Age-related decrease in paired-pulse intracortical inhibition in the
human primary motor cortex
Alexander Peinemann
a,
*
, Christian Lehner
a
,
Bastian Conrad
a
, Hartwig Roman Siebner
a,b
a
Department of Neurology, Technische Universita¨tMu¨nchen, Mo¨hlstrasse 28, D-81675Munich, Germany
b
Sobell Department of Neurophysiology, Institute of Neurology, University College of London, Queen Square, London WC1N 3BG, UK
Received 8 June 2001; received in revised form 22 August 2001; accepted 23 August 2001
Abstract
Using biphasic magnetic stimuli, paired-pulse transcranial magnetic stimulation (TMS) at short interstimulus intervals
(ISIs) was employed to investigate age-related changes in the balance between intracortical inhibition and facilitation. In
26 right-handed healthy individuals, motor evoked potentials were recorded from the relaxed right first dorsal inter-
osseus muscle after paired-pulse TMS of the left primary motor hand area. The magnitude of intracortical paired-pulse
inhibition at ISIs of 1–5 ms was markedly reduced in elderly individuals, whereas no age effect was observed for
intracortical paired-pulse facilitation at ISIs of 11–15 ms. This finding demonstrates that normal aging is associated
with a relative decrease in the excitability of intracortical inhibitory circuits. In conclusion, paired-pulse TMS provides a
non-invasive means of studying age-related functional changes in the motor cortex.q 2001 Elsevier Science Ireland Ltd.
All rights reserved.
Keywords: Aging; Excitability; Motor cortex; Intracortical facilitation; Intracortical inhibition; Motor cortex; Paired-pulse; Transcranial
magnetic stimulation
In healthy adults, the size of the electromyographic
response elicited by suprathreshold transcranial magnetic
stimulation (TMS) over the primary motor cortex (M1) is
consistently inhibited, if a subthreshold transcranial
magnetic stimulus is given 1–5 ms beforehand [4,13]. A
facilitatory effect of the subthreshold conditioning stimulus
(CS) on the motor evoked potential (MEP) prevails at inter-
stimulus intervals (ISIs) of 8–15 ms [4,13]. Although the
exact neuronal mechanisms leading to these inhibitory and
facilitatory phenomena are not known, there is convincing
evidence that the modulatory effects of the subthreshold CS
are mediated by separate populations of inhibitory and exci-
tatory intracortical interneurons, which control motor cortex
output to the spinal cord [1,6,14]. The relative magnitude of
intracortical paired-pulse inhibition (PPI) and facilitation
(PPF) at a given ISI is expressed by the ratio between the
size of the conditioned MEP and the unconditioned MEP
[4,13]. By plotting the normalized size of the conditioned
Electromyogram (EMG) response against the ISI, one can
generate an intracortical excitability curve, which reflects the
balance between intracortical PPI and PPF [4,13,14].
Since its initial description byKujirai et al. [4], this paired-
pulse paradigm has been extensively used as a means of
monitoring the excitability of intracortical facilitatory and
inhibitory circuitry of the M1 under different experimental
conditions [8,11,12,14]. For instance, it has been shown that
voluntary contraction is associated with a significant
decrease in the magnitude of PPI and PPF at short intervals
in healthy subjects [11] and that subthreshold 5-Hz repetitive
TMS of the primary sensorimotor cortex reduces PPI [8].
Moreover, several studies have demonstrated a decrease in
intracortical PPI at short ISIs in a variety of neurological
disorders, including Parkinson’s disease, focal dystonia and
amyotrophic lateral sclerosis [9,10,12,15].
In healthy subjects, intracortical PPI and PPF has mainly
been studied in young volunteers. Therefore, little is known
about the impact of normal aging on the balance between
intracortical inhibition and facilitation. Employing the
paired-pulse TMS paradigm as described by Kujirai et al.
[4], the present study investigated age-related changes in
intracortical PPI and PPF in healthy subjects.
Neuroscience Letters 313 (2001) 33–36
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved.
PII: S0304-3940(01)02239-X
www.elsevier.com/locate/neulet
* Corresponding author. Tel.: 149-89-4140-4628/4607; fax:
149-89-4140-4867.
E-mail address: alexander.peinemann@neuro.med.tu-mue
nchen.de (A. Peinemann).

Thirteen young healthy volunteers (eight men and five
women) with a mean age of 28 years (range: 20–38 years)
and thirteen elderly healthy volunteers (five men and eight
women) with a mean age of 51 years (range: 48–71 years)
participated in the study. All subjects were consistent right-
handers according to the Edinburgh Handedness Inventory
[7]. Written informed consent was obtained from all
subjects. Experimental procedures were approved by the
Ethics committee of the medical faculty of the Technische
Universita¨tMu¨nchen.
TMS was delivered over the left primary motor hand area
through a tangentially oriented eight-shaped coil (MC-B70,
Medtronic-neuromuscular, Skovlunde, Denmark). The
transducing coil was placed over the optimal site for eliciting
MEPs in the relaxed right first dorsal interosseus (FDI)
muscle. The coil was then fixed to a custom-built holder
and constant coil position was continuously monitored
during the entire experiment. The magnetic stimulus had a
biphasic waveform, a pulse width of 200ms and a rise time of
50 ms. During the first phase of the stimulus, the current
flowed in a posterior-to-anterior direction through the stimu-
lated cortex.
The intensity of resting motor threshold (rMT) was
defined as the lowest TMS intensity that evoked MEPs of
at least 50 mV peak-to-peak amplitude in five of ten succes-
sive trials.
PPI and PPF was assessed using a Dantec Maglite with
Twintop option (Medtronic-neuromuscular, Skovlunde,
Denmark). The stimulation intensity of the CS was set just
below active motor threshold, corresponding to approxi-
mately 75% of rMT. The stimulation intensity of the second
stimulus, which is referred to as the test stimulus (TS), was
adjusted to elicit MEPs of approximately 1–1.5 mV peak-to-
peak amplitudewhen given alone. The conditioning effects at
short ISIs of 1, 3, and 5ms were used to assess the magnitude
of PPI, whereas the conditioning effect at longer ISIs of 11,
13, and 15ms was taken as ameasure of PPF. At each ISI, we
recorded two blocks of five consecutive MEPs. In addition,
we sampled six blocks offive consecutiveMEPs, whichwere
elicited by the TS alone. The assignment of the blocks with
different ISI was pseudorandomized and counterbalanced
across subjects. Throughout the experiment the targetmuscle
was completely relaxed. Absence of background EMGactiv-
ity was monitored by audiovisual feedback.
Transcranially evoked MEPs were recorded from the
relaxed right FDI muscle, using pairs of Ag-AgCl surface
electrodes. EMG signals were amplified in the bandwidth of
20 Hz–2 kHz and digitized at 5 kHz (CED 1401 Laboratory
Interface, Cambridge Electronic Design, UK). Measure-
ment of the area was made on individual MEPs (SignalA-
verager software, Cambridge Electronic Design, UK). In
order to detect the relative amount of PPI and PPF, the
mean area of the conditioned responses, at each ISI, was
normalized to the mean area of the unconditioned test
responses (i.e. expressed as a percentage of the mean area
of the unconditioned response alone) [4,13]. The normal-
ized MEP amplitudes were plotted against the ISIs in order
to generate an intracortical excitability curve [4,13].
A two-factorial analysis of variance (ANOVA) with the
factors ‘GROUP’ and ‘ISI’ was employed to test for differ-
ences in PPI and PPF between age groups. ANOVA was
performed separately for short ‘inhibitory’ ISIs of 1–5ms
and longer ‘facilitatory’ ISIs of 11–15 ms. Conditional on a
significant F-value, post-hoc Student’s t-test for indepen-
dent samples was performed for each ISI. A two-tailed
Student’s t-test for independent samples was used to
compare the mean rMT and the mean MEP area evoked
by the unconditioned test response between both age groups.
Significance level was set at a P-value , 0:05. Data are
given as mean and one-fold SD.
For PPI at short ISIs, ANOVA revealed a main effect for
GROUP (F ¼ 11:0; P ¼ 0:001). There was neither a signif-
icant main effect of ISI nor a significant interaction term
between GROUP and ISI. The main effect for GROUP was
due to a marked decrease in PPI in elderly patients (Fig. 1).
Post-hoc Student’s t-test demonstrated that the age-related
decrease in PPI was evenly present at all ‘inhibitory’ ISIs.
Therefore, we calculated the mean PPI at short inhibitory
ISIs (1–5 ms) in order to quantify the magnitude of PPI in
each subject. The magnitude of mean PPI at ISIs of 1–5ms
demonstrated a considerable inter-individual variability in
both age groups (Fig. 2). When relating mean PPI and age,
linear regression analysis confirmed an age-related decrease
in PPI (r
2
¼ 0:234, P ¼ 0:012, Fig. 2).
For PPF, ANOVA revealed neither a significant main
effect for the two factors nor a significant interaction term
A. Peinemann et al. / Neuroscience Letters 313 (2001) 33–3634
Fig. 1. Intracortical excitability curves for the group of young
healthy adults (opened circles) and elderly healthy adults (closed
circles). The size of the conditioned MEP is expressed as a
percentage of the unconditioned MEP (ordinate) and plotted
against the interstimulus interval (abscissa). Vertical bars are
SEM. The horizontal line is the 100% level (i.e. mean size of the
unconditioned MEP). Note that there is a substantially stronger
PPI at short ISIs (1–5 ms) in young participants.