Utilising TMS-EEG to Assess the Response to Cerebellar-Brain Inhibition

Cerebellar-brain inhibition (CBI) is a transcranial magnetic stimulation (TMS) paradigm indexing excitability of cerebellar projections to motor cortex (M1). Stimulation involved with CBI is often considered to be uncomfortable, and alternative ways to index connectivity between cerebellum and the cortex would be valuable. We therefore sought to assess the utility of electroencephalography in conjunction with TMS (combined TMS-EEG) to record the response to CBI. A total of 33 volunteers (25.7 ± 4.9 years, 20 females) participated across three experiments. These investigated EEG responses to CBI induced with a figure-of-eight (F8; experiment 1) or double cone (DC; experiment 2) conditioning coil over cerebellum, in addition to multisensory sham stimulation (experiment 3). Both F8 and DC coils suppressed early TMS-evoked EEG potentials (TEPs) produced by TMS to M1 (P < 0.05). Furthermore, the TEP produced by CBI stimulation was related to the motor inhibitory response to CBI recorded in a hand muscle (P < 0.05), but only when using the DC coil. Multisensory sham stimulation failed to modify the M1 TEP. Cerebellar conditioning produced changes in the M1 TEP that were not apparent following sham stimulation, and that were related to the motor inhibitory effects of CBI. Our findings therefore suggest that it is possible to index the response to CBI using TMS-EEG. In addition, while both F8 and DC coils appear to recruit cerebellar projections, the nature of these may be different.

Comparing cortico-motor hotspot identification methods in the lower extremities post-stroke: MEP amplitude vs. latency

Transcranial magnetic stimulation (TMS) is a technique used to probe and measure cortico-motor responses of the nervous system. However, lower extremity (LE) specific methodology has been slow to develop. In this retrospective analysis, we investigated what motor evoked potential metric, amplitude (MEP_amp) or latency (MEP_lat), best distinguished the motor-cortical target, i.e. hotspot, of the tibialis anterior and soleus post-stroke. Twenty-three participants with stroke were included in this investigation. Neuronavigation was used to map hotspots, derived via MEP_amp and MEP_lat, over a 3cm × 5cm grid. Distances between points with the greatest response within a session and between days were compared. Both criterion, amplitude and latency, provided poor identification of locations between trials within a session, and between multiple visits. Identified hotspots were similar only 15 % and 8% of the time between two assessments within the same session, for amplitude and latency respectively. However, MEP_amp was more consistent in identifying hotspots, evidenced by locations being less spatially distant from each other (Amplitude: 1.4 cm (SD 0.10) Latency: 1.7 (SD 1.04), P = 0.008) within a session and between days (Amplitude: 1.3 cm (SD 0.95), Latency 1.9 cm (SD 1.14), P = 0.004). While more work is needed to develop LE specific methodology for TMS, especially as it applies to investigating gait impairments, MEP_amp appears to be a more consistent criterion for hotspot identification when compared to MEP_lat. It is recommended that future works continue to use MEP_amp when identifying tibialis anterior and soleus hotspots using neuronavigation.

The ACDC pilot trial: targeting the anterior cingulate by double cone coil rTMS for the treatment of depression

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) of the dorsolateral-prefrontal cortex (DLPFC) with conventional figure-of-8 (=butterfly) coils has been used as an antidepressant therapeutic tool for almost twenty years. Very recently, an innovative rTMS coil, the so-called double cone coil (DC), was introduced allowing the modulation of the anterior cingulate cortex (AC). We investigated safety and therapeutic effectiveness of this stimulation in a naturalistic clinical setting.

METHOD

Forty-five patients suffering a moderate to severe depressive episode were randomized to receive 15 sessions of either conventional rTMS of the left DLPFC ("butterfly-rTMS"; 10 Hz; 2000 stimuli/day, RMT 110%), mediofrontal double cone coil stimulation of the anterior cingulate cortex ("ACDC-rTMS" with equal parameters), or sham-stimulation. The primary outcome was the change in the 21-items Hamilton Rating Scale for Depression (HAMD) from baseline to the end of treatment. Secondary outcome measures were changes over the course of the trial regarding the HAMD, the Beck Depression Inventory (BDI), the Clinical Global Impression (CGI) and the Global Assessment of Functioning (GAF) scales.

RESULTS

There was a significant group × time interaction effect regarding the primary outcome (F = 3.269; df = 2,37; P = 0.049). Post-hoc t-testing revealed a significant effect for the comparison ACDC vs. butterfly at week 3/end of treatment (T = 2.646; df = 26; P = 0.014). No severe adverse events occurred during the study. ACDC-stimulation was well tolerated by the majority of patients similar like butterfly-rTMS and sham-stimulation.

CONCLUSION

This pilot study demonstrated the feasibility of ACDC-rTMS-stimulation as an add-on-treatment for depression. Its clinical effects warrant further investigation in the future.

Cerebellar transcranial magnetic stimulation: the role of coil geometry and tissue depth

BACKGROUND

While transcranial magnetic stimulation (TMS) coil geometry has important effects on the evoked magnetic field, no study has systematically examined how different coil designs affect the effectiveness of cerebellar stimulation.

HYPOTHESIS

The depth of the cerebellar targets will limit efficiency. Angled coils designed to stimulate deeper tissue are more effective in eliciting cerebellar stimulation.

METHODS

Experiment 1 examined basic input-output properties of the figure-of-eight, batwing and double-cone coils, assessed with stimulation of motor cortex. Experiment 2 assessed the ability of each coil to activate cerebellum, using cerebellar-brain inhibition (CBI). Experiment 3 mapped distances from the scalp to cerebellar and motor cortical targets in a sample of 100 subjects' structural magnetic resonance images.

RESULTS

Experiment 1 showed batwing and double-cone coils have significantly lower resting motor thresholds, and recruitment curves with steeper slopes than the figure-of-eight coil. Experiment 2 showed the double-cone coil was the most efficient for eliciting CBI. The batwing coil induced CBI only at higher stimulus intensities. The figure-of-eight coil did not elicit reliable CBI. Experiment 3 confirmed that cerebellar tissue is significantly deeper than primary motor cortex tissue, and we provide a map of scalp-to-target distances.

CONCLUSIONS

The double-cone and batwing coils designed to stimulate deeper tissue can effectively stimulate cerebellar targets. The double-cone coil was found to be most effective. The depth map provides a guide to the accessible regions of the cerebellar volume. These results can guide coil selection and stimulation parameters when designing cerebellar TMS studies.