Direct MRI mapping of neuronal activity evoked by electrical stimulation of the median nerve at the right wrist

Abstract

Magnetic source MRI (msMRI) has being developed recently for direct detections of neuronal magnetic fields to map brain activity. However, controversial results have been reported by different research groups. In this study, more evidence was provided to demonstrate that the neuronal current signal could be detected by MRI using a rapid median nerve stimulation paradigm. The experiments were performed on six normal human participants to investigate the temporal specificity of the effect, as well as inter-and intrasubject reproducibility. Significant activation of contralateral primary sensory cortex (S1) was detected 80 ms after stimulation onset (corresponding to the P80 evoked potential peak). The 80-ms latency S1 activation was observed over three independent sessions for one subject and for all six participants. The magnitude of the signal change was 0.2-0.3%. Coinciding with our expectations, no S1 activation was found when MRI data acquisitions were targeted at the N20 and P30 peaks because of mutual cancellation of magnetic fields generated by those peaks. The results demonstrated good reproducibility of S1 activations and indicated that the S1 activations most likely originated from neuronal magnetic field rather than hemodynamic response. Direct detections of transient neuronal magnetic fields using MRI offer several advantages over the conventional functional MRI (fMRI) techniques, which measure regional cerebral hemodynamic response induced by neural firing (1-4). Because it directly measures the effects of neuronal magnetic fields, the novel fMRI technique, termed here as magnetic source MRI (msMRI), could avoid the assumption of close coupling of regional cerebral hemodynamics and neuronal activity. It can potentially offer higher temporal resolution and better spatial localization. The feasibility of msMRI has been studied theoretically and experimentally by several research groups (5-25). Controversial results have been reported. It is currently still a matter of debate whether the technique is practical for mapping neuronal activity.The feasibility of directly mapping neuronal activity has been addressed theoretically. Divergent conclusions were reached by investigators considering different dipole models or measuring different parameters (i.e., phase and magnitude imaging). Initially, Singh (5) used an electrical current phantom to investigate the possibility of detecting the evoked neuromagnetic field. The neuronal magnetic field inside the brain was calculated on the basis of the field observed on the surface of the skull. It was reported that the required measurement of phase shifts was 20-fold smaller than the sensitivity of the scanner they used. However, it is possible that the local neuronal magnetic field is much larger than they predicted because opposite neuronal currents have a cancellation effect which results in a much smaller field on the skull. Konn et al. (6) modeled neuronal current flow as an extended current dipole located in a conducting sphere. They demonstrated that the min...

Detection of neuronal current MRI in human without BOLD contamination

“…However, using a detection threshold (0.2%) smaller than these signal changes, no ncMRI activation was detected in the present study without the task-induced BOLD activation. This suggests that the signal changes observed in the previous ncMRI studies (2,3,7,8,10) cannot be confidently attributed to the direct effect of neuronal currents. They may result from the contamination of the task-induced BOLD background.…”

“…Therefore, both of these BOLD artifact-free human and animal studies indicate that the ncMRI signal evoked by the physiological (visual) stimulation is too weak to be detected. In the previous ncMRI studies on human subjects (2,3,7,8,10), fast signal changes of $0.3-1% were observed in the magnitude images using steady-state stimulation tasks. However, using a detection threshold (0.2%) smaller than these signal changes, no ncMRI activation was detected in the present study without the task-induced BOLD activation.…”

“…Nonetheless, up to date, a consensus has not been reached on the detectability of ncMRI on human subjects. Multiple groups have reported a successful detection of ncMRI signal in human brain (2)(3)(4)(5)(6)(7)(8)(9)(10), while a failure in ncMRI detection has been claimed in other studies (11)(12)(13)(14)(15). Due to these conflicting results, there is still no conclusion on the question whether ncMRI signal is detectable or not.…”

Direct neural current imaging in an intact cerebellum with magnetic resonance imaging

“…The literature contains several reports of positive results (Kamei et al, 1999; Xiong et al, 2003; Bianciardi et al, 2004; Liston et al, 2004; Konn et al, 2004; Petridou et al, 2006; Truong and Song, 2006; Chow et al, 2006a; Chow et al, 2006b; Chow et al, 2007; Chow et al, 2008; Xue et al, 2009) which conflict with reports of negative results (Chu et al, 2004; Parkes et al, 2007; Mandelkow et al, 2007; Tang et al, 2008; Luo et al, 2009; Rodionov et al, 2010; Luo, Jiang & Gao 2011; Huang, 2014). …”

Magnetic resonance imaging of oscillating electrical currents

“…These studies have primarily been proof-ofprinciple demonstrations using artificial systems (6,(18)(19)(20) and theoretical examinations of the feasibility of neuronal current detection (20)(21)(22)(23)(24)(25). While work is being done to develop a viable and broadly applicable detection modality for in vivo experiments (26)(27)(28)(29), a number of studies (30)(31)(32)(33) have cast doubt as to the feasibility of a technique based on traditional current-imaging methods.…”