Dopamine influences primary motor cortex plasticity and dorsal premotor-to-motor connectivity in Parkinson's disease

Induced neural phase precession through exogenous electric fields

The gradual shifting of preferred neural spiking relative to local field potentials (LFPs), known as phase precession, plays a prominent role in neural coding. Correlations between the phase precession and behavior have been observed throughout various brain regions. As such, phase precession is suggested to be a global neural mechanism that promotes local neuroplasticity. However, causal evidence and neuroplastic mechanisms of phase precession are lacking so far. Here we show a causal link between LFP dynamics and phase precession. In three experiments, we modulated LFPs in humans, a non-human primate, and computational models using alternating current stimulation. We show that continuous stimulation of motor cortex oscillations in humans lead to a gradual phase shift of maximal corticospinal excitability by ~90°. Further, exogenous alternating current stimulation induced phase precession in a subset of entrained neurons (~30%) in the non-human primate. Multiscale modeling of realistic neural circuits suggests that alternating current stimulation-induced phase precession is driven by NMDA-mediated synaptic plasticity. Altogether, the three experiments provide mechanistic and causal evidence for phase precession as a global neocortical process. Alternating current-induced phase precession and consequently synaptic plasticity is crucial for the development of novel therapeutic neuromodulation methods.

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.

Parkinson's disease: Alterations of motor plasticity and motor learning

This chapter reviews the alterations in motor learning and motor cortical plasticity in Parkinson's disease (PD), the most common movement disorder. Impairments in motor learning, which is a hallmark of basal ganglia disorders, influence the performance of motor learning-related behavioral tasks and have clinical implications for the management of disturbance in gait and posture, and for rehabilitative management of PD. Although plasticity is classically induced and assessed in sliced preparation in animal models, in this review we have concentrated on the results from non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS), transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) in patients with PD, in addition to a few animal electrophysiologic studies. The chapter summarizes the results from different cortical and subcortical plasticity investigations. Plasticity induction protocols reveal deficient plasticity in PD and these plasticity measures are modulated by medications and deep brain stimulation. There is considerable variability in these measures that are related to inter-individual variations, different disease characteristics and methodological considerations. Nevertheless, these pathophysiologic studies expand our knowledge of cortical excitability, plasticity and the effects of different treatments in PD. These tools of modulating plasticity and motor learning improve our understanding of PD pathophysiology and help to develop new treatments for this disabling condition.

On the Use of TMS to Investigate the Pathophysiology of Neurodegenerative Diseases

Neurodegenerative diseases are a collection of disorders that result in the progressive degeneration and death of neurons. They are clinically heterogenous and can present as deficits in movement, cognition, executive function, memory, visuospatial awareness and language. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation tool that allows for the assessment of cortical function in vivo. We review how TMS has been used for the investigation of three neurodegenerative diseases that differ in their neuroanatomical axes: (1) Motor cortex-corticospinal tract (motor neuron diseases), (2) Non-motor cortical areas (dementias), and (3) Subcortical structures (parkinsonisms). We also make four recommendations that we hope will benefit the use of TMS in neurodegenerative diseases. Firstly, TMS has traditionally been limited by the lack of an objective output and so has been confined to stimulation of the motor cortex; this limitation can be overcome by the use of concurrent neuroimaging methods such as EEG. Given that neurodegenerative diseases progress over time, TMS measures should aim to track longitudinal changes, especially when the aim of the study is to look at disease progression and symptomatology. The lack of gold-standard diagnostic confirmation undermines the validity of findings in clinical populations. Consequently, diagnostic certainty should be maximized through a variety of methods including multiple, independent clinical assessments, imaging and fluids biomarkers, and post-mortem pathological confirmation where possible. There is great interest in understanding the mechanisms by which symptoms arise in neurodegenerative disorders. However, TMS assessments in patients are usually carried out during resting conditions, when the brain network engaged during these symptoms is not expressed. Rather, a context-appropriate form of TMS would be more suitable in probing the physiology driving clinical symptoms. In all, we hope that the recommendations made here will help to further understand the pathophysiology of neurodegenerative diseases.

The continuum between neurodegeneration, brain plasticity, and movement: a critical appraisal

While the “physiological” aging process is associated with declines in motor and cognitive features, these changes do not significantly impair functions and activities of daily living. Differently, motor and cognitive impairment constitute the most common phenotypic expressions of neurodegeneration. Both manifestations frequently coexist in the same disease, thus making difficult to detect “pure” motor or cognitive conditions. Movement disorders are often characterized by cognitive disturbances, and neurodegenerative dementias often exhibit the occurrence of movement disorders. Such a phenotypic overlap suggests approaching these conditions by highlighting the commonalities of entities traditionally considered distinct. In the present review, we critically reappraised the common clinical and pathophysiological aspects of neurodegeneration in both animal models and patients, looking at motricity as a trait d’union over the spectrum of neurodegeneration and focusing on synaptopathy and oscillopathy as the common pathogenic background. Finally, we discussed the possible role of movement as neuroprotective intervention in neurodegenerative conditions, regardless of the etiology. The identification of commonalities is critical to drive future research and develop novel possible disease-modifying interventions.

Evolving concepts on bradykinesia

Bradykinesia is one of the cardinal motor symptoms of Parkinson's disease and other parkinsonisms. The various clinical aspects related to bradykinesia and the pathophysiological mechanisms underlying bradykinesia are, however, still unclear. In this article, we review clinical and experimental studies on bradykinesia performed in patients with Parkinson's disease and atypical parkinsonism. We also review studies on animal experiments dealing with pathophysiological aspects of the parkinsonian state. In Parkinson's disease, bradykinesia is characterized by slowness, the reduced amplitude of movement, and sequence effect. These features are also present in atypical parkinsonisms, but the sequence effect is not common. Levodopa therapy improves bradykinesia, but treatment variably affects the bradykinesia features and does not significantly modify the sequence effect. Findings from animal and patients demonstrate the role of the basal ganglia and other interconnected structures, such as the primary motor cortex and cerebellum, as well as the contribution of abnormal sensorimotor processing. Bradykinesia should be interpreted as arising from network dysfunction. A better understanding of bradykinesia pathophysiology will serve as the new starting point for clinical and experimental purposes.

Randomized Controlled Trial of a Home-Based Action Observation Intervention to Improve Walking in Parkinson Disease

OBJECTIVE

To examine the feasibility and efficacy of a home-based gait observation intervention for improving walking in Parkinson disease (PD).

DESIGN

Participants were randomly assigned to an intervention or control condition. A baseline walking assessment, a training period at home, and a posttraining assessment were conducted.

SETTING

The laboratory and participants' home and community environments.

PARTICIPANTS

Nondemented individuals with PD (N=23) experiencing walking difficulty.

INTERVENTION

In the gait observation (intervention) condition, participants viewed videos of healthy and parkinsonian gait. In the landscape observation (control) condition, participants viewed videos of moving water. These tasks were completed daily for 8 days.

MAIN OUTCOME MEASURES

Spatiotemporal walking variables were assessed using accelerometers in the laboratory (baseline and posttraining assessments) and continuously at home during the training period. Variables included daily activity, walking speed, stride length, stride frequency, leg swing time, and gait asymmetry. Questionnaires including the 39-item Parkinson Disease Questionnaire (PDQ-39) were administered to determine self-reported change in walking, as well as feasibility.

RESULTS

At posttraining assessment, only the gait observation group reported significantly improved mobility (PDQ-39). No improvements were seen in accelerometer-derived walking data. Participants found the at-home training tasks and accelerometer feasible to use.

CONCLUSIONS

Participants found procedures feasible and reported improved mobility, suggesting that observational training holds promise in the rehabilitation of walking in PD. Observational training alone, however, may not be sufficient to enhance walking in PD. A more challenging and adaptive task, and the use of explicit perceptual learning and practice of actions, may be required to effect change.

Usefulness of Transcranial Magnetic Stimulation to Assess Motor Function in Patients With Parkinsonism

Objective

To investigate the clinical significance of upper and lower extremity transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) in patients with parkinsonism.

Methods

Twenty patients (14 men, 6 women; mean age 70.5±9.1 years) suffering from parkinsonism were included in this study. All participants underwent single-pulse TMS session to assess the corticospinal excitability of the upper and lower extremity motor cortex. The resting motor threshold (RMT) was defined as the lowest stimulus intensity able to evoke MEPs of an at least 50 µV peak-to-peak amplitude in 5 of 10 consecutive trials. Five sweeps of MEPs at 120% of the RMT were performed, and the mean amplitude and latency of the MEPs were calculated. Patients were also assessed using the Unified Parkinson's Disease Rating Scale part III (UPDRS-III) and the 5-meter Timed Up and Go (5m-TUG) test.

Results

There was a significant positive correlation between the RMTs of MEPs in the upper and lower extremities (r=0.612, p=0.004) and between the amplitude of MEPs in the upper and lower extremities (r=0.579, p=0.007). The RMT of upper extremity MEPs showed a significant negative relationship with the UPDRS-III score (r=–0.516, p=0.020). In addition, RMTs of lower extremity MEPs exhibited a negative relationship with the UPDRS-III score, but the association was not statistically significant (r=–406, p=0.075).

Conclusion

These results indicated that the RMT of MEPs reflect the severity of motor dysfunction in patients with parkinsonism. MEP is a potential quantitative, electrodiagnostic method to assess motor function in patients with parkinsonism.

Primary motor cortex LTP/LTD-like plasticity in probable corticobasal syndrome

Whether the primary motor cortex (M1) contributes to the pathophysiology of corticobasal syndrome (CBS) remains unclear. In this study in patients with probable CBS, we tested whether M1 plasticity contributes to the pathophysiology of symptoms in the contralateral “less affected” limb, manifesting only parkinsonism, and in the contralateral “more affected” limb, manifesting parkinsonism plus other motor and nonmotor symptoms. In Experiment 1, we applied intermittent/continuous theta-burst stimulation (iTBS/cTBS) over the M1 contralateral to the less affected limb in 17 patients. In Experiment 2, we applied iTBS/cTBS over the M1 contralateral to the more affected limb in 14 of the 17 patients. We measured iTBS/cTBS-induced plasticity as reflected by motor-evoked potential (MEP) changes. Data were compared with those obtained in 17 healthy subjects (HS). In Experiment 1, TBS over the M1 contralateral to the less affected limb disclosed reduced plasticity in patients than in HS. In Experiment 2, in 5 of 14 patients we recorded abnormally low-amplitude MEPs, preventing the evaluation of plasticity in the M1 contralateral to the more affected limb. In the remaining nine patients, TBS disclosed abnormal plasticity characterized by high intersubject variability. In these nine patients, the response to TBS correlated with specific patients' clinical features. In the present study in patients with probable CBS, we have demonstrated heterogeneous abnormalities of M1 that contribute to the pathophysiology of this condition.

Primary motor cortex of the parkinsonian monkey: altered encoding of active movement

Abnormalities in the movement-related activation of the primary motor cortex (M1) are thought to be a major contributor to the motor signs of Parkinson's disease. The existing evidence, however, variably indicates that M1 is under-activated with movement, overactivated (due to a loss of functional specificity) or activated with abnormal timing. In addition, few models consider the possibility that distinct cortical neuron subtypes may be affected differently. Those gaps in knowledge were addressed by studying the extracellular activity of antidromically-identified lamina 5b pyramidal-tract type neurons (n = 153) and intratelencephalic-type corticostriatal neurons (n = 126) in the M1 of two monkeys as they performed a step-tracking arm movement task. We compared movement-related discharge before and after the induction of parkinsonism by administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and quantified the spike rate encoding of specific kinematic parameters of movement using a generalized linear model. The fraction of M1 neurons with movement-related activity declined following MPTP but only marginally. The strength of neuronal encoding of parameters of movement was reduced markedly (mean 29% reduction in the coefficients from the generalized linear model). This relative decoupling of M1 activity from kinematics was attributable to reductions in the coefficients that estimated the spike rate encoding of movement direction (-22%), speed (-40%), acceleration (-49%) and hand position (-33%). After controlling for MPTP-induced changes in motor performance, M1 activity related to movement itself was reduced markedly (mean 36% hypoactivation). This reduced activation was strong in pyramidal tract-type neurons (-50%) but essentially absent in corticostriatal neurons. The timing of M1 activation was also abnormal, with earlier onset times, prolonged response durations, and a 43% reduction in the prevalence of movement-related changes beginning in the 150-ms period that immediately preceded movement. Overall, the results are consistent with proposals that under-activation and abnormal timing of movement-related activity in M1 contribute to parkinsonian motor signs but are not consistent with the idea that a loss of functional specificity plays an important role. Given that pyramidal tract-type neurons form the primary efferent pathway that conveys motor commands to the spinal cord, the dysfunction of movement-related activity in pyramidal tract-type neurons is likely to be a central factor in the pathophysiology of parkinsonian motor signs.

Relative quantification of amine-containing metabolites using isobaric N,N-dimethyl leucine (DiLeu) reagents via LC-ESI-MS/MS and CE-ESI-MS/MS

Tandem mass spectrometry (MS/MS)-based relative quantification by isobaric labeling is a useful technique to compare different metabolic expression levels in biological systems. For the first time, we have labeled primary and secondary amine-containing small molecules using 4-plex isobaric N,N-dimethyl leucine (DiLeu) to perform relative quantification. Good labeling efficiency and quantification accuracy were demonstrated with a mixture of 12 metabolite standards including amino acids and small molecule neurotransmitters. Labeling amine-containing metabolites with DiLeu reagents also enabled the separation of polar metabolites by nanoRPLC and improved the detection sensitivity by CE-ESI-MS. The 4-plex DiLeu labeling technique combined with LC-MS/MS and CE-MS/MS platforms were applied to profile and quantify amine-containing metabolites in mouse urine. The variability of concentrations of identified metabolites in urine samples from different mouse individuals was illustrated by the ratios of reporter ion intensities acquired from online data-dependent analysis.

TMS enhances retention of a motor skill in Parkinson's disease

BACKGROUND

In Parkinson's disease (PD), skill retention is poor, even when acquisition rate is generally preserved. Recent work in normal subjects suggests that 5 Hz-repetitive transcranial magnetic stimulation (5Hz-rTMS) may induce phenomena of long-term potentiation at the cortical level.

OBJECTIVE/HYPOTHESIS

We thus verified whether, in PD, 5Hz-rTMS enhances retention of a visuo-motor skill that involves the activity of the right posterior parietal cortex.

METHODS

A group of patients with PD was tested in two two-day sessions, separated by one week (treatment and placebo sessions). The first day of each session, they learned to adapt their movements to a step-wise 60° visual rotation. Immediately after the task, either real 5Hz-rTMS (treatment) or sham (placebo) stimulation was applied over the right posterior parietal cortex (P6). Retention of this motor skill was tested the following day.

RESULTS

In patients with PD, adaptation achieved at the end of training was comparable in the treatment and placebo sessions and was similar to that of a group of age-matched controls. However, retention indices tested on the following day were significantly lower in the placebo compared to the treatment session in which retention indices were restored to the level of the controls. Importantly, reaction and movement time as well as other kinematic measures were the same in the treatment and placebo sessions.

CONCLUSION

These results suggest that rTMS applied after the acquisition of a motor skill over specific areas involved in this process might enhance skill retention in PD.

Effects of dopaminergic treatment on functional cortico-cortical connectivity in Parkinson's disease

Interactions between dorsal premotor cortex (PMd) and primary motor cortex (M1) and interhemispheric inhibition (IHI) between M1 are impaired in Parkinson's disease (PD). We used dual-site transcranial magnetic stimulation to compare effects of first-time levodopa application with chronic dopaminergic therapy on these interactions in PD. Twelve untreated PD patients were studied before and after their first-ever intake of levodopa. The effects of chronic dopaminergic medication were evaluated in 11 patients who had received regular dopaminergic medication for approximately 3 years. Nine of these patients were also measured after overnight withdrawal of medication. For IHI, conditioning stimuli (CS) were applied to left M1 followed by test stimuli (TS) over right M1 and vice versa in separate blocks at interstimulus intervals (ISI) of 6-10 ms. Next, CS were applied to left PMd at subthreshold intensity followed by TS over left M1 at ISIs of 4 and 6 ms. Results were compared to 17 age- and gender-matched controls. In de novo PD patients, levodopa reduced left-to-right IHI, but did not alter PMd-M1 connectivity. In contrast, inhibitory PMd-M1 connectivity was present in early disease patients under chronic dopaminergic stimulation, but not in de novo PD patients at low stimulus intensities at an ISI of 4 ms. First-time exposure to levodopa exerts different effects on cortico-cortical pathways than chronic dopaminergic stimulation in PD, suggesting a change in the responsiveness of cortico-cortical circuits during the course of PD.

Parkinson's disease disrupts both automatic and controlled processing of action verbs

The problem of how word meaning is processed in the brain has been a topic of intense investigation in cognitive neuroscience. While considerable correlational evidence exists for the involvement of sensory-motor systems in conceptual processing, it is still unclear whether they play a causal role. We investigated this issue by comparing the performance of patients with Parkinson's disease (PD) with that of age-matched controls when processing action and abstract verbs. To examine the effects of task demands, we used tasks in which semantic demands were either implicit (lexical decision and priming) or explicit (semantic similarity judgment). In both tasks, PD patients' performance was selectively impaired for action verbs (relative to controls), indicating that the motor system plays a more central role in the processing of action verbs than in the processing of abstract verbs. These results argue for a causal role of sensory-motor systems in semantic processing.

Modulation of corticomotor excitability after maximal or sustainable-rate repetitive finger movement is impaired in Parkinson's disease and is reversed by levodopa

OBJECTIVES

In healthy subjects, fatiguing exercises induce a period of post-exercise corticomotor depression (PECD) that is absent in Parkinson's disease (PD). Our objective is to determine the time-course of corticomotor excitability changes following a 10-s repetitive index finger flexion-extension task performed at maximal voluntary rate (MVR) and a slower sustainable rate (MSR) in PD patients OFF and ON levodopa.

METHODS

In 11 PD patients and 10 healthy age-matched controls, motor evoked potentials (MEPs) were recorded from the extensor indicis proprius (EIP) and first dorsal interosseous (FDI) muscles of the dominant arm immediately after the two tasks and at 2-min intervals for 10 min.

RESULTS

In the OFF condition the PECD was absent in the two test muscles after both the MVR and MSR tasks. In the ON condition finger movement kinematics improved and a period of PECD comparable to that in controls was present after both tasks.

CONCLUSION

The absence of PECD in PD subjects off medication indicates a persisting increase in corticomotor excitability after non-fatiguing repetitive finger movement that is reversed by levodopa.

SIGNIFICANCE

Dopamine depletion is associated with impaired modulation of corticomotor excitability after non-fatiguing repetitive finger movement.

Motor cortical plasticity in Parkinson's disease

In Parkinson's disease (PD), there are alterations of the basal ganglia (BG) thalamocortical networks, primarily due to degeneration of nigrostriatal dopaminergic neurons. These changes in subcortical networks lead to plastic changes in primary motor cortex (M1), which mediates cortical motor output and is a potential target for treatment of PD. Studies investigating the motor cortical plasticity using non-invasive transcranial magnetic stimulation (TMS) have found altered plasticity in PD, but there are inconsistencies among these studies. This is likely because plasticity depends on many factors such as the extent of dopaminergic loss and disease severity, response to dopaminergic replacement therapies, development of l-DOPA-induced dyskinesias (LID), the plasticity protocol used, medication, and stimulation status in patients treated with deep brain stimulation (DBS). The influences of LID and DBS on BG and M1 plasticity have been explored in animal models and in PD patients. In addition, many other factors such age, genetic factors (e.g., brain derived neurotropic factor and other neurotransmitters or receptors polymorphism), emotional state, time of the day, physical fitness have been documented to play role in the extent of plasticity induced by TMS in human studies. In this review, we summarize the studies that investigated M1 plasticity in PD and demonstrate how these afore-mentioned factors affect motor cortical plasticity in PD. We conclude that it is important to consider the clinical, demographic, and technical factors that influence various plasticity protocols while developing these protocols as diagnostic or prognostic tools in PD. We also discuss how the modulation of cortical excitability and the plasticity with these non-invasive brain stimulation techniques facilitate the understanding of the pathophysiology of PD and help design potential therapeutic possibilities in this disorder.

Where is the action? Action sentence processing in Parkinson's disease

According to an influential view of conceptual representation, action concepts are understood through motoric simulations, involving motor networks of the brain. A stronger version of this embodied account suggests that even figurative uses of action words (e.g., grasping the concept) are understood through motoric simulations. We investigated these claims by assessing whether Parkinson's disease (PD), a disorder affecting the motor system, is associated with selective deficits in comprehending action-related sentences. Twenty PD patients and 21 age-matched controls performed a sentence comprehension task, where sentences belonged to one of four conditions: literal action, non-idiomatic metaphoric action, idiomatic action, and abstract. The same verbs (referring to hand/arm actions) were used in the three action-related conditions. Patients, but not controls, were slower to respond to literal and idiomatic action than to abstract sentences. These results indicate that sensory-motor systems play a functional role in semantic processing, including processing of figurative action language.

Safety of transcranial magnetic stimulation in Parkinson's disease: a review of the literature

BACKGROUND

Transcranial magnetic stimulation (TMS) has been used in both physiological studies and, more recently, the therapy of Parkinson's disease (PD). Prior TMS studies in healthy subjects and other patient populations demonstrate a slight risk of seizures and other adverse events. Our goal was to estimate these risks and document other safety concerns specific to PD patients.

METHODS

We performed an English-Language literature search through PudMed to review all TMS studies involving PD patients. We documented any seizures or other adverse events associated with these studies. Crude risks were calculated per subject and per session of TMS.

RESULTS

We identified 84 single pulse (spTMS) and/or paired-pulse (ppTMS) TMS studies involving 1091 patients and 77 repetitive TMS (rTMS) studies involving 1137 patients. Risk of adverse events was low in all protocols. spTMS and ppTMS risk per patient for any adverse event was 0.0018 (95% CI: 0.0002-0.0066) per patient and no seizures were encountered. Risk of an adverse event from rTMS was 0.040 (95% CI: 0.029-0.053) per patient and no seizures were reported. Other adverse events included transient headaches, scalp pain, tinnitus, nausea, increase in pre-existing pain, and muscle jerks. Transient worsening of Parkinsonian symptoms was noted in one study involving rTMS of the supplementary motor area (SMA).

CONCLUSION

We conclude that current TMS and rTMS protocols do not pose significant risks to PD patients. We would recommend that TMS users in this population follow the most recent safety guidelines but do not warrant additional precautions.

The pharmacology of neuroplasticity induced by non-invasive brain stimulation: building models for the clinical use of CNS active drugs

The term neuroplasticity encompasses structural and functional modifications of neuronal connectivity. Abnormal neuroplasticity is involved in various neuropsychiatric diseases, such as dystonia, epilepsy, migraine, Alzheimer's disease, fronto-temporal degeneration, schizophrenia, and post cerebral stroke. Drugs affecting neuroplasticity are increasingly used as therapeutics in these conditions. Neuroplasticity was first discovered and explored in animal experimentation. However, non-invasive brain stimulation (NIBS) has enabled researchers recently to induce and study similar processes in the intact human brain. Plasticity induced by NIBS can be modulated by pharmacological interventions, targeting ion channels, or neurotransmitters. Importantly, abnormalities of plasticity as studied by NIBS are directly related to clinical symptoms in neuropsychiatric diseases. Therefore, a core theme of this review is the hypothesis that NIBS-induced plasticity can explore and potentially predict the therapeutic efficacy of CNS-acting drugs in neuropsychiatric diseases. We will (a) review the basics of neuroplasticity, as explored in animal experimentation, and relate these to our knowledge about neuroplasticity induced in humans by NIBS techniques. We will then (b) discuss pharmacological modulation of plasticity in animals and humans. Finally, we will (c) review abnormalities of plasticity in neuropsychiatric diseases, and discuss how the combination of NIBS with pharmacological intervention may improve our understanding of the pathophysiology of abnormal plasticity in these diseases and their purposeful pharmacological treatment.

Dexamphetamine effects on separate constructs in the rubber hand illusion test

RATIONALE

Corporeal awareness is an integral component of self-consciousness and is distorted in several neurological and psychiatric disorders. Research regarding the neural underpinnings of corporeal awareness has made much progress recently using the rubber hand illusion (RHI) procedure. However, more studies are needed to investigate the possibility of several dissociable constructs related to the RHI specifically, and corporeal awareness generally.

OBJECTIVES

Considering dopamine's involvement in many perceptual-motor learning processes, as well as its apparent relationship with disorders such as schizophrenia that are linked to body ownership disturbances, we gave 0.45 mg/kg dexamphetamine (a dopamine transporter reverser) to 20 healthy participants to examine the effects of increased dopamine transmission on the RHI.

METHODS

The effect of dexamphetamine on separate quantitative constructs underlying RHI were examined including embodiment of rubber hand, loss of ownership of real hand, perception of movement, affect, deafference, and proprioceptive drift. The experiment was a double-blind, placebo-controlled, cross-over design.

RESULTS

Dexamphetamine increased participants' ratings of embodiment (particularly "ownership") of the rubber hand and was associated with the experience of loss of ownership of the person's real hand. There were significant increases from asynchronous to synchronous stroking for the measures of movement and proprioceptive drift after placebo but not dexamphetamine. There were no changes in the measures of other constructs.

CONCLUSIONS

These results show a novel pharmacological manipulation of separate constructs of the RHI. This finding may aid in our understanding of disorders that have overlapping disturbances in both dopamine activity and body representations, particularly schizophrenia.