Differential force scaling of fine‐graded power grip force in the sensorimotor network

Abstract

Force scaling in the sensorimotor network during generation and control of static or dynamic grip force has been the subject of many investigations in monkeys and human subjects. In human, the relationship between BOLD signal in cortical and subcortical regions and force still remains controversial. With respect to grip force, the modulation of the BOLD signal has been mostly studied for forces often reaching high levels while little attention has been given to the low range for which electrophysiological neuronal correlates have been demonstrated. We thus conducted a whole-brain fMRI study on the control of fine-graded force in the low range, using a power grip and three force conditions in a block design. Participants generated on a dynamometer visually guided repetitive force pulses (ca. 0.5 Hz), reaching target forces of 10%, 20%, and 30% of maximum voluntary contraction. Regions of interest analysis disclosed activation in the entire cortical and subcortical sensorimotor network and significant force-related modulation in several regions, including primary motor (M1) and somatosensory cortex, ventral premotor and inferior parietal areas, and cerebellum. The BOLD signal, however, increased monotonically with force only in contralateral M1 and ipsilateral anterior cerebellum. The remaining regions were activated with force in various nonlinear manners, suggesting that other factors such as visual input, attention, and muscle recruitment also modulate the BOLD signal in this visuomotor task. These findings demonstrate that various regions of the sensorimotor network participate differentially in the production and control of fine-graded grip forces.

Cerebellar lobules and dentate nuclei mirror cortical force‐related‐BOLD responses: Beyond all (linear) expectations

“…The main effect of movement results (i.e., the 0th order effect) confirmed these findings; especially in lobule V. A recent meta analysis of the different types of gripping (i.e., power or precision) or different patterns (static or dynamic) showed that lobule V was involved in all these motor tasks [King et al, 2014]; suggesting that this lobule is involved in the basic aspects of motor control during gripping. Within the medial region of lobule V, a positive linear relationship between force levels and activation was detected in our study, in line with previous findings with both low force [Keisker et al, 2009] and higher force ranges [Spraker et al, 2012]. We speculate that this indicates an increased recruitment of neurons in motor regions at the highest forces.…”

“…One of the interesting findings of this study is the bilateral activation in lobule VI seen in the main effect of movement analysis; this is in line with previous GF studies [Halder et al, 2007; Kuhtz‐Buschbeck et al, 2008], although it has rarely been discussed or possibly not detected [Keisker et al, 2009]. Our data, however, not only shows a contralateral activation of lobule VI (as well as the more familiar ipsilateral one) but also shows that the activation pattern of lobule VI is not homogenous.…”

“…For example, Keisker et al [2009] showed (without any parametric fitting) that the posterior cerebellum exhibited a greater response at low and high forces; whereas it responded with a lower BOLD signal at middle forces, indicating a match to the +2nd order fit of our study. However, Keisker's study used only three forces, precluding the characterization of higher‐order non‐linearities.…”

“…This contrasts with previous analyses at the whole brain level [Alahmadi et al, 2015, 2016d; Ehrsson et al, 2000; Halder et al, 2007; Keisker et al, 2009, 2010; King et al, 2014; Kuhtz‐Buschbeck et al, 2001, 2008; Neely et al, 2013; Noble et al, 2013; Spraker et al, 2012; Vaillancourt et al, 2003; Ward and Frackowiak, 2003]. Some of these studies report a linear association between GF strength and areas of the anterior cerebellum [Halder et al, 2007; Kuhtz‐Buschbeck et al, 2008] but not in the posterior cerebellum, despite the role of the posterior cerebellum in visual processing (when using visual cues) and motor planning [King et al, 2014; Stoodley and Schmahmann, 2010; Stoodley et al, 2012].…”

“…Some of these studies report a linear association between GF strength and areas of the anterior cerebellum [Halder et al, 2007; Kuhtz‐Buschbeck et al, 2008] but not in the posterior cerebellum, despite the role of the posterior cerebellum in visual processing (when using visual cues) and motor planning [King et al, 2014; Stoodley and Schmahmann, 2010; Stoodley et al, 2012]. Conversely, a recent dynamic power grip study found that the signal in the anterior cerebellum was linearly correlated with forces, whereas the posterior cerebellum evidenced higher signals at low and high forces but low signals at intermediate force levels [Keisker et al, 2009]. In contrast with all of these findings, a static precision grip fMRI study found that the signals in the whole (i.e., anterior and posterior) cerebellum tended to covary non‐linearly with force [Spraker et al, 2012].…”