MOTOR-EVOKED POTENTIALS FOLLOWING IMAGERY AND LIMB DISUSE

Short-term arm immobilization modulates excitability of inhibitory circuits within, and between, primary motor cortices

Previous research has suggested that short-term immobilization of the arm may be a low-cost, non-invasive strategy to enhance the capacity for long-term potentiation (LTP)-like plasticity in primary motor cortex (M1). Short-term immobilization reduces corticospinal excitability (CSE) in the contralateral M1, and interhemispheric inhibition (IHI) from ipsi- onto contralateral M1 is increased. However, it is unclear whether reduced CSE and increased IHI are associated with changes in intracortical inhibition, which has been shown to be important for regulating neuroplasticity in M1. The current study used transcranial magnetic stimulation to evaluate the effects of short-term (6 h) arm immobilization on CSE, IHI, and intracortical inhibition measured bilaterally in 43 neurotypical young adults (23 immobilized). We replicated previous findings demonstrating that immobilization decreased CSE in, and increased IHI onto, the immobilized hemisphere, but a significant change in intracortical inhibition was not observed at the group level. Across individuals, decreased CSE was associated with a decreased short-interval intracortical inhibition, an index of GABAA -ergic inhibition, within the immobilized hemisphere only in the immobilization group. Previous research has demonstrated that decreases in GABAA -ergic inhibition are necessary for the induction of LTP-like plasticity in M1; therefore, decreased intracortical inhibition after short-term arm immobilization may provide a novel mechanism to enhance the capacity for LTP-like plasticity within M1 and may be a potential target for strategies to augment plasticity capacity to enhance motor learning in health and disease.

Motor imagery practice benefits during arm immobilization

Motor imagery (MI) is known to engage motor networks and is increasingly used as a relevant strategy in functional rehabilitation following immobilization, whereas its effects when applied during immobilization remain underexplored. Here, we hypothesized that MI practice during 11 h of arm-immobilization prevents immobilization-related changes at the sensorimotor and cortical representations of hand, as well as on sleep features. Fourteen participants were tested after a normal day (without immobilization), followed by two 11-h periods of immobilization, either with concomitant MI treatment or control tasks, one week apart. At the end of each condition, participants were tested on a hand laterality judgment task, then underwent transcranial magnetic stimulation to measure cortical excitability of the primary motor cortices (M1), followed by a night of sleep during which polysomnography data was recorded. We show that MI treatment applied during arm immobilization had beneficial effects on (1) the sensorimotor representation of hands, (2) the cortical excitability over M1 contralateral to arm-immobilization, and (3) sleep spindles over both M1s during the post-immobilization night. Furthermore, (4) the time spent in REM sleep was significantly longer, following the MI treatment. Altogether, these results support that implementing MI during immobilization may limit deleterious effects of limb disuse, at several levels of sensorimotor functioning.

Motor Imagery Training During Arm Immobilization Prevents Corticomotor Idling: An EEG Resting-State Analysis

Limb disuse causes overt, measurable alterations in motor functions. Motor imagery (MI) practice has been used as a behavioral strategy to prevent motor impairments due to limb disuse or immobilization. Yet, how MI operates at the neural level in the context of short-term limb immobilization remains understudied. We hypothesized that MI treatment applied during 12 h of arm immobilization prevents immobilization-related changes in resting-state electroencephalographic (rsEEG) power and functional connectivity. Fourteen participants first underwent rsEEG after 12 h of normal motor activity (without immobilization). Then, rsEEG recording was performed after 12 h of arm immobilization either with MI treatment or without, each condition separated by 1 week, according to a randomized within-subjects design. MI treatment consisted in performing varied visual and kinaesthetic MI exercises (5 sessions of 15 min every two hours). The results showed that in the delta, theta, alpha and beta frequency bands, interhemispheric difference in sensors power over the motor cortex (i.e. C3 vs. C4) was reduced after arm immobilization, while it did not change when MI treatment was delivered during the immobilization period. Moreover, functional connectivity across the sensors-network in the delta (1-4 Hz) and alpha (8-12 Hz) frequency bands decreased after immobilization while it was restored by MI treatment. To conclude, MI counteracts functional neural changes within and between motor regions in the context of limb immobilization. Practical applications for motor rehabilitation strategies, particularly in stroke patients, are also discussed.

Beyond Jeannerod's motor simulation theory: An approach for improving post-traumatic motor rehabilitation

This paper depicts an approach aiming to allow individuals with orthopedic trauma injuries to optimize recovery and safely return to daily life. The core of this approach is motor simulation, used in complement to conventional physical rehabilitation methods. This paper provides recent scientific insights on the basis of motor simulation, discusses benefits of this approach in motor rehabilitation, and provides applied perspectives.

Neural plasticity during motor learning with motor imagery practice: Review and perspectives

In the last decade, many studies confirmed the benefits of mental practice with motor imagery. In this review we first aimed to compile data issued from fundamental and clinical investigations and to provide the key-components for the optimization of motor imagery strategy. We focused on transcranial magnetic stimulation studies, supported by brain imaging research, that sustain the current hypothesis of a functional link between cortical reorganization and behavioral improvement. As perspectives, we suggest a model of neural adaptation following mental practice, in which synapse conductivity and inhibitory mechanisms at the spinal level may also play an important role.

The embodied nature of motor imagery processes highlighted by short-term limb immobilization

We investigated the embodied nature of motor imagery processes through a recent use-dependent plasticity approach, a short-term limb immobilization paradigm. A splint placed on the participants' left-hand during a brief period of 24 h was used for immobilization. The immobilized participants performed two mental rotation tasks (a hand mental rotation task and a number mental rotation task) before (pre-test) and immediately after (post-test) the splint removal. The control group did not undergo the immobilization procedure. The main results showed an immobilization-induced effect on left-hand stimuli, resulting in a lack of task-repetition benefit. By contrast, accuracy was higher and response times were shorter for right-hand stimuli. No immobilization-induced effects appeared for number stimuli. These results revealed that the cognitive representation of hand movements can be modified by a brief period of sensorimotor deprivation, supporting the hypothesis of the embodied nature of motor simulation processes.

The power of the mind: the cortex as a critical determinant of muscle strength/weakness

We tested the hypothesis that the nervous system, and the cortex in particular, is a critical determinant of muscle strength/weakness and that a high level of corticospinal inhibition is an important neurophysiological factor regulating force generation. A group of healthy individuals underwent 4 wk of wrist-hand immobilization to induce weakness. Another group also underwent 4 wk of immobilization, but they also performed mental imagery of strong muscle contractions 5 days/wk. Mental imagery has been shown to activate several cortical areas that are involved with actual motor behaviors, including premotor and M1 regions. A control group, who underwent no interventions, also participated in this study. Before, immediately after, and 1 wk following immobilization, we measured wrist flexor strength, voluntary activation (VA), and the cortical silent period (SP; a measure that reflect corticospinal inhibition quantified via transcranial magnetic stimulation). Immobilization decreased strength 45.1 ± 5.0%, impaired VA 23.2 ± 5.8%, and prolonged the SP 13.5 ± 2.6%. Mental imagery training, however, attenuated the loss of strength and VA by ∼50% (23.8 ± 5.6% and 12.9 ± 3.2% reductions, respectively) and eliminated prolongation of the SP (4.8 ± 2.8% reduction). Significant associations were observed between the changes in muscle strength and VA (r = 0.56) and SP (r = -0.39). These findings suggest neurological mechanisms, most likely at the cortical level, contribute significantly to disuse-induced weakness, and that regular activation of the cortical regions via imagery attenuates weakness and VA by maintaining normal levels of inhibition.

Preventing functional loss during immobilization after osteoporotic wrist fractures in elderly patients: a randomized clinical trial

BACKGROUND

Distal radius fractures are among the most common fractures and account for approximately one-sixth of all fractures diagnosed. Therapy results after distal radius fracture, especially of elderly patients, are often suboptimal. The inevitable immobilization for several weeks leads to reduction in range of motion, deterioration of muscle strength, malfunction of fine motor skills as well as changes of motor and sensory representations in the brain. Currently, there are no strategies to counteract these immobilization problems. The overall aim of the study is to investigate the therapeutic potential of motor-cognitive approaches (mental practice or mirror therapy) on hand function after wrist fracture.

METHODS/DESIGN

This study is a controlled, randomized, longitudinal intervention study with three intervention groups. One experimental group imagines movements of the fractured upper extremity without executing them (mental practice). The second experimental group receives a mirror therapy program consisting of the performance of functional movement synergies using the unaffected forearm, wrist, and hand. The control group completes a relaxation training regime. Additionally, all patients receive usual care by the general practitioner. We include women aged 60 years and older having a distal radius fracture and sufficient cognitive function. All groups are visited at home for therapy sessions 5 times per week for the first 3 weeks and 3 times per week for weeks 4 to 6. Measurements are taken at therapy onset, and after 3, 6 and 12 weeks. The primary outcome measure will assess upper extremity functioning (Patient-Rated Wrist Evaluation [PRWE]), while secondary outcome measures cover subjective wrist function (Disabilities of the Arm and Shoulder; [DASH], objective impairment (range of motion, grip force) and quality of life (EuroQol-5D, [EQ5D]).

DISCUSSION

Results from this trial will contribute to the evidence on motor-cognitive approaches in the early therapy of distal radius fractures.

TRIAL REGISTRATION

The trial is registered at ClinicalTrials.gov with registration number NCT01394809 and was granted permission by the Medical Ethical Review Committee of the University of Tübingen in June 2011.

Training the motor cortex by observing the actions of others during immobilization

Limb immobilization and nonuse are well-known causes of corticomotor depression. While physical training can drive the recovery from nonuse-dependent corticomotor effects, it remains unclear if it is possible to gain access to motor cortex in alternative ways, such as through motor imagery (MI) or action observation (AO). Transcranial magnetic stimulation was used to study the excitability of the hand left motor cortex in normal subjects immediately before and after 10 h of right arm immobilization. During immobilization, subjects were requested either to imagine to act with their constrained limb or to observe hand actions performed by other individuals. A third group of control subjects watched a nature documentary presented on a computer screen. Hand corticomotor maps and recruitment curves reliably showed that AO, but not MI, prevented the corticomotor depression induced by immobilization. Our results demonstrate the existence of a visuomotor mechanism in humans that links AO and execution which is able to effect cortical plasticity in a beneficial way. This facilitation was not related to the action simulation, because it was not induced by explicit MI.

Effects of Practice, Visual Loss, Limb Amputation, and Disuse on Motor Imagery Vividness

Background. The ability to generate vivid images of movements is variable across individuals and likely influenced by sensorimotor inputs. Objectives. The authors examined (1) the vividness of motor imagery in dancers and in persons with late blindness, with amputation or an immobilization of one lower limb; (2) the effects of prosthesis use on motor imagery; and (3) the temporal characteristics of motor imagery. Methods. Eleven dancers, 10 persons with late blindness, 14 with amputation, 6 with immobilization, and 2 groups of age-matched healthy individuals (27 in control group A; 35 in control group B) participated. The Kinesthetic and Visual Imagery Questionnaire served to assess motor imagery vividness. Temporal characteristics were assessed with mental chronometry. Results. The late blindness group and dance group displayed higher imagery scores than respective control groups. In the amputation and immobilization groups, imagery scores were lower on the affected side than the intact side and specifically for imagined foot movements. Imagery scores of the affected limb positively correlated with the time since walking with prosthesis. Movement times during imagination and execution (amputation and immobilization) were longer on the affected side than the intact side, but the temporal congruence between real and imagined movement times was similar to that in the control group. Conclusions. The mental representation of actions is highly modulated by imagery practice and motor activities. The ability to generate vivid images of movements can be specifically weakened by limb loss or disuse, but lack of movement does not affect the temporal characteristics of motor imagery.

Reliability of motor-evoked potentials in the ADM muscle of older adults

The purpose of this study was to determine the reliability of motor-evoked potentials (MEPs) produced in the abductor digiti minimi (ADM) muscle of male and female older adults, both within and between sessions. The number of trials within a session required to obtain reliable results was also examined. The investigation was conducted on 30 elderly individuals (15 M, 15 F; mean age 76 years). With the ADM at rest, MEPs were evoked at intensities of 1.1, 1.3, and 1.5 times motor threshold (MT). Ten stimuli were delivered at each intensity, with 10-12s between stimuli. The MEP responses were blocked with two, three, four, and five-stimuli means in each block. An intraclass correlation (ICC) reliability analysis of variance model was used to assess reliability of the MEP amplitude, using a variable number of trials per block. A subset of 10 subjects repeated the protocol following 20min of rest to assess the reliability between sessions. As expected, MEP amplitudes were significantly higher as stimulus intensity increased. There were no significant differences between blocks, nor was there a significant gender effect. ICC reliability coefficients ranged between 0.09 with two trials per block and 0.97 with five trials per block. Between session reliability ranged from 0.65 to 0.83. Highly reliable MEP amplitudes can be obtained in older individuals using two blocks of TMS stimuli with five trials per block.