Corticospinal excitability and sleep: a motor threshold assessment by transcranial magnetic stimulation after awakenings from REM and NREM sleep

Transcranial magnetic stimulation (TMS) is a recently established technique in the neurosciences that allows the non-invasive assessment, among other parameters, of the excitability of motor cortex. Up to now, its application to sleep research has been very scarce and because of technical problems it provided contrasting results. In fact delivering one single suprathreshold magnetic stimulus easily awakes subjects, or lightens their sleep. For this reason, in the present study we assessed motor thresholds (MTs) upon rapid eye movement (REM) and non-rapid eye movement (NREM) sleep awakenings, both in the first and in the last part of the night. Taking into account that a full re-establishment of wake regional brain activity patterns upon awakening from sleep needs up to 20-30 min, it is possible to make inferences about the neurophysiological characteristics of the different sleep stages by analyzing the variables of interest immediately after provoked awakenings. Ten female volunteers slept in the lab for four consecutive nights. During the first night the MTs were collected, following a standardized procedure: 5 min before lights off, upon stage 2 awakening (second NREM period), upon REM sleep awakening (second REM period), upon the final morning awakening (always from stage 2). Results showed that MTs increased linearly from presleep wakefulness to REM sleep awakenings, and from the latter to stage 2 awakenings. There was also a time-of-night effect on MTs upon awakening from stage 2, indicating that MTs decreased from the first to the second part of the night. The increase in corticospinal excitability across the night, which parallels the fulfillment of sleep need, is consistent with the linear decrease of auditory arousal thresholds during the night. The maximal reduction of corticospinal excitability during early NREM sleep can be related to the hyperpolarization of thalamocortical neurons, and is in line with the decreased metabolic activity of motor cortices during this sleep stage. On the contrary, the increase of MTs upon REM sleep awakenings should reflect peripheral factors. We conclude that our findings legitimate the introduction of the TMS technique as a new proper tool in sleep research.     

Enhancement of memory by auditory stimulation during postlearning REM sleep in humans

REM sleep involvement in memory processes was demonstrated in animals and humans: 1) REM sleep deprivation impairs the memory fixation, 2) learning sessions are followed by modifications of REM sleep characteristics. Moreover, sleep patterns can be modified by applying auditory stimulations during REM sleep. We show that REM actual auditory stimulations significantly improve the retention of a Morse code learning task. These results are discussed in terms of brain activation.     

Cortical activation during temporal preparation assessed by transcranial magnetic stimulation

Preparatory modulations relative to the timing of upcoming stimuli may involve activation or suppression mechanisms. Here, we assessed the interplay between these mechanisms with transcranial magnetic stimulation (TMS) of the motor cortex. Single- or paired-pulse TMS with 3- or 15-ms interstimulus intervals was delivered during the interval between the warning and the imperative stimuli (i.e., the foreperiod) of a choice reaction time task. Temporal uncertainty was manipulated through between-block variation of the foreperiod duration (500 or 2500 ms). The shortening of reaction time for the short foreperiod was accompanied with a decrease in amplitude of the single-pulse motor evoked potential (MEP), indicating corticospinal suppression. The co-occurring increase in amplitude of both paired-pulse MEPs (3 and 15 ms) expressed relative to single-pulse MEPs reveals released short intracortical inhibition (SICI) and enhanced intracortical facilitation (ICF). These results suggest that temporal preparation is associated with both corticospinal suppression and cortical activation.     

Cortical activation during temporal preparation assessed by transcranial magnetic stimulation

Preparatory modulations relative to the timing of upcoming stimuli may involve activation or suppression mechanisms. Here, we assessed the interplay between these mechanisms with transcranial magnetic stimulation (TMS) of the motor cortex. Single- or paired-pulse TMS with 3- or 15-ms interstimulus intervals was delivered during the interval between the warning and the imperative stimuli (i.e., the foreperiod) of a choice reaction time task. Temporal uncertainty was manipulated through between-block variation of the foreperiod duration (500 or 2500 ms). The shortening of reaction time for the short foreperiod was accompanied with a decrease in amplitude of the single-pulse motor evoked potential (MEP), indicating corticospinal suppression. The co-occurring increase in amplitude of both paired-pulse MEPs (3 and 15 ms) expressed relative to single-pulse MEPs reveals released short intracortical inhibition (SICI) and enhanced intracortical facilitation (ICF). These results suggest that temporal preparation is associated with both corticospinal suppression and cortical activation.     

Reduced intra-cortical inhibition after sleep deprivation: a transcranial magnetic stimulation study

Sleep deprivation has multiple effects on brain function. It increases the risk for epileptic seizures both in healthy individuals and in patients with epilepsy. Furthermore it represents an effective antidepressive intervention with rapid onset. However, the mechanisms underlying these effects are still largely unknown. Transcranial magnetic stimulation (TMS) can be used as a non-invasive method for the measurement of motor cortex excitability. Here we used TMS for assessing sleep deprivation effects on cortical excitability in healthy individuals. Before and after 24 h of sleep deprivation, parameters of cortical excitability (resting motor threshold, short intracortical inhibition, intracortical facilitation, cortical silent period) were measured in a sample of 15 healthy volunteers (11 women, 4 men, aged between 21 and 30 years with a mean of 24.3±2.7 years). We detected a significant (p=0.042) reduction of short intracortical inhibition (SICI) after sleep deprivation. Motor threshold, intracortical facilitation and contralateral silent period remained unchanged. Our results confirm previous studies which have demonstrated changes of SICI after sleep deprivation. Our findings further suggest that the increased risk for epileptic seizures after sleep deprivation is mediated by a reduction of intracortical inhibition. Whether this mechanism is also involved in mediating the antidepressant effect of sleep deprivation has to be addressed by further studies in depressive patients.     

Modulation of slow cortical potentials by transcranial magnetic stimulation in humans

We studied the effects of transcranial magnetic stimulation (TMS) on slow cortical potentials (SCPs) of the brain elicited during performance of a feedback and reward task. Ten healthy participants were trained to self-regulate their SCP amplitude using visual feedback and reward for increased or decreased amplitudes. Subjects participated in 27 runs (each comprising 70 trials) under three different conditions: single-pulse TMS delivered with the coil centered over Cz (vertex), over a lateral scalp position (LSP), which increased task difficulty, and in the absence of stimulation. Cz stimulation led to a non-significant enhancement of negative SCPs, while LSP stimulation led to a significant increase of positive SCPs. These results are consistent with the idea that enhanced task difficulty, as in LSP stimulation, enhances cognitive processing load leading to an increase of positive SCPs. Additionally, the data raise the hypothesis that TMS delivered to bilateral midcentral regions could modulate the amplitude of negative SCPs.     

Hemispheric asymmetry in visuospatial attention assessed with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) was used to study visuospatial attention processing in ten healthy volunteers. In a forced choice recognition task the subjects were confronted with two symbols simultaneously presented during 120 ms at random positions, one in the left and the other in the right visual field. The subject had to identify the presented pattern out of four possible combinations and to press the corresponding response key within 2 s. Double-pulse TMS (dTMS) with a 100-ms interstimulus interval (ISI) and an intensity of 80% of the stimulator output (corresponding to 110-120% of the motor threshold) was applied by a non-focal coil over the right or left posterior parietal cortex (PPC, corresponding to P3/P4 of the international 10-20 system) at different time intervals after onset of the visual stimulus (starting at 120 ms, 270 ms and 520 ms). Double-pulse TMS over the right PPC starting at 270 ms led to a significant increase in percentage of errors in the contralateral, left visual field (median: 23% with TMS vs 13% without TMS, P=0.0025). TMS applied earlier or later showed no effect. Furthermore, no significant increase in contra- or ipsilateral percentage of errors was found when the left parietal cortex was stimulated with the same timing. These data indicate that: (1) parietal influence on visuospatial attention is mainly controlled by the right lobe since the same stimulation over the left parietal cortex had no significant effect, and (2) there is a vulnerable time window to disturb this cortical process, since dTMS had a significant effect on the percentage of errors in the contralateral visual hemifield only when applied 270 ms after visual stimulus presentation.     

Reduction of cortico-spinal excitability by transcranial magnetic stimulation at predictable timing

Electrophysiological studies have shown that cortico-spinal excitability increases during the motor preparation period in reaction time (RT) paradigms. However, there is a line of contradictory evidence with transcranial magnetic stimulation (TMS) showing that its excitability is reduced during the preparation period. In these studies, the subjects can predict the TMS timings. Thus we investigated how the predictability of TMS timing affects cortico-spinal excitability. A single-pulse TMS was delivered to the hand section of the left motor cortex while seven right-handed subjects relaxed their hands in a flexed position. We prepared three conditions: (i) in the semi-PREDICTABLE condition, two visual stimuli at 500 ms interval were presented and the TMS was delivered either 0, 125, 250, 375, or 500 ms after the first stimulus; (ii) in the PREDICTABLE condition, the TMS was provided only at 500 ms after the first stimulus; (iii) in the UNPREDICTABLE condition, no visual cue preceded the TMS. We recorded motor evoked potentials (MEPs) from the wrist flexor and extensor muscles. We found a significant reduction of MEP amplitude in the flexor muscles in both the PREDICTABLE and semi-PREDICTABLE conditions, but not in the UNPREDICTABLE condition. These results showed that the predictability of TMS per se, without the preparation of motor outputs, can reduce cortico-spinal excitability.     

Suppression of the transcallosal motor output: a transcranial magnetic stimulation study in healthy subjects

We tested the hypothesis that transcranial magnetic stimulation (TMS), in addition to its inhibitory action on the corticospinal output, can also exert some inhibitory effect on the transcallosal system connecting the two motor cortices. In seven normal subjects, instructed to keep their right opponens pollicis (OP) muscle fully relaxed and their left OP muscle voluntarily contracted, we used a paired-pulse TMS protocol, to stimulate the left motor cortex. We evaluated the effect of low-intensity conditioning stimulation on the ipsilateral silent period (iSP) elicited by the subsequent test stimulus. Compelling evidence exists to support that this iSP is mediated by the activation of transcallosal motor fibres. Simultaneously, the inhibition of the motor evoked potential (MEP) in the right OP muscle was also investigated. At the interstimulus interval (ISI) of 3 ms, the iSP was significantly (P<0.0001, repeated-measures ANOVA) suppressed by conditioning intensities ranging from 1.2 to 0.6 of MEP threshold. The assessment of the time-course showed that iSP inhibition was present in all the tested subjects only at ISIs of 2 and 3 ms (for each subject P<0.05, repeated-measures ANOVA). Several findings suggest that the suppression of iSP is brought about by the activation of inhibitory mechanisms operating in the stimulated (left) motor cortex. We propose that the assessment of iSP suppression could be a method to study the excitability of intracortical inhibitory circuits in the affected hemisphere of patients with unilateral damage of the corticospinal tract.     

Measures of cortical inhibition by paired-pulse transcranial magnetic stimulation in anesthetized rats

Paired-pulse transcranial magnetic stimulation (ppTMS) is a noninvasive method to measure cortical inhibition in vivo. Long interpulse interval (50-500 ms) ppTMS (LI-ppTMS) provokes intracortical inhibitory circuits and can reveal pathologically impaired cortical inhibition in disorders such as epilepsy. Adaptation of ppTMS protocols to rodent disease models is highly desirable to facilitate basic and translational research. We previously adapted single-pulse TMS (spTMS) methods to rats, but ppTMS has yet to be applied. Specifically, whether ppTMS elicits an inhibitory response in rodents is unknown. ppTMS in rats also requires anesthesia, a setting under which the preservation of these measures is undetermined. We therefore tested, in anesthetized rats, whether anesthetic choice affects spTMS-motor-evoked potentials (MEPs), LI-ppTMS in rats, as in humans, elicits intracortical inhibition of the MEP, and rat LI-ppTMS inhibition is acutely impaired in a seizure model. Rats were anesthetized with pentobarbital (PB) or ketamine-atropine-xylazine (KAX) and stimulated unilaterally over the motor cortex while recording bilateral brachioradialis MEPs. LI-ppTMS was applied analogous to human long interval intracortical inhibition (LICI) protocols, and acute changes in inhibition were evaluated following injection of the convulsant pentylenetetrazole (PTZ). We find that spTMS-evoked MEPs were reliably present under either anesthetic, and that LI-ppTMS elicits inhibition of the conditioned MEP in rats, similar to human LICI, by as much as 58 ± 12 and 71 ± 11% under PB and KAX anesthesia, respectively. LI-ppTMS inhibition was reduced to as much as 53% of saline controls following PTZ injection, while spTMS-derived measures of corticospinal excitability were unchanged. Our data show that regional inhibition, similar to human LICI, is present in rats, can be elicited under PB or KAX anesthesia, and is reduced following convulsant administration. These results suggest a potential for LI-ppTMS as a biomarker of impaired cortical inhibition in murine disease models.     

Auditory evoked responses upon awakening from sleep in human subjects

The hypothesis that a state of hypoarousal upon awakening should lead to a decrease in amplitude and an increase in latency of the N1-P2 components of the Auditory Evoked Potentials (AEPs) as compared to presleep wakefulness levels, was evaluated after two nocturnal awakenings and after the final morning awakening from a 7.5-h night of sleep. The amplitude of the N1-P2 complex was reduced upon awakening as compared to presleep wakefulness levels, but only following the first nocturnal awakening, scheduled after the first 2 h of sleep. This result is interpreted as indicating a link between slow wave sleep amount, mainly present during the first part of the night, and lowered levels of brain activation upon awakening. The reaction times, recorded concomitantly to AEPs, were more sensitive to the negative effects of sleep inertia.     

Selective suppression of the incorrect response implementation in choice behavior assessed by transcranial magnetic stimulation

Selecting the adequate alternative in choice situations may involve an inhibition process. Here we assessed response implementation during the reaction time of a between‐hand choice task with single‐ or paired‐pulse (3 or 15 ms interstimulus intervals [ISIs]) transcranial magnetic stimulation of the motor cortex. The amplitude of the single‐pulse motor evoked potential (MEP) initially increased for both hands. At around 130 ms, the single‐pulse MEP kept increasing for the responding hand and decreased for the nonresponding hand. The paired‐pulse MEP revealed a similar pattern for both ISIs with no effect on short intracortical inhibition and intracortical facilitation measures. The results suggest that the incorrect response implementation was selectively suppressed before execution of the correct response, preventing errors in choice context. The results favor models assuming that decision making involves an inhibition process.     

Short-term homeostasis of REM sleep assessed in an intermittent REM sleep deprivation protocol in the rat

An intermittent rapid eye movement (REM) sleep deprivation protocol was applied to determine whether an increase in REM sleep propensity occurs throughout an interval without REM sleep comparable with the spontaneous sleep cycle of the rat. Seven chronically implanted rats under a 12 : 12 light-dark schedule were subjected to an intermittent REM sleep deprivation protocol that started at hour 6 after lights-on and lasted for 3 h. It consisted of six instances of a 10-min REM sleep permission window alternating with a 20-min REM sleep deprivation window. REM sleep increased throughout the protocol, so that total REM sleep in the two REM sleep permission windows of the third hour became comparable with that expected in the corresponding baseline hour. Attempted REM sleep transitions were already increased in the second deprivation window. Attempted transitions to REM sleep were more frequent in the second than in the first half of any 20-min deprivation window. From one deprivation window to the next, transitions to REM sleep changed in correspondence to the amount of REM sleep in the permission window in-between. Our results suggest that: (i) REM sleep pressure increases throughout a time segment similar in duration to a spontaneous interval without REM sleep; (ii) it diminishes during REM sleep occurrence; and (iii) that drop is proportional to the intervening amount of REM sleep. These results are consistent with a homeostatic REM sleep regulatory mechanism that operates in the time scale of spontaneous sleep cycle.     

Effect of low-frequency repetitive transcranial magnetic stimulation on interhemispheric inhibition

We studied the effects of 1-Hz repetitive transcranial magnetic stimulation (rTMS) on the excitability of interhemispheric connections in 13 right-handed healthy volunteers. TMS was performed using figure-eight coils, and surface electromyography (EMG) was recorded from both first dorsal interosseous (FDI) muscles. A paired-pulse method with a conditioning stimulus (CS) to the motor cortex (M1) followed by a test stimulus to the opposite M1 was used to study the interhemispheric inhibition (ppIHI). Both CS and TS were adjusted to produce motor-evoked potentials of approximately 1 mV in the contralateral FDI muscles. After baseline measurement of right-to-left IHI (pre-RIHI) and left-to-right IHI (pre-LIHI), rTMS was applied over left M1 at 1 Hz with 900 stimuli at 115% of resting motor threshold. After rTMS, ppIHI was studied using both the pre-rTMS CS (post-RIHI and post-LIHI) and an adjusted post-rTMS CS set to produce 1-mV motor evoked potentials (MEPs; post-RIHI(adj) and post-LIHI(adj)). The TS was set to produce 1-mV MEPs. There was a significant reduction in post-LIHI (P = 0.0049) and post-LIHI(adj) (P = 0.0169) compared with pre-LIHI at both interstimulus intervals of 10 and 40 ms. Post-RIHI was significantly reduced compared with pre-RIHI (P = 0.0015) but pre-RIHI and post-RIHI(adj) were not significantly different. We conclude that 1-Hz rTMS reduces IHI in both directions but is predominantly from the stimulated to the unstimulated hemisphere. Low-frequency rTMS may be used to modulate the excitability of IHI circuits. Treatment protocols using low-frequency rTMS to reduce cortical excitability in neurological and psychiatric conditions need to take into account their effects on IHI.     

Story structure in verbal reports of mental sleep experience after awakening in REM sleep.

Four night reports obtained after awakening in rapid eye movement (REM) sleep and their corresponding morning reports were collected from 20 subjects in an experimental night. All of the reports were analyzed using a slightly modified version of Mandler and Johnson's story grammar. Values for a series of indicators were compared with respect to the factors "moment of reporting" (at night/in the morning) and "order of reporting" (first/second/third/fourth report). Story-like organization seems to be a feature of dream production and not merely due to reconstructive effects in recall: values of no indicator significantly differ in night and morning reports. The more extensive thematic progression and the increasing complexity in reports over the first half of the night show that the effectiveness of the system of dream production varies in different periods of REM sleep. These findings, while strengthening the view of a multilevel character of the system of dream production, still generate the problem of the direction of activation (bottom-up or top-down) of this system.     

Modulation of intracortical inhibition induced by low- and high-frequency repetitive transcranial magnetic stimulation

We studied the changes of duration of subsequent silent periods (SPs) during repetitive magnetic stimulation (rTMS) trains of ten stimuli delivered at low (1 Hz) and high (7 Hz) frequencies. The effects at different intensities of stimulation (motor threshold, MT, 115% and 130% above the MT) were also evaluated. rTMS was performed in eight healthy subjects with a figure-of-eight coil placed over the hand motor area. The SP was recorded from abductor pollicis brevis (APB) muscle during a voluntary contraction of 30% of maximum effort. rTMS at 1-Hz frequency progressively decreased the duration of SP, whereas an alternating pattern of smaller and larger values was observed during trains at 7-Hz frequency and higher stimulus intensity. The findings show that rTMS changes the duration of cortical SPs; the effect is probably due to the modulation of intracortical inhibitory interneurons depending on the frequency and intensity of stimulation.     

The effect of transcranial magnetic stimulation and peripheral nerve stimulation on corticomuscular coherence in humans

Cortex and muscle show coupled oscillations in the 15–35 Hz frequency band during voluntary movements. To obtain evidence of the neuronal network responsible for this rhythmicity we investigated the effect of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation on the coupling between eletcroencephalographic (EEG) activity recorded from the scalp over the motor cortex and electromyographic (EMG) activity recorded from the tibialis anterior (TA) muscle in 15 healthy human subjects. TMS over the leg area at intensities between 0.95 and 1.1 × threshold for a motor evoked potential (MEP) in the TA increased corticomuscular coherence in the 15–35 Hz frequency band. This effect lasted on average for 300 ms, but could last up to 600–800 ms in some subjects. Stimulation of motor nerves from the ankle muscles suppressed corticomuscular coherence in the 15–35 Hz frequency range between leg area EEG and TA EMG for a period up to 600–800 ms. In addition, increased coherence around 10 Hz was observed for a period up to 250 ms after the stimulation. Stimulation of motor nerves in the arm and motor nerves from the ankle muscles in the other leg had no effect. The findings indicate that TMS has direct access to the neuronal circuitry in the motor cortex, which generates the corticomuscular coherence. This effect was caused either by direct activation of corticospinal cells or by activation of local neuronal circuitries in the motor cortex. The effects of peripheral nerve stimulation suggest that an alternative rhythm generator may entrain the cortical cells into a lower 10 Hz rhythm and disrupt the 15–35 Hz rhythm.     

Cortical excitability in obstructive sleep apnea syndrome: transcranial magnetic stimulation study

STUDY OBJECTIVE: To investigate cortical motor area function in patients with obstructive sleep apnea syndrome (OSAS) during the daytime. DESIGN: The day after a nocturnal polysomnography, transcranial magnetic stimulation (TMS) of the motor cortex was performed recording Motor Evoked Potential from the first dorsal interosseous muscle of the dominant hand. We evaluated: 1) the relaxed motor threshold (RMT), 2) the threshold of the cortical silent period (CSP), 3) the duration of CSP elicited by five stimulus intensities (95%,100%,105%,130%, and 150% of RMT). To estimate the influence of waking on TMS, recordings were performed five times in a day. The Epworth Sleepiness Scale (ESS), and Stanford Sleepiness Scale (SSS) were also measured. SETTING: The study was carried out in the Sleep and Evoked Potentials laboratories of the Don C. Gnocchi Foundation (ONLUS IRCCS) Pozzola tico, (Florence), Italy. PATIENTS: 10 patients with OSAS and 10 healthy volunteers. Intervention: N/A Measurements and Results: In OSAS patients, ESS and SSS were significantly higher than in controls. Patients had a longer duration of CSP at 95%,100% and 105% RMT intensity at almost recording hours; with 130% of RMT stimuli intensity OSAS patients were significantly different at 10AM from controls and with 150% of RMT intensity the difference did not reach significativity. PaCO2 was significantly correlated with CSP duration elicited at 10AM with 95%, 100% and 105% of RMT stimulus intensities. CONCLUSIONS: We found alterations of motor cortical excitability in OSAS patients during the daytime. We believe that PaCO2 levels, acting probably on various ion channels or metabolic pathways, may change the excitability of motor cortex modifying excitatory and inhibitory cortical circuits.