Effects of pulse waveform/current direction on the response to TMS

We found significant differences in RMT, MEP latency, cSP, SICI, LICI, and ICF between the three conditions. The only TMS measure where the statistical analysis showed no influence of the explored waveforms/current directions was baseline MEP amplitude. MonoAP pulses yielded the highest RMT followed by monoPA and biAP-PA. These findings are consistent with the results of previous studies that compared monoPA and biphasic waveforms (Niehaus et al., 2000; Kammer et al., 2001; Sommer et al., 2006; Delvendahl, Gattinger, et al., 2014; Stephani et al., 2016) as well as monoPA and monoAP current directions (Sakai et al., 1997; Orth & Rothwell, 2004;

Delvendahl, Lindemann, et al., 2014). Together, these results support a model of current-cortex interactions whereby the corticospinal pathway is most efficiently stimulated with biAP-PA pulse waveforms followed by monoPA and monoAP current directions induced orthogonally to the central sulcus.

In contrast with our finding that RMT with biphasic pulses was the lowest among the three conditions, Orth and Rothwell (Orth & Rothwell, 2004) found the RMT to be higher with biphasic pulses than with either monoAP or monoPA pulses. These different results can be due to two factors:

(1) The induced current direction in the Orth and Rothwell study (PA-AP) was opposite to that in the present study (AP-PA) and, thus, could have altered the interactions of the phases. While at threshold intensities the PA component of the biphasic pulse is likely the primary contributor to the MEP, it is possible that the AP component may have an agonistic effect if it is first and an antagonistic effect if it is second. Future studies could resolve this by directly comparing the TMS measures obtained with biAP-PA and biPA-AP pulses using the same stimulator; (2) We used a MagVenture MagPro device, whereas Orth and Rothwell used a Magstim 200 stimulator (Magstim Co. Whitland, Dyfed, UK). As previously mentioned in State of the Art (Chapter 2 – Section 2.2), Kammer and colleagues (Kammer et al., 2001) compared devices from Magstim and MagVenture companies and obtained lower RMTs with monophasic than with biphasic pulses when using Magstim, whereas MagVenture led to lower biphasic thresholds regardless of current direction.

These results suggest that different devices may have different total stimulation strengths depending on the waveform.

Our results are in agreement with prior studies that found the MEP latencies to be shorter with monoPA than with monoAP pulses (Mills et al., 1992; Takahashi et al., 2005; Sommer et al., 2006; Ni et al., 2011; Delvendahl, Gattinger, et al., 2014; Delvendahl, Lindemann, et al., 2014;

D’Ostilio et al., 2016), probably because different current directions activate different neural components with distinct latencies within the corticospinal pathway (Di Lazzaro & Rothwell, 2014).

Moreover, our results show a difference between monoAP and monoPA latencies of about 1.2 ms.

This difference is in line with the results from Di Lazzaro (Di Lazzaro et al., 2001, 2003; Di Lazzaro et al., 2011) and probably reflects that monoAP pulses elicit later and more dispersed I-waves.

In contrast, discrepant results have been reported when comparing the latency of MEPs obtained with monophasic and biphasic pulses. While some studies found longer latencies with monoAP than with biphasic pulses (Sommer et al., 2006; Delvendahl, Gattinger, et al., 2014), other studies found no signifcant difference between the three waveform/current directions (Niehaus et al., 2000). Following Di Lazzaro and Rothwell’s theoretical model (Di Lazzaro & Rothwell, 2014),

we would expect that biphasic pulses elicit MEPs with shorter latencies, since at high-enough intensities, a biphasic pulse evokes a D-wave reflecting the direct activation of the PTNs. Our results, however, showed that biAP-PA MEP latencies were longer than monoPA, and comparable to monoAP, latencies. Although these results may appear contradictory, they could be due to several factors: (1) Our data show a difference in MEP latency between monoPA and biAP-PA of about 1.7 ms. This may be due to the fact that the neural components activated by biAP-PA had a longer latency, similar to the ones activated by monoAP (in our study the difference between monoPA and monoAP latencies was 1.2ms). (2) It is possible that the intensity of the biphasic pulse was not high enough to reach layer V of the motor cortex or to overcome the PTNs’ firing threshold and therefore, the activation of the PTN’s was indirect. Previous studies have shown that biphasic pulses at 120% of RMT might not activate the PTNs directly and hence do not elicit D-waves (Di Lazzaro et al., 2001) but elicit a complex group of I-waves with longer latencies. (3) At threshold levels, the PA phase as second component of the biAP-PA pulse has a greater importance, whereas the AP phase gains more relevance as the stimulation intensity is increased to suprathreshold levels. Considering that MEP latency was 1.7 ms longer with biAP-PA than with monoPA pulses, it is possible that in our study, the AP component played a more relevant role in the activation of the motor cortex. Therefore, the PA and AP components could have worked against each other in activating the inhibitory and excitatory interneuron networks, hence leading to longer latecies. We hasten to add that this hypothesis is based on insufficient evidence in the literature and needs to be investigated in future studies, for example by comparing the latencies of MEPs elicited with the different waveforms and current directions at different intensities in a input-ouput curve (see Chapter 4 – Section 4.2 for the description and diagram of an input-ouput curve). Lastly, when controlling for potential confounding factors, we found that gender significantly influenced MEP latencies. This relationship has been described in previous studies and is considered to be due to a difference in limbs length between genders (Livingston, Goodkin, & Ingersoll, 2010).

Contradictory results have also been reported regarding the effects of waveform and current direction on MEP amplitude (Mills et al., 1992; Takahashi et al., 2005; Sommer et al., 2006; Ni et al., 2011; Delvendahl, Gattinger, et al., 2014; Delvendahl, Lindemann, et al., 2014;

D’Ostilio et al., 2016). As previously argued in State of the Art (Chapter 2 – Section 2.5), an additional source of variability is differences in methodology among previous studies: while some studies used a fixed portion of MSO to elicit MEPs , other studies used a specific percentage of RMT to assess the effect of waveforms/current directions on MEP amplitude (Delvendahl, Gattinger, et al., 2014; Delvendahl, Lindemann, et al., 2014). Our finding that MEP amplitudes elicited at 120% of RMT were not significantly different between the three conditions is consistent with the results of previous studies that used similar TMS parameters (Delvendahl, Gattinger, et al., 2014; Delvendahl, Lindemann, et al., 2014).

With the FDI slightly contracted, biAP-PA yielded longer cSP durations than monoPA, with monoAP in between. These results are generally consistent with the findings of previous cSP studies (Orth & Rothwell, 2004; Sommer et al., 2006). Moreover, the similarity in cSP duration between monoPA and monoAP pulses reflected in our data is consistent with the results reported by Sommer and colleagues (Sommer et al., 2006), but contrasts with those reported by Orth and Rothwell (Orth & Rothwell, 2004), who observed shorter cSP durations with monoPA pulses than with either monoAP or biPA-AP pulses. These different results can be due to several factors: First, Orth and Rothwell used a Magstim 200 stimulator for monophasic pulses and a Magstim Super Rapid stimulator for biphasic pulses (Magstim Co., Whitland, Dyfed, UK), whereas both we and Sommer and colleagues used a MagPro X100 stimulator for all conditions. As mentioned above and in State of the Art, it has been shown that the maximal intensities of the induced magnetic field vary across stimulators (Kammer et al., 2001) and waveforms, which may influence the cSP duration. Second, Orth and Rothwell used 150% of active motor threshold as the stimulation intensity, whereas both the present study and that from Sommer and colleagues set the stimulation intensity based on RMT. Therefore, our results are in agreement with those of Sommer

et al.’s study, in which the technical TMS pulse parameters were mostly similar to ours but differ to some extent (cSP was not significantly different between monoAP and monoPA) from those studies in which the cSP was performed with a different stimulator and with different sti mulation parameters.

In sum, RMT was lowest with biAP-PA and highest with monoAP, latencies were shorter with monoPA, whereas MEP amplitudes were comparable in the three conditions. These findings indicate that different pulse waveforms/current directions may recruit different subgroups of interneurons at different intensities (Di Lazzaro & Rothwell, 2014). For example, biAP-PA pulses seem to be more efficient at threshold levels but elicit non-significantly smaller MEPs at higher intensities.

Paired-pulse protocols have been conventionally performed with monoPA pulses, probably due to historical reasons and technical availability when they were first described. Our results show that monophasic pulses resulted in stronger short intracortical inhibition (SICI), but weaker facilitation (ICF), when measured with monoPA. Interestingly, significant facilitation (compared to baseline) was only achieved in the two conditions that included an AP component (i.e., monoAP

and biAP-PA).

Although the physiological mechanisms responsible for the results of measures of intracortical balance of inhibition and facilitation (i.e., cSP and paired pulse TMS) cannot be directly inferred from the present study, some hypotheses can be formulated. The results suggest that monoPA waveforms may be more efficient in targeting short-interval inhibitory cortical mechanisms. Based on invasive epidural recordings showing a reduction of I2- and late I-wave amplitudes from LICI and SICI performed with monoPA pulses (Nakamura et al., 1996; Di Lazzaro, Restuccia, et al., 1998; Di Lazzaro et al., 2002, 2011), the present results are consistent with the hypothesis that monoPA pulses activate interneuron networks in layers II and III of the motor cortex that inhibit layer V PTNs. However, no effect on the amplitude of D- or I-waves was observed when performing ICF with monoPA. In our study, performing ICF with monoPA pulses induced a

small facilitation that was not significantly different from baseline. On the other hand, pulse waveforms with an AP component (monoAP and biAP-PA) led to significant facilitation. So far, the influence of AP currents on D- and I-waves during facilitatory protocols has only been studied in a single subject (Di Lazzaro et al., 2006) showing the influence of ICF on late I-waves (I4- and I5-waves). Additional insights to the relationship between AP currents and ICF may come from the results of cSP. Even though cSP is an inhibitory protocol conducted with a single suprathreshold pulse, it is dependent on voluntary muscle contraction, which may reflect the engagement of additional cortical (i.e., premotor or supplementary motor areas) and/or subcortical structures.

Similar to the results with ICF, cSP seems to be longer with pulses that include an AP component.

If AP-oriented currents target inputs to primary motor cortex from surrounding cortices or other brain structures, the present results support the hypothesis that these cortico-cortical connections may subserve the processes that underlie both cSP and ICF. Although, this hypothesis needs to be confirmed in future studies with epidural recordings.

Finally, we examined the associations between RMT and the other TMS measures, and found the baseline MEP amplitude to be the only TMS measure that was related to RMT. The negative association between these two measures was also observed in the other experiment of TMS reliability of the present work (see Chapter 8 for further information).

7.4.2 Effects of pulse waveform/current direction on the reliability of TMS