Bilateral somatosensory evoked potentials following intermittent theta-burst repetitive transcranial magnetic stimulation

<p>Abstract</p> <p>Background</p> <p>Intermittent theta-burst stimulation (iTBS) is a form of repetitive transcranial magnetic stimulation that may alter cortical excitability in the primary somatosensory cortex (SI). The present study investigated the effects of iTBS on subcortical and early cortical somatosensory evoked potentials (SEPs) recorded over left, iTBS stimulated SI and the right-hemisphere non-stimulated SI. SEPs were recorded before and at 5, 15, and 25 minutes following iTBS.</p> <p>Results</p> <p>Compared to pre-iTBS, the amplitude of cortical potential N20/P25 was significantly increased for 5 minutes from non-stimulated SI and for 15 to 25 minutes from stimulated SI. Subcortical potentials recorded bilaterally remained unaltered following iTBS.</p> <p>Conclusion</p> <p>We conclude that iTBS increases the cortical excitability of SI bilaterally and does not alter thalamocortical afferent input to SI. ITBS may provide one avenue to induce cortical plasticity in the somatosensory cortex.</p

Area 5 Influences Excitability within the Primary Motor Cortex in Humans

In non-human primates, Brodmann's area 5 (BA 5) has direct connectivity with primary motor cortex (M1), is largely dedicated to the representation of the hand and may have evolved with the ability to perform skilled hand movement. Less is known about human BA 5 and its interaction with M1 neural circuits related to hand control. The present study examines the influence of BA 5 on excitatory and inhibitory neural circuitry within M1 bilaterally before and after continuous (cTBS), intermittent (iTBS), and sham theta-burst stimulation (sham TBS) over left hemisphere BA 5. Using single and paired-pulse TMS, measurements of motor evoked potentials (MEPs), short interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were quantified for the representation of the first dorsal interosseous muscle. Results indicate that cTBS over BA 5 influences M1 excitability such that MEP amplitudes are increased bilaterally for up to one hour. ITBS over BA 5 results in an increase in MEP amplitude contralateral to stimulation with a delayed onset that persists up to one hour. SICI and ICF were unaltered following TBS over BA 5. Similarly, F-wave amplitude and latency were unaltered following cTBS over BA 5. The data suggest that BA 5 alters M1 output directed to the hand by influencing corticospinal neurons and not interneurons that mediate SICI or ICF circuitry. Targeting BA 5 via cTBS and iTBS is a novel mechanism to powerfully modulate activity within M1 and may provide an avenue for investigating hand control in healthy populations and modifying impaired hand function in clinical populations

Modulation of inhibitory and excitatory neural circuits in the primary motor cortex following theta-burst rTMS to area 5

Subcortical and cortical loci interact with the primary motor cortex (M1) and influence the neural circuitry mediating hand movement. Area 5 located within the superior parietal lobule has direct connectivity with M1, is largely dedicated to the representation of the hand, and is considered important for thumb opposition movements. The present study examined the modulation of inhibitory and excitatory neural circuits within bilateral M1 before and after continuous (cTBS), intermittent (iTBS), and sham theta-burst stimulation (TBS) over left-hemisphere area 5. Two experiments were performed to address the influence of area 5 on neural circuitry within M1. Specifically, inhibitory circuitry (short interval intracortical inhibition (SICI)) and excitatory circuitry (motor evoked potentials (MEPs), intracortical facilitation (ICF)), were examined for the representation of the first dorsal interosseous (FDI) muscle within bilateral M1. MEPs, SICI, and ICF were measured bilaterally before and at 5-20 minutes, 25-40 minutes, and 45-60 minutes after TBS cessation. The order for right versus left M1 recordings for MEPs, SICI, and ICF recordings were kept constant within subjects across each time block and this order of cortex stimulated (right, left) was randomized across subjects. The results of Experiment 1 and 2 demonstrate that area 5 selectively influences M1 circuitry such that MEPs are increased bilaterally following area 5 cTBS and increased in the right FDI following area 5 iTBS. Area 5 TBS does not modulate ICF or SICI. The novel findings from the Master’s thesis suggest area 5 is a cortical loci that influences M1 excitatory circuitry and possibly motor control of the hand

Figure 1

<p>A. TMS target locations. MRI from one participant demonstrating the targets used for cTBS within BA 5 and M1 bilaterally. A (anterior), P (posterior). Yellow lines indicate location of TMS coil placement. B. Experimental Time Course. Graphic representation depicting the order of data collection and experiment procedures. RFDI/LFDI (right, left first dorsal interosseous), MEPs (motor evoked potentials), SICI (short interval intracortical inhibition), ICF (intracortical facilitation), TBS (theta-burst stimulation).</p

ITBS over area 5 on MEPs, SICI and ICF.

<p>A. Group-averaged MEPs (with standard errors) obtained from the right (top) and left (bottom) FDI for the iTBS group. B. Group-averaged SICI obtained from the right (top) and left (bottom) FDI for the iTBS group. C. Group-averaged ICF obtained from the right (top) and left (bottom) FDI for the iTBS group. Time course denoted as T_o (before TBS), T₁ (5–20 min post TBS), T₂ (25–40 min post TBS), T₃ (45–60 min post TBS). *p≤0.05.</p