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

Abstract

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