Human Motor Corpus Callosum: Topography, Somatotopy, and Link between Microstructure and Function

Abstract

The corpus callosum (CC) is the principal white matter fiber bundle connecting neocortical areas of the two hemispheres. Although an object of extensive research, important details about the anatomical and functional organization of the human CC are still largely unknown. Here we focused on the callosal motor fibers (CMFs) that connect the primary motor cortices (M1) of the two hemispheres. Topography and somatotopy of CMFs were explored by using a combined functional magnetic resonance imaging/diffusion tensor imaging fiber-tracking procedure. CMF microstructure was assessed by fractional anisotropy (FA), and CMF functional connectivity between the hand areas of M1 was measured by interhemispheric inhibition using paired-pulse transcranial magnetic stimulation. CMFs mapped onto the posterior body and isthmus of the CC, with hand CMFs running significantly more anteriorly and ventrally than foot CMFs. FA of the hand CMFs but not FA of the foot CMFs correlated linearly with interhemispheric inhibition between the M1 hand areas. Findings demonstrate that CMFs connecting defined body representations of M1 map onto a circumscribed region in the CC in a somatotopically organized manner. The significant and topographically specific positive correlation between FA and interhemispheric inhibition strongly suggests that microstructure can be directly linked to functional connectivity. This provides a novel way of exploring human brain function that may allow prediction of functional connectivity from variability of microstructure in healthy individuals, and potentially, abnormality of functional connectivity in neurological or psychiatric patients.

Bilateral Transcutaneous Electrical Nerve Stimulation Improves Lower‐Limb Motor Function in Subjects With Chronic Stroke: A Randomized Controlled Trial

“…Based on the findings of neuroanatomical48 and clinical23, 24 studies, the underlying mechanisms that mediate the effects of Bi‐TENS might include the enhancement of interhemispheric interaction, possibly via the corpus callosum, which is the white matter in the human brain that connects the 2 hemispheres via more than 200 million axonal connections 48. The motor areas of the 2 hemispheres have been shown with functional magnetic resonance imaging and diffusion tensor imaging to be connected via the posterior body of the corpus callosum 48.…”

Evaluation of Callosal Motor Fiber Location in the Human Brain by Diffusion Tensor Tractography Combined with Functional MRI

“…Through prior assessment of CMFs in the CC, it is known that CMFs primarily cross through the anterior midbody of the CC, as shown in rhesus monkeys by Pandya et al and Schmahmann et al 11,14) . However, Meyer et al found that CMFs cross through the posterior body and isthmus of the CC utilizing transcranial magnetic stimulation (TMS) measurements, and Hofer et al, Zarei et al, and Wahl et al made the same observations using diffusion tensor imaging (DTI) 2,7,8,13) . In essence, prior studies aiming to obtain accurate anatomical and functional features of the human brain have reported controversial results.…”

“…Being the largest fiber bundle, it enables interconnection between the two cerebral hemispheres [1][2][3][4][5][6][7][8] . Previous studies have used magnetic resonance imaging (MRI) to determine several anatomical and morphological attributes of the CC, such as volume, gender difference, and handedness for the evaluation of changes between health and neurological disorders 1,4,6,[9][10][11][12] .…”

“…Previous studies have used magnetic resonance imaging (MRI) to determine several anatomical and morphological attributes of the CC, such as volume, gender difference, and handedness for the evaluation of changes between health and neurological disorders 1,4,6,[9][10][11][12] . The anatomical location and features of CC-related motor fibers, known as callosal motor fibers (CMFs), have been studied by many neurological researchers to assess and obtain more accurate characteristic information about the CC 2,7,[13][14][15] . Through prior assessment of CMFs in the CC, it is known that CMFs primarily cross through the anterior midbody of the CC, as shown in rhesus monkeys by Pandya et al and Schmahmann et al 11,14) .…”

“…Diffusion tensor tractography (DTT), based on DTI data, enables 3D neural fiber tract visualization, and has been used to identify major white matter pathways, such as the corticobulbar, corticospinal, and associated fiber tracts 8,16,18,19) . Moreover, combining DTT with functional magnetic resonance imaging (fMRI) provides more accurate information for in vivo analysis of the brain 7,[20][21][22] . This method enables identification of the locations of cortical activation-related motor outcomes and helps set up a region of interest (ROI) by avoiding user-dependent bias.…”