Exp Brain Res (2006) 173: 115–129
DOI 10.1007/s00221-006-0371-4
R ES E AR C H A RT I C L E
Rolf Verleger Æ Thomas Kötter Æ Piotr Jaśkowski
Andreas Sprenger Æ Hartwig Siebner
A TMS study on non-consciously triggered response tendencies
in the motor cortex
Received: 7 December 2005 / Accepted: 15 January 2006 / Published online: 28 February 2006
Springer-Verlag 2006
Abstract Non-consciously perceived arrow stimuli can
speed up responses to similar stimuli that are shortly
presented after a masked prime. Yet response facilitation may turn into a delay at particular intervals between masked primes and targets. In this case, the
lateralized readiness potential, as a measure of the time
course of differential activation between the primed and
the unprimed motor cortices, consistently yielded two
consecutive maxima of opposite polarity, at 250 and at
350 ms after prime onset. To further explore the
mechanisms underlying inverse priming, we used singlepulse transcranial magnetic stimulation (TMS) of the
left or right primary motor hand area (M1). Lateralized
changes in corticomotor excitability induced by the
masked prime were probed by assessing the effect of
priming on the amplitude of the TMS-induced motorevoked potentials (MEPs). In two experiments, MEPs
increased and decreased, respectively, in the hand
primed by the masked arrows when TMS was given at
250 and at 350 ms after prime onset, confirming the
expectation that MEP changes may indicate the response tendencies induced by the masked primes. Both
effects were more distinct with TMS of the left M1.
However, there were also some differences between the
patterns of results in the two experiments. We propose
that the left M1 is activated for preparation of both
right- and left-hand movements, and we relate the
R. Verleger (&) Æ T. Kötter Æ P. Jaśkowski Æ A. Sprenger
Department of Neurology,, University of Lübeck,
23538 , Lübeck, Germany
E-mail: [email protected]
Tel.: +49-451-5002916
Fax: +49-451-5002489
P. Jaśkowski
Department of Cognitive Psychology,
University of Finance and Management,
Warsaw, Poland
H. Siebner
Department of Neurology, Christian-Albrechts University,
Kiel, Germany
present results to current hypotheses about the nature
of inverse priming.
Keywords Non-conscious processing Æ Response
tendencies Æ Masking Æ TMS Æ LRP
Introduction
Non-consciously perceived stimuli can prime our perception and actions (Öğmen and Breitmeyer 2006).
Usually, non-consciously perceived stimuli facilitate responses to similar stimuli following the prime (Cheesman and Merikle 1984; Enns and Di Lollo 2000;
Jaśkowski et al. 2003; Klotz and Neumann 1999; Vorberg et al. 2003). An interesting new line of research was
opened by the findings of Eimer and Schlaghecken
(1998) showing that masked stimuli can have inverse
effects, delaying responses to similar stimuli. Rightward
and leftward arrows were used as imperative stimuli,
requiring corresponding right and left key-press responses. These arrow targets were preceded by masked
arrows, with a stimulus-onset asynchrony of 100 ms or
more. Stimuli are schematically depicted in Fig. 1. When
the masked arrows pointed to the same direction as the
target arrows, responses to targets were delayed,
whereas a facilitating effect was found when primes
pointed to the direction opposite to the visible targets.
Inverse priming with arrow stimuli proved reliable and
replicable (Eimer 1999; Eimer and Schlaghecken 2002;
Eimer et al. 2002; Klapp and Hinkley 2002; Lingnau and
Vorberg 2005; Lleras and Enns 2004; Praamstra and
Seiss 2005; Schlaghecken and Eimer 1997, 2000, 2001,
2002; Schlaghecken et al. 2003; Verleger et al. 2004).
Inverse priming is influenced by the design of the
mask (Jaśkowski and Przekoracka-Krawczyk 2005;
Lleras and Enns 2004; Verleger et al. 2004, 2005; Verleger et al., submitted for publication) and by the type of
prime and target stimuli (Verleger et al. 2005; Verleger
et al., submitted for publication) such that largest inverse priming was obtained when arrow primes were
116
Fig. 1 Stimuli used in the present experiments. Double arrows are
used as primes, masks, and targets, taken from Eimer and
Schlaghecken (1998). Targets may point to the right (here used as
example) or left, requiring a right or left key-press, respectively.
Primes are either identical to the target, neutral, or opposite to the
target. Primes are masked by a compound of leftwards and
rightwards arrows
masked by a double-arrows mask (as depicted in Fig. 1).
Further, inverse priming depends on the onset interval
between mask and target, with positive priming at very
brief intervals turning to inverse priming at intervals of
100 ms or more (Lingnau and Vorberg 2005; Mattler
2005; Schlaghecken and Eimer 2002; Seiss and Praamstra 2004; Verleger et al., submitted for publication).
Eimer and Schlaghecken (1998) recorded the lateralized readiness potential (LRP) during inverse priming to
study the neuronal underpinnings of this effect. The
LRP is the difference of EEG voltage between the scalp
sites over the two motor cortices, contra- and ipsilateral
to arrow direction. Thus, in tasks requiring a response
choice between the two hands, the LRP provides a
measure of response selection at any given moment between stimulus and response. Eimer and Schlaghecken
(1998) found that the LRP displays three distinct peaks
when primes and targets pointed to the same direction:
A first peak at about 250 ms after onset of the masked
arrow-prime, interpreted as response activation by that
prime; a second, opposite peak at 350 ms, interpreted by
Eimer and Schlaghecken (1998) as inhibition of this
activation, and a third peak at the time of overt response
to the target (about 500 ms after prime onset) reflecting
the execution of the response. These LRP results proved
replicable (Eimer 1999; Eimer and Schlaghecken 2003;
Praamstra and Seiss 2005; Verleger et al. 2004; Verleger
et al., submitted for publication). Critical to the interpretation of the inverse priming effect in terms of
underlying processes is the second LRP peak. Lleras and
Enns (2004) and Verleger et al. (2004) proposed that this
opposite peak of the LRP might reflect activation of the
opposite response rather than inhibition of the primed
response, and ascribed inverse priming to interactions of
mask elements with the prime. They argued that the
prime-mask interaction produces a second prime that
points to the opposite direction and thereby primes the
opposite response.
The question whether the LRP peak at 350 ms reflects inhibition of one motor cortex (henceforth called
‘‘inhibition hypothesis’’) or activation of the opposite
cortex (henceforth ‘‘opposite-activation hypothesis’’)
would be easily resolved if the LRP could be separated
into its contra- and ipsilateral constituents which might
then be separately compared to sequences with neutral
primes as control condition. However, this proved difficult with this sequence of stimuli, as any direct comparison between response-related potentials evoked by
arrow primes and neutral primes was confounded by
differences in stimulus-evoked potentials between sequences of arrow primes and targets versus sequences of
neutral primes and targets (Verleger et al. 2004). Of
note, this problem does not apply for the LRP because
these differences are readily cancelled by the LRP subtraction method (Coles 1989). Circumventing this
problem, Praamstra and Seiss (2005) computed current
source densities, i.e., they compared the contra- and
ipsilateral constituents of the LRP to the voltages recorded from the immediate topographical surroundings
of either recording. The obtained values were mirror
symmetric for contra- and ipsilateral sites, leading these
authors to conclude that all three LRP peaks reflect both
processes: simultaneous activation of one side and
inhibition of the other side (cf. Vidal et al. 2003;
Yordanova et al. 2004, for similar current-source-density results obtained in unprimed choice-response tasks).
To further explore the physiology that underlies the
inverse priming effect, we gave single-pulse transcranial
magnetic stimulation (TMS) to the primary motor hand
area (M1) at 250 and 350 ms after prime onset. When
given to the M1 at suprathreshold intensities, TMS is
able to evoke a muscle twitch in the contralateral hand.
The size of the motor response can be measured by
recording the surface electromyogram (EMG) from
hand muscles, referred to as ‘‘motor-evoked potential’’
(MEP). It has been shown that the MEP amplitude increases or decreases depending on the actual level of
cortical excitability in the M1 (Leocani et al. 2000;
Siebner and Rothwell 2003). Because the mean amplitude of the MEP provides a measure of the excitability
of the corticospinal motor output at the time of stimulation, suprathreshold TMS provides a sensitive means
of probing the temporal dynamics of activation in the
stimulated M1.
Our approach was inspired by three preceding studies. Stürmer et al. (2000) studied the Simon effect by
probing M1 activation by single-pulse TMS at 200 ms
after stimulus onset. The Simon effect denotes the finding that left–right choice-responses to any stimulus feature are speeded if the target stimulus is presented on the
side on which the response is made, and are delayed in
the other case (Simon 1990). The timing of TMS was
selected on the basis of previous LRP results (e.g.,
Wascher and Wauschkuhn 1996; Stürmer et al. 2002)
and indeed proved sensitive in distinguishing between
conditions in that study. Recently, Théoret et al. (2004)
applied the same rationale to masked priming, using
TMS pulses as a probe to test whether masked numbers
affect decisions made in response to subsequent visible
numbers (<5 vs. >5; cf. Dehaene et al. 1998) by directly
activating the M1. TMS impulses given at 360 ms after
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prime onset produced larger MEPs after congruent than
after incongruent primes, but the data did not allow for
a clear separation between prime-induced effects and
response-related effects. Finally, Schlaghecken et al.
(2003) were the first to use the present paradigm in
combination with TMS. But different from the approach
of using TMS as a probe, these authors applied repetitive TMS before the task to induce a lasting change in
the state of the neuronal processes involved in response
selection. However, only a general delay of response
times was obtained whereas the effect of primes was not
changed.
The main purpose of the present experiments was to
test whether MEP measurements would provide a sensitive measure of prime-induced changes in M1 excitability in this task, and to examine whether TMS to the
left or right M1 would interfere with task performance.
The timing of TMS was selected on the basis of the time
course of the LRP. In experiment 1, MEPs evoked at
three different time points after prime onset were compared to each other (100, 250, 350 ms), and behavioral
effects of TMS at these time points were compared to
trials without TMS within the same block and to an
entire block without TMS. Only two TMS time points
(250 and 350 ms) were used in experiment 2, but neutral
primes were added as a basis for comparison and the
LRP was simultaneously recorded. We deliberately
chose stimuli that would maximize the magnitude of
inverse priming: arrows as primes and targets and
overlaid arrows as masks (Verleger et al., submitted for
publication). This implies that the present study cannot
easily be generalized to sequences of other stimuli and
masks that produce weaker inverse priming of response
times [e.g., arrow primes followed by random-lines
masks, as used by Praamstra and Seiss (2005), Schlaghecken and Eimer (2002), Seiss and Praamstra (2004)]
and might therefore induce other processes than the ones
investigated here.
We expected that the direction of MEP changes
(i.e., facilitation or inhibition) at 350 ms after prime
onset would shed light on the neurophysiological
mechanism underlying the inverse priming effect. The
opposite-activation hypothesis predicts that the M1
opposite to prime direction will be activated, therefore
corticospinal excitability (as indexed by the MEP
amplitude) should increase in the opposite M1.
Accordingly, for masked arrows that pointed leftwards, the sequence of prime and mask should activate
a right-hand response and thus increase the MEPs in
right-hand muscles, indicating an activation of the left
(opposite) M1. The inhibition hypothesis predicts that
the primed M1 will be actively inhibited, therefore
MEP amplitudes elicited in the hand contralateral to
the primed M1 should become smaller. For instance,
masked arrows pointing rightwards should produce a
decrease in MEP amplitudes in right-hand muscles. A
third possibility is that the prime triggers both mechanisms, activation of the opposite M1 and inhibition
of the primed M1 (referred to as activation–inhibition
hypothesis), with both mechanisms either working in
parallel or being coordinated by reciprocal interactions
(Praamstra and Seiss 2005; Schlaghecken and Eimer
2002, 2004).
Experiment 1
This experiment had three blocks: TMS to the left M1,
TMS to the right M1, or no TMS. In the two blocks with
TMS, participants received single suprathreshold pulses
to the M1 at 100, 250, or 350 ms after prime onset, or no
TMS at all. We predicted that MEP amplitudes at
250 ms would be larger in the primed hand and smaller
in the unprimed hand. If the predicted effects at 250 ms
would not be obtained, the TMS approach would
obviously not work in the intended way. However, mean
MEP amplitude elicited with TMS at 350 ms was the
critical measure, as detailed above.
Methods
Participants
Twelve students (seven men and five women) of the
University of Lübeck participated for 7€ per hour. Their
ages ranged between 21 and 29 years (mean 25 years),
they had normal or corrected-to-normal vision, and
were all right-handed, as examined by the Edinburgh
Handedness Inventory. The experiment was approved
by the ethical committee of the medical faculty of the
University of Lübeck.
Stimuli
Stimuli modeled after those used by Eimer and Schlaghecken (1998, experiment 1) were presented at the center
of a 17-in. monitor at a distance of about 1.2 m. Primes
and targets were pairs of black open triangles, symbolizing arrow-heads, pointing left (<<) or right (>>).
The mask consisted of four arrow-heads, which were the
left and right primes (or targets) overlaid on each other.
Arrow-heads were 0.75 high at the extended side, 0.7
wide, and had 0.05 distance from the center, such that
pairs of arrow-heads were 1.5 wide. The lines forming
the arrows were 0.1 wide. The thin red fixation cross
was 0.15 wide and high. Screen background was white.
The graphics card driving the screen worked at a rate of
75 Hz.
Procedure and apparatus
A standard figure-of-eight shaped coil (type P/N 9925)
and a MagStim Rapid stimulator were used for TMS of
the M1 (The Magstim Company, Whitland, Dyfed, UK;
http://www.magstim.com). Each half-wing of the coil
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measured 9.5 cm in diameter. Before the experiment
proper, participants’ left and right M1 were functionally
localized using the motor response elicited by TMS
(Siebner and Rothwell 2003). The coil was moved over
the M1 ipsilateral to the stimulated hand until stimulation yielded maximal MEPs in the relaxed contralateral
first dorsal interosseus (FDI) muscle. This coil position
will be referred to as motor hotspot. Coil orientation
was always tangential to the skull with the handle
pointing backwards and laterally at an angle of
approximately 45 in the sagittal plane. Throughout the
experiment, participants wore a bathing cap. Left and
right hotspots, including coil orientation, were marked
on the cap with a pen to ensure constant stimulation
conditions. After determining the hotspots, we evaluated
the resting motor threshold (RMT) by gradually
increasing and reducing TMS intensities. RMT was defined as the intensity to elicit MEPs of at least 50 lV in
five of ten consecutive trials in the relaxed FDI muscle.
Stimulus intensities were adjusted to 110% of individual
RMT, resulting in a mean stimulus intensity of 71% of
maximum stimulator output for TMS to the left M1
(range 52–89%) and 69% for TMS to the right M1
(range 53–89%).
For the experiment, participants were seated in a
comfortable armchair in a darkened chamber in front of
the computer screen. The TMS coil was held in a stable
support. The experiment consisted of three blocks: a
block without effective TMS, with effective TMS to the
left M1, and with effective TMS to the right M1,
respectively. Each of the six permutations of these
blocks was used with two participants. In blocks with
TMS, one of the two experimenters stayed with the
participant and continuously monitored the correct
TMS position. In the block without effective TMS, the
coil was discharged 50 cm off the participant’s head. In
most participants, an inflatable head support was put
around the neck, providing a light and comfortable restraint of the head. Participants held a computer keyboard on their lap.
Any trial started with the fixation cross, which was
replaced after 0.6 s by prime, mask, and target. Primes
were presented for one frame (13 ms), followed by the
mask (93 ms), and the target (93 ms), using Presentation
software
(http://www.neurobehavioralsystems.com). In the blocks with TMS, a single TMS pulse
could occur either at target onset (=107 ms after prime
onset) or at 250 or at 350 ms after prime onset. For
convenience, the 107-ms TMS is rounded to 100 ms
throughout this paper. Left- and right-pointing targets
required a left- and right-hand key-press, respectively.
The keys were the outer low left and right keys on the
keyboard (strg and num-pad Enter), to be pressed with
the index fingers. The next trial followed 1 s after the
response. Targets pointed left and right in random order, and so did primes, such that 50% of trials had
identical primes, and 50% had opposite primes. There
were 160 trials in the block without TMS and 432 trials
in each of the two blocks with TMS. In each TMS block,
single-pulse TMS was given at 100 ms (72 trials), 250 ms
(144 trials), or 350 ms (144 trials) after prime onset.
These trials were intermingled with 72 trials during
which no TMS was applied.
Recording and data analysis
For the key-press responses, percentages of wrong responses and mean latencies of correct responses (relative
to target onset) were calculated separately for each
condition. MEPs were recorded with Ag/AgCl surface
electrodes in a bipolar montage from the left- and rightFDI muscle. Ground electrodes were located at either
forearm over the processus styloideus radii. Voltages
were amplified within 1.5–1,500 Hz (Nihon-Kohden
Neurotop) and stored at 3,000 Hz per channel on a
computer. Data were rectified and averaged for correctly
responded trials, separately for each condition. Statistical analysis was performed by analysis of variance for
repeated measurements (SPSS 11.0, procedure GLM).
Effects obtained from repeated-measurement factors
with more than two levels were corrected by HuynhFeldt’s e.
Results and discussion
Behavior
Response times within the TMS blocks were analyzed by
ANOVA with four factors: responding hand (left, right),
stimulated hemisphere (left, right), timing of TMS (no
TMS, TMS at 100, at 250, or at 350 ms), and prime
congruency (identical, opposite to target). Inverse
priming was reliably obtained (main effect of prime
congruency: F(1,11)=9.7, P=0.01) with responses to
identical primes delayed by 31 ms on average (Fig. 2,
top panel). Thus, inverse priming still occurred when
TMS was given during the interval between target onset
and the response. The timing of TMS had an effect on
response times (F(3,33)=4.4, P=0.03) with TMS given
100 ms after prime onset (i.e., at target onset) resulting
in faster responses. Probably, the concurrent click produced by TMS acted as an unspecific coactivating
stimulus (Burle et al. 2002). Of note, response times did
not differ between the no-TMS trials and trials with
TMS at 250 and 350 ms. Furthermore, response times
did not depend on the side of the stimulated hemisphere
nor on its interaction with side of the responding hand
(P>0.10 for all effects involving this factor), suggesting
that TMS did not have a consistent impact on the process of responding. Responses with the left hand were
generally faster than responses with the right hand, by
19 ms on average (F(1,11)=23.3, P=0.001). The only
other effect was an interaction of TMS timing · prime
congruency · responding hand (F(3,33)=3.7, P=0.03)
because of a smaller effect of prime congruency for lefthand responses without TMS (Fig. 2, top panel).
119
Fig. 2 Response times and
error rates in experiment 1.
Means over the 12 participants.
Black lines are for right-hand
responses, gray lines for lefthand responses. Lines are bold
when the responding hand is
contralateral to the hemisphere
stimulated by TMS, and thin in
the other case. Note that these
definitions apply to the
correctly responding hand.
Thus, for example, the large
error value at TMS-350 is from
the black thin line, i.e., the
response would have been
correct for the right hand, not
stimulated by TMS, so an error
means that the left hand,
stimulated by TMS, pressed its
key instead
Response times in the block without TMS, depicted
on the right side of the top panel of Fig. 2, were obviously similar to the values obtained in trials without
TMS in blocks with TMS. For brevity, these data will
therefore not be considered in detail.
Error rates were submitted to ANOVA using the
same factorial design (Fig. 2, bottom panel). More errors were made when TMS was applied at 100 ms than
at any other time point (F(3,33)=4.4, P=0.02), in line
with the above interpretation of an unspecific activating
effect. Likewise corresponding to response times, identical primes led to higher error rates than opposite
primes (main effect of prime congruency: F(1,11)=6.8,
P=0.02). An exception from this regularity occurred
with left-hand responses during right-hemisphere stimulation
(responding
hand · prime
congruency:
F(1,11)=13.3,
P=0.004;
stimulated
hemisphere · responding
hand · prime
congruency:
F(1,11)=5.1, P=0.046). In this condition, errors were
not increased after identical primes. In other words,
when prime and target pointed leftwards and the right
hemisphere was stimulated the number of erroneous
right-hand responses was not increased. (Effects of
prime congruency separately by stimulated hemisphere
and responding hand: left hemisphere, left
hand: F(1,11)=3.7, P=0.08; left hemisphere, right
hand: F(1,11)=6.5, P=0.03; right hemisphere, left
hand: F(1,11)=0.0, n.s.; right hemisphere, right hand:
F(1,11)=9.4, P=0.01). Figure 2 suggests (bold gray
lines) that this effect was most marked with TMS at
100 ms (where the prime congruency effect was even
reversed) and at 350 ms, but the interaction of this effect with timing of TMS (i.e., the interaction among
stimulated hemisphere · responding hand · prime
congruency · timing of TMS) did not reach significance. Rather, ANOVA attributed parts of these effects
to the following effects that also involved the factor of
stimulated hemisphere. More errors were made in the
block in which the right hemisphere was stimulated
than in the left-hemisphere block (bold gray and thin
black lines in lower panel of Fig. 2) with TMS at
350 ms and in trials without TMS (stimulated hemisphere · TMS timing: F(3,33)=3.7, P=0.02; effects of
stimulated hemisphere separately for no TMS:
F(1,11)=5.33, P=0.04; for TMS at 100 ms:
F(1,11)=1.4, n.s.; for TMS at 250 ms: F(1,11)=0.5,
n.s.; for TMS at 350 ms: F(1,11)=5.29, P=0.04) and
with TMS at 100 ms when primes were opposite
(stimulated hemisphere · TMS timing · prime congruency: F(3,33)=3.2, P=0.04; effects of stimulated
hemisphere · prime congruency separately for no TMS:
F(1,11)=0.0, n.s.; for TMS at 100 ms: F(1,11)=6.9,
P=0.02; for TMS at 250 ms: F(1,11)=0.0, n.s.; for
TMS at 350 ms: F(1,11)=0.2, n.s.).
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Grand means of the rectified MEPs from the hand
stimulated by TMS (contralateral to the coil) are displayed in Fig. 3. Mean amplitudes of integrated epochs
30–50 ms after stimulation are listed in Table 1. The
ANOVA design differed slightly from the design used
for response times and errors. First, no MEPs could be
analyzed when there was no TMS, so the factor timing
of TMS was reduced to three levels. Second, because of
the obvious relationship of MEPs to their contralateral
TMS impulse the levels of the two factors side of
responding hand and direction of prime were redefined
from left versus right and identical versus opposite to
target, to become both contralateral versus ipsilateral to
the stimulated hemisphere.
Mean MEP amplitudes were larger at 350 ms than at
250 or 100 ms (timing of TMS: F(2,22)=17.1,
P=0.001). In addition, timing of TMS interacted with
prime side (F(2,22)=15.7, P=0.001), with side of
responding hand (F(2,22)=22.4, P<0.001), and with
prime side · side of responding hand (F(2,22)=7.7,
P=0.02). To clarify these effects, separate analyses were
performed for the three TMS time points.
No effect was significant at time point 100 ms, indicating that mean MEP amplitudes were comparable in
size across conditions.
At 250 ms, right-hand MEPs (evoked by stimulation
of the left M1) were larger when also the primes had
pointed rightwards than when they did not (bold vs. thin
lines in upper panel of Fig. 3; prime side: F(1,11)=8.3,
P=0.02;
prime
side · stimulated
hemisphere:
F(1,11)=6.4, P=0.03; prime side for stimulation of left
M1: F(1,11)=10.9, P=0.007; prime side for stimulation
of right M1: F(1,11)=1.9, n.s.). MEPs also tended to be
larger at the hand to which the targets pointed (side of
responding hand: F(1,11)=4.5, P=0.06; black vs. gray
lines in Fig. 3).
Fig. 3 Rectified motor-evoked potentials (MEPs) from the hand
contralateral to TMS in experiment 1. Grand means over the 12
participants. As symbolized by the schematic head and hands, these
MEPs were recorded from the right hand, evoked by TMS applied
to the left hemisphere in the upper panel, and from the left hand,
evoked by TMS applied to the right hemisphere in the lower panel.
Time point zero is prime onset. Displayed are 100 ms epochs after
each stimulation (107, 250, 350 ms after prime onset). Bold lines are
for trials where primes point to the hand stimulated by TMS, thin
lines in the other case. Lines are black when targets point to the
hand stimulated by TMS (i.e., when this hand is also the
responding hand) and gray in the other case
Motor cortex excitability
121
Table 1 Mean MEP amplitudes ± onefold SD (in lV) in experiment 1
Time point of TMS (ms)
100
250
350
Target contralateral to coil
Target ipsilateral to coil
Prime contralateral
to coil
Prime ipsilateral
to coil
Prime contralateral
to coil
Prime ipsilateral
to coil
L TMS
R hand
R TMS
L hand
L TMS
R hand
R TMS
L hand
L TMS
R hand
R TMS
L hand
L TMS
R hand
R TMS
L hand
101±95
147±133
206±161
81±73
86±82
219±198
118±117
101±104
439±322
77±68
72±65
308±249
109±107
131±131
98±90
98±92
80±82
68±78
92±95
93±96
135±107
92±84
68±58
94±77
‘‘Target contralateral to coil’’ means, for example for the TMS coil at the left M1, that the target arrow pointed to the right. The same
holds for ‘‘prime contralateral to the coil’’. ‘‘L TMS, R Hand’’ means that the TMS coil is at the left M1, stimulating the right hand
At 350 ms, MEPs were consistently larger in the hand
to which the targets pointed (side of responding hand:
F(1,11)=24.2, P<0.001; black vs. gray lines in Fig. 3)
and, of most interest, were smaller in the hand to which
the primes had pointed (prime side: F(1,11)=13.4,
P=0.004; bold vs. thin lines in Fig. 3). Both factors
interacted (F(1,11)=8.2, P=0.02) because the prime
effect was stronger for the responding hand (Fig. 3).
Nevertheless, the prime effect was significant in separate
analyses both for the responding hand (F(1,11)=11.2,
P=0.007; black lines in Fig. 3) and for the nonresponding hand (F(1,11)=10.4, P=0.008; gray lines in
Fig. 3). These MEP effects did not differ between stimulation of the left and the right hemispheres (upper vs.
lower panel of Fig. 3).
Thus, changes in MEP amplitude provided a sensitive
probe of dynamic changes in the excitability of the M1
during a RT paradigm that used masked priming (with a
few minor problems, see General discussion). But results
were not decisive yet, because the changes in MEP
amplitudes induced by TMS at 350 were in accordance
with all hypotheses: The inhibition hypothesis predicted
that MEPs would be smaller for the hand towards which
the prime had pointed (bold lines smaller than thin lines
in Fig. 3), the opposite-activation hypothesis predicted
that MEPs would be larger for the hand toward which
the prime had not pointed (thin lines larger than bold
lines in Fig. 3) and the activation–inhibition hypothesis
predicted that both outcomes would occur. These three
predictions are not conflicting. This ambiguity can be
attributed to the fact that the experimental design did
not include a neutral stimulus as a point of reference, to
which MEP increases or decreases could be related. We
had expected that MEPs at 100 ms would serve as a
reference. However, obviously, MEPs were generally
larger at 350 ms than at 100 ms, reflecting increased
readiness to respond.
Experiment 2
To overcome these problems, we performed a second
experiment in which we introduced trials with neutral
primes as reference stimuli. We expected that these trials
would enable us to distinguish whether the MEP decrease at 350 ms reflects inhibition of the response cued
by the prime or activation of the opposite response. If
inhibition is the only effective mechanism then MEPs of
neutral and other-hand primes will not differ, both being
larger than the inhibited MEP with same-hand primes.
In contrast, if activation of a response with the opposite
hand is the only effective mechanism then MEPs of
neutral and same-hand primes will not differ, both being
smaller than the activated MEP with other-hand primes.
Finally, if the prime triggers both tendencies then MEPs
evoked by neutral primes will be in-between same- and
other-hand primes.
In two blocks, suprathreshold TMS was applied to
the left or to the right M1. In each trial, single-pulse
TMS was given at 250 or 350 ms after prime onset, or no
TMS pulse was applied. Simultaneous EEG recordings
enabled the measurement of the LRP in trials without
TMS, to verify whether the time points of TMS at 250
and 350 ms corresponded to the latencies of maximal
differences in activity between the motor cortices.
Methods
Participants
Initially, we had screened 20 students of the University
of Lübeck. Because we wanted to record the LRP during
the TMS experiment, the EEG cap was attached to the
participants’ head, and we then assessed the motor hot
spot as well as the RMT. The electrode cap increased the
distance between the TMS coil and the stimulated M1,
thereby a higher intensity of TMS was needed for
effective stimulation of the M1. Participants were included if the RMT of both FDI muscles was below 80%
of maximum stimulator output while participants wore
the EEG cap. Only nine participants met the TMS criteria. Another participant was excluded because of an
error rate of >25%. In total, simultaneous TMS and
LRP recordings were possible in eight out of the 20
individuals. To reach sufficient statistical power, we
decided to study additional volunteers but without
concurrent EEG recording.
122
In total, 12 students (five females, seven males) participated in experiment 2, and in eight of the 12 participants we simultaneously recorded the LRP. Their ages
ranged between 22 and 28 years, they had normal or
corrected-to-normal vision, were all right-handed, as
examined by the Edinburgh Handedness Inventory, and
were paid 7 € per hour. None had participated in
experiment 1.
gets, and these two differences were averaged to yield the
LRP as the general difference contralateral–ipsilateral
(denoted as |C3 C4|) relative to target-arrow direction
(= relative to the responding hand).
Results and discussion
Behavior
Stimuli, procedure, and apparatus
Stimuli were the same as in experiment 1, but in addition
to left- and right-pointing primes, neutral primes were
presented. These neutral primes consisted of two inwards-pointing arrow-heads (><). Procedures for
localizing the left and right M1 and determining motor
thresholds were identical to experiment 1. TMS intensities were again adjusted to 10% above individual
RMT, which corresponded to 77% of maximum stimulator output for the left hemisphere (range 66–88%)
and 76% for the right hemisphere (range 55–86%). Since
the block without TMS was omitted, the experiment
consisted of two blocks: TMS on the left side, TMS on
the right side. Order of these blocks alternated among
participants.
The TMS pulse was given at 250 or at 350 ms after
prime onset. Primes pointed left or right or were neutral
with 33% probability each. There were 540 trials in either block: 180 trials each with TMS at 250 ms after
prime onset, with TMS at 350 ms after prime onset, and
without TMS.
Recording and data analysis
Additionally to manual responses and MEPs, EEG was
recorded from four scalp positions: C3, C4, PO7, PO8.
An electrode at the nose tip was used as an online EEG
reference, and further electrodes were placed above and
below the left eye and at the outer canthi of both eyes for
bipolar measurement of vertical and horizontal EOG, in
order to control for EOG artifacts transmitted to EEG.
An electrode placed at the forehead was used as ground
for EEG and EOG. All the recordings (EMG, EEG,
EOG) were done with Ag/AgCl electrodes within DC to
2,500 Hz by a BrainAmp MR plus and stored at 2,500 Hz
per channel. Off-line, EMG data were filtered to a range
of 2–1,250 Hz, rectified and averaged for correctly responded trials, separately for each experimental condition. EEG data were low-pass filtered at 30 Hz, edited for
artifacts (rejecting trials with zero lines, followed by
correcting ocular artifacts, followed by rejecting trials
with voltage differences more than 200 lV or voltage
steps more than 20 lV), and the LRP was formed from
the average for correctly responded trials: Trials with
right- and left-pointing target arrows were separately
averaged, the difference left minus right recording
(C3 C4) was formed in the average of right targets, right
minus left recording (C4 C3) in the average of left tar-
Response times (Fig. 4, top panel) were analyzed by
ANOVA with the factors responding hand (left, right),
stimulated hemisphere (left, right), timing of TMS (no
TMS, TMS at 250 or 350 ms), and prime congruency
(identical, opposite to target, neutral). In agreement with
experiment 1, there was reliable inverse priming (prime
congruency: F(2,22)=9.5, P=0.008), with an average
difference of 30 ms between opposite and identical
primes. Mean response times to neutral primes were
20 ms faster than responses to identical primes
(F(1,11)=13.2, P=0.004, in an ANOVA on neutral and
identical primes only) and 10 ms slower than responses
to opposite primes (F(1,11)=3.8, P=0.08, in an ANOVA on neutral and opposite primes only). Like in
experiment 1, left-hand responses were slightly faster
than right-hand responses (mean difference in response
time: 11 ms, F(1,11)=4.6, P=0.06). No other effect was
significant (P>0.1).
The ANOVA of error rates (Fig. 4, bottom panel)
showed
main
effects
of
prime
congruency
(F(2,22)=8.8, P=0.01) because most errors were made
when primes were identical, and of timing of TMS
(F(2,22)=12.5, P<0.001) because least errors were
made without TMS and most errors with TMS at
350 ms. Several interactions were obtained: stimulated
hemisphere · responding hand (F(1,11)=8.3, P=0.02),
stimulated hemisphere · responding hand · timing of
TMS (F(2,22)=5.3, P=0.01), and stimulated hemisphere · responding hand · timing of TMS · prime
congruency (F(4,44)=3.7, P=0.02). Separate analysis
at the three TMS time-points resolved those interactions as follows.
When no TMS was applied, the only significant effect
was a main effect of prime congruency (F(2,22)=8.2,
P=0.003), with more errors committed with identical
primes than with either neutral or opposite primes. This
effect tended to differ between right and left responses
(black vs. gray lines in Fig. 4; responding hand · prime
congruency, F(2,22)=3.4, P=0.05).
When TMS was given 250 ms after prime onset, no
effect was significant (P>0.1).
At 350 ms after prime onset, TMS provoked
more erroneous responses with the stimulated hand
than with the non-stimulated hand (stimulated hemisphere · responding hand: F(1,11)=14.1, P=0.003).
This was above all true when primes were identical
and neutral but not when primes were opposite
(stimulated hemisphere · responding hand · prime
congruency: F(2,22)=4.0, P=0.04; effect of stimulated
123
Fig. 4 Response times and
error rates in experiment 2.
Means over the 12 participants.
Black lines are for right-hand
responses, gray lines for lefthand responses. Lines are bold
when the responding hand is
also the one stimulated by
TMS, and thin in the other case.
Note that these definitions
apply to the correctly
responding hand. Thus, for
example, large error values are
reached at TMS-350 by the thin
lines, meaning that the response
would have been correct for the
hand not stimulated by TMS,
so an error means that the hand
stimulated by TMS pressed its
key instead
hemisphere · responding hand for identical, neutral,
opposite primes: F(1,11)=11.2, P=0.006; F(1,11)=7.5,
P=0.02; F(1,11)=0.3, n.s.). The main effect of prime
congruency (F(2,22)=9.7, P=0.007) reflected the generally higher error rate for identical than for opposite
primes. As indicated by the mentioned interaction of
prime congruency · stimulated hemisphere · responding hand, this effect of prime congruency was more
pronounced when the correct hand was ipsilateral to the
site of TMS (such that TMS induced wrong responses),
F(2,22)=8.8, P=0.008, and less marked, though still
significant (F(2,22)=7.5, P=0.008), when the correct
hand was contralateral to the site of TMS.
Motor cortex excitability
Grand means of the rectified MEPs contralateral to the
stimulated M1 are displayed in Fig. 5 for the right hand
and in Fig. 6 for the left hand. Mean amplitudes were
determined for epochs 25–37 ms after stimulation (Table 2) and were analyzed by ANOVA with the factors
timing of TMS (250, 350 ms), stimulated hemisphere
(left or right M1), responding hand (contralateral or
ipsilateral to stimulated hemisphere), and prime (pointing to the hand contralateral or ipsilateral to the stimulated hemisphere, or neutral).
Motor-evoked potentials were larger at 350 ms than
at 250 ms (timing of TMS: F(1,11)=17.6, P=0.001).
Furthermore, timing of TMS interacted with prime side
(F(2,22)=10.4, P=0.004), with responding hand
(F(1,11)=12.4, P=0.005), with prime side · responding
hand (F(2,22)=6.5, P=0.02), and with prime
side · stimulated hemisphere (F(2,22)=4.8, P=0.03).
To clarify these effects, separate analyses were performed for the two TMS time-points.
At 250 ms, MEPs were slightly larger when primes
pointed to the stimulated hand (bold lines in Figs. 5 and
6) but, in contrast to experiment 1, this effect was far
from significance (prime side: F(2,22)=1.6, P=0.23). In
accordance with experiment 1, MEPs were larger at the
hand to which the targets pointed (side of responding
hand: F(1,11)=7.6, P=0.02; upper vs. lower panels in
Figs. 5 and 6; note the different scales). Other effects
were not significant.
At 350 ms, mean MEP amplitudes were considerably
larger in the responding hand than in the non-responding hand (F(1,11)=14.0, P=0.003; upper vs. lower
panels in Figs. 5 and 6; note the different scales). Replicating the results of experiment 1, MEP amplitudes
decreased in the hand to which the primes had pointed
(prime side: F(2,22)=14.6, P=0.001; bold lines in
Figs. 5 and 6). This relative decrease was more pronounced
for
the
responding
hand
(prime
side · responding hand: F(2,22)=3.5, P=0.058; upper
panels of Figs. 5 and 6) and depended on the stimulated
M1 (prime side · stimulated hemisphere: F(2,22)=4.88,
P=0.02; prime side · responding hand · stimulated
hemisphere: F(2,22)=4.90, P=0.02). To account for
these effects and simultaneously clarify the role of
124
Fig. 5 Rectified motor-evoked
potentials (MEPs) from the
right hand in experiment 2.
Grand means over the 12
participants. As symbolized by
the schematic head and hands,
these MEPs were recorded from
the right hand, evoked by TMS
applied to the left hemisphere.
The stimulated right hand is
also the responding hand in the
upper panel, and the nonresponding hand in the lower
panel. Time point zero is prime
onset. Displayed are 60-ms
epochs after either stimulation
(250 and 350 ms after prime
onset). Bold lines are for trials
where primes point to the right
hand stimulated by TMS, thin
lines are for trials where primes
point to the other hand, and
dotted lines are for neutral trials
Fig. 6 Rectified motor-evoked
potentials (MEPs) from the left
hand in experiment 2. Grand
means over the 12 participants.
As symbolized by the schematic
head and hands, these MEPs
were recorded from the left
hand, evoked by TMS applied
to the right hemisphere. The
stimulated left hand is also the
responding hand in the upper
panel, and the non-responding
hand in the lower panel. Time
point zero is prime onset.
Displayed are 60-ms epochs
after either stimulation (250 and
350 ms after prime onset). Bold
lines are for trials where primes
point to the left hand stimulated
by TMS, thin lines are for trials
where primes point to the other
hand, and dotted lines are for
neutral trials
neutral primes, MEP amplitudes evoked by the three
types of primes were compared to one another by paired
t tests, separately for TMS stimulations of the left
and right hemisphere and of the responding and the
non-responding hand.
We first report the results for TMS stimulating the
non-responding hand. When TMS probed the right nondominant M1 (Fig. 6, lower panel) MEPs did not differ
between trials with neutral primes and primes pointing
contralateral to the stimulated M1 (t(11)=0.4, n.s.) and
125
Table 2 Mean MEP amplitudes ± onefold SD (in lV) in experiment 2
Time point Target contralateral to coil
of TMS
Prime contralateral Prime neutral
(ms)
to coil
L TMS
R hand
250
350
R TMS
L hand
L TMS
R hand
R TMS
L hand
Target ipsilateral to coil
Prime ipsilateral
to coil
Prime contralateral Prime neutral
to coil
Prime ipsilateral
to coil
L TMS
R hand
L TMS
R hand
L TMS
R hand
R TMS
L hand
R TMS
L hand
L TMS
R hand
R TMS
L hand
R TMS
L hand
223±202 332±550 182±123 298±471 190±167 287±474 185±171 277±432 181±141 225±524 170±131 288±480
416±388 489±710 487±400 540±542 717±636 554±610 115±93 272±425 152±117 263±414 189±127 326±475
were smaller in these trials than in trials with primes
pointing ipsilateral to the stimulated M1 (t(11)=2.4,
P=0.03, and 2.1, P=0.06). In contrast, when TMS
probed the left dominant M1 (Fig. 5, lower panel) trials
with neutral primes had intermediate MEP amplitudes.
They were larger than MEPs associated with primes
pointing contralateral to the stimulated M1 (t(11)=2.2,
P=0.05) and were smaller than MEPs associated with
primes pointing ipsilateral to the stimulated M1(t(11)=
3.4, P=0.006).
When TMS stimulated the responding hand, results
were similar to the non-responding hand for probing the
left dominant M1 but were less clear when probing the
right non-dominant M1. When TMS was applied to the
right M1 (Fig. 6, upper panel) MEPs did not differ between trials with the three types of primes (P>0.20). By
contrast, when TMS was applied to the left M1 (Fig. 5,
upper panel) MEPs had intermediate amplitudes in trials
with neutral primes, larger than with primes pointing
contralateral to the stimulated M1 (t(11)=3.0, P=0.01)
and smaller than with primes pointing ipsilateral to the
stimulated M1 (t(11)=2.4, P=0.04).
intervals were analyzed by separate ANOVAs, starting
at 1–25 ms, ending at 375–400 ms (where effects started
becoming trivial, reflecting the differences in overt response times). ANOVAs had the factors prime (identical, neutral, opposite) and stimulated hemisphere (left or
right) although TMS was not applied in these trials.
Consequently, the latter factor indeed did not have any
effect. Figure 7 indicates that LRPs were affected by
primes, main effects being significant in the epoch from
201 to 250 ms (two 25 ms intervals: F(2,14)=11.0,
P=0.001, and 9.1, P=0.003) and in the epoch from 301
to 375 ms (three 25 ms intervals: F(2,14)=5.8, P=0.04;
11.4, P=0.005; 7.4, P=0.008). Replicating earlier findings quoted in Introduction, the former epoch reflects
activation evoked by the prime, with a peak latency of
230 ms in the grand mean, and the latter epoch reflects
the critical process under study (positivity of the contraipsilateral difference), with a peak latency of 330 ms in
the grand mean.
General discussion
Methodology: TMS as a probe in masked priming
Lateralized readiness potentials
Lateralized readiness potentials are displayed in Fig. 7,
recorded from |C3 C4| in eight participants in the
TMS-free trials. Mean amplitudes of successive 25-ms
Transcranial magnetic stimulation was used to probe
dynamic changes in the excitability of the M1 during a
two-alternative choice-response task in which non-consciously perceived information primed response choice.
Fig. 7 Lateralized readiness potentials (LRPs). Grand means of
contra-ipsilateral difference potentials recorded from C3 and C4
(sites overlying the motor cortices) over eight participants in
experiment 2. Time point zero is prime onset, target onset was at
107 ms (from which time-point response times were measured),
bold black lines are from trials with identical primes, thin black lines
from trials with neutral primes, and gray lines from trials with
opposite primes
126
The time course of the LRP was used to select the
appropriate timing of TMS during the time between
prime onset and overt manual response.
Our results underscore that simultaneous LRP
measurements are important to check whether the time
points selected for TMS were adequate. For instance,
the lack of a prime effect in experiment 2 on the MEPs
evoked at 250 ms might be ascribed to the somewhat
early occurrence of the first LRP peak. Since the peak
of LRP had reached its maximum already at 230 ms
after prime onset, it is conceivable that TMS given
20 ms after the LRP peak was suboptimal to probe the
prime effect. Perhaps the TMS impulse at 350 ms was
also given too late to be an optimal probe for the
prime-mask effect, since the critical LRP reversal became significant already at 300 ms after prime onset. In
future studies, it might thus be preferable to measure
the individual LRPs in an experimental block immediately before TMS and then adjust the time points of
TMS to the latency of the individual LRP peaks. This
temporal adjustment would correspond to the topographical adjustment of individual participants’ TMS
positioning based on activation foci measured in previous fMRI (e.g., Koski et al. 2005).
Because previous TMS studies have shown that TMS
to the M1 can produce specific effects on response time
(Day et al. 1991; Romaiguère et al. 1997; Sawaki et al.
1999) and may influence the selection of responses
(Brasil-Neto et al. 1992) we were concerned that TMS,
intended to serve as an external probe of the responsepriming process, would actively interfere with the very
process to be measured (Koski et al. 2005). If only response times were considered, TMS appeared to have no
influence on the response-priming effect. In blocks with
TMS, we observed the usual inverse priming effect, with
response times being virtually identical to blocks without TMS. However, error rates indicated that the TMS
probe did indeed interfere with response selection. In
both the experiments, this was most evident when TMS
was applied 350 ms after prime onset and the response
ipsilateral to the site of TMS would have been appropriate. Then, TMS of the M1 increased the probability
of a false response with the contralateral hand. Though
this was an unwanted side effect, revealing TMS-induced
interference with task performance, this finding was
interesting because error rate was only increased by
TMS when primes were identical to targets. We will
return to this result below.
Additional behavioral effects of TMS were obtained
when TMS was given 100 ms after prime onset (i.e., at
target onset). First, TMS unspecifically speeded responses and increased errors. We attribute this unspecific effect of TMS to the concomitant auditory and
somatosensory stimulation resulting in intersensory
facilitation (Terao et al. 1997; Romaigùere et al. 1997;
Sawaki et al. 1999). Second, if TMS was given at 100 ms
to the right M1 and the priming arrow also pointed to
the right side then the right key was pressed more frequently. This may be explained by a stimulus–response
compatibility effect produced by the lateral auditory and
somatosensory input generated by TMS, which occurred
precisely at the time of target presentation when given at
100 ms after prime onset. Since the TMS pulse represents a lateralized sensory stimulus, it is not surprising
that responses ipsilaterally to TMS were facilitated (Simon 1990; Wascher et al. 2001; cf. for similar effects of
TMS Koski et al. 2005).
We conclude that the TMS pulse actively interfered
with task performance and thus was not just a passive
probe of changes in cortical excitability. However, these
behavioral effects were modest and did not principally
alter the inverse priming effect which was of primary
interest to the present study.
Motor-evoked potentials were less extended in time
and occurred slightly earlier in experiment 2 than in
experiment 1. The most probable reason for these differences is that different amplifiers were used in the two
experiments. Consequently, the window for analysis was
selected to have a width of 21 ms in experiment 1 (30–
50 ms) and of 13 ms in experiment 2 (25–37 ms).
On the mechanisms contributing to inverse priming
In order to elucidate the neurophysiological mechanism
responsible for inverse priming, the excitability of the
primary motor hand areas was probed at 350 ms after
prime onset. This is the time point at which the LRP gets
reversed in this paradigm, as was verified in experiment 2.
Indeed, a correlate of the LRP reversal was found,
with MEPs being smaller in the hand originally primed
and being correspondingly larger in the other hand. This
result remained inconclusive in experiment 1 as it was
compatible with all three hypotheses: With inhibition
hypothesis (the originally primed hand was inhibited),
with opposite-activation hypothesis (the opposite hand
was activated), and with the activation–inhibition
hypothesis (both the originally primed hand was inhibited and the opposite hand was activated). This ambiguity motivated us to perform experiment 2, introducing
neutral primes as a control condition to arrive at a
decision between the alternative hypotheses.
Another finding, mentioned above in the methodological discussion, was relevant with respect to the tested
hypotheses: Errors increased with TMS at 350 ms but
only if primes were identical to imperative stimuli. For
example, when a right-hand response was required then
TMS to the right M1 at 350 ms could trigger an erroneous left-hand response if the prime had prompted a
right-hand response. This finding does not fit the inhibition hypothesis: According to this hypothesis, the
rightward prime activates the left M1 at 250 ms and
inhibits the same left M1 at 350 ms but does not increase
activation of the right M1 at 350 ms. Precisely such an
increase is postulated by the activation hypothesis and by
inhibition–activation hypothesis.
Motor-evoked potential results of experiment 2 were
complex, differing between right and left hand.
127
When the corticospinal output to the left, non-dominant hand was stimulated (TMS coil placed over the
right M1) no effects of primes on MEP amplitudes were
obtained in trials requiring a response with the left hand.
This negative finding is at variance with the clear effects
obtained in experiment 1. Possibly, priming effects were
obscured by neuronal processes subserving response
generation. Note that response times were shorter for
left-hand responses and participants tended to respond
faster in experiment 2 than in experiment 1.
Left-hand MEPs were uncontaminated by actual response generation in trials requiring a response with the
right hand. In this condition, MEP amplitudes were
significantly smaller when primes were neutral and when
primes pointed contralateral to the stimulated M1 than
when primes pointed ipsilateral to the stimulated M1,
and neutral- and contralateral-prime MEPs did not
differ from each other. This finding favors the oppositeactivation hypothesis over the two other hypotheses.
When the right, dominant hand was stimulated (TMS
coil placed over the left M1) MEP amplitudes recorded
during neutral-prime trials were of intermediate size between MEP amplitudes obtained in trials with primes
pointing contralateral and ipsilateral to the stimulated
M1. This finding was consistent for both right- and lefthand responses. The latter condition is more relevant
because these trials were not contaminated by preparation of the actual response. This pattern of results, with
neutral-trial MEPs lying in-between the other two conditions, is predicted by the activation–inhibition hypothesis. It might either indicate that both mechanisms work
in parallel: Triggered by the prime and protected by the
mask from further influences, the primed M1 is inhibited.
At the same time, the prime-mask complex induces
activation in the contralateral M1. Alternatively, this
pattern might reflect dynamic reciprocal interhemispheric inhibition between the motor cortices. If so,
temporary evidence for one response always produces a
concurrent suppression of the other response.
Both variants of the activation–inhibition hypothesis,
however, need to explain why inhibition (MEPs being
smaller when primes pointed to the stimulated M1 than
with neutral primes) was only found during left-hemispheric stimulation. This finding would imply that the
dominance of the right hand/left hemisphere is associated with a greater responsiveness to inhibition. This
implication, however, is at variance with previous TMS
studies which probed the time course of activation between stimulus and response (Leocani et al. 2000) or
measured the inhibitory effect of a conditioning TMS
impulse on the MEP evoked by TMS to the other
hemisphere (Netz et al. 1995). Both studies found that
the left M1 had a stronger inhibitory influence on the
right non-dominant M1 rather than being more susceptible to inhibitory drive from the non-dominant M1.
Thus, the left M1 seems to be more easily activated than
more easily inhibited.
This latter interpretation, that the left M1 might become more easily activated, may indeed account for the
larger MEPs at 350 ms after neutral primes than after
primes pointing contralateral to the stimulated M1
specifically with left M1 stimulation. This result might
indicate that the left dominant M1 is not only activated
by left-pointing primes but also, though to a lesser extent, by neutral primes. In contrast, neutral primes
would hardly activate the right M1. Therefore, the difference between left and right M1 may reflect their
responsiveness to prime activation: Both M1s are activated at 350 ms by ipsilateral primes, but only the left
M1 is activated by neutral primes. This interpretation
would be in line with the opposite-activation hypothesis.
Two arguments may be put forward to further support
this interpretation. First, the prime effects at 250 ms
were asymmetric in experiment 1: prime effects were
significant only when the left M1 was stimulated. This
result might reflect an enhanced tendency of the left M1
to be activated. Second, the present difference between
hemispheres is in good agreement with the results obtained by Stürmer et al. (2000) who used TMS to probe
excitability changes associated with the Simon effect
(where stimulus position, though irrelevant, may prime
the M1 contralateral to the stimulus). In their study,
MEPs obtained in neutral trials (when stimuli were
presented at fixation) were of intermediate size between
compatible and incompatible trials when the left dominant hemisphere was stimulated and were as small as in
incompatible trials when the right non-dominant hemisphere was stimulated. Stürmer et al. favored an interpretation of these results in terms of a decisive role of
activation rather than in terms of activation and inhibition: in the non-dominant right hemisphere, there is
only activation when the stimulus is contralateral,
whereas in the dominant left hemisphere, activation is
evoked both by contralateral and by neutral stimuli.
Several differences were noted between results of
experiments 1 and 2. In experiment 2, there was no
significant prime effect on MEPs when TMS was given
at 250 ms, and also the prime effects at 350 ms were less
marked than in experiment 1, at least when the right
hemisphere was stimulated. These differences might be
ascribed to random variation between participants of the
two experiments. But there might also exist more systematic reasons for these differences. Comparing the
response times suggests that responses were faster in
experiment 2 and error rates were somewhat increased.
Thus, there might be slight differences in speed-accuracy
trade-off between both experiments (which probably is a
reflection of our view that MEP effects would be clearer
when participants respond fast, so participants in
experiment 2 probably felt encouraged to respond fast).
These differences might be related to the differences in
MEP effects. There were also more objective differences
in experimental design which might have led to different
MEP patterns. First, the density of TMS trials differed:
there were 17% trials without TMS in experiment 1, but
33% in experiment 2. Possibly, the efficiency of TMS as
a probe was affected by this difference in frequency.
Second, there were 33% trials with neutral primes in
128
experiment 2, and none in experiment 1. Possibly, the
overall effect of primes was therefore reduced in experiment 2 (cf. Jaśkowski et al. 2003, for effects of probability of masked primes).
Taken together, the present results favor the oppositeactivation hypothesis since it may be applied both to lefthand responses and to right-hand responses. Alternatively, the activation–inhibition hypothesis might be true
for right-hand responses. The implication is that, at least
in right-handed subjects, the neuronal mechanisms
underlying the inverse priming effect depend on hemispheric dominance. However, in view of the noted differences between experiments 1 and 2, further studies are
needed to validate these conclusions.
Acknowledgments This study was supported by a grant from the
Deutsche Forschungsgemeinschaft to Rolf Verleger and Hartwig
Siebner (Ve 110/14–1). Piotr Jaśkowski was supported by a grant
from the Deutsche Forschungsgemeinschaft to Rolf Verleger (Ve
110/7-4). Hartwig Siebner was supported by a grant from the
Volkswagenstiftung (I/79 932). Thanks are due to Michaela
Fritzmannova for her valuable help with MEP and EEG recordings. Many helpful suggestions were provided by two reviewers
(Friederike Schlaghecken and an anonymous one) of a previous
version of this paper.
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