Multimodal finger-printing of the human precentral cortex forming the motor hand knob

TLDR: A sulcus-aligned MRI-informed TMS mapping approach is used to determine the optimal site (hotspot) for evoking MEPs in the precentral hand knob and provides first-time evidence for behaviourally relevant, structural and functional phenotypic variation in the crown of human precentral motor hand knob.

Abstract: Transcranial magnetic stimulation (TMS) of the precentral hand knob can evoke motor evoked potentials (MEP) in contralateral hand muscles. Biophysical modelling points to the dorsal premotor cortex (PMd) in the superficial crown-lip region as primary site of TMS-induced neuronal excitation. Here, we used a sulcus-aligned MRI-informed TMS mapping approach to determine the optimal site (hotspot) for evoking MEPs in the precentral hand knob. Individual precentral hotspot location varied along the rostro-caudal axis. Individuals with a more rostral location had longer MEP latencies. Spatiotemporal hotspot rostrality was associated with higher precentral myelin-related signals, stronger movement-related activation of PMd in the precentral crown, and higher temporal precision during paced finger tapping. Together, our multimodal mapping approach provides first-time evidence for behaviourally relevant, structural and functional phenotypic variation in the crown of human precentral motor hand knob. The results have important implications for physiological and interventional TMS studies targeting the precentral hand knob.

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Frequently asked questions

Q1. What future works have the authors mentioned in the paper "Multimodal finger-printing of the human precentral cortex forming the motor hand knob" ?

Future work needs to systematically address how the inter-individual differences in hotspot rostrality are influenced by the orientation of the TMS-induced currents in the precentral hand knob, for instance by using monophasic current configurations. 

Q2. What software was used for the analysis of functional MRI data?

The authors used the Statistical Parametric Mapping (SPM) software package (SPM8; Wellcome Department of Imaging Neuroscience, London, UK, http://www.fil.ion.ucl.ac.uk) for pre-processing and statistical analysis of the functional MRI data. 

Q3. What is the main reason why TMS is primarily used in the posterior M1HAND?

TMS with a caudal hotspot in the posterior lip region may primarily stimulate the rostral part of the anterior M1HAND (BA4a), whereas TMS in individuals with a rostral hotspot in the precentral crown may primarily stimulate PMd (BA6). 

Q4. Where is the bulk of cortico-motoneuronal output neurons located?

Even if TMS preferentially targets the rostral part of anterior M1HAND (BA4a) in the posterior crown-lip region, the bulk of cortico-motoneuronal pyramidal output neurons can be expected to be located deeper in the sulcal wall, in the posterior M1HAND (BA4p). 

Q5. How was the normal distribution of all variables verified?

Before applying parametric statistical tests, the normal distribution of all variables was verified by means of a Kolmogorov and Smirnov test. 

Q6. What was the way to determine the position of the left FDI muscle?

Prior to sulcus-aligned TMS mapping, the authors located the motor hotspot position of the left FDI muscle by trial-and-error with the handle of the coil angled at 45-degree relative to the midsagittal line. 

Q7. What is the correlation between the M1HAND and PMd groups?

Since the rostral M1HAND is confined to the posterior lip region of the precentral gyrus (11), the authors hypothesized that the M1HAND group would display a higher electrical field magnitude in the posterior lip region relative to the PMd group. 

Q8. What is the sulcus-aligned TMS mapping of the motor hand?

Sulcus-aligned TMS mapping of the motor hand knob Standard grid-based TMS mapping of corticomotor representations keeps the coil orientation of the TMS coil identical across all grid sites (21–26, 31). 

Q9. What is the way to improve the focality of the mapping procedure?

the authors could use lower stimulus intensities which improved the focality of their mapping procedure by using a small-size dedicated coil.