Median nerve stimulation modulates extracellular signals in the primary motor area of a macaque monkey

Passive mapping of hand motor cortex across altered states of consciousness

OBJECTIVE

To evaluate the ability of median nerve stimulation (MNS)-induced high gamma band (HGB) activity in mapping the hand motor cortex at different states of consciousness.

METHODS

Five patients undergoing awake craniotomy were recruited. MNS-induced electrocorticographic signals were recorded from awake to anesthetic states, with the loss of consciousness (LOC) session divided into three stages (LOC1, LOC2, and LOC3) based on conscious level. HGB signals were analyzed to localize hand motor cortex. Linear models were applied to analyze HGB dynamics during LOC.

RESULTS

The sensitivity of hand motor cortex mapping based on HGB average envelope at short-latency period was 100%, 96.67%±3.33%, 83.47%±8.19%, and 82.22%±11.44% for the awake, LOC1, LOC2 and LOC3 stages. The sensitivity for HGB average envelope at long-latency period was 92.67%±4.52%, 90.85%±4.13%, 72.27%±17.07%, and 40.53%±12.82% across the same stages. The sensitivity based on HGB average power at short-latency period decreased from 100% in awake stage to 72.83%±12.95%, 48.11%±15.95%, and 21.12%±5.70% across LOC stages. The sensitivity for HGB average power at long-latency period dropped from 92.67%±4.52% in awake stage to 70.94%±10.79%, 58.37%±17.49%, and 25.71%±14.95% in the subsequent LOC stages. The slope coefficient of the simple linear model for long-latency average envelope was significantly smaller than that for short-latency. In the linear mixed effects model, the Condition × Sliding Window estimate coefficient was -0.794.

CONCLUSION

In awake state, HGB average envelope and average power both effectively localized hand motor cortex. With declining consciousness, the mapping ability of average power significantly deteriorated, while the mapping ability of short-latency average envelope remained relatively stable.

iTBS-induced LTP-like plasticity parallels oscillatory activity changes in the primary sensory and motor areas of macaque monkeys

Recently, neuromodulation techniques based on the use of repetitive transcranial magnetic stimulation (rTMS) have been proposed as a non-invasive and efficient method to induce in vivo long-term potentiation (LTP)-like aftereffects. However, the exact impact of rTMS-induced perturbations on the dynamics of neuronal population activity is not well understood. Here, in two monkeys, we examine changes in the oscillatory activity of the sensorimotor cortex following an intermittent theta burst stimulation (iTBS) protocol. We first probed iTBS modulatory effects by testing the iTBS-induced facilitation of somatosensory evoked potentials (SEP). Then, we examined the frequency information of the electrocorticographic signal, obtained using a custom-made miniaturised multi-electrode array for electrocorticography, after real or sham iTBS. We observed that iTBS induced facilitation of SEPs and influenced spectral components of the signal, in both animals. The latter effect was more prominent on the θ band (4–8 Hz) and the high γ band (55–90 Hz), de-potentiated and potentiated respectively. We additionally found that the multi-electrode array uniformity of β (13–26 Hz) and high γ bands were also afflicted by iTBS. Our study suggests that enhanced cortical excitability promoted by iTBS parallels a dynamic reorganisation of the interested neural network. The effect in the γ band suggests a transient local modulation, possibly at the level of synaptic strength in interneurons. The effect in the θ band suggests the disruption of temporal coordination on larger spatial scales.