Subtle hemispheric asymmetry of motor cortical inhibitory tone

Subtle hemispheric asymmetry of motor cortical inhibitory tone

Tihomir V. Ilic, Patrick Jung, Ulf Ziemann*

Clinic of Neurology, J.W. Goethe-University Frankfurt, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main, Germany

Accepted 25 September 2003

Abstract

Objective: To test whether a novel paired transcranial magnetic stimulation (TMS) protocol (J Physiol 545.1 (2002) 153) detects

hemispheric differences in motor cortical inhibition.

Methods: Nine right-handers and 8 left-handers participated. Focal paired TMS was applied to the hand area of the dominant (M1-D) or

non-dominant motor cortex (M1-ND). Motor evoked potentials (MEP) were recorded from the relaxed contralateral abductor digiti minimi.

The first (S1) and second pulse (S2) were separated by 1.5 or 2.1 ms. Nine stimulus intensities of S1 and S2 (i.e. 9 £ 9 intensity conditions)

ranging from 60 to 140% of resting motor threshold (RMT) were tested. The interaction between S1 and S2 was expressed by

MEPS1þS2/(MEPS1 þ MEPS2) p 100%. Values below and above 100% indicate short-interval intracortical inhibition (SICI) and facilitation

(SICF), respectively.

Results: In right-handers, RMT was lower, SICI was present with fewer intensity conditions and the magnitude of SICI was less in M1-D

than M1-ND. No hemispheric asymmetry was found for SICF. Left-handers showed no hemispheric difference for any of these measures.

Conclusions: Findings suggest that, in right-handers, M1-D is controlled by less inhibitory tone than M1-ND. This may put the M1-D to an

advantage for processes that are associated with a reduction of SICI, such as voluntary activation and use-dependent plasticity.

q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Keywords: Short-interval intracortical inhibition; Motor evoked potential; Transcranial magnetic stimulation; Motor cortex; Hemispheric asymmetry

1. Introduction

Many cerebral functions are associated with hemispheric

specialisation. These include speech, sensory cognition, and

motor function. Anatomical and electrophysiological

studies demonstrated a larger volume of the dominant

(M1-D) than non-dominant motor cortex (M1-ND) (Amunts

et al., 1996; Volkmann et al., 1998). The hand represen-

tation is greater in the M1-D than M1-ND. More extensive

neuronal connectivity in M1-D might constitute a neural

substrate which favours the formation of experience-

dependent excitatory and inhibitory interactions between

elementary movement representations in M1-D over M1-

ND (Hammond, 2002).

It is now possible to assess some of these excitatory and

inhibitory interactions in motor cortex non-invasively by

transcranial magnetic stimulation (TMS). For instance,

long-lasting inhibition influenced by activity through

the GABAB receptor can be tested by the duration of the

cortical silent period (Fuhr et al., 1991; Cantello et al., 1992;

Inghilleri et al., 1993; Wilson et al., 1993; Hallett, 1995;

Werhahn et al., 1999). One study found that the duration of

the cortical silent period is shorter in M1-D compared to

M1-ND (Priori et al., 1999) but this asymmetry was not

confirmed by another study (Cicinelli et al., 1997). Short-

lasting inhibition is influenced by activity through the

GABAA receptor and can be assessed by the short-interval

intracortical inhibition (SICI) as measured by paired TMS

protocols (Kujirai et al., 1993; Ziemann et al., 1996a,c; Di

Lazzaro et al., 1998, 2000). In contrast to the findings on the

cortical silent period, no hemispheric asymmetry was found

for the SICI (Cicinelli et al., 2000; Civardi et al., 2000;

Maeda et al., 2002). However, SICI testing in the classical

protocols (Kujirai et al., 1993; Ziemann et al., 1996c) is

limited by the possible occurrence of ‘floor effects’ because

these classical protocols use a conditioning first pulse that is

already optimal for producing maximum SICI (Ziemann

et al., 1996b). One novel paired TMS protocol uses a

systematic variation of the intensities of the first and second

pulse to prevent ‘floor effects’ (Ilic et al., 2002). That

Clinical Neurophysiology 115 (2004) 330–340

www.elsevier.com/locate/clinph

1388-2457/$30.00 q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

doi:10.1016/j.clinph.2003.09.017

* Corresponding author. Tel.: þ49-69-6301-5739; fax: þ49-69-6301-

6842.

E-mail address: [email protected] (U. Ziemann).

http://www.elsevier.com/locate/clinph

protocol revealed an increase of SICI by a single oral dose

of the GABAA receptor agonist diazepam which was not

apparent in the classical protocols (Ilic et al., 2002).

A reduction of SICI occurs prior to or during voluntary

activation of a motor representation (Ridding et al., 1995b;

Reynolds and Ashby, 1999; Zoghi et al., 2003). A reduction

of SICI enhances use-dependent plasticity (Ziemann et al.,

2001) while an increase in SICI by a GABAA receptor

agonist suppresses use-dependent plasticity in human motor

cortex (Tegenthoff et al., 1999; Butefisch et al., 2000). The

aim of the present study was to revisit the question of

possible hemispheric differences in SICI by taking the

advantages of the novel paired TMS protocol (Ilic et al.,

2002). We will demonstrate that, in right-handers, SICI is

less in the M1-D compared to the M1-ND. This hemispheric

asymmetry supports the idea that M1-D may have an

advantage over M1-ND with processes that are associated

with a reduction of SICI, such as voluntary activation of

movement representations or use-dependent plasticity.

2. Methods and subjects

2.1. Subjects

Seventeen healthy volunteers (mean age, 29.7 ^ 1.1

years; range, 22–36 years; 6 females, 11 males) participated

in the main experiments. Nine subjects were right-handed

(RH, mean laterality index, 92.5 ^ 2.4) and 8 left-handed

(LH, mean laterality index, 268.7 ^ 3.9) when tested with

the Edinburgh Inventory (Oldfield, 1971). Another 8 RH

subjects (mean age, 31.4 ^ 1.6 years; 5 males; mean

laterality index, 97.1 ^ 1.9) took part in a control

experiment (see below, Section 2.3) Written informed

consent was obtained from all subjects. The study was

approved by the Ethics Committee of the J.W. Goethe

University of Frankfurt, Germany, and conformed to the

Declaration of Helsinki.

2.2. Recording and stimulation procedures

Subjects were seated comfortably in a reclining chair.

Surface EMG was recorded from the abductor digiti minimi

(ADM) muscle, using surface Ag-AgCl cup electrodes in a

belly-tendon montage. The raw EMG was amplified and

filtered (10 Hz–2 kHz, Counterpoint Electromyograph,

Dantec Electronics, Skovlunde, Denmark), passed through a

CED micro 1401 laboratory interface (Cambridge Elec-

tronic Design, Cambridge, UK) for digitisation (sampling

rate, 4 kHz) and then fed into a personal computer for online

display and offline analysis, using customised data collec-

tion and conditional averaging software (Spike 2 for

Windows, Version 3.05, Cambridge Electronic Design,

Cambridge, UK).

Focal TMS was applied over the hand area of the motor

cortex through a figure-of-8 coil (outer diameter of each

wing, 9 cm; peak magnetic field, ,1.5 T) using two

MAGSTIM 200 magnetic stimulators (Magstim, Whitland,

UK) connected to the BiStim module (Magstim) throughout

all measurements. The stimulating coil was placed flat on

the skull with the handle pointing backwards and rotated 458

away from the midline. Thus, the current induced in the

brain was directed from lateral-posterior to medial-anterior,

approximately perpendicular towards the assumed line of

the central sulcus. This is the optimal orientation for a

predominantly trans-synaptic activation of the corticospinal

neurones (Kaneko et al., 1996). The optimal coil position for

activating the contralateral ADM was determined as the site

where stimulation produced consistently the largest motor

evoked potential (MEP) at a slightly supra-threshold

stimulus intensity. This site was marked on the scalp with

a pen in order to assure a constant placement of the coil

throughout the experiment. Resting motor threshold (RMT)

was determined in the resting ADM to the nearest 1% of

maximum stimulator output using single-pulse TMS. RMT

was defined as the lowest stimulus intensity which elicited

MEP .50 mV in at least 5 of 10 consecutive trials (Rossini

et al., 1994). Throughout this paper, RMT will be reported

in percent of the maximum stimulator output. All exper-

iments were performed in the relaxed ADM. The raw EMG

was displayed continuously at a high gain (50 mV/D) of the

recording device on the computer screen and played through

a loudspeaker for acoustic feedback. Trials contaminated by

EMG activity were discarded from analysis.

For the paired pulse experiments, two different inter-

stimulus intervals (ISI) of 1.5 and 2.1 ms were tested twice

in each subject in pseudo-randomised order and in separate

sessions (i.e. 4 sessions per subject). Each session consisted

of successive testing of M1-D and M1-ND at a given ISI.

The order of testing M1-D and M1-ND in a given session

was reversed in the second session of the same ISI to

exclude possible order effects. Whether M1-D or M1-ND

was tested first in the first session at a given ISI was set

pseudo-randomly, with a balanced order across subjects.

Since statistical analysis did not reveal any order effects,

results from the two sessions at a given ISI were pooled in

the final statistical analysis (see below).

The ISI of 1.5 and 2.1 ms were selected because they

showed different interactions between the first (S1) and

second pulse (S2) in previous paired-pulse TMS exper-

iments when the target muscle was studied at rest. Marked

short-latency intracortical facilitation (SICF) occurred at the

ISI of 1.5 ms, if a supra-threshold S1 and a sub-threshold S2

were used (Ziemann et al., 1998; Ilic et al., 2002), or if both

stimuli were approximately equal to RMT (Tokimura et al.,

1996), while relatively little or no SICI occurred when S1

was sub-threshold and S2 supra-threshold (Fisher et al.,

2002; Ilic et al., 2002). In contrast, at ISI of around 2 ms,

SICF was relatively weak (Tokimura et al., 1996; Ziemann

et al., 1998; Ilic et al., 2002) while SICI was prominent (Di

Lazzaro et al., 1998; Fisher et al., 2002; Ilic et al., 2002).

Therefore, it may be expected that these intervals are

T.V. Ilic et al. / Clinical Neurophysiology 115 (2004) 330–340 331

particularly suitable for testing hemispheric differences in

SICF and SICI, respectively.

It is currently thought that SICI reflects the summation of

strong GABAergic inhibitory post-synaptic potentials at the

cortico-spinal neurone elicited by the sub-threshold S1, and

excitatory post-synaptic potentials elicited by the supra-

threshold S2 (Kujirai et al., 1993; Ilic et al., 2002). SICF, on

the other hand, most likely originates non-synaptically

through direct excitation of the axon initial segment of

excitatory interneurones by the sub-threshold S2 which

were made hyper-excitable through excitatory post-synaptic

potentials by a supra-threshold S1 (Hanajima et al., 2002;

Ilic et al., 2002).

In all experiments, S1 and S2 varied in steps of 10%

of RMT between 60 and 140% RMT (i.e. 9 intensity

steps). The paired-pulse conditions consisted of all

possible combinations of S1 and S2 intensities (i.e.

9 £ 9 ¼ 81 conditions). In addition, single-pulse con-

ditions were applied at each of the 9 different intensities.

Five trials were performed for each condition (i.e. 5

repeats £ 90 conditions ¼ 450 trials per hemisphere; two

hemispheres £ 450 conditions ¼ 900 trials per session).

The different intensity conditions were applied in pseudo-

randomised order. The intensities of the two magnetic

stimulators were set automatically by customised soft-

ware (Spike 2) at a minimum step width of 1% of

maximum stimulator output via the CED 1401 laboratory

interface and the remote port of the MAGSTIM

stimulators. The inter-trial interval varied randomly in a

range of 6 ^ 2 s. One session lasted on average

approximately 2 h.

2.3. Control experiment

The principal finding of this study was slightly less SICI

in the M1-D than M1-ND of right-handed subjects at the ISI

of 2.1 ms (see Section 3). Concomitantly, the RMT was

significantly lower in M1-D than M1-ND (see Table 1).

Since stimulus intensity was related to RMT for the SICI

measurements, absolute stimulus intensities were lower for

M1-D than M1-ND. This may have contributed to the

finding of less SICI in M1-D because it was not known

whether, similar to the difference in RMT, the threshold for

SICI is also lower in M1-D than M1-ND. In order to test

this, RMT and SICI threshold were determined in a control

experiment in 8 consistent RH subjects (laterality index

.80; (Habibet al., 1995). RMT was measured as described

above. SICI threshold was determined at the ISI of 2.1 ms in

the voluntarily relaxed ADM. The intensity of S1 was varied

in steps of 3% of maximum stimulator output between 30

and 100% RMT. The intensity of S2 was kept constant and

adjusted to produce a MEP of 0.5–1.5 mV when given

alone. This resulted in a mean S2 intensity of 128.8 ^ 8.3%

RMT in M1-D, and 130.0 ^ 12.0% in M1-ND. Paired and

single pulse (S2 alone) conditions were applied in

randomised order. Ten repetitions were delivered perTab

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T.V. Ilic et al. / Clinical Neurophysiology 115 (2004) 330–340332

condition. The inter-trial interval was 6 ^ 2 s. M1-D and

M1-ND were tested consecutively in the same session in

randomised order which was balanced across the subjects.

The effect of S1 on the MEP elicited by S2 alone was

expressed as 100% 2 MEPS1þS2/MEPS2 p 100%. For each

subject and hemisphere, this effect was plotted as a

dependent variable against the stimulus intensity of S1

(given in % of maximum stimulator output). A 3rd

order sigmoid polynomial regression function was fitted,

and SICI threshold was defined as the stimulus intensity of

S1 which resulted in a 25% diminution of MEPS1þS2

compared to MEPS2.

2.4. Data analysis

RH and LH subjects were analysed separately. A paired

two-tailed t test was used to assess hemispheric differences

in RMT. MEP intensity curves were constructed from the

single pulse data of the paired-pulse experiments. For each

subject and ISI, MEP amplitude was normalised to the

largest MEP of M1-D, which was assigned a value of 1

(Fig. 1). MEP intensity curves were analysed by a two-way

ANOVA with the within-subject factors of hemisphere

(M1-D vs. M1-ND) and stimulus intensity (9 levels, 60%

RMT – 140% RMT).

For analysis of the paired-pulse data, conditional

averages of the single trial peak-to-peak MEP amplitudes

were calculated, and the interaction between S1 and S2 was

expressed as the ratio of the MEP amplitude elicited by

paired TMS (MEPS1þS2) over the arithmetic sum of the

MEP amplitudes produced by the single pulses (MEPS1 þ

MEPS2) (Ilic et al., 2002):

Interaction ¼ MEPS1þS2=ðMEPS1 þ MEPS2Þ p 100%:

Fig. 1. MEP intensity curves in RH and LH subjects in M1-D and M1-ND. MEP amplitude was normalised for each subject and inter-stimulus interval to the

maximum MEP in the M1-D which was assigned a value of 1. Normalised MEP amplitudes are plotted against stimulus intensity (expressed in % of resting

motor threshold, RMT). The 4 plots refer to right-handed (A, B); and left-handed subjects (C, D) and to the sessions where, for the paired-pulse experiments,

inter-stimulus intervals of 1.5 ms (A, C); and 2.1 ms (B, D) were tested. M1-D and M1-ND are indicated by black and white circles, respectively. All data are

means ^ SEM.


Those conditions where S1 and S2 were ,RMT (16/81

conditions) were discarded from analysis because both

single pulse conditions resulted in no MEP, and therefore,

no meaningful interaction could be calculated. Interactions

were plotted as contour lines separately for M1-D and M1-

ND, and separately for the ISI of 1.5 and 2.1 ms (RH

subjects: Fig. 2; LH subjects: Fig. 3). In order to run a

statistical analysis on the ‘extent’ of SICI and SICF,

conditions with non-overlapping interactions ,25, 25–50,

50–75 and 75–100% (SICI), or 100–125, 125–150,

150–300, 300–600% and .600% (SICF) were counted

(Fig. 4). Furthermore, in order to evaluate the ‘magnitude’

of SICI and SICF, an average of the 10 lowest SICI values

(SICI-10) and the 10 highest SICF values (SICF-10) was

calculated from each subject (Fig. 5). Finally, from each

experiment those 3 intensity conditions were selected which

Fig. 2. Short-interval paired-pulse inhibition and facilitation in M1-D and M1-ND as a function of stimulus intensity and inter-stimulus interval in the resting

ADM of RH subjects. (A–D) Refer to inter-stimulus intervals (ISI) of 1.5 ms (A, B); and 2.1 ms (C, D). Stimulus intensity of the first stimulus (S1, x-axis), and

the second stimulus (S2, y-axis) are plotted relative to the ADM RMT. As there were 9 different stimulus intensities for S1 and S2, each diagram consists of 81

conditions. Those 16/81 conditions with S1 and S2 ,RMT were discarded from analysis because both single pulses did not elicit a MEP, and therefore no

meaningful interaction between S1 and S2 could be calculated. For each of the other conditions, the interaction between S1 and S2 is expressed as

MEPS1þS2/(MEPS1 þ MEPS2) p 100%. Data are grand averages from 9 RH subjects and shown as contour plots. The thick line in each diagram indicates no

interaction (100%), dashed lines show inhibitory (,100%) and thin solid lines facilitatory (.100%) interactions. The numbers on the contour lines provide the

interaction values (in %). Note that, at the ISI of 2.1 ms, the area of inhibition (inhibitory interaction ,50% coloured in red) was less extensive in M1-D than

M1-ND.


best approximated the conditions in the classical paired-

pulse TMS protocols (Kujirai et al., 1993; Tokimura et al.,

1996; Ziemann et al., 1998) (cf. Table 1). This was done

to appreciate the superior sensitivity of the novel paired-

pulse protocol (Ilic et al., 2002) in detecting subtle

hemispheric differences in paired-pulse excitability. Paired

t tests corrected for multiple comparisons were applied to

test for hemispheric differences (M1-D vs. M1-ND) of SICI

and SICF extent and magnitude, and for SICI and SICF

according to the classical paired-pulse TMS protocols.

For all comparisons, a significance level of P , 0:05 was

adopted. Descriptive statistics will always be reported as

mean ^ 1 standard error of the mean (SEM).

3. Results

3.1. Main experiment (hemispheric asymmetry of SICI and

SICF)

In RH subjects, RMT was lower in M1-D than M1-ND

(P , 0:05). This difference was consistent for both ISI

(Table 1). No hemispheric difference in RMT was observed

in LH subjects (Table 1).

In RH and LH subjects, MEP intensity curves were not

different between hemispheres (Fig. 1; RH subjects, ISI 1.5

ms: Fð1; 8Þ ¼ 0:87, P ¼ 0:78; RH subjects, ISI 2.1 ms:

Fð1; 8Þ ¼ 0:74, P ¼ 0:41; LH subjects, ISI 1.5 ms:

Fig. 3. Short-interval paired-pulse inhibition and facilitation in M1-D and M1-ND as a function of stimulus intensity and inter-stimulus interval in the resting

ADM of LH subjects. Data are grand averages from 8 LH subjects. Otherwise, same arrangement and conventions as in Fig. 2.


Fig. 4. ‘Extent’ of SICI and SICF. Data are means ^ SEM from 9 RH subjects (A, B); and 8 LH subjects (C, D) at ISI of 1.5 ms (A, C); and 2.1 ms (B, D).

Frequency plots (given as percentage of the analysed 65/81 conditions, y-axis) show conditions at discrete, non-overlapping interaction levels of

MEPS1þS2/(MEPS1 þ MEPS2) £ 100% as indicated on the x-axis. Filled and open columns refer to M1-D and M1-ND, respectively. Note that, in RH subjects

and the ISI of 2.1 ms, SICI conditions ,25% were less frequent in M1-D than M1-ND (*P ¼ 0:0019).

Fig. 5. ‘Magnitude’ of SICI and SICF. Data are means ^ SEM from the 10 lowest values of inhibitory interactions (A, SICI-10) and the 10 highest values of

facilitatory interactions (B, SICF-10) in the resting ADM from 9 RH subjects and 8 LH subjects. Filled and open columns refer to M1-D and M1-ND,

respectively. Significant differences between hemispheres are indicated (paired t tests). Note that, in RH subjects at the ISI of 2.1 ms, the magnitude of

inhibition was less (i.e. higher SICI-10 values) in M1-D than M1-ND.


Fð1; 6Þ ¼ 1:89, P ¼ 0:22; LH subjects, ISI 2.1 ms:

Fð1; 5Þ ¼ 5:62, P ¼ 0:06). In addition, none of the inter-

actions of hemisphere with stimulus intensity was signifi-

cant. These are important negative results because

single-pulse MEP amplitude may affect the interaction

between S1 and S2 (Kujirai et al., 1993; Ziemann et al.,

1996c; Ilic et al., 2002).

The main results are displayed in Figs. 2–5. The

interaction plots between S1 and S2 (Figs. 2 and 3) suggest

that, in RH subjects and at the ISI of 2.1 ms, the number of

conditions with inhibitory interactions is lower in M1-D

than M1-ND (Figs. 2C,D). No such difference was observed

at the ISI of 1.5 ms (Figs. 2A,B). LH subjects showed no

conspicuous difference at either ISI (Fig. 3). Statistical

analysis confirmed that, in RH subjects and at the ISI of 2.1

ms, the extent of SICI was less in M1-D than M1-ND for

SICI ,25% (Fig. 4). No significant differences occurred in

RH subjects at the ISI of 1.5 ms, or in LH subjects at either

interval (Fig. 4).

The magnitude of inhibitory interaction (SICI-10) in RH

subjects was significantly less in M1-D than M1-ND for the

ISI of 2.1 ms but not at the ISI of 1.5 ms (Fig. 5A) while

there was no such hemispheric difference in LH subjects at

either interval (Fig. 5A). SICF-10 showed a trend towards

higher values in the M1-D of LH subjects at the ISI of 1.5 ms

but this difference was not statistically significant (Fig. 5B,

P ¼ 0:06).

Finally, SICI according to the classical paired-pulse

protocol (Kujirai et al., 1993) in RH and LH subjects

showed a trend towards less inhibition in M1-D than

M1-ND, but this did not reach statistical significance

(Table 1). SICF (Tokimura et al., 1996; Ziemann et al.,

1998) in RH and LH subjects was also not significantly

different between hemispheres (Table 1).

3.2. Control experiment (hemispheric asymmetry of SICI

threshold)

The control experiment replicated the finding of the main

experiment that, in RH subjects, RMT was significantly

lower in M1-D (39.1 ^ 1.5%) compared to M1-ND

(43.1 ^ 2.7%, P ¼ 0:042) (Fig. 6). In addition, SICI

threshold was also significantly lower in M1-D

(22.3 ^ 1.5%) than M1-ND (25.6 ^ 1.4%, P ¼ 0:0045)

(Fig. 6). MEP size elicited by S2 alone did not differ

between hemispheres (M1-D: 1.19 ^ 0.16 mV; M1-ND:

1.00 ^ 0.22 mV). Finally, the ratio of SICI threshold over

RMT was also not different between hemispheres (M1-D:

0.57 ^ 0.03; M1-ND: 0.60 ^ 0.04, P ¼ 0:26).

These findings indicate that the lower stimulus intensities

used for M1-D when testing the paired-pulse measures in

the main experiment cannot account for the observed

hemispheric difference in SICI extent and magnitude (see

above, Figs. 2C,D, 4B and 5A) because of similar

hemispheric differences in RMT and SICI threshold.

4. Discussion

The main novel finding is significantly less SICI in the

M1-D compared to the M1-ND of RH subjects. In addition,

this study confirms that, in RH subjects, RMT is lower in

M1-D compared to M1-ND (Macdonell et al., 1991; Triggs

et al., 1994, 1997) although this hemispheric differences

was not unanimously found in previous reports (Cicinelli

et al., 1997; Mills and Nithi, 1997; Semmler and Nordstrom,

1998; Cicinelli et al., 2000; Civardi et al., 2000; Ziemann

and Hallett, 2001).

4.1. Physiological interpretation

The cortical origin of SICI was proven by epidural

recordings of the descending cortico-spinal discharge at the

level of the cervical spinal cord which showed a reduction in

the size and number of I-waves when a sub-threshold

conditioning pulse preceded the supra-threshold test pulse by

1–5 ms (Nakamura et al., 1997; Di Lazzaro et al., 1998).

Application of agonists at the GABAA receptor, such as

benzodiazepines, leads to an increase of SICI (Ziemann et al.,

1996a; Di Lazzaro et al., 2000; Ilic et al., 2002). Therefore,

convergent evidence points to the idea that SICI measures

inhibition at the level of the human motor cortex which is

influenced by activation through the GABAA receptor.

To the best of our knowledge, only 3 previous studies

tested for hemispheric differences in SICI (Cicinelli et al.,

2000; Civardi et al., 2000; Maeda et al., 2002). Contrasting

with the present report, none of those studies found

Fig. 6. Relationship between resting motor threshold (RMT) and short-

interval intracortical inhibition threshold (SICI threshold). RMT (y-axis)

and SICI threshold (inter-stimulus interval of 2.1 ms, x-axis) of M1-D and

M1-ND of 8 RH subjects are expressed as percentage of the maximum

stimulator output (% MSO). Grey circles with error bars indicate

means ^ SEM in M1-D (dark grey) and M1-ND (pale grey). SICI

threshold was positively correlated with RMT (Pearson’s r ¼ 0:581,

P , 0:02). RMT was significantly lower in M1-D than M1-ND (P , 0:05).

SICI threshold was also significantly lower in M1-D than M1-ND

(P , 0:01). Thus, the lower inhibitory tone in M1-D than M1-ND observed

in the main experiment (cf. Figs. 2C,D, 4B and 5A) was not an artefact due

to lower stimulus intensities used for testing of M1-D.


a significant hemispheric asymmetry. Several reasons may

account for this disagreement. Most importantly, all previous

studies measured SICI according to the classical Kujirai

protocol which uses a conditioning pulse of 80% RMT

intensity. This is in the optimal range of intensities for

producing maximum SICI (Kujirai et al., 1993; Ziemann

et al., 1996c). Therefore, small SICI differences may escape

detection with this method due to saturation or ‘floor’ effects.

In contrast, a range of non-optimal intensities of the

conditioning pulse was used in the present protocol. This

revealed, in RH subjects, significantly less SICI in the M1-D

than M1-ND, while testing of SICI according to the Kujirai

protocol did not show a significant difference (Table 1).

Another point is that the degree of handedness of the subjects

was less well defined in two of the previous studies (Cicinelli

et al., 2000; Maeda et al., 2002). In fact, RH and LH subjects

were mixed in one study (Maeda et al., 2002). Finally, it is

now clear that both inhibitory and excitatory interactions can

be obtained at short ISI of 1–5 ms. The net effect depends on

the exact ISI and the intensities of the first and second pulse

(Kujirai et al., 1993; Tokimura et al., 1996; Ziemann et al.,

1996c, 1998; Awiszus et al., 1999; Fisher et al., 2002; Ilic

et al., 2002). The present study showed that the hemispheric

difference in SICI in RH subjects occurred at the ISI of 2.1 ms

but not at 1.5 ms. An ISI of around 2 ms was not tested in two

of the previous studies (Cicinelli et al., 2000; Maeda et al.,

2002) which may be another reason why those authors did not

find a significant hemispheric difference in SICI. Why the

hemispheric asymmetry of SICI in the present study was

observed only at the ISI of 2.1 ms is not entirely clear but one

may argue that relatively pure SICI is being tested at that

interval without much contamination by SICF (Tokimura

et al., 1996; Ziemann et al., 1998; Fisher et al., 2002; Ilic et al.,

2002).

SICI differences may be explained by differences in the

amplitude of the MEP elicited by S2 (Stefan et al., 2002),

and by stimulus intensity (Ilic et al., 2002). These effects

were carefully controlled for in the present experiments.

First, the MEP intensity curves were not different (cf.

Fig. 1B). Second, slightly lower absolute stimulus inten-

sities were used for testing of M1-D compared to M1-ND

because stimulus intensity was related to the slightly

asymmetric RMT. However, this did not contribute to the

observed hemispheric difference in SICI because the SICI

threshold was, similarly to RMT, significantly lower in

M1-D than M1-ND (see Section 2.3, Fig. 6). Therefore, it is

very likely that the observed hemispheric difference in SICI

of RH subjects is a true biological finding.

Our failure to detect a significant hemispheric difference

in any measure of motor excitability in LH subjects is most

likely explained by the fact that left-handedness is on

average associated with a less pronounced and more

variable cortical asymmetry of both structure and function

compared to right-handedness (for review, LeMay, 1992).

In the motor system, further evidence was provided that

right-hemispheric motor dominance in LH subjects is less

consistent than left-hemispheric motor dominance in RH

subjects (Netz et al., 1995). In addition, the lower mean

laterality index in our LH compared to the RH subjects

possibly impeded a detection of significant side differences

in intracortical motor inhibition. It was shown previously

that a lower degree of asymmetry of hand preference was no

longer associated with a hemispheric difference in motor

excitability (Triggs et al., 1994, 1997).

4.2. Functional significance

GABA is the major inhibitory neurotransmitter in the

human cerebral cortex (McCormick, 1989). GABAergic

neurons constitute approximately 25–30% of the neuronal

population (Jones, 1993). The role of GABA in motor control

was assessed by iontophoretic injection of the GABAA

receptor agonist muscimol or the GABAA receptor antago-

nist bicuculline into monkey M1. Muscimol resulted in a

severe disturbance of manual dexterity such as the ability to

pick up small objects (Matsumura et al., 1991; Brochier et al.,

1999), individuated finger movements (Schieber and Polia-

kov, 1998), prolonged reaction and movement times

(Matsumura et al., 1991; Kubota, 1996) and even a paretic

hand (Kubota, 1996). These deficits were associated with a

decrease in movement-related activity of neurones in M1

(Matsumura et al., 1992). Low cortical GABA may also lead

to uncontrolled motor activity in the form of seizures (for

review, (Olsen and Avoli, 1997). Application of bicuculline

also produced a severe disruption of manual dexterity in a

small object pick-up task, and in addition resulted in co-

contraction of muscles which are normally not activated

during a particular movement (Matsumura et al., 1991).

These abnormalities were associated with an increase in

movement-related activity of neurones in M1, a decrease of

direction selectivity of movement-related activity and the

movement-related recruitment of neurones which were not

activated prior to bicuculline injection (Matsumura et al.,

1992). The muscular co-contractions under bicuculline are

reminiscent of those in dystonia, a neurological disorder

associated with gross deficits in SICI (Ridding et al., 1995a;

Siebner et al., 1999; Sommer et al., 2002) and a reduced

content of GABA in the sensori-motor cortex (Levy and

Hallett, 2002). These findings demonstrate that the amount of

GABA related inhibition in motor cortex is extremely

important for the fine tuning of movement-related neuronal

activity which in turn is essential for the level of dexterity of

complex or individuated finger movements (Keller, 1993).

Therefore, it appears that there exists an optimal level of

GABAergic inhibitory tone to permit optimal motor

performance. Too much inhibitory tone blocks required

neuronal activity while too low inhibitory tone may result in

dystonic or seizure activity. We showed here for the first time

that, in RH subjects, the M1-D displays less inhibitory tone

than the M1-ND. However, this difference was rather subtle

so that it is likely that the inhibitory tone in both M1 operates

closely to the proposed optimum for motor performance.


Slightly less SICI in M1-D than M1-ND may provide some

advantage for the readiness and ease to carry out movements

with the dominant compared to the non-dominant hand

because a reduction of SICI occurs before and during

voluntary contraction of hand and forearm muscles (Ridding

et al., 1995b; Reynolds and Ashby, 1999; Zoghi et al., 2003).

Another important role of GABA related inhibition in

motor cortex is its regulatory function on plasticity (Jones,

1993; Keller, 1993). Synaptic plasticity such as long-term

potentiation can be induced clearly less reliably in motor

cortex than in other areas of the cerebral neocortex

(Castro-Alamancos et al., 1995). Therefore, in animal

preparations, it was often necessary to reduce the GABA

related cortical inhibition in order to permit induction of

long-term potentiation (Hess and Donoghue, 1994; Hess

et al., 1996). GABA related inhibition is reduced in the area

surrounding a photothrombotic lesion of sensori-motor

cortex in rats (Domann et al., 1993) and long-term

potentiation can be induced more readily in this disinhibited

area compared to control cortex (Hagemann et al., 1998).

Recovery of sensori-motor function after cortical lesion in

rats is significantly delayed or even permanently disrupted

by administration of the GABAA receptor agonist diazepam

(Schallert et al., 1986; Hernandez and Schallert, 1990).

Similarly, there exists anecdotal knowledge that the intake

of GABAergic drugs in patients after stroke is detrimental

for functional outcome (Goldstein, 1998). Finally, use-

dependent plasticity in healthy subjects can be blocked by

the GABAA receptor agonist lorazepam (Tegenthoff et al.,

1999; Butefisch et al., 2000) but enhanced by disinhibition

(Ziemann et al., 2001). These data strongly suggest that an

increase in GABA related inhibition has a disruptive effect

on plasticity in motor cortex. In this context, the subtle

hemispheric asymmetry in inhibitory tone demonstrated in

this study may put a small advantage to the M1-D in terms

of its readiness to undergo functionally relevant plasticity.

This contention can be tested in future experiments. It

should be kept in mind, however, that use-dependent

plasticity may rely on a more complex multitude of

excitatory and inhibitory cortical processes that goes

beyond of what is being tested by SICI.

Acknowledgements

Dr Ilic was a research fellow of the Alexander von

Humboldt Foundation.

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Subtle hemispheric asymmetry of motor cortical inhibitory tone