Eur J Appl Physiol DOI 10.1007/s00421-014-2930-x
Original Article
The effect of muscle vibration on short latency intracortical inhibition in humans Kapka Mancheva · Christoph Schrader · Lilia Christova · Reinhard Dengler · Andon R. Kossev
Received: 21 November 2013 / Accepted: 31 May 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose The purpose of the present study was to investigate the effect of muscle vibration (MV) on short latency intracortical inhibition (SICI) and facilitation (ICF) assessed by paired-pulse transcranial magnetic stimulation (TMS). Methods Nineteen right-handed healthy subjects were investigated without and with MV of the right extensor carpi radialis (ECR), using single- and paired-pulse TMS with interstimulus interval (ISI) of 3 and 13 ms. Intensities of the conditioning and test stimulus were 70 and 120 % of the motor threshold at rest. The motor-evoked potentials (MEPs) were recorded simultaneously from the vibrated ECR and its antagonist flexor carpi radialis (FCR). Results In all the subjects a SICI of similar strength could be observed at 3 ms, at rest and during MV both in the vibrated muscle as well as in its antagonist. The subjects were divided in two groups according to the changes in MEP response to paired-pulse TMS with 13 ms ISI observed during MV. In nine subjects SICI was evident also at 13 ms when vibration was applied, while in another ten subjects vibration induced ICF at 13 ms. Conclusions The effect of MV is not just a facilitation of SICI, but a stronger prolongation of the effect of
Communicated by Toshio Moritani. K. Mancheva · L. Christova · A. R. Kossev (*) Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, “Acad. G. Bonchev” Str., bl. 21, 1113 Sofia, Bulgaria e-mail: [email protected] C. Schrader · R. Dengler Department of Neurology and Clinical Neurophysiology, Medical School of Hannover, Hannover, Germany
intracortical inhibition to an ISI at which ICF is well pronounced, when the intensity of the conditioning stimulus exceeds the threshold for intracortical facilitation. Keywords Muscle vibration (MV) · Transcranial magnetic stimulation (TMS) · Intracortical inhibition (ICI) · Intracortical facilitation (ICF) Abbreviations ECR Extensor carpi radialis muscle FCR Flexor carpi radialis muscle ICF Intracortical facilitation SICI Short latency intracortical inhibition ISI Interstimulus interval MEP Motor-evoked potential MT Motor threshold MV Muscle vibration TMS Transcranial magnetic stimulation TVR Tonic vibration reflex
Introduction Transcranial magnetic stimulation (TMS) of the primary motor cortex can elicit motor-evoked potentials (MEPs) in the contralateral target muscle, and the parameters of MEPs are used for pyramidal tract excitability assessment. Excitability of intracortical mechanisms can be assessed by using paired-pulse TMS introduced by Kujirai et al. (1993). In this stimulation paradigm, the first conditioning stimulus modifies the response to the following test stimulus. The effect of paired-pulse TMS depends on the intensity of both conditioning and test pulses, as well as on the interval between the paired stimuli (ISI). Kujirai et al. (1993), using subthreshold conditioning and suprathreshold test stimuli,
13
described short latency intracortical inhibition (SICI) at short ISI intervals (10 ms). There is a strong evidence that SICI is of cortical origin (Kujirai et al. 1993; DiLazzaro et al. 1998; Ziemann et al. 1996a). In fact, SICI is “one of the most popular tools in human motor neurophysiology” (Rothwell et al. 2009). There is an evidence that different intracortical mechanisms are responsible for SICI and ICF (Ziemann et al. 1996a, b; Kossev et al. 2003). The size of conditioned MEPs in response to paired-pulse TMS depends on the balance between the excitatory and inhibitory effects of both ICF and SICI, and the impact effect depends on the interstimulus interval. A large number of studies have used TMS to evaluate the impact of afferent sensory input on the excitability of human motor cortex (for review see Rosenkranz and Rothwell 2003). Muscle vibration (MV) is a powerful tool for modifying MEPs (Claus et al. 1988a, b; Kossev et al. 1999; Siggelkow et al. 1999; Rosenkranz et al. 2000). Siggelkow et al. (1999) have shown that the effect of MV on motor cortex excitability depends on vibration frequency. A significant increase of motor cortex excitability without an increase in tonic background muscle activity was found at low-amplitude MV. Such facilitation of excitability was stronger at 80 Hz MV than at 120 Hz and is not evident at 160 Hz (Claus et al. 1988a, b). Using lower MV frequencies, Steyvers et al. (2003) have confirmed that the most effective MV frequency is in the 75 Hz range. A comparison of MEPs evoked by TMS during MV and transcranial electrical stimulation has suggested a cortical level of MVinduced facilitation (Kossev et al. 1999). There is a strong evidence that the vibration stimuli mainly excite the muscle spindle afferents (Burke et al. 1976a, b; Gandevia 1985; Roll et al. 1989; Collins and Prochazka 1996; Martin and Park 1997). Microneurographic studies in humans have shown that the primary muscle spindle endings preferentially respond to low amplitude MV (Burke et al. 1976a, b) and that they follow with optimal one-to-one discharge rate in humans only under frequencies of about 80–100 Hz (Roll et al. 1989). These findings suggest that the primary muscle spindle messages (Ia fibers) represent the major sensory input underlying the MV-related facilitation of motor cortex excitability. Rosenkranz et al. (2003) suggested that MV induces a focused activation of the motor cortex associated with a reduced activity of intracortical inhibitory circuits, selectively targeting the vibrated muscle. The thresholds of SICI and ICF, assessed by intensity of conditioning stimulus at which the inhibition and facilitation of MEPs become significant at short (3 ms) and longer (13 ms) ISI, respectively, are different as the threshold of SICI is significantly lower than that of ICF (Kossev et al. 2003). Apparently, the simultaneous action of both mechanisms can allow an incorrect
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Eur J Appl Physiol
assessment of MV effect on the intracortical circuits conducting short latency influences. The purpose of the present study was to investigate more carefully the effect of MV, especially on SICI.
Methods Subjects Nineteen right-handed healthy subjects aged from 24 to 63 years (mean ± SD 36.7 ± 9.8 years) took part in the investigations. The study was approved by the local ethics committee. Subjects were informed on the research purpose and gave their written informed consent to the experimental procedure. Subjects were seated on adjustable chair with both arms in slight abduction from the trunk (20°) and flexion in the elbow (110°). Muscle vibration Low-amplitude muscle vibration (frequency 80 Hz, amplitude 0.5 mm, duration of 4 s), subthreshold for the tonic vibration reflex (TVR), was applied to the right extensor carpi radialis muscle (ECR) by means of an electromagnetic mechanical stimulator (Model V100, Ling Dynamic Systems, Wernau, Germany), with a disk surface (2 cm in diameter) (Kossev et al. 1999). Duration of the MV trains was 4 s, with random intertrial intervals of 12–22 s. Transcranial magnetic stimulation Transcranial magnetic stimulation was provided by two MagStim200 stimulators (MagStim Co., London, UK), producing pulses of 100 μs duration and up to 2.0 T intensity. The stimulators were connected to a BiStim module for the adjustment of the intervals between TMS pulses. The circular coil (diameter 9 cm) was adjusted over the vertex to evoke optimal MEPs in the right ECR at the lowest possible threshold. In such position of the coil the amplitudes and areas of MEPs recorded from flexor carpi radialis muscle (FCR) in control condition were similar to the corresponding values of the target ECR (Kossev et al. 1999, 2002; Siggelkow et al. 1999). Stimulus intensity of single-pulse TMS was 120 % of the motor threshold (MT) at rest. The resting motor threshold for each subject was defined as the lowest stimulus intensity which evoked at least three MEPs with minimum peak-to-peak amplitude of 50 μV in four stimulations. In application of paired-pulse TMS for the assessment of SICI (3 ms ISI) and ICF (13 ms ISI), the conditioning stimulus had an intensity of 70 % and the testing pulse one of 120 % of MT at rest (Kujirai et al. 1993, Kossev et al.
Eur J Appl Physiol
2002, 2003). Single- or paired-pulse TMS was applied at 3 s after the start of MV. Electromyographic (EMG) recordings Motor-evoked potentials were simultaneously recorded from the vibrated right ECR and its non-vibrated antagonist FCR by using Ag/AgCl disk surface EMG electrodes placed over these muscles in a belly-tendon montage. The EMG amplitude was amplified using a conventional EMG machine (Counterpoint; Dantec Electronics, Skovlunde, Denmark). The myoelectric activity of both muscles was continuously monitored for absence of voluntary background activity and of TVR. The EMG signals were amplified (band pass 5 Hz to 10 kHz), digitized via an analogue-to-digital convertor (CED 1401 power, Cambridge Electronic Design Ltd, Cambridge, UK) and stored on disk. Experimental procedure After evaluation of MT at rest, the first part of the experimental protocol was carried out without MV. Five singlepulse TMS were followed by 10 paired-pulse TMS with ISI of 3 and 13 ms (5 trials for each ISI) in random order. The first part of the experiment was completed by another five single-pulse TMS and testing of MT at rest in order to confirm the optimal position of the circular coil (Fig. 1). In the second part of the investigation, MV was applied and the experimental protocol was repeated. Single- and paired-TMS pulses were applied of 3 ms after the onset of MV. At the end of the experimental sessions, the optimal position of the circular coil was checked by five singlepulse TMS without MV and testing of MT at rest. During the experiments, the myoelectric activity of both muscles was monitored continuously to ensure absence of voluntary background activity and TVR (Lance et al. 1966; Hagbarth and Eklund 1968; Marsden et al. 1969). Data processing The values of resting motor thresholds for all investigated subjects were expressed as percentage of the maximal stimulator output. Epochs of 300 ms duration (100 ms prior and 200 ms after the stimulus) were stored on disk for offline analysis by Spike 2, version 4.15 software (Cambridge Electronic Design Ltd). The MEP parameter of interest was MEP size, as assessed by the total voltage–time integral (area) calculated from the rectified signal in the time interval between the onset and the end of MEP. The average values of MEP area (n = 5) were calculated for each condition (without and with MV) and the type of TMS stimuli (single and paired pulse with ISIs of 3 and 13 ms). For the evaluation of the effect of MV the obtained data were normalized
Fig. 1 Experimental design. For details refer to the text in “Methods”
and expressed as percentage of the unconditioned baseline values (i.e. the response to single-pulse TMS at 120 % of MT without MV). To assess furthermore the effect of the conditioning stimulus, the MEP areas (the average of 5 MEPs) obtained in response to paired-pulse TMS during MV were normalized to those MEPs evoked by singlepulse TMS during MV. The effects of MV on ISI/ICF for both investigated muscles were evaluated by two-way repeated measures ANOVA (general linear model, GLM) (Statistic 7.0, Stat Soft Inc., USA, 2004), with “TMS condition” (control, MV) and “interval” (single pulse, 3 ms ISI, 13 ms ISI). Duncan’s test was applied where appropriate as post hoc analysis. The two groups were divided according to the ratio: values of MEP response during MV to paired pulse TMS with 13 ms ISI to corresponding MEP values at single pulse TMS −> or < of 1. Nonparametric Mann–Whitney U test was used to compare responses to MV in the two groups.
Results There was no evidence of a tonic vibration reflex in the EMG, and none of the investigated subjects reported any perception of illusory movement during the applied MV.
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The mean value (±SD) of the motor threshold at rest (without MV) was 52.7 ± 39 % of the maximal stimulator output. The two-way repeated ANOVA measures, with “TMS condition” and “interval” factors showed a significant effect of the “TMS condition” (F1,72 = 6.14, p = 0.0018) and “interval” (F2,72 = 20.82, p = 0.00000) factors for the vibrated ECR. Interaction between the factors was insignificant (F2,72 = 2.14, p = 0.124). For non-vibrated FCR, the two-way repeated ANOVA measures revealed a significant effect of the “interval” (F2,72 = 27.45, p = 0.0000) factor and an insignificant effect of the “TMS condition” (F1,72 = 0.47, p = 0.496) factor, which was also insignificant for the interaction between the factors (F2,72 = 0.40, p = 0.670). The mean values of MEP areas recorded from ECR and FCR in response to single-pulse TMS without MV were 7.64 ± 3.28 and 5.83 ± 3.02 mV ms, respectively, for the group of all investigated subjects (n = 19). Figure 2 illustrates the simultaneous recordings of MEPs from both muscles (vibrated ECR and its antagonist FCR), without MV (a) and with MV (b). During MV, the mean value of MEP area in response to single pulse TMS for ECR was significantly greater (Duncan’s test, p = 0.0065), 15.81 ± 14.16 mV ms, than without MV, whereas the parameters for FCR were not affected by MV (mean MEP area 6.12 ± 3.17 mV ms) (see also Fig. 3, upper plots). Without MV, the SICI was well expressed (significant reduction of MEPs) in both muscles ECR and FCR (p = 0.0364 and p = 0.0001, respectively), while ICF was not evident (Fig. 3, open bars). During MV, the SICI was expressed in both muscles (p = 0.0001 and p = 0.0007), while ICF was not evident (Fig. 3, lower plots, closed bars). In nine of the investigated subjects, during MV, the normalized MEP areas recorded in response to paired-pulse TMS were significantly suppressed with respect to single-pulse MEP in both muscles, and not only at 3 ms ISI (Duncan’s test, p = 0.0003 and p = 0.0002 for ECR and FCR, respectively), but also at 13 ms ISI (p = 0.0450 and p = 0.0182). The late depression was not observed in the control condition (Fig. 4, lower plots, open bars). The differences between control and MV condition in response to single pulse TMS was significant (p = 0.045) only in the vibrated ECR (Fig. 4, upper plots, closed bars). The effect of MV in the other ten subjects was different: during MV the SICI was significant only in the vibrated ECR (p = 0.0036) and at 13 ms ISI the ICF was significant in both muscles (Duncan’s test, p = 0.0073 and p = 0.0444 for ECR and FCR, respectively) (Fig. 5, lower plots, closed bars). Furthermore, augmentation of MEPs recorded from the vibrated ECR during MV was significant at singlepulse TMS (p = 0.0418), as well as at paired-pulse TMS
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Eur J Appl Physiol
Fig. 2 MEPs recorded in response to single pulse TMS without MV (a) and with MV (b) for the vibrated ECR muscle and its antagonist FCR
with 13 ms ISI (p = 0.0004) (Fig. 5, upper plots, closed bars). There was a significant (Mann–Whitney U test, z = −1.965, p = 0.049) difference between the two groups concerning the effect of MV at paired pulse TMS and 13 ms ISI.
Discussion The paired-pulse TMS paradigm, originally suggested by Kujirai et al. (1993), has been used for the assessment of SICI and ICF. This technique presumably tests two independent GABAA-mediated intracortical mechanisms (Sanger et al. 2001; Ziemann et al. 1996b). In our earlier experiments (Kossev et al. 2003), we have studied the effect of the conditioning stimulus intensity on MEP parameters recorded from ECR in response to paired-pulse TMS using the same ISIs (3 and 13 ms) and the same intensity of the test stimulus (120 % of MT) as in the present study. At very low intensities (
Original Article
The effect of muscle vibration on short latency intracortical inhibition in humans Kapka Mancheva · Christoph Schrader · Lilia Christova · Reinhard Dengler · Andon R. Kossev
Received: 21 November 2013 / Accepted: 31 May 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose The purpose of the present study was to investigate the effect of muscle vibration (MV) on short latency intracortical inhibition (SICI) and facilitation (ICF) assessed by paired-pulse transcranial magnetic stimulation (TMS). Methods Nineteen right-handed healthy subjects were investigated without and with MV of the right extensor carpi radialis (ECR), using single- and paired-pulse TMS with interstimulus interval (ISI) of 3 and 13 ms. Intensities of the conditioning and test stimulus were 70 and 120 % of the motor threshold at rest. The motor-evoked potentials (MEPs) were recorded simultaneously from the vibrated ECR and its antagonist flexor carpi radialis (FCR). Results In all the subjects a SICI of similar strength could be observed at 3 ms, at rest and during MV both in the vibrated muscle as well as in its antagonist. The subjects were divided in two groups according to the changes in MEP response to paired-pulse TMS with 13 ms ISI observed during MV. In nine subjects SICI was evident also at 13 ms when vibration was applied, while in another ten subjects vibration induced ICF at 13 ms. Conclusions The effect of MV is not just a facilitation of SICI, but a stronger prolongation of the effect of
Communicated by Toshio Moritani. K. Mancheva · L. Christova · A. R. Kossev (*) Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, “Acad. G. Bonchev” Str., bl. 21, 1113 Sofia, Bulgaria e-mail: [email protected] C. Schrader · R. Dengler Department of Neurology and Clinical Neurophysiology, Medical School of Hannover, Hannover, Germany
intracortical inhibition to an ISI at which ICF is well pronounced, when the intensity of the conditioning stimulus exceeds the threshold for intracortical facilitation. Keywords Muscle vibration (MV) · Transcranial magnetic stimulation (TMS) · Intracortical inhibition (ICI) · Intracortical facilitation (ICF) Abbreviations ECR Extensor carpi radialis muscle FCR Flexor carpi radialis muscle ICF Intracortical facilitation SICI Short latency intracortical inhibition ISI Interstimulus interval MEP Motor-evoked potential MT Motor threshold MV Muscle vibration TMS Transcranial magnetic stimulation TVR Tonic vibration reflex
Introduction Transcranial magnetic stimulation (TMS) of the primary motor cortex can elicit motor-evoked potentials (MEPs) in the contralateral target muscle, and the parameters of MEPs are used for pyramidal tract excitability assessment. Excitability of intracortical mechanisms can be assessed by using paired-pulse TMS introduced by Kujirai et al. (1993). In this stimulation paradigm, the first conditioning stimulus modifies the response to the following test stimulus. The effect of paired-pulse TMS depends on the intensity of both conditioning and test pulses, as well as on the interval between the paired stimuli (ISI). Kujirai et al. (1993), using subthreshold conditioning and suprathreshold test stimuli,
13
described short latency intracortical inhibition (SICI) at short ISI intervals (10 ms). There is a strong evidence that SICI is of cortical origin (Kujirai et al. 1993; DiLazzaro et al. 1998; Ziemann et al. 1996a). In fact, SICI is “one of the most popular tools in human motor neurophysiology” (Rothwell et al. 2009). There is an evidence that different intracortical mechanisms are responsible for SICI and ICF (Ziemann et al. 1996a, b; Kossev et al. 2003). The size of conditioned MEPs in response to paired-pulse TMS depends on the balance between the excitatory and inhibitory effects of both ICF and SICI, and the impact effect depends on the interstimulus interval. A large number of studies have used TMS to evaluate the impact of afferent sensory input on the excitability of human motor cortex (for review see Rosenkranz and Rothwell 2003). Muscle vibration (MV) is a powerful tool for modifying MEPs (Claus et al. 1988a, b; Kossev et al. 1999; Siggelkow et al. 1999; Rosenkranz et al. 2000). Siggelkow et al. (1999) have shown that the effect of MV on motor cortex excitability depends on vibration frequency. A significant increase of motor cortex excitability without an increase in tonic background muscle activity was found at low-amplitude MV. Such facilitation of excitability was stronger at 80 Hz MV than at 120 Hz and is not evident at 160 Hz (Claus et al. 1988a, b). Using lower MV frequencies, Steyvers et al. (2003) have confirmed that the most effective MV frequency is in the 75 Hz range. A comparison of MEPs evoked by TMS during MV and transcranial electrical stimulation has suggested a cortical level of MVinduced facilitation (Kossev et al. 1999). There is a strong evidence that the vibration stimuli mainly excite the muscle spindle afferents (Burke et al. 1976a, b; Gandevia 1985; Roll et al. 1989; Collins and Prochazka 1996; Martin and Park 1997). Microneurographic studies in humans have shown that the primary muscle spindle endings preferentially respond to low amplitude MV (Burke et al. 1976a, b) and that they follow with optimal one-to-one discharge rate in humans only under frequencies of about 80–100 Hz (Roll et al. 1989). These findings suggest that the primary muscle spindle messages (Ia fibers) represent the major sensory input underlying the MV-related facilitation of motor cortex excitability. Rosenkranz et al. (2003) suggested that MV induces a focused activation of the motor cortex associated with a reduced activity of intracortical inhibitory circuits, selectively targeting the vibrated muscle. The thresholds of SICI and ICF, assessed by intensity of conditioning stimulus at which the inhibition and facilitation of MEPs become significant at short (3 ms) and longer (13 ms) ISI, respectively, are different as the threshold of SICI is significantly lower than that of ICF (Kossev et al. 2003). Apparently, the simultaneous action of both mechanisms can allow an incorrect
13
Eur J Appl Physiol
assessment of MV effect on the intracortical circuits conducting short latency influences. The purpose of the present study was to investigate more carefully the effect of MV, especially on SICI.
Methods Subjects Nineteen right-handed healthy subjects aged from 24 to 63 years (mean ± SD 36.7 ± 9.8 years) took part in the investigations. The study was approved by the local ethics committee. Subjects were informed on the research purpose and gave their written informed consent to the experimental procedure. Subjects were seated on adjustable chair with both arms in slight abduction from the trunk (20°) and flexion in the elbow (110°). Muscle vibration Low-amplitude muscle vibration (frequency 80 Hz, amplitude 0.5 mm, duration of 4 s), subthreshold for the tonic vibration reflex (TVR), was applied to the right extensor carpi radialis muscle (ECR) by means of an electromagnetic mechanical stimulator (Model V100, Ling Dynamic Systems, Wernau, Germany), with a disk surface (2 cm in diameter) (Kossev et al. 1999). Duration of the MV trains was 4 s, with random intertrial intervals of 12–22 s. Transcranial magnetic stimulation Transcranial magnetic stimulation was provided by two MagStim200 stimulators (MagStim Co., London, UK), producing pulses of 100 μs duration and up to 2.0 T intensity. The stimulators were connected to a BiStim module for the adjustment of the intervals between TMS pulses. The circular coil (diameter 9 cm) was adjusted over the vertex to evoke optimal MEPs in the right ECR at the lowest possible threshold. In such position of the coil the amplitudes and areas of MEPs recorded from flexor carpi radialis muscle (FCR) in control condition were similar to the corresponding values of the target ECR (Kossev et al. 1999, 2002; Siggelkow et al. 1999). Stimulus intensity of single-pulse TMS was 120 % of the motor threshold (MT) at rest. The resting motor threshold for each subject was defined as the lowest stimulus intensity which evoked at least three MEPs with minimum peak-to-peak amplitude of 50 μV in four stimulations. In application of paired-pulse TMS for the assessment of SICI (3 ms ISI) and ICF (13 ms ISI), the conditioning stimulus had an intensity of 70 % and the testing pulse one of 120 % of MT at rest (Kujirai et al. 1993, Kossev et al.
Eur J Appl Physiol
2002, 2003). Single- or paired-pulse TMS was applied at 3 s after the start of MV. Electromyographic (EMG) recordings Motor-evoked potentials were simultaneously recorded from the vibrated right ECR and its non-vibrated antagonist FCR by using Ag/AgCl disk surface EMG electrodes placed over these muscles in a belly-tendon montage. The EMG amplitude was amplified using a conventional EMG machine (Counterpoint; Dantec Electronics, Skovlunde, Denmark). The myoelectric activity of both muscles was continuously monitored for absence of voluntary background activity and of TVR. The EMG signals were amplified (band pass 5 Hz to 10 kHz), digitized via an analogue-to-digital convertor (CED 1401 power, Cambridge Electronic Design Ltd, Cambridge, UK) and stored on disk. Experimental procedure After evaluation of MT at rest, the first part of the experimental protocol was carried out without MV. Five singlepulse TMS were followed by 10 paired-pulse TMS with ISI of 3 and 13 ms (5 trials for each ISI) in random order. The first part of the experiment was completed by another five single-pulse TMS and testing of MT at rest in order to confirm the optimal position of the circular coil (Fig. 1). In the second part of the investigation, MV was applied and the experimental protocol was repeated. Single- and paired-TMS pulses were applied of 3 ms after the onset of MV. At the end of the experimental sessions, the optimal position of the circular coil was checked by five singlepulse TMS without MV and testing of MT at rest. During the experiments, the myoelectric activity of both muscles was monitored continuously to ensure absence of voluntary background activity and TVR (Lance et al. 1966; Hagbarth and Eklund 1968; Marsden et al. 1969). Data processing The values of resting motor thresholds for all investigated subjects were expressed as percentage of the maximal stimulator output. Epochs of 300 ms duration (100 ms prior and 200 ms after the stimulus) were stored on disk for offline analysis by Spike 2, version 4.15 software (Cambridge Electronic Design Ltd). The MEP parameter of interest was MEP size, as assessed by the total voltage–time integral (area) calculated from the rectified signal in the time interval between the onset and the end of MEP. The average values of MEP area (n = 5) were calculated for each condition (without and with MV) and the type of TMS stimuli (single and paired pulse with ISIs of 3 and 13 ms). For the evaluation of the effect of MV the obtained data were normalized
Fig. 1 Experimental design. For details refer to the text in “Methods”
and expressed as percentage of the unconditioned baseline values (i.e. the response to single-pulse TMS at 120 % of MT without MV). To assess furthermore the effect of the conditioning stimulus, the MEP areas (the average of 5 MEPs) obtained in response to paired-pulse TMS during MV were normalized to those MEPs evoked by singlepulse TMS during MV. The effects of MV on ISI/ICF for both investigated muscles were evaluated by two-way repeated measures ANOVA (general linear model, GLM) (Statistic 7.0, Stat Soft Inc., USA, 2004), with “TMS condition” (control, MV) and “interval” (single pulse, 3 ms ISI, 13 ms ISI). Duncan’s test was applied where appropriate as post hoc analysis. The two groups were divided according to the ratio: values of MEP response during MV to paired pulse TMS with 13 ms ISI to corresponding MEP values at single pulse TMS −> or < of 1. Nonparametric Mann–Whitney U test was used to compare responses to MV in the two groups.
Results There was no evidence of a tonic vibration reflex in the EMG, and none of the investigated subjects reported any perception of illusory movement during the applied MV.
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The mean value (±SD) of the motor threshold at rest (without MV) was 52.7 ± 39 % of the maximal stimulator output. The two-way repeated ANOVA measures, with “TMS condition” and “interval” factors showed a significant effect of the “TMS condition” (F1,72 = 6.14, p = 0.0018) and “interval” (F2,72 = 20.82, p = 0.00000) factors for the vibrated ECR. Interaction between the factors was insignificant (F2,72 = 2.14, p = 0.124). For non-vibrated FCR, the two-way repeated ANOVA measures revealed a significant effect of the “interval” (F2,72 = 27.45, p = 0.0000) factor and an insignificant effect of the “TMS condition” (F1,72 = 0.47, p = 0.496) factor, which was also insignificant for the interaction between the factors (F2,72 = 0.40, p = 0.670). The mean values of MEP areas recorded from ECR and FCR in response to single-pulse TMS without MV were 7.64 ± 3.28 and 5.83 ± 3.02 mV ms, respectively, for the group of all investigated subjects (n = 19). Figure 2 illustrates the simultaneous recordings of MEPs from both muscles (vibrated ECR and its antagonist FCR), without MV (a) and with MV (b). During MV, the mean value of MEP area in response to single pulse TMS for ECR was significantly greater (Duncan’s test, p = 0.0065), 15.81 ± 14.16 mV ms, than without MV, whereas the parameters for FCR were not affected by MV (mean MEP area 6.12 ± 3.17 mV ms) (see also Fig. 3, upper plots). Without MV, the SICI was well expressed (significant reduction of MEPs) in both muscles ECR and FCR (p = 0.0364 and p = 0.0001, respectively), while ICF was not evident (Fig. 3, open bars). During MV, the SICI was expressed in both muscles (p = 0.0001 and p = 0.0007), while ICF was not evident (Fig. 3, lower plots, closed bars). In nine of the investigated subjects, during MV, the normalized MEP areas recorded in response to paired-pulse TMS were significantly suppressed with respect to single-pulse MEP in both muscles, and not only at 3 ms ISI (Duncan’s test, p = 0.0003 and p = 0.0002 for ECR and FCR, respectively), but also at 13 ms ISI (p = 0.0450 and p = 0.0182). The late depression was not observed in the control condition (Fig. 4, lower plots, open bars). The differences between control and MV condition in response to single pulse TMS was significant (p = 0.045) only in the vibrated ECR (Fig. 4, upper plots, closed bars). The effect of MV in the other ten subjects was different: during MV the SICI was significant only in the vibrated ECR (p = 0.0036) and at 13 ms ISI the ICF was significant in both muscles (Duncan’s test, p = 0.0073 and p = 0.0444 for ECR and FCR, respectively) (Fig. 5, lower plots, closed bars). Furthermore, augmentation of MEPs recorded from the vibrated ECR during MV was significant at singlepulse TMS (p = 0.0418), as well as at paired-pulse TMS
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Eur J Appl Physiol
Fig. 2 MEPs recorded in response to single pulse TMS without MV (a) and with MV (b) for the vibrated ECR muscle and its antagonist FCR
with 13 ms ISI (p = 0.0004) (Fig. 5, upper plots, closed bars). There was a significant (Mann–Whitney U test, z = −1.965, p = 0.049) difference between the two groups concerning the effect of MV at paired pulse TMS and 13 ms ISI.
Discussion The paired-pulse TMS paradigm, originally suggested by Kujirai et al. (1993), has been used for the assessment of SICI and ICF. This technique presumably tests two independent GABAA-mediated intracortical mechanisms (Sanger et al. 2001; Ziemann et al. 1996b). In our earlier experiments (Kossev et al. 2003), we have studied the effect of the conditioning stimulus intensity on MEP parameters recorded from ECR in response to paired-pulse TMS using the same ISIs (3 and 13 ms) and the same intensity of the test stimulus (120 % of MT) as in the present study. At very low intensities (
