the motor system is a dynamic network of cortical and subcortical areas interacting through excitatory and inhibitory circuits , modulated by somatosensory input .
modifications of cortical excitability enable recovery and reorganization of the remaining motor areas both in animal models [ 4 , 5 ] and in humans [ 1 , 6 ] .
transcranial magnetic stimulation ( tms ) and magnetoencephalography ( meg ) have both been applied in stroke patients to reveal cortical excitability changes .
motor threshold ( mt ) , that is , the minimal tms intensity eliciting motor evoked potentials ( meps ) , is related to membrane excitability as it is influenced by drugs affecting neuronal ion channels .
paired pulse tms ( pp - tms ) reveals short - interval intracortical inhibition ( sici ) , related to the activation of gaba - a receptors and intracortical facilitation ( icf ) attributed mainly to glutamatergic nmda receptor activation ( for references , see ) . in acute stroke
, the mt is increased in the lesioned hemisphere ( lh ) whereas in the nonlesioned ( nh ) hemisphere it is normal   or decreased one month after stroke .
sici is decreased in both hemispheres early after stroke ; icf is stronger in nh in severe than mild strokes [ 1 , 6 ] .
finger movements and median nerve   or finger stimulation   modify spontaneous meg oscillations over the sensorimotor cortex in the beta band ( 1525  hz ) .
they are suppressed at 100300  ms after tactile stimulation ( event - related desynchronization ; erd ) , reflecting increased excitability , and increased at 5001000  ms ( event - related synchronization ; ers ) , reflecting removal of excitation [ 12 , 13 ] or reduced excitability .
inhibitory gaba - a agonist diazepam increases meg beta activity [ 15 , 16 ] .
combined meg and magnetic resonance spectroscopy showed a linear relationship between the beta ers strength and gaba concentration .
beta ers has been shown to be significantly weaker in the lh than nh ; stronger ers amplitude was correlated with a better affected hand function up to 3 months after stroke .
reduction of beta ers , however , correlated with clinical improvement after physiotherapy of patients with chronic stroke .
movement - related beta erd was significantly reduced in lh of stroke patients . the hand representation in the somatosensory cortex ( s1 ) , estimated by somatosensory evoked fields ( sefs ) , is the largest one month after stroke and its increase was correlated with the affected hand function . in tms mapping
, hand motor representation expands in the lh up to 1 month . in animal models ,
somatosensory reorganization is activated immediately after the lesion by diminished gaba - a - related inhibition and by glutamatergic activation after one month .
we hypothesized to see correlations between erd and mt related to the early cortical excitability and between ers and sici , both attributed to gaba - a - related processes .
moreover , we expected that icf , reflecting glutamatergic activity , would correlate with the somatosensory modifications in meg , as glutamate is associated with late somatosensory plasticity .
meg and ntms were recorded from thirteen patients ( age 67.3  11 years , 8 women ) , with first - ever ischemic stroke in the middle cerebral artery territory one ( t1 ) and three months after stroke ( t2 ) .
six patients had a subcortical and seven patients had a subcorticocortical or cortical stroke ( table 1 ) . at t1 ,
one patient used amitriptyline 10  mg / day and another used citalopram 10  mg / day .
one patient used zopiclone 7.5  mg for occasional sleeplessness . at t2 , citalopram 10  mg / day was used by one patient .
data from these patients has been presented previously [ 18 , 21 , 23 , 24 ] .
as the patients having both tms and meg recordings are a subset of the previous patient groups , we recalculated the group - level electrophysiological parameters to show that the present patient group is sufficiently similar to those reported previously ( see supplementary tables 1 and 2 in supplementary material available online at http://dx.doi.org/10.1155/2015/309546 ) .
an eximia navigated magnetic stimulator with a coplanar figure - of - eight coil of 70  mm wing radius ( nexstim ltd . , helsinki , finland ) was used for ntms .
patients ' mris , required for navigation , were scanned at t1 and used also at t2 .
the site was then stimulated by single tms pulses at 110% of mt and by the pp - tms at 90% for conditioning and 110% of mt for test pulses .
fifteen single pulses or pairs of conditioning and test pulses were delivered in each stimulation session .
the interval between stimuli was 3.3  s and the intersession interval varied between 1 and 3  min .
the isi selected for the paired - pulse stimuli was 2  ms for sici and 12  ms for icf .
the peak - to - peak amplitudes of meps elicited by pp - tms were normalized by dividing them by the corresponding single - pulse mep amplitude to simplify subject - to - subject comparisons .
meg during rest and tactile stimulation of the thumb ( d1 ) , index ( d2 ) , and little finger ( d5 ) were recorded with a 306-sensor neuromagnetometer ( elekta neuromag , helsinki , finland ) in biomag laboratory , right before the ntms measurement .
time - frequency representations ( tfr ) in the 1030  hz band were calculated to define the frequency range of beta modulation , which was quantified by the temporal spectral evolution method ( tse ) from signals of 2 to 4 meg sensors showing the strongest reactivity . only the contralateral beta modifications ( the affected hand stimulation for the lh and the unaffected hand for nh ) were analyzed .
onset and offset of the erd and ers were defined as a time point when the signal differed 2 sds from the baseline .
the absolute erd and ers strengths were calculated from the peak amplitudes and converted into relative values in relation to the 300  ms prestimulus baseline . for sefs , about 120 responses were averaged for stimulation of d2 ( isi 3005  ms ) , and d1 and d5 ( isi 1005  ms ) in separate sessions .
the size of the hand representation in the si was determined by calculating the euclidean distance in xyz - space between the equivalent current dipoles ( ecds ) of the earliest responses to d1 and d5 stimulation .
multiple comparison correction was carried out according to the number of tests ( n = 32 ) suggested by the four prior hypotheses ( t1 and t2 were tested separately ; lh and nh variations lead to four tests in each case ; n = 442 = 32 ) .
we also present significances of correlation values without multiple comparisons and supply all correlation coefficients and corresponding p values ( cf .
[ 1 , 9 , 28 ] for a similar approach and   for its statistical aspects ) .
the differences between ntms and meg values obtained at t1 and t2 were tested with student 's t - test .
in the lh , meps were found in 11 patients both at t1 and at t2 and were present in the nh in all patients .
mt was higher for lh than nh in 9 patients at t1 ( p < 0.05 , binomial test ) and in 10 patients at t2 ( p < 0.05 , binomial test ) .
the mts in the lh and nh correlated strongly both at t1 ( r = .82 , p < .01 ) and at t2 ( r = .78 , p < .01 ) .
pp2ms stimulations of nh did not inhibit meps ( disinhibition ; diminished sici ) in 5 patients at t1 and in 3 patients at t2 .
pp12ms stimulation of lh facilitated meps ( icf ) in 10 out of 11 patients at t1 and in all patients at t2 . in nh
mep amplitudes elicited by pp12ms stimulations were correlated between the lh and nh at t1 ( r = .62 ; p < .05 ) but not at t2 ( supplementary table 2 ) .
erd started 140  10  ms after tactile stimulation and peaked at 250  10  ms .
the subsequent ers started at 520  40  ms and peaked at 900  70  ms . at t1 ,
erd was absent in one patient and ers in two patients in the lh ; ers was missing from the nh in one patient . at t2 ,
ers was smaller in the lh than nh at t1 ( 46  31% versus 63  32% ; p < .05 ) ; at t2 , the difference was nonsignificant .
sefs from both hemispheres were detected in 12 patients at t1 and in all patients at t2 .
they were smaller in the lh than nh at t1 ( 25 nam versus 32 nam ; p < .04 ) but not at t2 .
the si hand representation area was larger in the lh than nh at t1 ( 12  3  mm versus 10  3  mm p < .003 ) but not at t2 ( supplementary table 3 ) .
the plots of the most relevant correlations are depicted in figure 1 to show that they were not driven by outliers .
all correlations are displayed in table 2 to enable evaluation of significance of our hypotheses against general effects of the lesions . 

the mt and erd were correlated in the lh at t1 ( r = .66 , p < .03 ) , indicating that small erd was associated with a high mt ( figure 1(a ) ) . at t2 , this correlation was weaker ( r = .58 , p < .06 ) . however , the mt of the lh correlated with the erd of nh ( r = .62 , p < .04 ) , and the mt of the nh correlated with erd of the lh ( r = .65 , p < .02 ) , suggesting that high mt was associated with a small erd in the opposite hemisphere at t2 ( table 2 ) . 

sici and the ers did not correlate at t1 or in lh at t2 . in the nh ,
high ers was associated with a strong sici ( r = .59 , p < .04 ; figure 1(b ) ) .
in addition to hypothesized correlations , sici of the nh and erd of the lh at t2 were correlated ( r = .82 , p < .001 ) , indicating that strong erd in the lh was associated with a strong sici in the nh .
sici in the lh was correlated also with the si amplitude of the lh ( r = .64 , p < .04 ) , indicating that small si amplitude was associated with a weak sici ( table 2 ) . 

icf and sef parameters did not correlate at t1 . at t2 , icf in the lh correlated with the s1 amplitude of lh ( r = .65 , p
in addition , icf in the nh correlated ( r = .82 , p < .001 ) with the si finger representation area of the lh ; this correlation remained significant also after bonferroni correction ( table 2 ) .
moreover , icf in the lh at t1 was correlated ( r = .83 ; p < .002 ) with the si finger representation area of lh at t2 ; high icf at t1 resulted in a small hand representation area at t2 ( figure 1(c ) ) .
our study is the first to compare meg and ntms excitability parameters during stroke recovery .
navigated tms , not applied previously in longitudinal studies of stroke patients , enabled the precise replication of the stimulus site between separate measurements , adding reliability to the follow - up .
we found correlations between cortical excitability estimates derived from ntms and meg . as expected , we found correlations between mt and erd , but in only one of the four possible intrahemispheric and two out of four interhemispheric conditions .
the sici and beta ers , both attributed to gabaergic mechanisms , were correlated in one of their four possible intrahemispheric conditions ( in the nh at t2 ) .
one reason for this may be that various gaba - a receptor subtypes contribute to sici .
nonselective gaba - a receptor activators modify sici whereas the gaba - a1 receptor specific zolpidem did not .
nonselective gaba - a agonist zopiclone increased meg beta activity whereas zolpidem suppressed beta activity in the vicinity of stroke lesion .
moreover , in healthy subjects , diazepam increased meg erd but did not affect ers when the increase of baseline beta activity was taken into account .
this nonspecificity could contribute to the correlations of sici with the erd and mt as well .
correlations between ntms parameters and meg erd / ers were stronger at t2 than at t1 .
analogously , most tms intracortical excitability measures did not correlate with the hand function acutely but did so 3 months after stroke .
recovery of sensorimotor fmri activation to digit stimulation from 1 to 3 months was correlated with final motor function , emphasizing the importance of this time period for stroke recovery .
is attributed mainly to glutaminergic mechanisms , glutamate may contribute to stroke - induced plasticity .
somewhat surprisingly , high icf at t1 correlated with small si hand area at t2 ( figure 1(c ) ) ; thus , the narrowing towards normal hand representation size may be supported by glutaminergic activity .
icf did not correlate with the meg erd / ers , and the si hand area and beta erd / ers were not correlated ; this suggests different mechanisms underlying sici and icf ( see   for a detailed discussion ) .
for example , high mt was associated with a small erd in the opposite hemisphere , and strong erd in the lh was associated with a strong sici in the nh .
increased excitability within the unaffected motor cortex may cause imbalance between the homologous cortical motor areas and worsen also the ipsilesional hand coordination ( for references , see ) .
thus , some functional correlations may relate to the modified general excitability properties of the motor system , instead of effects in the immediate vicinity of the stroke .
tms results give direct information of the changes in the motor output and the immediate effects of tms are relatively local .
however , also subcortical and spinal processes affect the meps used to evaluate the tms effects .
meg reveals the activity of the whole cortical mantle and enables mapping of network effects generated by stroke .
meg source analysis suggests mainly motor cortex origin of beta ers [ 13 , 33 , 34 ] .
however , in electrocorticography , recorded directly from the cortex , beta erd and ers appear outside of pre- and postcentral gyri , in supplementary motor cortex , or broadly from pre- and postcentral gyri , frontolateral and medial cortex [ 37 , 38 ]
. the widespread cortical generation of the erd and ers may make them resilient to small cortical strokes .
multitude of generators may contribute to considerable variability of source locations of beta erd in stroke patients ( cf . ) .
multiple sources underlying meg signals may also explain resilience of auditory evoked fields after small strokes .
stronger correlations between icf and si parameters than between mt and sici and ers / erd may , in part , result from spatially more limited source areas of s1 responses than those of erd / ers .
it can be expected that meg and ntms produce complementary information about the effects of stroke on cortical networks . moreover ,
meg parameters in the affected hemisphere and ntms indices in the unaffected hemisphere were correlated with the motor performance of the affected hand ( cf . [ 18 , 21 , 24 ] ) .
erd in the 822  hz band may reflect downregulation of intracortical inhibition in the human motor cortex , as tms delivered during erd is associated with increased mep amplitudes and reduced sici .
however , 1  hz repetitive tms over the motor cortex reduces meps to subsequent single tms pulses , indicating inhibition , but decreases postmovement beta ers , and intermittent theta burst tms facilitates meps but increases postmovement beta ers .
beta ers is reduced in patients with myoclonus epilepsy , indicating increased cortical excitability . in line
, sici is decreased in myoclonus epilepsy patients ; however , mt is increased [ 44 , 45 ] , and the silent period after the tms pulse , reflecting motor cortical postsynaptic inhibition , is prolonged , indicating prevailing inhibitory cortical tonus .
thus , only some aspects of the cortical excitability may be shared in excitability estimates obtained by tms and meg .
our results suggest that some tms and meg excitability measures reflect the activity of the same transmitter systems .
however , high mt and absence of erd / ers may also correlate because of severely affected sensorimotor connections between the periphery and the cortex .
we detected sefs in 12/13 patients and meps in 11/13 patients already at t1 , indicating that both somatosensory and motor pathways were conveying signals .
motor function can be maintained despite significant damage to the corticospinal tract , as estimated from mt of stroke patients .
moreover , we observed fewer correlations at t1 , when the sensorimotor pathways probably were more affected , than at t2 .
however , such spurious correlations should remain stable or decrease during recovery from t1 to t2 but not increase , as in our data .
the limitations of our study include the small size of the patient group as the precise features of structural and functional changes may differ among the patients .
cortical excitability is modified differently in cortical and subcortical strokes [ 46 , 47 ] .
this , however , should not alter our correlations between the tms and meg , as both were recorded from the same patients .
possible effects of medication on excitability should go in parallel for meg and ntms , as the patients were tested sequentially during the same day .
the patients were not tested in the acute stage with tms , and meg recordings showed most dramatic erd and ers modifications between the acute phase and t1 . although mt in the lh is correlated with the paretic hand function in acute stroke , this correlation , however , weakens during recovery , and tms intracortical excitability parameters correlated with the clinical performance best at 3 months .
the 2  ms or 12  ms isis , selected for our paired - pulse stimuli , produce clear sici and icf in healthy subjects   and in stroke patients , but we did not test other parameters , which could have produced stronger correlations between ntms and meg .
icf and si response amplitude and area size , mt and the erd of the hemisphere harboring the stroke lesion , and sici and ers of the nonlesioned hemisphere are correlated in stroke patients .
numerous correlations of the excitability parameters between the lh and nh emphasize the importance of the hemispheric balance of the excitatory - inhibitory properties of the sensorimotor system in analyzing the stroke - related dysfunction during stroke recovery .
 
 objective . 
 stroke alters cortical excitability both in the lesioned and in the nonlesioned hemisphere . 
 stroke recovery has been studied using transcranial magnetic stimulation ( tms ) . 
 spontaneous brain oscillations and somatosensory evoked fields ( sefs ) measured by magnetoencephalography ( meg ) are modified in stroke patients during recovery . 
 methods . 
 we recorded sefs and spontaneous meg activity and motor threshold ( mt ) short intracortical inhibition ( sici ) and intracortical facilitation ( icf ) with navigated tms ( ntms ) at one and three months after first - ever hemispheric ischemic strokes . 
 changes of meg and ntms parameters attributed to gamma - aminobutyrate and glutamate transmission were compared . 
 results . 
 icf correlated with the strength and extent of sef source areas depicted by meg at three months . 
 the ntms mt and event - related desynchronization ( erd ) of beta - band meg activity and sici and the beta - band meg event - related synchronization ( ers ) were correlated , but less strongly . conclusions . 
 this first report using sequential ntms and meg in stroke recovery found intra- and interhemispheric correlations of ntms and meg estimates of cortical excitability . 
 icf and sef parameters , mt and the erd of the lesioned hemisphere , and sici and ers of the nonlesioned hemisphere were correlated . 
 covarying excitability in the lesioned and nonlesioned hemispheres emphasizes the importance of the hemispheric balance of the excitability of the sensorimotor system . 