Attentional influences on short-interval intracortical inhibition

Emerging concepts on bradykinesia in non-parkinsonian conditions

BACKGROUND AND PURPOSE

Bradykinesia is one of the cardinal motor symptoms of Parkinson's disease. However, clinical and experimental studies indicate that bradykinesia may also be observed in various neurological diseases not primarily characterized by parkinsonism. These conditions include hyperkinetic movement disorders, such as dystonia, chorea, and essential tremor. Bradykinesia may also be observed in patients with neurological conditions that are not seen as "movement disorders," including those characterized by the involvement of the cerebellum and corticospinal system, dementia, multiple sclerosis, and psychiatric disorders.

METHODS

We reviewed clinical reports and experimental studies on bradykinesia in non-parkinsonian conditions and discussed the major findings.

RESULTS

Bradykinesia is a common motor abnormality in non-parkinsonian conditions. From a pathophysiological standpoint, bradykinesia in neurological conditions not primarily characterized by parkinsonism may be explained by brain network dysfunction.

CONCLUSION

In addition to the pathophysiological implications, the present paper highlights important terminological issues and the need for a new, more accurate, and more widely used definition of bradykinesia in the context of movement disorders and other neurological conditions.

Stop Signal Task Training Strengthens GABA-mediated Neurotransmission within the Primary Motor Cortex

We have recently shown that the efficiency in stopping a response, measured using the stop signal task, is related to GABAA-mediated short-interval intracortical inhibition (SICI) in the primary motor cortex. In this study, we conducted two experiments on humans to determine whether training participants in the stop signal task within one session (Experiment 1) and across multiple sessions (Experiment 2) would increase SICI strength. For each experiment, we obtained premeasures and postmeasures of stopping efficiency and resting-state SICI, that is, during relaxed muscle activity (Experiment 1, n = 45, 15 male participants) and SICI during the stop signal task (Experiment 2, n = 44, 21 male participants). In the middle blocks of Experiment 1 and the middle sessions of Experiment 2, participants in the experimental group completed stop signal task training, whereas control participants completed a similar task without the requirement to stop a response. After training, the experimental group showed increased resting-state SICI strength (Experiment 1) and increased SICI strength during the stop signal task (Experiment 2). Although there were no overall behavioral improvements in stopping efficiency, improvements at an individual level were correlated with increases in SICI strength at rest (Experiment 1) and during successful stopping (Experiment 2). These results provide evidence of neuroplasticity in resting-state and task-related GABAA-mediated SICI in the primary motor cortex after response inhibition training. These results also suggest that SICI and stopping efficiency are temporally linked, such that a change in SICI between time points is correlated with a change in stopping efficiency between time points.

The effect of experimental pain on short-interval intracortical inhibition with multi-locus transcranial magnetic stimulation

Chronic neuropathic pain is known to alter the primary motor cortex (M1) function. Less is known about the normal, physiological effects of experimental neurogenic pain on M1. The objective of this study is to determine how short-interval intracortical inhibition (SICI) is altered in the M1 representation area of a muscle exposed to experimental pain compared to SICI of another muscle not exposed to pain. The cortical representation areas of the right abductor pollicis brevis (APB) and biceps brachii (BB) muscles of 11 subjects were stimulated with a multi-locus transcranial magnetic stimulation device while the resulting motor-evoked potentials (MEPs) were recorded with electromyography. Single- and paired-pulse TMS was administered in seven conditions, including one with the right hand placed in cold water. The stimulation intensity for the conditioning pulses in the paired-pulse examination was 80% of the resting motor threshold (RMT) of the stimulated site and 120% of RMT for both the test and single pulses. The paired-pulse MEP amplitudes were normalized with the mean amplitude of the single-pulse MEPs of the same condition and muscle. SICI was compared between conditions. After the cold pain, the normalized paired-pulse MEP amplitudes decreased in APB, but not in BB, indicating that SICI was potentially increased only in the cortical area of the muscle subjected to pain. These data suggest that SICI is increased in the M1 representation area of a hand muscle shortly after exposure to pain has ended, which implies that short-lasting pain can alter the inhibitory balance in M1.

Repetitive Passive Movement Modulates Corticospinal Excitability: Effect of Movement and Rest Cycles and Subject Attention

Repetitive passive movement (PM) affects corticospinal excitability; however, it is unknown whether a duty cycle which repeats movement and rest, or subjects’ conscious attention to movements, affects corticospinal excitability. We aimed to clarify the effect of the presence or absence of a duty cycle and subjects’ attention on corticospinal excitability. Three experiments were conducted. In Experiment 1, PM of the right index finger was performed for 10 min. Three conditions were used: (1) continuous PM (cPM) at a rate of 40°/s; (2) intermittent PM (iPM) with a duty cycle at 40°/s; and (3) iPM at 100°/s. In conditions 1 and 3, motor evoked potential (MEP) amplitude was significantly reduced. In Experiment 2, PM was performed for 30 min: condition 1 comprised cPM at a rate of 40°/s and Condition 2 comprised iPM at 40°/s. MEP amplitude significantly decreased in both conditions. In Experiment 3, PM was performed for 10 min: condition 1 comprised paying attention to the moving finger during iPM and Condition 2 was similar to Condition 1 but while counting images on a monitor without looking at the movement finger, and Condition 3 comprised counting images on a monitor without performing PM. MEP amplitude significantly increased only under Condition 1. Thus, afferent input from movements above a certain threshold may affect corticospinal excitability reduction. Furthermore, corticospinal excitability increases when paying attention to passive finger movement.

Methylphenidate promotes the interaction between motor cortex facilitation and attention in healthy adults: A combined study using event-related potentials and transcranial magnetic stimulation

OBJECTIVE

This study investigated simultaneously the impact of methylphenidate (MPH) on the interaction of inhibitory and facilitative pathways in regions processing motor and cognitive functions.

METHOD

Neural markers of attention and response control (event-related potentials) and motor cortical excitability (transcranial magnetic stimulation) and their pharmacological modulation by MPH were measured simultaneously in a sample of healthy adults (n = 31) performing a cued choice reaction test.

RESULTS

Methylphenidate modulated attentional gating and response preparation processes (increased contingent negative variation) and response inhibition (increased nogo P3). N1, cue- and go-P3 were not affected by MPH. Motor cortex facilitation, measured with long-interval cortical facilitation, was increased under MPH in the nogo condition and was positively correlated with the P3 amplitude.

CONCLUSION

Methylphenidate seems particularly to enhance response preparation processes. The MPH-induced increased motor cortex facilitation during inhibitory task demands was accompanied by increased terminal response inhibition control, probably as a compensatory process.

Cortical excitability following passive movement

In brain injury rehabilitation, passive movement exercises are frequently used to maintain or improve mobility and range of motion. They can also induce beneficial and sustained neuroplastic changes. Neuroimaging studies have revealed that passive movements without motor commands activate not only the primary somatosensory cortex but also the primary motor cortex, supplementary motor area, and posterior parietal cortex as well as the secondary somatosensory cortex (S2) in healthy subjects. Repetitive passive movement has also been reported to induce increases or decreases in cortical excitability. In this review, we focused on the following: cortical activity following passive movement; cortical excitability during passive movement; and changes in cortical excitability after repetitive passive movement.

Functional dystonia and the borderland between neurology and psychiatry: New concepts

Mind-brain dualism has dominated historical commentary on dystonia, a dichotomous approach that has left our conceptual grasp of it stubbornly incomplete. This is particularly true of functional dystonia, most diagnostically challenging of all functional movement disorders, in which the question of inherent psychogenicity remains a focus of debate. Phenomenological signs considered in isolation lack the specificity to distinguish organic and nonorganic forms, and dystonia's variability has frustrated attempts to develop objective laboratory-supported standards. Diagnostic criteria for functional dystonia that place emphasis on psychiatric symptoms perform poorly in studies of reliability, partly explained by the high frequency of psychopathology in organic dystonia. Novel approaches from the cognitive neurosciences may offer a way forward. Theory on Bayesian statistical prediction in cognitive processing is supported by sufficient experimental evidence for this model to be taken seriously as a way of reconciling contradictory notions about voluntary and unconscious motor control in functional movement disorders. In a Bayesian formulation of functional dystonia, misallocation of attention and abnormal predictive beliefs generate movements that are executed without a sense of agency. Building on this framework, there is a consensus that a biopsychosocial approach is required and that a unified philosophy of brain and mind is the best way to locate dystonia in the neurology-psychiatry borderland. At a practical level, movement disorder neurologists are best placed to differentiate organic from functional dystonia. The main role of psychiatrists is in the diagnosis and management of the primarily psychiatric disorders that often accompany dystonia. © 2016 International Parkinson and Movement Disorder Society.

Neuronal mechanisms of motor learning and motor memory consolidation in healthy old adults

It is controversial whether or not old adults are capable of learning new motor skills and consolidate the performance gains into motor memory in the offline period. The underlying neuronal mechanisms are equally unclear. We determined the magnitude of motor learning and motor memory consolidation in healthy old adults and examined if specific metrics of neuronal excitability measured by magnetic brain stimulation mediate the practice and retention effects. Eleven healthy old adults practiced a wrist extension-flexion visuomotor skill for 20 min (MP, 71.3 years), while a second group only watched the templates without movements (attentional control, AC, n = 11, 70.5 years). There was 40 % motor learning in MP but none in AC (interaction, p < 0.001) with the skill retained 24 h later in MP and a 16 % improvement in AC. Corticospinal excitability at rest and during task did not change, but when measured during contraction at 20 % of maximal force, it strongly increased in MP and decreased in AC (interaction, p = 0.002). Intracortical inhibition at rest and during the task decreased and facilitation at rest increased in MP, but these metrics changed in the opposite direction in AC. These neuronal changes were especially profound at retention. Healthy old adults can learn a new motor skill and consolidate the learned skill into motor memory, processes that are most likely mediated by disinhibitory mechanisms. These results are relevant for the increasing number of old adults who need to learn and relearn movements during motor rehabilitation.

An unavoidable modulation? Sensory attention and human primary motor cortex excitability

The link between basic physiology and its modulation by cognitive states, such as attention, is poorly understood. A significant association becomes apparent when patients with movement disorders describe experiences with changing their attention focus and the fundamental effect that this has on their motor symptoms. Moreover, frequently used mental strategies for treating such patients, e.g. with task-specific dystonia, widely lack laboratory-based knowledge about physiological mechanisms. In this largely unexplored field, we looked at how the locus of attention, when it changed between internal (locus hand) and external (visual target), influenced excitability in the primary motor cortex (M1) in healthy humans. Intriguingly, both internal and external attention had the capacity to change M1 excitability. Both led to a reduced stimulation-induced GABA-related inhibition and a change in motor evoked potential size, i.e. an overall increased M1 excitability. These previously unreported findings indicated: (i) that cognitive state differentially interacted with M1 physiology, (ii) that our view of distraction (attention locus shifted towards external or distant location), which is used as a prevention or management strategy for use-dependent motor disorders, is too simple and currently unsupported for clinical application, and (iii) the physiological state reached through attention modulation represents an alternative explanation for frequently reported electrophysiology findings in neuropsychiatric disorders, such as an aberrant inhibition.

The Phenomenology of Functional (Psychogenic) Dystonia

From the very first descriptions of dystonia, there has been a lack of agreement on the differentiation of organic from functional (psychogenic) dystonia. This lack of agreement has had a significant effect on patients over the years, most particularly in the lack of access to appropriate management, whether for those with organic dystonia diagnosed as having a functional cause or vice versa. However, clinico-genetic advances have led to greater certainty about the phenomenology of organic dystonia and therefore recognition of atypical forms. The diagnosis of functional dystonia rests on recognition of its phenomenology and should not be, as far as possible, a diagnosis of exclusion. Here, we present an overview of the phenomenology of functional dystonia, concentrating on the three main phenotypic presentations: functional cranial dystonia; functional fixed dystonia; and functional paroxysmal dystonia. We hope that this review of phenomenology will aid in the positive diagnosis of functional dystonia and, through this, will lead to more rapid access to appropriate management.

Modulation of intracortical inhibition in response to acute psychosocial stress is impaired among individuals with chronic neck pain

OBJECTIVE

Psychosocial stress has been associated with a variety of chronic pain disorders although the mechanisms responsible for this relationship are unknown. The purpose of this study was to compare the excitability of intracortical and corticospinal pathways to the trapezius muscle in individuals with and without chronic neck pain during exposure to low and high levels of psychosocial stress.

METHODS

Single and paired-pulse transcranial magnetic stimulation was used to assess motor evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) during mental math performed in the presence and absence of social evaluative threat.

RESULTS

All participants demonstrated higher amplitude MEPs in the high stress compared to the low stress condition (p < 0.01). Participants with chronic neck pain had significantly greater SICI than healthy participants in the low stress condition (p = 0.03). During exposure to the stressor, healthy participants showed an increase in SICI, whereas participants with neck pain showed no change (group difference for change in SICI, p < 0.01).

CONCLUSIONS

These findings suggest that individuals with chronic neck pain inhibit motor output to the trapezius in the presence of minor stressors, and are unable to compensate for additional stress-evoked increases in corticospinal excitability through further modulation of SICI. This observation has potential implications for the management of patients who have difficulty relaxing painful muscles during times of stress.

Task-free functional MRI in cervical dystonia reveals multi-network changes that partially normalize with botulinum toxin

Cervical dystonia is characterized by involuntary, abnormal movements and postures of the head and neck. Current views on its pathophysiology, such as faulty sensorimotor integration and impaired motor planning, are largely based on studies of focal hand dystonia. Using resting state fMRI, we explored whether cervical dystonia patients have altered functional brain connectivity compared to healthy controls, by investigating 10 resting state networks. Scans were repeated immediately before and some weeks after botulinum toxin injections to see whether connectivity abnormalities were restored. We here show that cervical dystonia patients have reduced connectivity in selected regions of the prefrontal cortex, premotor cortex and superior parietal lobule within a distributed network that comprises the premotor cortex, supplementary motor area, primary sensorimotor cortex, and secondary somatosensory cortex (sensorimotor network). With regard to a network originating from the occipital cortex (primary visual network), selected regions in the prefrontal and premotor cortex, superior parietal lobule, and middle temporal gyrus areas have reduced connectivity. In selected regions of the prefrontal, premotor, primary motor and early visual cortex increased connectivity was found within a network that comprises the prefrontal cortex including the anterior cingulate cortex and parietal cortex (executive control network). Botulinum toxin treatment resulted in a partial restoration of connectivity abnormalities in the sensorimotor and primary visual network. These findings demonstrate the involvement of multiple neural networks in cervical dystonia. The reduced connectivity within the sensorimotor and primary visual networks may provide the neural substrate to expect defective motor planning and disturbed spatial cognition. Increased connectivity within the executive control network suggests excessive attentional control and while this may be a primary trait, perhaps contributing to abnormal motor control, this may alternatively serve a compensatory function in order to reduce the consequences of the motor planning defect inflicted by the other network abnormalities.

Assessment of post-excitatory long-interval cortical inhibition in adult attention-deficit/hyperactivity disorder

In order to further examine cortical impairment in adult ADHD patients and to test the hypothesis of a disturbed neuronal inhibition in adults with ADHD, late auditory evoked potentials were measured. By using paired-chirp auditory late responses, we compared 15 adults with ADHD with 15 control subjects, focusing on the inhibition elicited by the stimuli. Besides amplitude measurements, a time-frequency phase coherence study using the wavelet phase synchronization stability (WPSS) was performed. ADHD was diagnosed according to DSM-IV criteria. All ADHD subjects were without medication and did not suffer from any further axis I disorder. WPSS analysis revealed impaired auditory inhibition for ADHD patients for interstimulus intervals (ISI) between 500 and 1,100 ms as compared with healthy controls. By analyzing the WPSS in the interval from 80 ms to 220 ms, mean inhibition of the test chirp was found to be 6% in the ADHD group and 38.5% in the control subjects (p = 0.01). Moreover, overall smaller amplitudes in the N100 and P200 waves at all ISI were found (p = 0.04 and p = 0.02). However, reproducibility indices in the amplitude measurements were low, thus supporting the use of the instantaneous phase-based analysis method. The results support the hypothesis of reduced intracortical inhibition as a correlate of disturbed brain function in adults with ADHD.

A near infra-red spectroscopy study of the effects of pre-frontal single and paired pulse transcranial magnetic stimulation

OBJECTIVE

Concentration changes in hemoglobin following single and paired pulse (2 and 15 ms inter-stimulus interval) transcranial magnetic stimulation (TMS) was investigated using near infra-red spectroscopy (NIRS).

METHODS

TMS was delivered to left pre-frontal cortex at typical intensities used in neuroscience research and concentrations of deoxyhemoglobin (Hb), oxyhemoglobin (HbO) and total hemoglobin (HbT) were measured.

RESULTS

Significant drops in concentration of HbO and HbT were observed and while there was no effect of the different pulse types on amplitude, there was a difference in the time taken to return to baseline.

CONCLUSIONS

The changes observed imply that in pre-frontal cortex, the different TMS pulse types result in differential effects on oxygen consumption.

SIGNIFICANCE

This study aids our understanding of the physiological effects of single and paired pulse TMS.

Reward-related activity in the human motor cortex

The human primary motor cortex (M1) participates in motor learning and response selection, functions that rely on feedback on the success of behavior (i.e. reward). To investigate the possibility that behavioral contingencies alter M1 activity in humans, we tested intracortical inhibition with single and paired (subthreshold/suprathreshold) transcranial magnetic stimulation during a slot machine simulation that delivered variable money rewards for three-way matches and required no movement. A two-way match before the third barrel had stopped (increased reward expectation) was associated with more paired-pulse inhibition than no match. Receiving a large reward on the preceding trial augmented this effect. A control task that manipulated attention to the same stimuli produced no changes in excitability. The origin of this reward-related activity is not clear, although dopaminergic ventral tegmental area neurons project to M1, where they are thought to inhibit output neurons and could be the source of the finding. Transcranial magnetic stimulation of M1 may be useful as a quantitative measure of reward-related activity.