Visual feedback-related changes in ipsilateral cortical excitability during unimanual movement: Implications for mirror therapy.

Provision of a mirror image of a hand undertaking a motor task (i.e., mirror therapy) elicits behavioural improvements in the inactive hand. A greater understanding of the neural mechanisms underpinning this phenomenon is required to maximise its potential for rehabilitation across the lifespan, e.g., following hemiparesis or unilateral weakness. Young and older participants performed unilateral finger abductions with no visual feedback, with feedback of the active or passive hands, or with a mirror image of the active hand. Transcranial magnetic stimulation was used to assess feedback-related changes in two neurophysiological measures thought to be involved in inter-manual transfer of skill, namely corticospinal excitability (CSE) and intracortical inhibition (SICI) in the passive hemisphere. Task performance led to CSE increases, accompanied by decreases of SICI, in all visual feedback conditions relative to rest. However, the changes due to mirror feedback were not significantly different to those observed in the other (more standard) visual conditions. Accordingly, the unimanual motor action itself, rather than modifications in visual feedback, appears more instrumental in driving changes in CSE and SICI. Therefore, changes in CSE and SICI are unlikely to underpin the behavioural benefits of mirror therapy. We discuss implications for rehabilitation and directions of future research.

The influence of the BDNF Val66Met genotype on emotional recognition memory in post-traumatic stress disorder.

Dysregulated consolidation of emotional memories is a core feature of posttraumatic stress disorder (PTSD). Brain Derived Neurotrophic Factor (BDNF) influences synaptic plasticity and emotional memory consolidation. The BDNF Val66Met polymorphism has been associated with PTSD risk and memory deficits respectively, although findings have been inconsistent, potentially due to a failure to control for important confounds such as sex, ethnicity, and the timing/extent of previous trauma experiences. Furthermore, very little research has examined the impact of BDNF genotypes on emotional memory in PTSD populations. This study investigated the interaction effects of Val66Met and PTSD symptomatology in an emotional recognition memory task in 234 participants divided into healthy control (n = 85), trauma exposed (TE: n = 105) and PTSD (n = 44) groups. Key findings revealed impaired negative recognition memory in PTSD compared to control and TE groups and in participants with the Val/Met compared to the Val/Val genotype. There was a group × genotype interaction showing no Met effect in the TE group despite significant effects in PTSD and controls. Results suggest that people previously exposed to trauma who do not develop PTSD may be protected from the BDNF Met effect, however more research is needed to replicate findings and to explore the epigenetic and neural processes involved.

Age-related differences in corticomotor excitability and inhibitory processes during a visuomotor RT task.

This study tested the postulation that change in the ability to modulate corticospinal excitability and inhibitory processes underlie age-related differences in response preparation and generation during tasks requiring either rapid execution of a motor action or actively withholding that same action. Younger (n = 13, mean age = 26.0 years) and older adults (n = 13, mean age = 65.5 years) performed an RT task in which a warning signal (WS) was followed by an imperative signal (IS) to which participants were required to respond with a rapid flexion of the right thumb (go condition) or withhold their response (no-go condition). We explored the neural correlates of response preparation, generation, and inhibition using single- and paired-pulse TMS, which was administered at various times between WS and IS (response preparation phase) and between IS and onset of response-related muscle activity in the right thumb (response generation phase). Both groups exhibited increases in motor-evoked potential amplitudes (relative to WS onset) during response generation; however, this increase began earlier and was more pronounced for the younger adults in the go condition. Moreover, younger adults showed a general decrease in short-interval intracortical inhibition during response preparation in both the go and no-go conditions, which was not observed in older adults. Importantly, correlation analysis suggested that for older adults the task-related increases of corticospinal excitability and intracortical inhibition were associated with faster RT. We propose that the declined ability to functionally modulate corticospinal activity with advancing age may underlie response slowing in older adults.

Subthreshold repetitive transcranial magnetic stimulation drives structural synaptic plasticity in the young and aged motor cortex.

BACKGROUND: Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive tool commonly used to drive neural plasticity in the young adult and aged brain. Recent data from mouse models have shown that even at subthreshold intensities (0.12 T), rTMS can drive neuronal and glial plasticity in the motor cortex. However, the physiological mechanisms underlying subthreshold rTMS induced plasticity and whether these are altered with normal ageing are unclear. OBJECTIVE: To assess the effect of subthreshold rTMS, using the intermittent theta burst stimulation (iTBS) protocol on structural synaptic plasticity in the mouse motor cortex of young and aged mice. METHODS: Longitudinal in vivo 2-photon microscopy was used to measure changes to the structural plasticity of pyramidal neuron dendritic spines in the motor cortex following a single train of subthreshold rTMS (in young adult and aged animals) or the same rTMS train administered on 4 consecutive days (in young adult animals only). Data were analysed with Bayesian hierarchical generalized linear regression models and interpreted with the aid of Bayes Factors (BF). RESULTS: We found strong evidence (BF > 10) that subthreshold rTMS altered the rate of dendritic spine losses and gains, dependent on the number of stimulation sessions and that a single session of subthreshold rTMS was effective in driving structural synaptic plasticity in both young adult and aged mice. CONCLUSION: These findings provide further evidence that rTMS drives synaptic plasticity in the brain and uncovers structural synaptic plasticity as a key mechanism of subthreshold rTMS induced plasticity.

Unilateral contractions modulate interhemispheric inhibition most strongly and most adaptively in the homologous muscle of the contralateral limb.

We investigated how volitional contractions affect interhemispheric inhibition (IHI) from the active to the passive hemisphere. Younger and older adults isometrically contracted their dominant thumb (abductor pollicis brevis, APB) to various force targets. In ballistic contraction trials, transcranial magnetic stimulation (TMS) was administered very shortly after the onset of APB activity. In tonic contraction trials, TMS was delivered while the target force was maintained. In control trials both thumbs remained quiescent. In all trials, a test stimulus (TS) was directed to the APB hotspot in the non-dominant hemisphere (130% left APB resting motor threshold, RMT). In half the trials, a conditioning stimulus (130% right APB RMT) was applied to the APB hotspot in the dominant hemisphere 10 ms prior to the TS. Targeted ballistic contractions of the right APB were found to modulate (increase) IHI measured in the left APB, as previously reported for tonic contractions. Furthermore, the extent of the IHI increase was found to scale with the strength of the contralateral ballistic or tonic contraction. Less pronounced, but statistically significant, IHI increases were also observed in the left abductor digiti minimi and extensor carpi radialis during right APB contraction. For these muscles, however, the extent of the IHI modulation was independent of APB contraction strength. The capacity to modulate inhibition during contractions was unaffected by advancing age. During volitional actions, the ability to modulate IHI most adaptively in the homologous muscle of the resting limb may contribute to the prevention of mirror movements.

The effect of ballistic thumb contractions on the excitability of the ipsilateral motor cortex.

We investigated how ballistic contractions of the left thumb affect the excitability of the ipsilateral motor cortex using transcranial magnetic stimulation (TMS). TMS was applied at the motor hotspot for the right abductor pollicis brevis (APB) muscle. In 'self-triggered' trials, participants made targeted, isometric, contractions of the left APB. The right APB was either relaxed or maintained a tonic contraction. TMS was administered as soon as possible after electromyographic onset in the left APB. In 'control' trials, the left thumb remained quiescent and TMS was triggered by the computer. In each condition, 20-24 trials were conducted. Half these trials involved a single test stimulus, TS (130% APB resting motor threshold, RMT). In the other trials, short-interval intracortical inhibition (SICI) was investigated by applying a conditioning stimulus (70% APB RMT) 3 ms prior to the TS. SICI ratios were not significantly different in self-triggered and control trials. However, when the right APB was active, significantly shorter silent periods (SPs) were observed in self-triggered trials when compared with control trials. Our results support the view that SICI and SP are mediated by different inhibitory circuits, and that ipsilateral GABA(B)-ergic circuits (assessed by SP), but not GABA(A)-ergic circuits (assessed by SICI), are affected in the period immediately following voluntary ballistic contractions.

Cognitive outcomes associated with long-term, regular, recreational cannabis use in adults: A meta-analysis.

Despite research advances, it remains unclear if long-term, regular cannabis use harms cognition once intoxication has passed. Our meta-analysis aimed to investigate the association between cognitive functioning and long-term (mean ≥2 years), regular (mean ≥4 days/week), recreational cannabis use in adults during abstinence (mean ≥12 hr). We searched PubMed, PsycINFO, CINAHL, Scopus, and Dissertations and Theses International for English-language articles from the date each database began until May 22, 2019. We identified study inclusion by completing abstract and full text screening using predetermined criteria and Preferred Reporting Items for Systematic Review and Meta-Analysis guidelines. We classified cognitive performance into 6 cognitive domains (attention, executive function, learning and memory, decision making, information processing, and working memory), and included a global measure. Effect sizes were calculated for each domain using univariate meta-analyses. There were 30 studies with a total 849 participants who used cannabis (M = 30.7-years-old, SD = 5.5-years-old) and 764 control participants (M = 30.3-years-old, SD = 5.9-years-old). Cannabis was associated with significant but small-magnitude deficits in executive function, learning and memory, and global cognition, while decision making had moderate deficits. There were small-magnitude and nonsignificant group differences for information processing, working memory, and attention. Cannabis use duration, age of onset, and prolonged abstinence (≥25 days) did not influence outcomes, except group differences in executive function were nonsignificant in analyses of prolonged abstinence. Our results suggest that long-term, regular cannabis use is associated with small to moderate deficits in some cognitive domains. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Attentional influences on short-interval intracortical inhibition.

OBJECTIVE: The allocation of attention to sensory stimulation and movement might influence cortical activity. Two experiments were conducted to investigate the effect of variation of intensity of attention (Experiment 1) and direction of attention (Experiment 2) on cortical excitability and short-interval intracortical inhibition (SICI) during performance of a simple index finger abduction task. METHODS: Subjects responded to subtle cutaneous electrical stimulation delivered to the index finger while single and paired TMS pulses were delivered during muscle relaxation between successive responses. In Experiment 1, attentional resources allocated to the task were manipulated using a dual task paradigm involving a backward-counting task. In Experiment 2, spatial attention was varied by delivering cutaneous stimuli to the responding or the non-responding index finger. RESULTS: In Experiment 1, SICI was reduced during performance, but was unaffected by variation in the intensity of attention. The results of Experiment 2, however, showed that SICI was significantly lower when attention was directed to the responding hand compared with when it was directed to the non-responding hand. CONCLUSIONS: While SICI was not affected by variation of attentional resources, it was influenced by spatial attention. SIGNIFICANCE: These findings may be relevant in future investigations of the underlying neurophysiology of plasticity.

Low intensity repetitive transcranial magnetic stimulation modulates skilled motor learning in adult mice.

Repetitive transcranial magnetic stimulation (rTMS) is commonly used to modulate cortical plasticity in clinical and non-clinical populations. Clinically, rTMS is delivered to targeted regions of the cortex at high intensities (>1 T). We have previously shown that even at low intensities, rTMS induces structural and molecular plasticity in the rodent cortex. To determine whether low intensity rTMS (LI-rTMS) alters behavioural performance, daily intermittent theta burst LI-rTMS (120 mT) or sham was delivered as a priming or consolidating stimulus to mice completing 10 consecutive days of skilled reaching training. Relative to sham, priming LI-rTMS (before each training session), increased skill accuracy (~9%) but did not alter the rate of learning over time. In contrast, consolidating LI-rTMS (after each training session), resulted in a small increase in the rate of learning (an additional ~1.6% each day) but did not alter the daily skill accuracy. Changes in behaviour with LI-rTMS were not accompanied with long lasting changes in brain-derived neurotrophic factor (BDNF) expression or in the expression of plasticity markers at excitatory and inhibitory synapses for either priming or consolidation groups. These results suggest that LI-rTMS can alter specific aspects of skilled motor learning in a manner dependent on the timing of intervention.

Bilateral and unilateral movement training on upper limb function in chronic stroke patients: A TMS study.

The use of activity-dependent interventions has shown some success in promoting recovery of upper limb function in chronic stroke patients. This study compared the neurophysiological and behavioural changes associated with two such rehabilitation protocols: unilateral and bilateral movement training. Twelve chronic stroke patients were randomly assigned to the two training protocols involving six daily practice sessions. Each session consisted of 50 trials of a dowel placement task performed either with both impaired and unimpaired arm moving synchronously (bilateral training group) or with only the impaired arm moving (unilateral training). Kinematic measurements of upper limb movements were made in four unilateral test trials performed prior to and following each practice session. Functional assessments of the impaired upper limb and neurophysiological assessments, using transcranial magnetic stimulation (TMS), of the affected and non-affected cortical hemispheres were made prior to and following the intervention sessions. Individuals receiving bilateral training showed a reduction in movement time of the impaired limb and increased upper limb functional ability compared to individuals receiving unilateral training. In some patients changes to upper limb function were associated with changes to the cortical representation of a target muscle in the non-affected hemisphere. Overall, these findings suggest that a short-term bilateral training intervention may be effective in facilitating upper limb motor function in chronic stroke patients.

Systematic Review of Cognitive Function in Euthymic Bipolar Disorder and Pre-Surgical Temporal Lobe Epilepsy.

BACKGROUND: Bipolar disorder (BD) and temporal lobe epilepsy (TLE) overlap in domains including epidemiology, treatment response, shared neurotransmitter involvement and temporal lobe pathology. Comparison of cognitive function in both disorders may indicate temporal lobe mediated processes relevant to BD. This systematic review examines neuropsychological test profiles in euthymic bipolar disorder type I (BD-I) and pre-surgical TLE and compares experimental designs used. METHODS: A search of PubMed, PsychINFO, and Scopus using Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines was conducted. Inclusion criteria were comparison group or pre- to post-surgical patients; reported neuropsychological tests; participants aged 18-60 years. Fifty six studies met criteria: 27 BD-I; 29 TLE. RESULTS: Deficits in BD-I compared to healthy controls (HC) were in executive function, attention span and verbal memory. Deficits in TLE compared to HC were in executive function and memory. In the pre- to post-surgical comparisons, verbal memory in left temporal lobe (LTL) and, less consistently, visuospatial memory in right temporal lobe (RTL) epilepsy declined following surgery. BD-I studies used comprehensive test batteries in well-defined euthymic patients compared to matched HC groups. TLE studies used convenience samples pre- to post-surgery, comparing LTL and RTL subgroups, few included comparisons to HC (5 studies). TLE studies typically examined a narrow range of known temporal lobe-mediated neuropsychological functions, particularly verbal and visuospatial memory. CONCLUSION: Both disorders exhibit deficits in executive function and verbal memory suggestive of both frontal and temporal lobe involvement. However, deficits in TLE are measured pre- to post-surgery and not controlled at baseline pre-surgery. Further research involving a head-to-head comparison of the two disorders on a broad range of neuropsychological tests is needed to clarify the nature and extent of cognitive deficits and potential overlaps.

Neural correlates of performance trade-offs and dual-task interference in bimanual coordination: an ERP investigation.

Previous behavioural studies have provided a framework for understanding coordination dynamics using traditional dual-task methodology. The central cost associated with stabilising bimanual coordination patterns has been inferred from performance trade-offs during the concurrent performance of a probe reaction time (RT) task. The present study aimed to provide a direct measure of central cost by assessing electrophysiological correlates of performance trade-offs under dual-task conditions. Event-related potentials (ERPs) were recorded from 16 participants while an anti-phase bimanual coordination task and a visual three-stimulus task were performed under single task conditions and under dual-task conditions in which either task was prioritised. The visual task required a foot response to low probability target stimuli, while low probability distracter and high probability standard stimuli were ignored. Consistent with previous research, there was a performance trade-off between pattern stability and RT to visual targets when the coordination task was prioritised relative to when the visual task was prioritised. This was accompanied by a significant reduction in central P3a amplitude elicited by distracter stimuli and parietal P3b amplitude elicited by target stimuli. These findings indicate that prioritisation and thus stabilisation of the motor task reduced the amount of central/perceptual and automatic attentional resources available to perform the visual task providing insight into CNS mechanisms that constrain the coordination of movement through the allocation of attentional resources.

Construction and Evaluation of Rodent-Specific rTMS Coils.

Rodent models of transcranial magnetic stimulation (TMS) play a crucial role in aiding the understanding of the cellular and molecular mechanisms underlying TMS induced plasticity. Rodent-specific TMS have previously been used to deliver focal stimulation at the cost of stimulus intensity (12 mT). Here we describe two novel TMS coils designed to deliver repetitive TMS (rTMS) at greater stimulation intensities whilst maintaining spatial resolution. Two circular coils (8 mm outer diameter) were constructed with either an air or pure iron-core. Peak magnetic field strength for the air and iron-cores were 90 and 120 mT, respectively, with the iron-core coil exhibiting less focality. Coil temperature and magnetic field stability for the two coils undergoing rTMS, were similar at 1 Hz but varied at 10 Hz. Finite element modeling of 10 Hz rTMS with the iron-core in a simplified rat brain model suggests a peak electric field of 85 and 12.7 V/m, within the skull and the brain, respectively. Delivering 10 Hz rTMS to the motor cortex of anaesthetized rats with the iron-core coil significantly increased motor evoked potential amplitudes immediately after stimulation (n = 4). Our results suggest these novel coils generate modest magnetic and electric fields, capable of altering cortical excitability and provide an alternative method to investigate the mechanisms underlying rTMS-induced plasticity in an experimental setting.

Age-Specific Effects of Mirror-Muscle Activity on Cross-Limb Adaptations Under Mirror and Non-Mirror Visual Feedback Conditions.

Cross-limb transfer (CLT) describes the observation of bilateral performance gains due to unilateral motor practice. Previous research has suggested that CLT may be reduced, or absent, in older adults, possibly due to age-related structural and functional brain changes. Based on research showing increases in CLT due to the provision of mirror visual feedback (MVF) during task execution in young adults, our study aimed to investigate whether MVF can facilitate CLT in older adults, who are known to be more reliant on visual feedback for accurate motor performance. Participants (N = 53) engaged in a short-term training regime (300 movements) involving a ballistic finger task using their dominant hand, while being provided with either visual feedback of their active limb, or a mirror reflection of their active limb (superimposed over the quiescent limb). Performance in both limbs was examined before, during and following the unilateral training. Furthermore, we measured corticospinal excitability (using TMS) at these time points, and assessed muscle activity bilaterally during the task via EMG; these parameters were used to investigate the mechanisms mediating and predicting CLT. Training resulted in significant bilateral performance gains that did not differ as a result of age or visual feedback (both p > 0.1). Training also elicited bilateral increases in corticospinal excitability (p < 0.05). For younger adults, CLT was significantly predicted by performance gains in the trained hand (ß = 0.47), whereas for older adults it was significantly predicted by mirror activity in the untrained hand during training (ß = 0.60). The present study suggests that older adults are capable of exhibiting CLT to a similar degree to younger adults. The prominent role of mirror activity in the untrained hand for CLT in older adults indicates that bilateral cortical activity during unilateral motor tasks is a compensatory mechanism. In this particular task, MVF did not facilitate the extent of CLT.

The Influence of Mirror-Visual Feedback on Training-Induced Motor Performance Gains in the Untrained Hand.

The well-documented observation of bilateral performance gains following unilateral motor training, a phenomenon known as cross-limb transfer, has important implications for rehabilitation. It has recently been shown that provision of a mirror image of the active hand during unilateral motor training has the capacity to enhance the efficacy of this phenomenon when compared to training without augmented visual feedback (i.e., watching the passive hand), possibly via action observation effects [1]. The current experiment was designed to confirm whether mirror-visual feedback (MVF) during motor training can indeed elicit greater performance gains in the untrained hand compared to more standard visual feedback (i.e., watching the active hand). Furthermore, discussing the mechanisms underlying any such MVF-induced behavioural effects, we suggest that action observation and the cross-activation hypothesis may both play important roles in eliciting cross-limb transfer. Eighty participants practiced a fast-as-possible two-ball rotation task with their dominant hand. During training, three different groups were provided with concurrent visual feedback of the active hand, inactive hand or a mirror image of the active hand with a fourth control group receiving no training. Pre- and post-training performance was measured in both hands. MVF did not increase the extent of training-induced performance changes in the untrained hand following unilateral training above and beyond those observed for other types of feedback. The data are consistent with the notion that cross-limb transfer, when combined with MVF, is mediated by cross-activation with action observation playing a less unique role than previously suggested. Further research is needed to replicate the current and previous studies to determine the clinical relevance and potential benefits of MVF for cases that, due to the severity of impairment, rely on unilateral training programmes of the unaffected limb to drive changes in the contralateral affected limb.

Comparison of precipitating factors for mania and partial seizures: Indicative of shared pathophysiology?

OBJECTIVES: Mania in bipolar disorder (BD) and partial (focal) seizures (PS) arising from the temporal lobes, have a number of similarities. Typically, a chronic course of the disorders is punctuated by acute illness episodes. Common features of episodes may include sensory, perceptual, cognitive and affective changes. Both respond to anticonvulsant treatment. Common mechanisms imputed include neurotransmitters and kindling processes. Further investigation may improve understanding of the occurrence of both mania and PS, casting light on the relevance of temporal lobe mediated processes and pathology. One avenue of investigation is to compare aetiological factors and determine the extent of overlap which may indicate shared brain localization or pathophysiology. Aetiology includes predisposing, precipitating or perpetuating factors. This paper examines the literature on precipitating factors of mania, first or subsequent episode, and of PS in diagnosed epilepsy, which is the second or subsequent seizure, to identify the extent and nature of their overlap. METHOD: Narrative review based on a literature search of PubMed and Google Scholar. RESULTS: Precipitating factors for both mania and PS were stress, sleep deprivation, antidepressant medication and, tentatively, emotion. For mania alone, goal-attainment events, spring and summer season, postpartum, and drugs include steroids and stimulants. For PS alone, winter season, menstruation and specific triggers in complex reflex epilepsies. Those not substantiated include lunar phase and menopause. A wide range of chemicals may provoke isolated seizures but by definition epilepsy requires at least two seizures. CONCLUSIONS: The overlap of precipitating factors in mania and PS imply that common brain processes may contribute to both, consistent with findings from neuroscience research.

Inter- and Intra-individual variability following intermittent theta burst stimulation: implications for rehabilitation and recovery.

BACKGROUND: The continued refinement of non-invasive brain stimulation (NBS) techniques is indicative of promising clinical and rehabilitative interventions that are able to modulate cortical excitability. Intermittent theta burst stimulation (iTBS) is one such technique that can increase cortical excitability, purportedly via LTP-like mechanisms. While iTBS may have the capacity to promote recovery after neurological injury, and to combat cognitive and motor decline, recent reports observed highly variable effects across individuals, questioning the efficacy of iTBS as a clinical tool. OBJECTIVE: The aim of this study was to examine intra-individual reliability and inter-individual variability in responses to iTBS. METHODS: Thirty healthy participants completed two experimental sessions of the iTBS protocol 1-3 weeks apart. Motor evoked potentials in response to single pulse TMS were used to assess corticospinal excitability prior to, and up to 36 min following, iTBS. RESULTS: At the group level, iTBS evoked statistically significant increases in motor cortical excitability across both sessions (P < 0.001), with 22 out of 30 participants exhibiting increases in excitability in both sessions. A strong intraclass correlation demonstrated that both the direction, and magnitude of the plastic changes were reliable at the individual level. CONCLUSIONS: Overall, our results suggest that iTBS is capable of inducing relatively robust and consistent effects within and between young individuals. As such, the capacity for iTBS to be exploited in clinical and rehabilitative interventions should continue to be explored.

Decision making and action implementation: evidence for an early visually triggered motor activation specific to potential actions.

To make a decision may rely on accumulating evidence in favor of one alternative until a threshold is reached. Sequential-sampling models differ by the way of accumulating evidence and the link with action implementation. Here, we tested a model's prediction of an early action implementation specific to potential actions. We assessed the dynamics of action implementation in go/no-go and between-hand choice tasks by transcranial magnetic stimulation of the motor cortex (single- or paired-pulse TMS; 3-ms interstimulus interval). Prior to implementation of the selected action, the amplitude of the motor evoked potential first increased whatever the visual stimulus but only for the hand potentially involved in the to-be-produced action. These findings suggest that visual stimuli can trigger an early motor activation specific to potential actions, consistent with race-like models with continuous transmission between decision making and action implementation.

Age-related differences in corticospinal excitability and inhibition during coordination of upper and lower limbs.

The ability to coordinate upper and lower limbs--a prerequisite for many everyday activities--is known to decline with age. Here we report 2 experiments in which transcranial magnetic stimulation (TMS) was used to assess corticospinal excitatory and inhibitory processes in younger and older adults during cyclical hand-foot movements. In experiment 1, motor evoked potentials (MEP) and silent period (SP) durations were measured from the active right extensor carpi radialis (ECR) muscle while it executed rhythmic oscillations in conjunction with the right or left foot. Younger adults exhibited increased SP with ipsilateral limb combinations and decreased SP with contralateral limb combinations, relative to a baseline hand only condition. Strikingly, older adults exhibited a reduced SP when ipsilateral limbs moved in opposite directions. This effect was found to be most pronounced in those older adults who exhibited poor coordination performance, suggesting that the inability to regulate inhibitory processes may underlie age-related degradation of task performance. Experiment 2 examined motor evoked potentials and SP duration in the left extensor carpi radialis which maintained a tonic contraction while the coordination task was undertaken by the right arm and right or left foot. For younger adults, coordination of ipsilateral limbs was accompanied by increased inhibition in the ipsilateral motor cortex than during the coordination of contralateral limbs. No differences in SP between conditions were noted for the older adults. In summary, older adults' reduced ability to coordinate upper and lower limbs may be related to the capacity to regulate inhibitory function in both hemispheres. This study suggests for the first time a direct link between age-related differences in interlimb coordination and the control of corticospinal inhibitory processes.

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

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