Schibli, L. | Search bioRxiv

bioRxiv2022

Towards Detailed Cortical Maps of the Human Back with Representational Similarity Analysis

Cortical reorganization and its potential pathological significance are being increasingly studied in musculoskeletal disorders such as chronic low back pain (CLBP) patients. However, detailed sensory-topographic maps of the human back are lacking, and a baseline characterization of such representations, reflecting the somatosensory organization of the healthy back, is needed before exploring potential sensory map reorganization. To this end, a novel pneumatic vibrotactile stimulation method was used to stimulate paraspinal sensory afferents, while studying their cortical representations in unprecedented detail. In 41 young healthy participants, vibrotactile stimulations at 20 Hz and 80 Hz were applied bilaterally at nine lo cations along the thoracolumbar axis while functional magnetic resonance imaging (fMRI) was performed. Model-based whole-brain searchlight representational similarity analysis (RSA) was used to investigate the organizational structure of brain activity patterns evoked by thoracolumbar sensory inputs. A model based on seg mental distances best explained the similarity structure of brain activity patterns that were located in different areas of sensorimotor cortices, including the primary somatosensory and motor cortices and parts of the superior parietal cortex, suggesting that these brain areas process sensory input from the back in a "dermatomal" manner. The current findings provide a sound basis for testing the "cortical map reorganization theory" and its pathological relevance in CLBP. HighlightsO_LIFine-grained cortical activation patterns of paraspinal vibrotactile sensory input were obtained using whole-brain representational similarity analysis. C_LIO_LIThe patterns were well explained using a model reflecting segmental distances along the thoracolumbar axis. C_LIO_LIThe current results provide a solid basis for revisiting the "cortical map reorganization theory" and its pathological significance in chronic low back pain. C_LI

neuroscience↗

bioRxiv2021

In the back of your mind: Cortical mapping of tactile and proprioceptive paraspinal afferent inputs

Topographic organization is a hallmark of vertebrate cortex architecture, characterized by ordered projections of the bodys sensory surfaces onto brain systems. High-resolution functional magnetic resonance imaging (fMRI) has proven itself as a valuable tool to investigate the cortical landscape and its (mal-)adaptive plasticity with respect to various body part representations, in particular extremities such as the hand and fingers. Less is known, however, about the cortical representation of the human back. We therefore validated a novel, MRI-compatible method of mapping cortical representations of sensory afferents of the back, using vibrotactile stimulation at varying frequencies and paraspinal locations, in conjunction with fMRI. We expected high-frequency stimulation to be associated with differential neuronal activity in the primary somatosensory cortex (S1) compared to low-frequency stimulation and that somatosensory representations would differ across the thoracolumbar axis. We found significant differences between neural representations of high- and low-frequency stimulation and between representations of thoracic and lumbar paraspinal locations, in several bilateral S1 sub-regions, and in regions of the primary motor cortex (M1). High-frequency stimulation preferentially activated Brodmann Area (BA) regions BA3a and BA4p, while low-frequency stimulation was more encoded in BA3b and BA4a. Moreover, we found clear topographic differences in S1 for representations of the upper and lower back during high-frequency stimulation. We present the first neurobiological validation of a method for establishing detailed cortical maps of the human back, which might serve as a novel tool to evaluate the pathological significance of neuroplastic changes in clinical conditions such as chronic low back pain. Key pointsO_LIDetailed investigations of cortical representations of somatosensory paraspinal afferents along the thoracolumbar axis are lacking. C_LIO_LIUsing fMRI combined with a novel vibrotactile stimulation device ("pneuVID") we investigated different sensorimotor cortical representations of the back and explored topographic differences between the upper and lower back. C_LIO_LIWe found differential sub-regional sensorimotor neural representations of high- and low-frequency stimulation, as well as revealing initial evidence of the somatotopy of upper and lower paraspinal representations. C_LIO_LIThe current approach might serve as a promising tool to elucidate the role of cortical reorganisation in the pathophysiology of clinical conditions such as chronic low back pain. C_LI

neuroscience↗