Background The cortical motor threshold (CMT) is often used to individually adjust the intensity ... more Background The cortical motor threshold (CMT) is often used to individually adjust the intensity of transcranial magnetic stimulation (TMS). It reflects the integrated excitability of the corticomotor projection, including the excitability at the spinal level. The CMT reflects axonal and synaptic excitability as CMT can be modified by drugs that voltage-gated sodium channel blockers or non-NMDA glutamatergic drugs. Aims and hypothesis The CMT is not a static measure but is subject to state-dependent fluctuations. To test this hypothesis we used an adaptive stair-case procedure to track acute changes in CMT associated with motor imagery (MI) or shifts in visuospatial attention (VA). Method Eight healthy subjects were asked to imagine a continuous thumb-index movement with the left or right hand (MI-left and MI-right), to shift their visuospatial attention to the left or right FDI (VA-left and VA-right) or to fixate a cross in the central visual field. Using a fast ( Results The number of pulses needed to determine rMT did not significantly differ across conditions (average: 13.5). Using the normalized CMT values as dependent variable, we computed a repeated measures ANOVA with the factors Current (AP/PA), Task (MI/VA) and Site (Left/Right). We found a significant Task-by-Site interaction ( p = 0.004). Post hoc testing confirmed that CMT during MI-right was significantly lower than during MI-left or during VA-right. This was the case for AP and PA current direction. Discussion The resting CMT is subject to rapid state-dependent changes in the absence of overt motor activity. We show that these acute CMT changes can be captured with a threshold-hunting procedure. The “CMT tracking” approach might be particularly useful to assess the temporal dynamics of corticomotor excitability changes induced by interventional brain stimulation protocols.
Functional magnetic resonance imaging (fMRI) studies on the sense of agency (SoA) have yielded he... more Functional magnetic resonance imaging (fMRI) studies on the sense of agency (SoA) have yielded heterogeneous findings identifying regional brain activity during tasks that probed SoA. In this review, we argue that the reason behind this between-study heterogeneity is a "synecdochic" way the field conceptualizes and studies SoA. Typically, a single feature is experimentally manipulated and then this is interpreted as covering all aspects of SoA. The purpose of this paper is to give an overview of the fMRI studies of SoA and attempt to provide meaningful categories whereby the heterogeneous findings may be classified. This classification is based on a separation of the experimental paradigms (Feedback Manipulations of ongoing movements, Action-Effect, and Sensory Attenuation) and type of report employed (implicit, explicit reports of graded or dichotic nature, and whether these concern self-other distinctions or sense of control). We only find that Feedback Manipulation and Action-Effect share common activation in supplementary motor area, insula and cerebellum in positive SoA and inferior frontal gyrus in the negative SoA, but observe large networks related to SoA only in Feedback Manipulation studies. To illustrate the advantages of this approach, we discuss the findings from an fMRI study which we conducted, within this framework.
Background The cortical motor threshold (CMT) is often used to individually adjust the intensity ... more Background The cortical motor threshold (CMT) is often used to individually adjust the intensity of transcranial magnetic stimulation (TMS). It reflects the integrated excitability of the corticomotor projection, including the excitability at the spinal level. The CMT reflects axonal and synaptic excitability as CMT can be modified by drugs that voltage-gated sodium channel blockers or non-NMDA glutamatergic drugs. Aims and hypothesis The CMT is not a static measure but is subject to state-dependent fluctuations. To test this hypothesis we used an adaptive stair-case procedure to track acute changes in CMT associated with motor imagery (MI) or shifts in visuospatial attention (VA). Method Eight healthy subjects were asked to imagine a continuous thumb-index movement with the left or right hand (MI-left and MI-right), to shift their visuospatial attention to the left or right FDI (VA-left and VA-right) or to fixate a cross in the central visual field. Using a fast ( Results The number of pulses needed to determine rMT did not significantly differ across conditions (average: 13.5). Using the normalized CMT values as dependent variable, we computed a repeated measures ANOVA with the factors Current (AP/PA), Task (MI/VA) and Site (Left/Right). We found a significant Task-by-Site interaction ( p = 0.004). Post hoc testing confirmed that CMT during MI-right was significantly lower than during MI-left or during VA-right. This was the case for AP and PA current direction. Discussion The resting CMT is subject to rapid state-dependent changes in the absence of overt motor activity. We show that these acute CMT changes can be captured with a threshold-hunting procedure. The “CMT tracking” approach might be particularly useful to assess the temporal dynamics of corticomotor excitability changes induced by interventional brain stimulation protocols.
Functional magnetic resonance imaging (fMRI) studies on the sense of agency (SoA) have yielded he... more Functional magnetic resonance imaging (fMRI) studies on the sense of agency (SoA) have yielded heterogeneous findings identifying regional brain activity during tasks that probed SoA. In this review, we argue that the reason behind this between-study heterogeneity is a "synecdochic" way the field conceptualizes and studies SoA. Typically, a single feature is experimentally manipulated and then this is interpreted as covering all aspects of SoA. The purpose of this paper is to give an overview of the fMRI studies of SoA and attempt to provide meaningful categories whereby the heterogeneous findings may be classified. This classification is based on a separation of the experimental paradigms (Feedback Manipulations of ongoing movements, Action-Effect, and Sensory Attenuation) and type of report employed (implicit, explicit reports of graded or dichotic nature, and whether these concern self-other distinctions or sense of control). We only find that Feedback Manipulation and Action-Effect share common activation in supplementary motor area, insula and cerebellum in positive SoA and inferior frontal gyrus in the negative SoA, but observe large networks related to SoA only in Feedback Manipulation studies. To illustrate the advantages of this approach, we discuss the findings from an fMRI study which we conducted, within this framework.
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