On the Homology of the Dominant and Non-Dominant Corticospinal Tracts: A Novel Neurophysiological Assessment

Abstract

Objectives: The homology of hemispheric cortical areas plays a crucial role in brain functionality. Here, we extend this concept to the homology of the dominant and non-dominant hemi-bodies, investigating the relationship of the two corticospinal tracts (CSTs). The evoked responses provide an estimate of the number of in-phase recruitments via their amplitude as a suitable indicator of the neuronal projections' integrity. An innovative concept derived from experience in the somatosensory system is that their morphology reflects the recruitment pattern of the whole circuit.

Methods: CST homology was assessed via the Fréchet distance between the morphologies of motor-evoked potentials (MEPs) using a transcranial magnetic stimulation (TMS) in the homologous left- and right-hand first dorsal interosseous muscles of 40 healthy volunteers (HVs). We tested the working hypothesis that the inter-side Fréchet distance was higher than the two intra-side distances.

Results: In addition to a clear confirmation of the working hypothesis (p < 0.0001 for both hemi-bodies) verified in all single subjects, we observed that the intra-side Fréchet distance was higher for the dominant than the non-dominant one. Interhemispheric morphology similarity increased with right-handedness prevalence (p = 0.004).

Conclusions: The newly introduced measure of circuit recruitment patterning represents a potential benchmark for the evaluation of inter-lateral mechanisms expressing the relationship between homologous hemilateral structures subtending learning and suggests that variability in recruitment patterning physiologically increases in circuits expressing greater functionality.

Keywords: corticospinal tract; handedness; hemi-body homology; novel-concept physiological measures; on-center off-surround.

Figure 1

Experimental setting. Experimental procedure in each healthy volunteer (HV). Top: In each hemisphere, once the TMS FDI hotspot was identified, the position of the coil was monitored using a neuronavigational system. Approximately 20 MEPs were collected from each side, shown here superimposed in the 0–70 ms window of a representative HV. Bottom: CST homology was estimated as Fréchet distances between each pair of left and right MEP (homologous) morphologies (400 values in each subject) and between each pair of MEPs from the same side (190 values for each subject and each side).

Figure 2

Inter-side and intra-side MEP morphology similarity. Boxplots of individual Fréchet distances between inter-lateral MEP morphologies (DxSn) (average of about 400 pairs in each HV, x-axis) versus intra-lateral distances (average of about 190 values). It is evident that the inter-lateral distance is higher than both the right intra-lateral distance (A) and the left intra-lateral distance (B) in all subjects. In the right intra-side comparison, a prevalence of greater distances emerges compared to the left (C). The overall values and standard deviation with subjects are shown in (D).

Figure 3

Sex-dependent MEP morphology similarity. The overall values and standard deviation of Fréchet distances between inter-side (DxSn) MEP morphologies, the intra-side distances in the right (DxDx) and left (SnSn) distinct for Sex.

Figure 4

Inter-side and intra-side MEP amplitudes. For each healthy volunteer (HV), the difference between each pair of MEP amplitudes for left-right comparison (first row) and intra-side comparison (right side, second row; left third row).