Short-interval intracortical inhibition blocks long-term potentiation induced by paired associative stimulation

Effects of Paired Associative Stimulation on Cortical Plasticity in Agonist–Antagonist Muscle Representations

Paired associative stimulation (PAS) increases and decreases cortical excitability in primary motor cortex (M1) neurons, depending on the spike timing-dependent plasticity, i.e., long-term potentiation (LTP)- and long-term depression (LTD)-like plasticity, respectively. However, how PAS affects the cortical circuits for the agonist and antagonist muscles of M1 is unclear. Here, we investigated the changes in the LTP- and LTD-like plasticity for agonist and antagonist muscles during PAS: 200 pairs of 0.25-Hz peripheral electric stimulation of the right median nerve at the wrist, followed by a transcranial magnetic stimulation of the left M1 with an interstimulus interval of 25 ms (PAS-25 ms) and 10 ms (PAS-10 ms). The unconditioned motor evoked potential amplitudes of the agonist muscles were larger after PAS-25 ms than after PAS-10 ms, while those of the antagonist muscles were smaller after PAS-25 ms than after PAS-10 ms. The γ-aminobutyric acid A (GABAA)- and GABAB-mediated cortical inhibition for the agonist and antagonist muscles were higher after PAS-25 ms than after PAS-10 ms. The cortical excitability for the agonist and antagonist muscles reciprocally and topographically increased and decreased after PAS, respectively; however, GABAA and GABAB-mediated cortical inhibitory functions for the agonist and antagonist muscles were less topographically decreased after PAS-10 ms. Thus, PAS-25 ms and PAS-10 ms differentially affect the LTP- and LTD-like plasticity in agonist and antagonist muscles.

The neurophysiological aftereffects of brain stimulation in human primary motor cortex: a Sham-controlled comparison of three protocols

Paired associative stimulation (PAS), transcranial direct current stimulation (tDCS), and transcranial alternating current stimulation (tACS) are non-invasive brain stimulation methods that are used to modulate cortical excitability. Whether one technique is superior to the others in achieving this outcome and whether individuals that respond to one intervention are more likely to respond to another remains largely unknown. In the present study, the neurophysiological aftereffects of three excitatory neurostimulation protocols were measured with transcranial magnetic stimulation (TMS). Twenty minutes of PAS at an ISI of 25 ms, anodal tDCS, 20-Hz tACS, and Sham stimulation were administered to 31 healthy adults in a repeated measures design. Compared with Sham, none of the stimulation protocols significantly modulated corticospinal excitability (input/ouput curve and slope, TMS stimulator intensity required to elicit MEPs of 1-mV amplitude) or intracortical excitability (short- and long-interval intracortical inhibition, intracortical facilitation, cortical silent period). Sham-corrected responder analysis estimates showed that an average of 41 (PAS), 39 (tDCS), and 39% (tACS) of participants responded to the interventions with an increase in corticospinal excitability. The present data show that three stimulation protocols believed to increase cortical excitability are associated with highly heterogenous and variable aftereffects that may explain a lack of significant group effects.

TMS and neocortical neurons: an integrative review on the micro-macro connection in neuroplasticity

Tian D, Izumi S. TMS and neocortical neurons: an integrative review on the micro-macro connection in neuroplasticity. Jpn J Compr Rehabil Sci 2023; 14: 1-9. Neuroplasticity plays a pivotal role in neuroscience and neurorehabilitation as it bridges the organization and reorganization properties of the brain. Among the numerous neuroplastic protocols, transcranial magnetic stimulation (TMS) is a well-established non-invasive protocol to induce plastic changes in the brain. Here, we review the findings of four plasticity-inducing TMS protocols in the human motor cortex with relatively evident mechanisms: conventional repetitive TMS (rTMS), theta-burst stimulation (TBS), quadripulse stimulation (QPS) and paired associative stimulation (PAS). Based on the reviewed evidence and a preliminary TMS neurocytological model proposed in our previous report, we further integrate the neurophysiological evidence and plasticity rules of these protocols to present an updated micro-macro connection model between neocortical neurons and the neurophysiological evidence in TMS. This prototypical model will guide further efforts to understand the neural circuit of the motor cortex, the mechanisms of TMS, and the advance of neuroplasticity technologies and their outcomes.

Transcranial magnetic stimulation as a tool to induce and explore plasticity in humans

Activity-dependent synaptic plasticity is the main theoretical framework to explain mechanisms of learning and memory. Synaptic plasticity can be explored experimentally in animals through various standardized protocols for eliciting long-term potentiation and long-term depression in hippocampal and cortical slices. In humans, several non-invasive protocols of repetitive transcranial magnetic stimulation and transcranial direct current stimulation have been designed and applied to probe synaptic plasticity in the primary motor cortex, as reflected by long-term changes in motor evoked potential amplitudes. These protocols mimic those normally used in animal studies for assessing long-term potentiation and long-term depression. In this chapter, we first discuss the physiologic basis of theta-burst stimulation, paired associative stimulation, and transcranial direct current stimulation. We describe the current biophysical and theoretical models underlying the molecular mechanisms of synaptic plasticity and metaplasticity, defined as activity-dependent changes in neural functions that modulate subsequent synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), in the human motor cortex including calcium-dependent plasticity, spike-timing-dependent plasticity, the role of N-methyl-d-aspartate-related transmission and gamma-aminobutyric-acid interneuronal activity. We also review the putative microcircuits responsible for synaptic plasticity in the human motor cortex. We critically readdress the issue of variability in studies investigating synaptic plasticity and propose available solutions. Finally, we speculate about the utility of future studies with more advanced experimental approaches.

Low-intensity repetitive paired associative stimulation targeting the motor hand area at theta frequency causes a lasting reduction in corticospinal excitability

OBJECTIVE

Sub-motor threshold 5 Hz repetitive paired associative stimulation (5 Hz-rPAS_25ms) produces a long-lasting increase in corticospinal excitability. Assuming a spike-timing dependent plasticity-like (STDP-like) mechanism, we hypothesized that 5 Hz-rPAS at a shorter inter-stimulus interval (ISI) of 15 ms (5 Hz-rPAS_15ms) would exert a lasting inhibitory effect on corticospinal excitability.

METHODS

20 healthy volunteers received two minutes of 5 Hz-rPAS_15ms. Transcranial magnetic stimulation (TMS) was applied over the motor hotspot of the right abductor pollicis brevis muscle at 90% active motor threshold. Sub-motor threshold peripheral electrical stimulation was given to the left median nerve 15 ms before each TMS pulse. We assessed changes in mean amplitude of the unconditioned motor evoked potential (MEP), short-latency intracortical inhibition (SICI), intracortical facilitation (ICF), short-latency afferent inhibition (SAI), long-latency afferent inhibition (LAI), and cortical silent period (CSP) before and for 60 minutes after 5-Hz rPAS_15ms.

RESULTS

Subthreshold 5-Hz rPAS_15ms produced a 20-40% decrease in mean MEP amplitude along with an attenuation in SAI, lasting at least 60 minutes. A follow-up experiment revealed that MEP facilitation was spatially restricted to the target muscle.

CONCLUSIONS

Subthreshold 5-Hz rPAS_15ms effectively suppresses corticospinal excitability. Together with the facilitatory effects of subthreshold 5-Hz rPAS_25ms (Quartarone et al., J Physiol 2006;575:657-670), the results show that sub-motor threshold 5-Hz rPAS induces STDP-like bidirectional plasticity in the motor cortex.

SIGNIFICANCE

The results of the present study provide a new short-time paradigm of long term depression (LTD) induction in human sensory-motor cortex.

Motor learning enhanced by combined motor imagery and noninvasive brain stimulation is associated with reduced short-interval intracortical inhibition

BACKGROUND

Motor imagery (MI) improves motor skill learning, which is further enhanced when MI is paired with primary motor cortex transcranial brain stimulation or with electrical stimulation of the peripheral median nerve. Applying both stimulation types (here with 25 ms intervals) is called paired associative stimulation (PAS25). The final primary motor cortex output is determined by combined excitatory and intracortical inhibitory circuits, and reducing the latter is associated with enhanced synaptic transmission and efficacy. Indeed, short-interval intracortical inhibition (SICI) inhibits motor evoked potentials (MEPs), and motor learning has been associated with decreased SICI and increased cortical excitability. Here, we investigated whether cortical excitability and SICI are altered by PAS25 applied after MI-induced modulation of motor learning.

METHODS

Peak acceleration of a hand-grasping movement and MEPs and SICI were measured before and after MI alone, PAS25 alone, and MI followed by PAS25 in 16 healthy participants to evaluate changes in motor learning, corticospinal excitability, and intracortical inhibition.

RESULTS

After PAS25 alone, MEP amplitude increased while peak acceleration was unchanged. However, PAS25 applied following MI not only significantly enhanced both peak acceleration (p = 0.011) and MEP amplitude (p = 0.004) but also decreased SICI (p = 0.011). Moreover, we found that this decrease in SICI was significantly correlated with both the peak acceleration (r = 0.49, p = 0.029) and the MEP amplitude (r = 0.56, p = 0.013).

CONCLUSIONS

These results indicate that brain function altered by PAS25 of the motor cortex enhances MI-induced motor learning and corticospinal excitability and decreases SICI, suggesting that SICI underlies, at least in part, PAS25 modulation of motor learning.

Modulatory Effects of Motor State During Paired Associative Stimulation on Motor Cortex Excitability and Motor Skill Learning

Repeated pairing of electrical stimulation of a peripheral nerve with transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) representation for a target muscle can induce neuroplastic adaptations in the human brain related to motor learning. The extent to which the motor state during this form of paired associative stimulation (PAS) influences the degree and mechanisms of neuroplasticity or motor learning is unclear. Here, we investigated the effect of volitional muscle contraction during PAS on: (1) measures of general corticomotor excitability and intracortical circuit excitability; and (2) motor performance and learning. We assessed measures of corticomotor excitability using TMS and motor skill performance during a serial reaction time task (SRTT) at baseline and at 0, 30, 60 min post-PAS. Participants completed a SRTT retention test 1 week following the first two PAS sessions. Following the PAS intervention where the hand muscle maintained an active muscle contraction (PAS_ACTIVE), there was lower short interval intracortical inhibition compared to PAS during a resting motor state (PAS_REST) and a sham PAS condition (PAS_CONTROL). SRTT performance improved within the session regardless of PAS condition. SRTT retention was greater following both PAS_ACTIVE and PAS_REST after 1 week compared to PAS_CONTROL. These findings suggest that PAS may enhance motor learning retention and that motor state may be used to target different neural mechanisms of intracortical excitation and inhibition during PAS. This observation may be important to consider for the use of therapeutic noninvasive brain stimulation in neurologic patient populations.

Is There a Future for Non-invasive Brain Stimulation as a Therapeutic Tool?

Several techniques and protocols of non-invasive transcranial brain stimulation (NIBS), including transcranial magnetic and electrical stimuli, have been developed in the past decades. These techniques can induce long lasting changes in cortical excitability by promoting synaptic plasticity and thus may represent a therapeutic option in neuropsychiatric disorders. On the other hand, despite these techniques have become popular, the fragility and variability of the after effects are the major challenges that non-invasive transcranial brain stimulation currentlyfaces. Several factors may account for such a variability such as biological variations, measurement reproducibility, and the neuronal state of the stimulated area. One possible strategy, to reduce this variability is to monitor the neuronal state in real time using EEG and trigger TMS pulses only at pre-defined state. In addition, another strategy under study is to use the spaced application of multiple NIBS protocols within a session to improve the reliability and extend the duration of NIBS effects. Further studies, although time consuming, are required for improving the so far limited effect sizes of NIBS protocols for treatment of neurological or psychiatric disorders.

Differences between motor execution and motor imagery of grasping movements in the motor cortical excitatory circuit

Background

Both motor imagery (MI) and motor execution (ME) can facilitate motor cortical excitability. Although cortical excitability is modulated by intracortical inhibitory and excitatory circuits in the human primary motor cortex, it is not clear which intracortical circuits determine the differences in corticospinal excitability between ME and MI.

Methods

We recruited 10 young healthy subjects aged 18-28 years (mean age: 22.1 ± 3.14 years; five women and five men) for this study. The experiment consisted of two sets of tasks involving grasp actions of the right hand: imagining and executing them. Corticospinal excitability and short-interval intracortical inhibition (SICI) were measured before the interventional protocol using transcranial magnetic stimulation (baseline), as well as at 0, 20, and 40 min (T0, T20, and T40) thereafter.

Results

Facilitation of corticospinal excitability was significantly greater after ME than after MI in the right abductor pollicis brevis (APB) at T0 and T20 (p < 0.01 for T0, and p < 0.05 for T20), but not in the first dorsal interosseous (FDI) muscle. On the other hand, no significant differences in SICI between ME and MI were found in the APB and FDI muscles. The facilitation of corticospinal excitability at T20 after MI correlated with the Movement Imagery Questionnaire (MIQ) scores for kinesthetic items (Rho = -0.646, p = 0.044) but did not correlate with the MIQ scores for visual items (Rho = -0.265, p = 0.458).

Discussion

The present results revealed significant differences between ME and MI on intracortical excitatory circuits of the human motor cortex, suggesting that cortical excitability differences between ME and MI may be attributed to the activation differences of the excitatory circuits in the primary motor cortex.

Involvement of different neuronal components in the induction of cortical plasticity with associative stimulation

BACKGROUND

Paired associative stimulation (PAS), with stimulus interval of 21.5 or 25 ms, using transcranial magnetic stimulation in the posterior-anterior (PA) current direction, produces a long-term-potentiation-like effect. Stimulation with PA directed current generates both early and late indirect (I)-waves while that in anterior-posterior (AP) current predominantly elicits late I-waves. Short interval intracortical inhibition (SICI) inhibits late I-waves but not early I-waves.

OBJECTIVE

To investigate how cortical inhibition modulates the effects of PAS.

METHODS

PAS at stimulus interval of 21.5 ms conditioned by SICI (SICI-PAS) was compared to PAS alone with both PA and AP directed currents.

RESULTS

PAS with both current directions increased cortical excitability. SICI-PAS increased cortical excitability in the PA but not the AP current direction.

CONCLUSIONS

Both early and late I-waves circuits can mediate cortical PAS plasticity under different conditions. Plasticity induction with the late but not the early I-wave circuits is blocked by SICI.

Spinal Excitability Changes after Transspinal and Transcortical Paired Associative Stimulation in Humans

Paired associative stimulation (PAS) produces enduring neuroplasticity based on Hebbian associative plasticity. This study established the changes in spinal motoneuronal excitability by pairing transcortical and transspinal stimulation. Transcortical stimulation was delivered after (transspinal-transcortical PAS) or before (transcortical-transspinal PAS) transspinal stimulation. Before and after 40 minutes of each PAS protocol, spinal neural excitability was assessed based on the amplitude of the transspinal-evoked potentials (TEPs) recorded from ankle muscles of both legs at different stimulation intensities (recruitment input-output curve). Changes in TEPs amplitude in response to low-frequency stimulation and paired transspinal stimuli were also established before and after each PAS protocol. TEP recruitment input-output curves revealed a generalized depression of TEPs in most ankle muscles of both legs after both PAS protocols that coincided with an increased gain only after transcortical-transspinal PAS. Transcortical-transspinal PAS increased and transspinal-transcortical PAS decreased the low-frequency-dependent TEP depression, whereas neither PAS protocol affected the TEP depression observed upon paired transspinal stimuli. These findings support the notion that transspinal and transcortical PAS has the ability to alter concomitantly cortical and spinal synaptic activity. Transspinal and transcortical PAS may contribute to the development of rehabilitation strategies in people with bilateral increased motoneuronal excitability due to cortical or spinal lesions.

Changes in cortical excitability during paired associative stimulation in Parkinson's disease patients and healthy subjects

Paired associative stimulation (PAS) combines repetitive peripheral nerve stimulation with motor cortex (M1) transcranial magnetic stimulation (TMS), to induce plastic-like changes of cortical excitability. While much attention has been dedicated to post-PAS effects little is known about processes during PAS. We compared the time-course of changes in M1 excitability during standard facilitatory PAS intervention among patients with Parkinson's disease (PD), known to have diminished post-PAS response, and healthy subjects. Compared to baseline pre-PAS MEPs, conditioned MEPs during PAS decreased significantly in both groups. The decrease was significantly larger in healthy subjects than in PD patients, regardless whether patients were drug-naïve or not. Although post-PAS excitability increase was also larger in healthy subjects than in PD patients, there was no significant correlation between the two phenomena, i.e. the extent of MEP decrease during PAS and the extent of the post-PAS excitability increase. The results highlight an apparent physiological paradox that repetitive application of an inhibitory stimulation pattern leads to subsequent prolonged facilitation, thus broadening the understanding of the phenomenology of PAS response. Results also suggest that in PD cortical circuits involved in conveying inhibition during PAS, are impaired at the clinical onset of the disease and are not influenced by subsequent PD treatment.

Evaluating developmental motor plasticity with paired afferent stimulation

AIM

Brain plasticity mechanisms are probably different in children but remain poorly understood. Paired afferent stimulation (PAS) combines peripheral sensory stimulation with transcranial magnetic stimulation (TMS) of primary motor cortex to induce rapid, reversible, topographically specific increases in primary motor cortex excitability suggestive of long-term potentiation in adults. Our aim was to determine frequency, characteristics, age effects, and reproducibility of PAS in school-age children.

METHOD

Typically developing right-handed children (6-18y) were recruited. Median nerve stimulation was delivered 25ms before suprathreshold primary motor cortex stimulation (0.2Hz, 7.5min). Primary outcome was changed in the amplitude of motor evoked potentials (MEPs) at five time points after PAS (0, 15, 30, 45, 75min) expressed as area under the curve. Reproducibility was evaluated. Secondary outcomes included stimulus response curves and safety/tolerability.

RESULTS

Of 28 children (20 males, mean age 12y), 64% demonstrated PAS effects (11 definite, seven probable). PAS effects were sustained across all time points to 75min (p=0.004). Stimulus response curve scores increased after PAS (n=9, p=0.02). PAS effect and age were not correlated. PAS was highly reproducible (p=0.925, r=0.283). Tolerability was favorable without adverse events.

INTERPRETATION

PAS effects are present and reproducible in children. Pediatric PAS paradigms appear safe and tolerable. PAS may provide insight into endogenous developmental plasticity, informing future studies in children with cerebral palsy and other motor disorders.

Associative plasticity in surround inhibition circuits in human motor cortex

Surround inhibition is a physiological mechanism that is hypothesised to improve contrast between signals in the central nervous system. In the human motor system, motor surround inhibition (mSI) can be assessed using transcranial magnetic stimulation (TMS). We evaluated whether it is possible to modulate mSI, using a paradigm able to induce plastic effects in primary motor cortex (M1). Fifteen healthy volunteers participated in the experiments. To assess mSI, we delivered single pulses at rest and at the onset of a right thumb abduction. TMS pulses over abductor digiti minimi (ADM; surround muscle) hotspot were delivered when EMG activity in right abductor pollicis brevis (APB; active muscle) > 100 μV was detected. Paired associative stimulation (PAS) was delivered using peripheral median nerve electric stimulation and TMS over APB M1 area at an interstimulus interval of 21.5 ms for the real PAS (PAS21.5) and 100 ms for the sham PAS (PAS100). To verify the effect of PAS21.5 on mSI we collected 20 MEPs from ADM at rest and during APB movements before (T0) and 5 (T1), 15 (T2) and 30 (T3) minutes after PAS21.5. mSI from APB to ADM was present at baseline. PAS21.5 increased the amount of mSI compared with baseline whereas there was no effect after PAS100. Our results suggest that mSI is an adaptable phenomenon depending on prior experience.

Acute exercise enhances the response to paired associative stimulation-induced plasticity in the primary motor cortex

There is evidence that a single session of aerobic exercise can modulate intracortical inhibition. While decreases in inhibition appear to be a necessary precursor to the induction of long-term potentiation (LTP)-like plasticity, it is not known whether aerobic exercise can enhance the response to LTP induction. We investigated whether the addition of a preceding bout of exercise would modulate the response to paired associative stimulation (PAS) of the upper limb. It was hypothesized that exercise would enhance motor cortical (M1) excitability following PAS compared to a session of PAS alone. Ten healthy individuals underwent a control session involving PAS alone and an exercise session where PAS was preceded by 20 min of moderate-intensity stationary biking. PAS involved 180 pairs of stimuli (right median nerve, left M1) delivered at 0.1 Hz to the right abductor pollicis brevis representation. Excitability changes were measured by the area under a stimulus-response curve, and intracortical circuits were probed by testing short-interval intracortical inhibition (SICI), long-interval intracortical inhibition and intracortical facilitation. Two-way ANOVAs were conducted to compare excitability changes between sessions. PAS-induced increases in M1 excitability were enhanced in the exercise session (p < 0.026). In addition, SICI was differentially modulated between the two sessions, with greater decreases in SICI observed immediately after PAS when it was preceded by the exercise session (p < 0.03). Aerobic exercise enhances the effectiveness of PAS and may be a useful adjunct to traditional therapies and interventions that aim to promote neuroplasticity in cortical networks.

Heterosynaptic modulation of motor cortical plasticity in human

Inductions of long-term potentiation (LTP) and depression (LTD) are modulated if they are preceded by a priming protocol, in a manner consistent with metaplasticity. Depotentiation refers to reversal of LTP by a subsequent protocol that has no effect by itself. Paired associative stimulation (PAS) at interstimulus interval of 25 ms (PAS25) and 10 ms (PAS10) produces spike timing-dependent LTP-like and LTD-like effects in human primary motor cortex. Continuous theta burst stimulation (cTBS) with 600 pulses produces an LTD-like effect, whereas cTBS with 150 pulses (cTBS150) has no effect by itself. We investigated whether cortical plasticity induced by PAS can be modulated by heterosynaptic inputs of cTBS150. PAS25 and PAS10 primed and followed by cTBS150 were compared withPAS25 and PAS10 alone. Motor evoked potential (MEP) amplitude, recruitment curve, and intracortical circuits including short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), intracortical facilitation, and short-latency afferent inhibition were measured before and after the interventions. After PAS25 alone, MEP amplitude increased while intracortical circuits did not change. A priming cTBS150 enhanced the effects of PAS25 with further increase in MEP amplitude and led to reduction in SICI and LICI. PAS25 followed by cTBS150 led to reduced MEP amplitude and increased LICI and SICI. Both priming and following cTBS150 reversed the LTD-like effect produced by PAS10 with little change in intracortical circuits. We conclude that cortical plasticity induced by PAS and cTBS interacts in a heterosynaptic and bidirectional manner. The order of the interventions determines whether the underlying mechanisms are related to metaplasticity or depotentiation.

Altered synaptic plasticity in Tourette's syndrome and its relationship to motor skill learning

Gilles de la Tourette syndrome is a neuropsychiatric disorder characterized by motor and phonic tics that can be considered motor responses to preceding inner urges. It has been shown that Tourette patients have inferior performance in some motor learning tasks and reduced synaptic plasticity induced by transcranial magnetic stimulation. However, it has not been investigated whether altered synaptic plasticity is directly linked to impaired motor skill acquisition in Tourette patients. In this study, cortical plasticity was assessed by measuring motor-evoked potentials before and after paired associative stimulation in 14 Tourette patients (13 male; age 18–39) and 15 healthy controls (12 male; age 18–33). Tic and urge severity were assessed using the Yale Global Tic Severity Scale and the Premonitory Urges for Tics Scale. Motor learning was assessed 45 minutes after inducing synaptic plasticity and 9 months later, using the rotary pursuit task. On average, long-term potentiation-like effects in response to the paired associative stimulation were present in healthy controls but not in patients. In Tourette patients, long-term potentiation-like effects were associated with more and long-term depression-like effects with less severe urges and tics. While motor learning did not differ between patients and healthy controls 45 minutes after inducing synaptic plasticity, the learning curve of the healthy controls started at a significantly higher level than the Tourette patients' 9 months later. Induced synaptic plasticity correlated positively with motor skills in healthy controls 9 months later. The present study confirms previously found long-term improvement in motor performance after paired associative stimulation in healthy controls but not in Tourette patients. Tourette patients did not show long-term potentiation in response to PAS and also showed reduced levels of motor skill consolidation after 9 months compared to healthy controls. Moreover, synaptic plasticity appears to be related to symptom severity.

Dose-response curve of associative plasticity in human motor cortex and interactions with motor practice

Associative plasticity is hypothesized to be an important neurophysiological correlate of memory formation and learning with potentials for applications in neurorehabilitation and for the development of new electrophysiological measures to study disorders of cortical plasticity. We hypothesized that the magnitude of the paired associative stimulation (PAS)-induced long-term potentiation (LTP)-like effect depends on the number of pairs in the PAS protocol. We also hypothesized that homeostatic interaction of PAS with subsequent motor learning is related to the magnitude of the PAS-induced LTP-like effect. We studied 10 healthy subjects. In experiment 1a, subjects received 90 (PAS90), 180 (PAS180), or 270 (PAS270) pairs of stimuli, followed by a dynamic motor practice (DMP) 1 h after the end of the PAS protocols. In experiment 1b, the DMP preceded the PAS protocol. In experiment 2, the time course of PAS270 was studied. We found that PAS270 resulted in greater increase in motor evoked potential (MEP) amplitude compared with protocols with fewer pairs of stimuli. Moreover, the interaction between PAS protocols with motor learning differed depending on the number of stimulus pairs used to induce PAS. While DMP alone increased MEP amplitudes, DMP during the LTP-like effects induced by PAS270 led to a long-term depression (LTD)-like effect (homeostatic interaction). This homeostatic interaction did not occur after PAS90 and PAS180. In conclusion, we found a dose-dependent effect of the number of stimulus pairs used in the PAS protocol on cortical plasticity. Homeostatic interaction between PAS and DMP was observed only after PAS270.

Induction of cortical plasticity for reciprocal muscles by paired associative stimulation

BACKGROUND

Paired associative stimulation (PAS) is widely used to induce plasticity in the human motor cortex. Although reciprocal inhibition of antagonist muscles plays a fundamental role in human movements, change in cortical circuits for reciprocal muscles by PAS is unknown.

METHODS

We investigated change in cortical plasticity for reciprocal muscles during PAS. PAS consisted of 200 pairs of peripheral electric stimulation of the right median nerve at the wrist at a frequency of 0.25 Hz followed by transcranial magnetic stimulation of the left M1 at the midpoint between the center of gravities of the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles. Measures of motor cortical excitability included resting motor threshold (RMT), GABAA-mediated short-interval intracortical inhibition (SICI), and GABAB-mediated long-interval intracortical inhibition (LICI).

RESULTS

Motor evoked potential amplitude-conditioned LICI for the FCR muscle was significantly decreased after PAS (P = 0.020), whereas that for the ECR muscle was significantly increased (P = 0.033). Changes in RMT and SICI for the FCR and ECR muscles were not significantly different before and after PAS. Corticospinal excitability for both reciprocal muscles was increased during PAS, but GABAB-mediated cortical inhibitory functions for the agonist and antagonist muscles were reciprocally altered after PAS.

CONCLUSION

These results implied that the cortical excitability for reciprocal muscles including GABAB-ergic inhibitory systems within human M1 could be differently altered by PAS.

Mechanisms underlying the rules for associative plasticity at adult human neocortical synapses

The neocortex in our brain stores long-term memories by changing the strength of connections between neurons. To date, the rules and mechanisms that govern activity-induced synaptic changes at human cortical synapses are poorly understood and have not been studied directly at a cellular level. Here, we made whole-cell recordings of human pyramidal neurons in slices of brain tissue resected during neurosurgery to investigate spike timing-dependent synaptic plasticity in the adult human neocortex. We find that human cortical synapses can undergo bidirectional modifications in strength throughout adulthood. Both long-term potentiation and long-term depression of synapses was dependent on postsynaptic NMDA receptors. Interestingly, we find that human cortical synapses can associate presynaptic and postsynaptic events in a wide temporal window, and that rules for synaptic plasticity in human neocortex are reversed compared with what is generally found in the rodent brain. We show this is caused by dendritic L-type voltage-gated Ca2+ channels that are prominently activated during action potential firing. Activation of these channels determines whether human synapses strengthen or weaken. These findings provide a synaptic basis for the timing rules observed in human sensory and motor plasticity in vivo, and offer insights into the physiological role of L-type voltage-gated Ca2+ channels in the human brain.

Effects of short-latency afferent inhibition on short-interval intracortical inhibition

Peripheral nerve stimulation inhibits the motor cortex, and the process has been termed short-latency afferent inhibition (SAI) at interstimulus intervals (ISIs) of ∼20 ms. The objective of the present study was to test how SAI interacts with short-interval intracortical inhibition (SICI) under different stimulation conditions. We studied 20 healthy volunteers. Surface electromyogram was recorded from the first dorsal interosseous muscle. Using paired- and triple-pulse paradigms, we investigated how SAI interacts with SICI under these different conditions. The effects of different conditioning stimulus (CS) intensities (0.6-0.9 active motor threshold), SAI latencies (23 and 25 ms), and ISIs (2 and 3 ms) for SICI were examined in rest and active conditions. SAI had inhibitory interactions with SICI at different CS intensities for rest or active SICI, at SAI latencies of 23 and 25 ms. This interaction occurred at weak CS intensities for SICI when there was no inhibition, and SICI became facilitatory in the presence of SAI. This can be explained by SICI inhibiting SAI and not by saturation of inhibition. The interaction between SAI and SICI was greater for SICI at ISI of 3 ms than for ISI of 2 ms, suggesting that different circuits may be activated at these ISIs. We conclude that SAI and SICI have inhibitory interactions that are influenced by factors such as ISI and muscle activities, which should be considered in design and interpretation of cortical interaction studies.

Modulation of human corticospinal excitability by paired associative stimulation

Paired Associative Stimulation (PAS) has come to prominence as a potential therapeutic intervention for the treatment of brain injury/disease, and as an experimental method with which to investigate Hebbian principles of neural plasticity in humans. Prototypically, a single electrical stimulus is directed to a peripheral nerve in advance of transcranial magnetic stimulation (TMS) delivered to the contralateral primary motor cortex (M1). Repeated pairing of the stimuli (i.e., association) over an extended period may increase or decrease the excitability of corticospinal projections from M1, in manner that depends on the interstimulus interval (ISI). It has been suggested that these effects represent a form of associative long-term potentiation (LTP) and depression (LTD) that bears resemblance to spike-timing dependent plasticity (STDP) as it has been elaborated in animal models. With a large body of empirical evidence having emerged since the cardinal features of PAS were first described, and in light of the variations from the original protocols that have been implemented, it is opportune to consider whether the phenomenology of PAS remains consistent with the characteristic features that were initially disclosed. This assessment necessarily has bearing upon interpretation of the effects of PAS in relation to the specific cellular pathways that are putatively engaged, including those that adhere to the rules of STDP. The balance of evidence suggests that the mechanisms that contribute to the LTP- and LTD-type responses to PAS differ depending on the precise nature of the induction protocol that is used. In addition to emphasizing the requirement for additional explanatory models, in the present analysis we highlight the key features of the PAS phenomenology that require interpretation.

Hebbian and anti-Hebbian spike-timing-dependent plasticity of human cortico-cortical connections

Learning of new skills may occur through Hebbian associative changes in the synaptic strength of cortical connections [spike-timing-dependent plasticity (STDP)], but how the precise temporal relationship of the presynaptic and postsynaptic inputs determines the STDP effects in humans is poorly understood. We used a novel paired associative stimulation protocol to repeatedly activate the short-latency connection between the posterior parietal cortex and the primary motor cortex (M1) of the left-dominant hemisphere. In different experiments, we systematically varied the temporal relationships between the stimuli and the preferential activation of different M1 neuronal populations by applying transcranial magnetic stimulation over M1 with different coil orientations and in different states of cortical excitability (rest vs muscular contraction). We found evidence for the existence of both Hebbian and anti-Hebbian STDP in human long-range connections. The induction of bidirectional long-term potentiation or depression in M1 depended not only on the relative timing between the stimuli but, crucially, on the stimulation of specific neuronal populations and the activity state of the cortex. Our findings demonstrate that these mechanisms are not fixed but susceptible to rapid adaptations. This sudden transition from anti-Hebbian to Hebbian plasticity likely involves local dynamics of interaction with different populations of postsynaptic neurons.

Microcircuit mechanisms involved in paired associative stimulation-induced depression of corticospinal excitability

Synaptic weight changes induced by temporal correlations between the spikes of pre- and postsynaptic neurons are referred to as spike-timing-dependent plasticity (STDP). Transcranial magnetic stimulation (TMS) induces long-lasting effects on corticospinal excitability, if it is repetitively paired with stimulation of afferents from a corresponding contralateral hand region at short intervals (paired associative stimulation, PAS). PAS-induced plasticity has been linked with synaptic STDP. We aimed to investigate which elements of the cortical microcircuitry sustain and govern PAS-induced depression of corticospinal excitability in the target muscle representation (and enhancement of excitability in its functional surround). We show that the time window during which the interaction between both stimulus-induced cortical events leads to immediate post-interventional depression is short (<4.5 ms). The depressant PAS effects at the target representation were completely blocked by applying a subthreshold magnetic pulse 3 ms before the principal TMS pulse, even when the strength of the latter was adjusted to generate a motor-evoked potential of similar amplitude to that with the unconditioned magnetic pulse. Epidural recordings from the cervical cord of a patient showed that under this condition late TMS-evoked I-waves remain suppressed. When the intensity of the TMS component during PAS was lowered - sufficient to allow activation of inhibitory neurons, but insufficient to activate corticospinal neurons - excitability of short-latency intracortical inhibition remained unchanged. PAS-induced facilitation in the functional surround followed the same pattern as the centre-depressant effects. These findings may suggest that excitability-depressant PAS-induced effects are due to weakening of excitatory synapses between upper cortical layer principal neurons, but not those located on the corticospinal neuron, or inhibitory synapses. Inhibitory interneurons involved in short-latency intracortical inhibition are gate-keepers to producing centre-depressant/surround-facilitatory PAS effects. Based on these and earlier findings we propose a model specifying the composition and laminar location of the involved microcircuit of PAS-induced plasticity that may enhance its utility as a model of STDP in humans.