal and even distal portions, although they branch more extensively at the spinal cord.r It is currently unclear to what extent these ipsilateral secondary motor areas and subsequent cortical projections may contribute to hand function following stroke-induced damage to one hemisphere.r In the present study, we provide both structural and functional evidence indicating that individuals increasingly rely on ipsilateral secondary motor areas, although at the detriment of hand function.r Increased activity in ipsilateral secondary motor areas was associated with increased involuntary coupling between shoulder abduction and finger flexion, most probably as a result of the low resolution of these pathways, making it increasingly difficult to open the hand.r These findings suggest that, although ipsilateral secondary motor areas may support proximal movements, they do not have the capacity to support distal hand function, particularly for hand opening.Abstract Recent findings have shown connections of ipsilateral cortico-reticulospinal tract (CRST), predominantly originating from secondary motor areas to not only proximal, but also distal muscles of the arm. Following a unilateral stroke, CRST from the ipsilateral side remains intact and thus has been proposed as a possible backup system for post-stroke rehabilitation even for the hand. We argue that, although CRST from ipsilateral secondary motor areas can provide control for proximal joints, it is insufficient to control either hand or coordinated shoulder and hand movements as a result of its extensive spinal branching compared to contralateral Kevin Wilkins holds a BS in Psychology from Boston College and a PhD in Neuroscience from Northwestern University. Currently, he is a postdoctoral scholar at Stanford University. He seeks to use a mechanistic approach to better understand and treat movement disorders. J Physiol 598.11 corticospinal tract. To address this issue, we combined magnetic resonance imaging, high-density EEG, and robotics in 17 individuals with severe chronic hemiparetic stroke and 12 age-matched controls. We tested for changes in structural morphometry of the sensorimotor cortex and found that individuals with stroke demonstrated higher grey matter density in secondary motor areas ipsilateral to the paretic arm compared to controls. We then measured cortical activity when participants were attempting to generate hand opening either supported on a table or when lifting against a shoulder abduction load. The addition of shoulder abduction during hand opening increased reliance on ipsilateral secondary motor areas in stroke, but not controls. Crucially, the increased use of ipsilateral secondary motor areas was associated with decreased hand opening ability when lifting the arm as a result of involuntary coupling between the shoulder and wrist/finger flexors. Taken together, this evidence implicates a compensatory role for ...

Hybrid dedicated and distributed coding in PMd/M1 provides separation and interaction of bilateral arm signals

“…One candidate hypothesis for the presence of ipsilateral activity is that it supplies an independent control signal. There is some evidence that ipsilateral cortex plays an increased role in movement following hemispheric damage (Brinkman and Kuypers, 1973; Hummel and Cohen, 2006; Dancause, 2006; Wilkins et al, 2020), though not necessarily a beneficial or compensatory one. The magnitude of ipsilateral encoding increases with the degree of movement complexity (Verstynen et al, 2005) and may involve spatially distinct neural populations (Ziemann et al, 1999; Chen et al, 2003).…”

“…However, the corticospinal tract (CST) is almost entirely contralateral, and the effectiveness of the ipsilateral component has been debated (Lacroix et al, 2004; Rosenzweig et al, 2009; Soteropoulos et al, 2011; Baker et al, 2015). Ipsilateral cortex may also exert its influence via connections made in the reticular formation (Alagona et al, 2001; Baker et al, 2015; Wilkins et al, 2020), which projects bilaterally to the spinal cord. Our results showed a small amount of independent ipsilateral activity (monkey O more so than monkey W), with more of the ipsilateral signal coming from non-dedicated units (Figure 5).…”