Exploring Behavioral Correlates of Afferent Inhibition

Fluctuation of fine motor skills throughout the menstrual cycle in women

The effect of the menstrual cycle on fine motor skills is unclear. This study determined whether the menstrual cycle affected fine motor skills and related neural activities. Nineteen women with regular menstrual cycles were tested for fine motor skills using two types of tasks: grooved pegboard task (GPT), which evaluates motor control with high freedom of movements, and force modulation task (FMT), which evaluates more complex and fine motor control with low freedom of movements. We also assessed primary motor cortex intracortical circuits and sensorimotor integration using paired-pulse transcranial magnetic stimulation to reveal why the menstrual cycle affects fine motor skills. The present study indicated that fine motor skills assessed by FMT varied throughout the menstrual cycle while those measured by GPT did not. These results suggest that fine motor skills requiring more complex and fine control may be affected by the menstrual cycle. Additionally, changes in fine motor skills throughout the menstrual cycle may be associated with the severity of menstruation-related symptoms.

Cortical Sensorimotor Integration as a Neural Origin of Impaired Grip Force Direction Control following Stroke

BACKGROUND

Stroke is a major cause of disability worldwide. Upper limb impairment is prevalent after stroke. One of the post-stroke manifestations is impaired grip force directional control contributing to diminished abilities to grip and manipulate objects necessary for activities of daily living. The objective of this study was to investigate the neural origin of the impaired grip force direction control following stroke. Due to the importance of online adjustment of motor output based on sensory feedback, it was hypothesized that grip force direction control would be associated with cortical sensorimotor integration in stroke survivors.

METHODS

Ten chronic stroke survivors participated in this study. Cortical sensorimotor integration was quantified by short latency afferent inhibition (SAI), which represents the responsiveness of the primary motor cortex to somatosensory input. Grip force direction control was assessed during paretic grip.

RESULTS

Grip force direction control was significantly associated with SAI. This relationship was independent of sensory impairment level.

CONCLUSIONS

Cortical sensorimotor integration may play a significant role in the grip force direction control important for gripping and manipulating objects with the affected hand following stroke. This knowledge may be used to inform personalized rehabilitation treatment. For example, for patients with impaired grip force direction control, behavioral therapy focusing on feedback motor control, augmented by use of brain stimulation to reinforce cortical sensorimotor integration such as paired associative stimulation, may be applied.

Tactile sensorimotor training does not alter short- and long-latency afferent inhibition

Sensorimotor integration refers to the process of combining incoming sensory information with outgoing motor commands to control movement. Short-latency afferent inhibition (SAI), and long-latency afferent inhibition (LAI) are neurophysiological measures of sensorimotor integration collected using transcranial magnetic stimulation. No studies to date have investigated the influence of tactile discrimination training on these measures. This study aimed to determine whether SAI and LAI are modulated following training on a custom-designed tactile discrimination maze task. Participants performed a 'high difficulty' and 'low difficulty' maze training condition on separate visits. On an additional visit, no maze training was performed to serve as a control condition. Despite evidence of performance improvements during training, there were no significant changes in SAI or LAI following training in either condition. The total number of errors during maze training was significantly greater in the high-difficulty condition compared with the low-difficulty condition. These findings suggest that sensorimotor maze training for 30 min is insufficient to modify the magnitude of SAI and LAI.

The distribution and reliability of TMS-evoked short- and long-latency afferent interactions

Short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) occur when the motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) is reduced by the delivery of a preceding peripheral nerve stimulus. The intra-individual variability in SAI and LAI is considerable, and the influence of sample demographics (e.g., age and biological sex) and testing context (e.g., time of day) is not clear. There are also no established normative values for these measures, and their reliability varies from study-to-study. To address these issues and facilitate the interpretation of SAI and LAI research, we pooled data from studies published by our lab between 2014 and 2020 and performed several retrospective analyses. Patterns in the depth of inhibition with respect to age, biological sex and time of testing were investigated, and the relative reliability of measurements from studies with repeated baseline SAI and LAI assessments was examined. Normative SAI and LAI values with respect to the mean and standard deviation were also calculated. Our data show no relationship between the depth of inhibition for SAI and LAI with either time of day or age. Further, there was no significant difference in SAI or LAI between males and females. Intra-class correlation coefficients (ICC) for repeated measurements of SAI and LAI ranged from moderate (ICC = 0.526) to strong (ICC = 0.881). The mean value of SAI was 0.71 ± 0.27 and the mean value of LAI was 0.61 ± 0.34. This retrospective study provides normative values, reliability estimates, and an exploration of demographic and testing influences on these measures as assessed in our lab. To further facilitate the interpretation of SAI and LAI data, similar studies should be performed by other labs that use these measures.

The Interactions Between Primary Somatosensory and Motor Cortex during Human Grasping Behaviors

Afferent inputs to the primary somatosensory cortex (S1) are differentially processed during precision and power grip in humans. However, it remains unclear how S1 interacts with the primary motor cortex (M1) during these two grasping behaviors. To address this question, we measured short-latency afferent inhibition (SAI), reflecting S1-M1 interactions via thalamo-cortical pathways, using paired-pulse transcranial magnetic stimulation (TMS) during precision and power grip. The TMS coil over the hand representation of M1 was oriented in the posterior-anterior (PA) and anterior-posterior (AP) direction to activate distinct sets of corticospinal neurons. We found that SAI increased during precision compared with power grip when AP, but not PA, currents were applied. Notably, SAI tested in the AP direction were similar during two-digit than five-digit precision grip. The M1 receives movement information from S1 through direct cortico-cortical pathways, so intra-hemispheric S1-M1 interactions using dual-site TMS were also evaluated. Stimulation of S1 attenuated M1 excitability (S1-M1 inhibition) during precision and power grip, while the S1-M1 inhibition ratio remained similar across tasks. Taken together,our findings suggest that distinct neural mechanisms for S1-M1 interactions mediate precision and power grip, presumably by modulating neural activity along thalamo-cortical pathways.

Possible Contribution of T-pattern Detection and Analysis to the Study of the Behavioral Correlates of Afferent Inhibition

A pivotal tenet in modern behavioral sciences is that the study of behavior, in its most intimate structure, necessarily deals with time and, for this reason, behavioral dynamics are not intuitively perceivable and/or detectable (Eibl-Eibesfeldt, 1970). In reality, the possibility to describe a given behavior in terms of its structural/temporal features makes available new and detailed information otherwise unavailable. The aim of the present paper is to discuss the possible application of T-pattern detection and analysis, i.e., a multivariate approach specifically developed to describe the temporal structure of behavior, to the study of an important and still scantly investigated issue, namely the detection of behavioral correlates of the neurophysiological phenomenon known as afferent inhibition.

Fitness Level Influences White Matter Microstructure in Postmenopausal Women

Aerobic exercise has both neuroprotective and neurorehabilitative benefits. However, the underlying mechanisms are not fully understood and need to be investigated, especially in postmenopausal women, who are at increased risk of age-related disorders such as Alzheimer’s disease and stroke. To advance our understanding of the potential neurological benefits of aerobic exercise in aging women, we examined anatomical and functional responses that may differentiate women of varying cardiorespiratory fitness using neuroimaging and neurophysiology. A total of 35 healthy postmenopausal women were recruited (59 ± 3 years) and cardiorespiratory fitness estimated (22–70 mL/kg/min). Transcranial magnetic stimulation was used to assess -aminobutyric acid (GABA) and glutamate (Glu) receptor function in the primary motor cortex (M1), and magnetic resonance spectroscopy (MRS) was used to quantify GABA and Glu concentrations in M1. Magnetic resonance imaging was used to assess mean cortical thickness (MCT) of sensorimotor and frontal regions, while the microstructure of sensorimotor and other white matter tracts was evaluated through diffusion tensor imaging. Regression analysis revealed that higher fitness levels were associated with improved microstructure in pre-motor and sensory tracts, and the hippocampal cingulum. Fitness level was not associated with MCT, MRS, or neurophysiology measures. These data indicate that, in postmenopausal women, higher cardiorespiratory fitness is linked with preserved selective white matter microstructure, particularly in areas that influence sensorimotor control and memory.

Somatosensory changes associated with motor skill learning

Trial-and-error motor adaptation has been linked to somatosensory plasticity and shifts in proprioception (limb position sense). The role of sensory processing in motor skill learning is less understood. Unlike adaptation, skill learning involves the acquisition of new movement patterns in the absence of perturbation, with performance limited by the speed-accuracy trade-off. We investigated somatosensory changes during motor skill learning at the behavioral and neurophysiological levels. Twenty-eight healthy young adults practiced a maze-tracing task, guiding a robotic manipulandum through an irregular two-dimensional track featuring several abrupt turns. Practice occurred on days 1 and 2. Skill was assessed before practice on day 1 and again on day 3, with learning indicated by a shift in the speed-accuracy function between these assessments. Proprioceptive function was quantified with a passive two-alternative forced-choice task. In a subset of 15 participants, we measured short-latency afferent inhibition (SAI) to index somatosensory projections to motor cortex. We found that motor practice enhanced the speed-accuracy skill function ( F4,108 = 32.15, P < 0.001) and was associated with improved proprioceptive sensitivity at retention ( t22 = 24.75, P = 0.0031). Furthermore, SAI increased after training ( F1,14 = 5.41, P = 0.036). Interestingly, individuals with larger increases in SAI, reflecting enhanced somatosensory afference to motor cortex, demonstrated larger improvements in motor skill learning. These findings suggest that SAI may be an important functional mechanism for some aspect of motor skill learning. Further research is needed to test what parameters (task complexity, practice time, etc.) are specifically linked to somatosensory function.

NEW & NOTEWORTHY Somatosensory processing has been implicated in motor adaptation, where performance recovers from a perturbation such as a force field. We investigated somatosensory function during motor skill learning, where a new motor pattern is acquired in the absence of perturbation. After skill practice, we found changes in proprioception and short-latency afferent inhibition (SAI), signifying somatosensory change at both the behavioral and neurophysiological levels. SAI may be an important functional mechanism by which individuals learn motor skills.

Effect of the brain-derived neurotrophic factor gene Val66Met polymorphism on sensory-motor integration during a complex motor learning exercise

The brain-derived neurotrophic factor (BDNF) gene Val66Met polymorphism may cause impairment in short-term motor learning by reducing activity-dependent BDNF expression, which causes alterations in synaptic plasticity by changing glutamatergic and GABAergic synaptic transmissions. Sensory-motor integration (SMI) plays an important role in motor learning. In this study, we investigated the role of this polymorphism on SMI during a complex motor learning practice. Forty-three healthy participants performed standardized 5-day basketball shooting exercises under supervision. Electrophysiologic SMI studies were performed before the first day exercise (T0) and after the first and fifth day exercises (T1 and T2, respectively). SMI was studied using electrical median nerve stimulation at the wrist, followed by transcranial magnetic stimulation (TMS) of the contralateral motor cortex with various inter-stimulus intervals (ISIs). Recordings were made from the thenar and forearm flexor muscles. Participants were divided into two groups according to their BDNF genotype. Group 1 consisted of 26 subjects with the Val66Val genotype and group 2 included 17 subjects with the BDNF Met allele. Group 2 had a lower increase in basketball scores at day 5. Moreover, they had higher afferent facilitation for the responses recorded from both thenar and forearm flexor muscles at T1, but these changes could not be maintained until T2. This non-persistent early hyper-responsivity of the sensory-motor cortex in subjects with the BDNF Met allele might be explained by a transient upsurge of cortical excitability to compensate the insufficient cortical plasticity during motor learning, which could be considered as a sign of lower performance in motor skill learning.

The Impact of Glucose on Corticospinal and Intracortical Excitability

Neurotransmission is highly dependent on the availability of glucose-derived energy, although it is unclear how glucose availability modulates corticospinal and intracortical excitability as assessed via transcranial magnetic stimulation (TMS). In this double-blinded placebo-controlled study, we tested the effect of acute glucose intake on motor-evoked potential (MEP) recruitment curves, short-interval intracortical inhibition (SICI), short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI). Eighteen healthy males participated in four sessions. Session 1 involved acquisition of an individualized blood glucose response curve. This allowed measurements to be time-locked to an individualized glucose peak after consuming one of three drinks during the subsequent three sessions. Participants were administered a 300 mL concealed solution containing 75 g of glucose, sucralose, or water in separate sessions. Dependent measures were assessed at baseline and twice after drinking the solution. Secondary measures included blood glucose and mean arterial pressure. Corticospinal excitability and blood pressure increased following the drink across all treatments. No changes were observed in SICI, SAI or LAI. There was no rise in corticospinal excitability that was specific to the glucose drink, suggesting that acute changes in glucose levels do not necessarily alter TMS measures of corticospinal or intracortical excitability.

Fast Intracortical Sensory-Motor Integration: A Window Into the Pathophysiology of Parkinson's Disease

Parkinson's Disease (PD) is a prototypical basal ganglia disorder. Nigrostriatal dopaminergic denervation leads to progressive dysfunction of the cortico-basal ganglia-thalamo-cortical sensorimotor loops, causing the classical motor symptoms. Although the basal ganglia do not receive direct sensory input, they are important for sensorimotor integration. Therefore, the basal ganglia dysfunction in PD may profoundly affect sensory-motor interaction in the cortex. Cortical sensorimotor integration can be probed with transcranial magnetic stimulation (TMS) using a well-established conditioning-test paradigm, called short-latency afferent inhibition (SAI). SAI probes the fast-inhibitory effect of a conditioning peripheral electrical stimulus on the motor response evoked by a TMS test pulse given to the contralateral primary motor cortex (M1). Since SAI occurs at latencies that match the peaks of early cortical somatosensory potentials, the cortical circuitry generating SAI may play an important role in rapid online adjustments of cortical motor output to changes in somatosensory inputs. Here we review the existing studies that have used SAI to examine how PD affects fast cortical sensory-motor integration. Studies of SAI in PD have yielded variable results, showing reduced, normal or even enhanced levels of SAI. This variability may be attributed to the fact that the strength of SAI is influenced by several factors, such as differences in dopaminergic treatment or the clinical phenotype of PD. Inter-individual differences in the expression of SAI has been shown to scale with individual motor impairment as revealed by UPDRS motor score and thus, may reflect the magnitude of dopaminergic neurodegeneration. The magnitude of SAI has also been linked to cognitive dysfunction, and it has been suggested that SAI also reflects cholinergic denervation at the cortical level. Together, the results indicate that SAI is a useful marker of disease-related alterations in fast cortical sensory-motor integration driven by subcortical changes in the dopaminergic and cholinergic system. Since a multitude of neurobiological factors contribute to the magnitude of inhibition, any mechanistic interpretation of SAI changes in PD needs to consider the group characteristics in terms of phenotypical spectrum, disease stage, and medication.

Looking toward predicting functional recovery in disorders of consciousness: can sensorimotor integration help us?

PRIMARY OBJECTIVE

Only a few objective prognostic markers are available for patients with disorders of consciousness (DoC). We assessed whether the magnitude of short-latency afferent inhibition (SAI) might be a useful predictor of responsiveness recovery and functional outcome in patients with DoC.

RESEARCH DESIGN

We enrolled 40 patients with prolonged Minimally Conscious State (MCS) and Unresponsive Wakefulness Syndrome (UWS) in a longitudinal, observational study.

METHODS AND PROCEDURES

Clinical features (including Coma Recovery Scale-Revised, CRS-R, and Glasgow Outcome Scale, GOS) and SAI were collected at the study entry and after 18 months from study inclusion, to assess a correlation between SAI and the clinical outcome.

MAIN OUTCOMES AND RESULTS

At the follow-up, 19 patients remained in their baseline condition, whereas 7 UWS evolved into MCS or emerged-from-MCS (EMCS), eight MCS evolved into EMCS, and two MCS- evolved into MCS+. Two UWS and one MCS+ died for cardiopulmonary complications. The patients who showed the highest GOS, the highest CRS-R and the lowest SAI strength at study entry, improved at the follow-up.

CONCLUSIONS

Our findings suggest that an objective and simple neurophysiologic measure as SAI strength could provide useful information to predict the outcome and the behavioral responsiveness of patients with DoC.

Effects of lorazepam and baclofen on short- and long-latency afferent inhibition

KEY POINTS

Short-latency afferent inhibition (SAI) is modulated by GABA_A receptor activity, whereas the pharmacological origin of long-latency afferent inhibition remains unknown. This is the first study to report that long-latency afferent inhibition (LAI) is reduced by the GABA_A positive allosteric modulator lorazepam, and that both SAI and LAI are not modulated by the GABA_B agonist baclofen. These findings advance our understanding of the neural mechanisms underlying afferent inhibition.

ABSTRACT

The afferent volley evoked by peripheral nerve stimulation has an inhibitory influence on transcranial magnetic stimulation induced motor evoked potentials. This phenomenon, known as afferent inhibition, occurs in two phases: short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI). SAI exerts its inhibitory influence via cholinergic and GABAergic activity. The neurotransmitter receptors that mediate LAI remain unclear. The present study aimed to determine whether LAI is contributed by GABA_A and/or GABA_B receptor activity. In a double-blinded, placebo-controlled study, 2.5 mg of lorazepam (GABA_A agonist), 20 mg of baclofen (GABA_B agonist) and placebo were administered to 14 males (mean age 22.7 ± 1.9 years) in three separate sessions. SAI and LAI, evoked by stimulation of the median nerve and recorded from the first dorsal interosseous muscle, were quantified before and at the peak plasma concentration following drug ingestion. Results indicate that lorazepam reduced LAI by ∼40% and, in support of previous work, reduced SAI by ∼19%. However, neither SAI, nor LAI were altered by baclofen. In a follow-up double-blinded, placebo-controlled study, 10 returning participants received placebo or 40 mg of baclofen (double the dosage used in Experiment 1). The results obtained indicate that SAI and LAI were unchanged by baclofen. This is the first study to show that LAI is modulated by GABA_A receptor activity, similar to SAI, and that afferent inhibition does not appear to be a GABA_B mediated process.