Introduction
Repetitive Transcranial Magnetic Stimulation (rTMS) is increasingly used to treat
stroke, Parkinson's disease and depression (Fregni et al., 2005; Loo and Mitchell,
2005; Hallet, 2007; O'Reardon et al., 2007; Ridding and Rothwell, 2007). rTMS uses
bursts of magnetic pulses to change the excitability and connection strengths of cortical
neurons. However, the evidence to inform clinical application is highly inconsistent
(Thut and Pascual-Leone, 2010; Hamada et al., 2013) and substantially based on trial
and error. Systematic theory is lacking. Typically, in rTMS research, measurements
of motor-evoked potential (MEP) are made, often in terms of the strength of the MEP
and the length of the cortical silent period that follows. However, the MEP is probably
a poor and certainly an indirect measure of changes in the brain (Nicolo et al., 2015),
clouding our understanding of rTMS mechanisms. In practice, therefore, particular
amplitudes and timing of pulses in an rTMS sequence are selected because they show
promise in small subsets of people. However, even basics such as the sign of any change
in the outcome measure (e.g., does the MEP increase or decrease?) is debated. Many
results show a wide spread in responses. It has become common to talk about “responders”
and “non-responders” although evidence for a binary distinction in these two groups
is lacking—in reality there is usually a continuum of response often including potentiation
in some and depression in others (Nettekoven et al., 2015). Moreover, Héroux et al.
(2015) provide evidence that the irreproducibility of results may be due to small
sample sizes, unscientific screening of subjects and data, and selective reporting
of results.
In rTMS a regular train of pulses is applied. There is considerable variation in possible
stimulation sequences, leading to many parameters that could potentially affect the
results. Pulses are applied at a particular amplitude (normally recorded as percent
motor threshold, (%RMT), or sometimes percent machine output, themselves imprecise
measures), at a given rate (pulses per second, or hertz, Hz), until a particular number
of pulses have been applied. There are three numerical parameters here. Additionally,
one can consider variation in coil shape, orientation and place of application. Different
waveforms for the magnetic pulse are also available. Thus, for an ostensibly straightforward
pulse sequence such as non-bursting rTMS, the parameter space is considerable. Fitzgerald
et al. (2006) carried out a comprehensive review of rTMS effects. They concluded that
low frequency (0.9–5 Hz) stimulation generally results in a decreased MEP, while high
frequency stimulation (5–20 Hz) results in an increased MEP. Very low frequency stimulation
gave no effect. The “<5 Hz, depression; >5 Hz, potentiation” mantra is now well established
in rTMS literature.
More recently, bursting protocols have become popular. The quadripulse protocol (Tsutsumi
et al., 2014) is one example—four pulses are applied in quick succession, with this
pattern repeated at regular time intervals. Theta-burst stimulation (TBS) is another;
bursts of pulses are applied at theta-band (4–8 Hz) frequency. A continuous theta-burst
stimulation (cTBS) protocol adds two more parameters, the number of pulses in a burst
and the burst frequency. Intermittent theta-burst stimulation (iTBS) protocols require
a further two parameters. Here, the bursting protocol is applied for a given time
(denoted ON time, often 2 s), then removed for a given time (denoted OFF time, often
8 s) before being active for another 2 s period, etc. Thus, the parameter space for
describing intermittent bursting pulse sequences becomes vast. Experiments performed
to date do not come close to spanning it.
Descriptions of previous results
Recently, there has been some focus on developing good numerical models of the effects
of rTMS (Robinson, 2011; Fung et al., 2013; Wilson et al., 2014). That has been our
primary motivation for revisiting previous rTMS experiments (particularly cTBS and
iTBS) with a view to identifying overall trends. Which parameters are the most important?
What, on average, are their effects? Various forms of statistical regression, such
as Principal Component Analysis (PCA) should potentially be of benefit here. However,
it quickly became clear that for bursting sequences (e.g., cTBS and iTBS) there is
insufficient variation in data for such an analysis to be meaningful. Few protocols
have been evaluated in the large parameter space. Recently, for example, the cTBS
and iTBS protocols of Huang et al. (2005) have come to the fore and are being used
to the substantial exclusion of other possibilities.
In the case of repetitive, but not bursting protocols, there is more variation, particularly
in earlier studies. This has allowed us to tackle an analysis with regression and
PCA. We have identified 92 publications looking at rTMS protocols (from 1994 to 2006)
containing results of 164 different experiments. However, in many of these cases data
were not systematically reported and key information was missing. From these publications
we produced a subset of 35 publications containing data from 79 different experiments
in which we could unambiguously identify the frequency of the rTMS, the number of
pulses applied in the protocol, the intensity of the applied pulse (in terms of %RMT)
and the overall effect on the MEP. These experiments covered 1064 different subjects
(although some of these subjects are likely to be the same person). The number of
subjects in these experiments ranged from four to 45, with a median of ten. Nineteen
publications reported on “one-off” experiments with a single protocol, as opposed
to multiple protocols used on the same set of subjects. Various clinical populations
were covered, including Parkinson's disease, epilepsy, major depression and focal
hand dystonia, in addition to healthy populations. We have not looked at the effect
of population on results.
A few publications recorded the effect on the MEP in a quantitative manner; most did
not. Rather, it has been usually recorded qualitatively as “increase,” “decrease,”
“no change,” or, unhelpfully, “variable.” Again, there is a problem of interpretation
here. What is it about the MEP that “increases”? Is it the amplitude of the MEP, the
time-scale over which it occurs, or the integrated area of the MEP? Many publications
are vague on this point. We have taken a pragmatic approach and simply left the effect
on the MEP, for the purposes of analysis, as being “increase,” “decrease” or “no change.”
We assigned each of these results the numeric values of 1, 0, and −1, respectively.
Those protocols which recorded “variable” results, were assigned 0, although the results
of analyses do not change much when these experiments are excluded.
We thus constructed a four-dimensional dataset recording frequency of rTMS (f), number
of pulses applied (N), amplitude of each pulse (A), and (tri-valued) effect on the
MEP, M. We then carried out two analyses. First, we used linear regression to find
the overall effect of f, N, and A on M. Secondly, we used PCA to determine which variable
combinations were most important for influencing M and their overall effect. To eliminate
some bias, we weighted each experiment by the number of participants.
The regression analysis showed that it was only the pulse frequency f that had any
overall effect on the size of the MEP. For frequency, the analysis is shown in Figure
1A. Here, the result is plotted against the applied frequency. The area of the circles
is proportional to the number of subjects in each experiment. The solid line shows
the results of linear regression (with each experiment weighted by the number of participants);
the dashed line shows regression where the more questionable “one-off” experiments
have been excluded. We acknowledge that linear regression is dubious since we have
made the results tri-valued only and there is no a priori reason for believing a linear
relationship should apply.
Figure 1
(A) The effect of repetition frequency f on the size of the MEP. The circles denote
different experiments; the area of the circles is proportional to the number of participants
in each experiment. The solid line is a result of linear regression; the dashed line
is a result of linear regression where “one-off” experiments have been excluded. (B)
A two-dimensional rendering of the rTMS data set using Principal Component Analysis.
The first and second components form the x- and y-axes respectively. The points mark
the individual experiments. The f, N, A, and M axes are shown in terms of the first
two principal components. Data are taken from: Pascual-Leone et al., 1994; Jennum
et al., 1995; Wassermann et al., 1996; Chen et al., 1997a,b; Berardelli et al., 1998,
1999; Siebner et al., 1999a,b; Maeda et al., 2000a,b; Muellbacher et al., 2000; Rollnik
et al., 2000; Romeo et al., 2000; Siebner et al., 2000; Wu et al., 2000; Fierro et
al., 2001; Lorenzano et al., 2002; Romero et al., 2002; Sommer et al., 2002; Cincotta
et al., 2003; Gorsler et al., 2003; Grunhaus et al., 2003; Modugno et al., 2003; Schambra
et al., 2003; Fitzgerald et al., 2004; Peinemann et al., 2004; Stinear and Byblow,
2004; Brighina et al., 2005; Fitzgerald et al., 2005; Murase et al., 2005; Quartarone
et al., 2005a,b; Daskalakis et al., 2006; Inghilleri et al., 2006.
Results of PCA are summarized in Figure 1B. From a four-dimensional data-set we obtain
four principal components. The components, in order, explain 42, 31, 16, and 11% of
the variation in the data. We plot the data points in terms of the first two principal
components, and show the direction of the f, N, A and M axes on the figure.
It is clear that frequency is the major driver of the change in the MEP. Figure 1A
shows the effect is for low frequencies to reduce the MEP, and high frequencies to
increase it, in line with the much-assumed f < 5 Hz, depression; f > 5 Hz, potentiation.
However, the relationship is weak, with results widespread. This is supported by Figure
1B, which indicates that the M axis lies in a similar direction to the f axis, meaning
that an increase in f leads in general to an increase in the end result M. Results
show little link between N and A and the end result. On the PCA plot of Figure 1B,
the N and A axes lie roughly perpendicular to the end result. Linear regression (not
shown) gives no apparent trend.
A request for more systematic data
We do not wish to draw too much from the above results, other than to say that broadly
speaking they support the established rTMS dogma around the effect of repetition rate
on MEP. That in itself is unsurprising given that Fitzgerald et al. (2006) comprehensively
tabulated a large number of experiments, and there is considerable overlap between
our datasets. Too often, however, the mantra “<5 Hz, depression; >5 Hz, potentiation”
is stated without acknowledging the extent of the variation in results. Although Héroux
et al. (2015) have exposed some questionable research practices, excluding the “one-off”
data makes little difference to this conclusion.
The main focus of our comments is the difficulty we experienced in performing such
analyses, notably our inability to analyze the cTBS and iTBS literature in a quantitative
manner.
First, we note that the typical output measure of rTMS, the MEP, is poorly defined.
It is clear that different authors mean different things by this. It is not just a
case that electrical activity of different muscles are measured; rather that there
is no quantitative or even consistent qualitative definition of the change in the
MEP. We are left mostly with vague terminology “increase,” “decrease,” “no change,”
or even “fluctuates.” If we are to start understanding the results, drivers and mechanisms
of rTMS, we need to start by robustly defining outcome measures. A minority of publications
have focused on the length of the cortical silent period (CSP, the quiet period of
the electromyogram following a MEP) rather than the “size” of the MEP. This is more
easily quantifiable and is a more direct measure of cortical effects (Ziemann et al.,
2015), therefore is possibly a better place to start. Unlike the MEP, which depends
on a network of cortical and non-cortical excitatory and inhibitory processes, the
CSP originates in the cortex and is mediated by activation of GABAA and GABAB (Ziemann
et al., 2015).
If one persists with measuring the MEP, one needs also to relate changes in the MEP
to changes in the brain. The response to TMS depends not only on cortical excitability
but also to excitability at a spinal level and the properties of corticospinal-motoneurone
connections. Without suitable models of the process, it is difficult to relate changes
in MEP to changes in the cortex, and start to untangle the effects of plasticity,
neural excitability and gene expression that all could affect the results of a TMS
(Pell et al., 2011). One is therefore left with costly and difficult animal experiments
(Vahabzadeh-Hagh et al., 2012).
Next, we must become more rigorous in the planning, execution, recording and publication
of our experiments. Too often, key details have been left out. This has particularly
been evident in some of the earlier experiments. In compiling our data set, we had
to make assumptions about particular experiments because data were not unambiguously
given. Indeed, our data set for producing the Figure used only around half of the
literature we looked at, because we could not reliably identify the data we required.
Moreover, our analysis relies on published data; we cannot analyze unpublished results.
Finally, we note that in the last 10 years human experiments have focused on a very
limited range of pulse sequences, such as the cTBS and iTBS of Huang et al. (2005).
Most of the high dimensioned parameter space that bursting rTMS sequences provide
has been largely ignored. A much larger range of protocols is needed in order to investigate
effects systematically. It is true that the focus on these protocols is a result of
their promise in clinical applications, but in terms of science, we are left little
the wiser about what rTMS is actually doing.
Author contributions
MW conceived the research, processed some of the data and wrote the majority of the
article. LS identified the data set, carried out most of the data processing, identified
the major issues and wrote the minority of the article.
Conflict of interest statement
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.