End-stopping in the visual cortex: excitation or inhibition?

In the visual cortex some neurons respond more strongly to short stimuli than to long ones. This is referred to as "end-stopping" and has been generally attributed to inhibition. The role of inhibition, however, has been difficult to demonstrate. Moreover, modeling has shown that end-stopping can be created solely from excitation. The roles of excitation and inhibition were investigated using intracellular recordings (Anderson et al., 2001, J. Neurosci. 21: 2104-2112). The results of that study were interpreted in favor of inhibition. The present report re-examines these results and finds that they may be in good, perhaps even better, agreement with an excitation model of end-stopping.     

Is the human primary motor cortex involved in motor imagery?

Participation of the primary motor cortex (M1) in motor imagery was addressed using functional magnetic resonance imaging at 2.0 T and 2 x 2 x 4 mm3 resolution in six right-handed subjects. Paradigms comprised visually cued execution and imagination of a sequential finger-to-thumb opposition task (12 s) contrasted with motor rest and visual imagery (18 s), respectively. Motor execution activated M1 as well as other parts of the motor system including supplementary motor area (SMA) and premotor areas (PM). In contrast, motor imagery did not lead to activations in M1 except for 1/6 subjects but involved SMA and PM bilaterally as well as the anterior intraparietal cortex. Moreover, a region-of-interest analysis revealed a weak initial MRI signal increase in M1 in 4/6 subjects. This novel finding of a transient response reflecting the onset of imagination which does not lead to sustained M1 activation may explain previous contradictory reports.     

Does induction of plastic change in motor cortex improve leg function after stroke?

Combined peripheral nerve and brain stimulation ("dual stimulation") induces changes in the excitability of normal motor cortex. The authors sought to establish whether dual stimulation would also induce motor cortex plasticity and associated functional improvements in nine stroke patients with chronic stable hemiparesis. Following 4 weeks of daily dual stimulation, improvements were seen in some neurophysiological and functional measures. This technique may offer therapeutic opportunities in some stroke patients.     

Lateral inhibition in the auditory cortex: an EEG index of tinnitus?

Auditory ERPs were recorded from eight tinnitus patients and 12 controls. Tone pips of 1000 and 2000 Hz, as well as the patient's tinnitus pitch (around 4000 Hz) were used. Controls received tone pips at 1000, 2000, and 4000 Hz. Tones were presented at 30, 36, 42, 48 and 54 dB/SL. The intensity dependence of the auditory N100 was calculated for each frequency in each group. Patients showed a steeper response to the tinnitus frequency than responses to the 4000 Hz tone in controls. In contrast, intensity-dependence to the 2000 Hz tones was significantly decreased in patients (two-tailed Wilcoxon-Mann-Whitney U-test, p < 0.05). Responses to the 1000 Hz tones were similar for both groups. This reduced intensity dependence is hypothesized to result from lateral inhibition arising from tinnitus related activity in the 4000 Hz isofrequency region.     

Does the menstrual cycle influence the motor and phosphene thresholds in migraine?

Background and purpose:  The excitability of the visual and motor cortical areas is altered in migraineurs. Controversial results of previous studies on cortical excitability may depend on the hormonal status of female subjects. The present study aimed to determine whether the different phases of the menstrual cycle influence the phosphene thresholds (PT) and resting motor thresholds (RMT) in migraineurs.
Methods:  Thirty-two migraine patients participated in this study. Three to six PT and RMT measurements were done in headache-free intervals during the follicular, middle and luteal phases of the female cycle, or in active dosage and withdrawal phases in patients who were taking low dosage oral contraceptives.
Results:  Generally, PTs showed higher individual variabilities than RMTs. Additionally, we have observed that the RMTs and PTs were significantly independent from hormonal changes. However, patients who were taking a low dosage of oral contraceptives had lower PTs compared with patients who were not taking oral contraceptives. RMTs show the opposite tendency.
Conclusion:  The results imply that PTs and RMTs can be reliably measured independently from the menstrual hormone status in female migraineurs.     

Does post-movement beta synchronization reflect an idling motor cortex?

After the completion of a voluntary movement, a synchronization of cortical beta rhythms is recorded over the contralateral central region, which is assumed to reflect the termination of the motor command. In order to test this hypothesis, we compared in eight healthy subjects the synchronization of EEG beta rhythms following active and passive index extension. The passive movement was also performed after deafferentation by ischaemic nerve block in three subjects. Beta synchronization was present in all subjects after both active and passive movements, and disappeared under ischaemia in all three subjects. Post-movement beta synchronization can not solely be explained by an idling motor cortex. It may also, at least in part, reflect a movement-related somatosensory processing.     

Does transcranial electrical stimulation enhance corticospinal excitability of the motor cortex in healthy individuals? A systematic review and meta‐analysis

Numerous studies have explored the effects of transcranial electrical stimulation (tES) – including anodal transcranial direct current stimulation (a‐tDCS), cathodal transcranial direct current stimulation (c‐tDCS), transcranial alternative current stimulation (tACS), transcranial random noise stimulation (tRNS) and transcranial pulsed current stimulation (tPCS) – on corticospinal excitability (CSE) in healthy populations. However, the efficacy of these techniques and their optimal parameters for producing robust results has not been studied. Thus, the aim of this systematic review was to consolidate current knowledge about the effects of various parameters of a‐tDCS, c‐tDCS, tACS, tRNS and tPCS on the CSE of the primary motor cortex (M1) in healthy people. Leading electronic databases were searched for relevant studies published between January 1990 and February 2017; 126 articles were identified, and their results were extracted and analysed using RevMan software. The meta‐analysis showed that a‐tDCS application on the dominant side significantly increases CSE (P < 0.01) and that the efficacy of a‐tDCS is dependent on current density and duration of application. Similar results were obtained for stimulation of M1 on the non‐dominant side (P = 0.003). The effects of a‐tDCS reduce significantly after 24 h (P = 0.006). Meta‐analysis also revealed significant reduction in CSE following c‐tDCS (P < 0.001) and significant increases after tRNS (P = 0.03) and tPCS (P = 0.01). However, tACS effects on CSE were only significant when the stimulation frequency was ≥140 Hz. This review provides evidence that tES has substantial effects on CSE in healthy individuals for a range of stimulus parameters.     

Response inhibition or salience detection in the right ventrolateral prefrontal cortex?

This study addresses the question of whether frontal activation in response-inhibition tasks is specifically associated with the suppression of a motor response. An alternative model suggests a role in the detection of behaviorally relevant or salient events. For this purpose, we used functional MRI with an auditory go/no-go paradigm. This paradigm allowed the disentangling of inhibition-related from salience-related effects, which were associated with different frontal regions. Importantly, the right ventrolateral prefrontal cortex consistently showed sensitivity for salience but not for inhibition requirements. This reflects a more general salience-detection mechanism, which is not specific for response-inhibition tasks.     

Prolonged disynaptic inhibition in the cortex mediated by slow, non-α7 nicotinic excitation of a specific subset of cortical interneurons

Cholinergic activation of nicotinic receptors in the cortex plays a critical role in arousal, attention, and learning. Here we demonstrate that cholinergic axons from the basal forebrain of mice excite a specific subset of cortical interneurons via a remarkably slow, non-α7 nicotinic receptor-mediated conductance. In turn, these inhibitory cells generate a delayed and prolonged wave of disynaptic inhibition in neighboring cortical neurons, altering the spatiotemporal pattern of inhibition in cortical circuits.     

TMS: a navigator for NIRS of the primary motor cortex?

Near-infrared spectroscopy (NIRS) is a non-invasive optical imaging technique, which is increasingly used to measure hemodynamic responses in the motor cortex. The location at which the NIRS optodes are placed on the skull is a major factor in measuring the hemodynamic responses optimally. In this study, the validity of using transcranial magnetic stimulation (TMS) in combination with a 3D motion analysis system to relocate the TMS derived position was tested. In addition, the main goal was to quantify the advantage of using TMS to locate the optimal position in relation to the most commonly used EEG C3 position. Markers were placed on the TMS coil and on the head of the subject. In eleven subjects, a TMS measurement was performed to determine the individual motor-evoked potential center-of-gravity (MEP-CoG). This procedure was repeated in nine subjects to test the validity. Subsequently, hemodynamic responses were measured at the MEP-CoG position and at the C3 position during a thumb abduction and adduction task. On average, the MEP-CoG location was located 19.2 mm away from the C3 position. The reproducibility study on the MEP-CoG relocation procedure revealed no systematic relocations. No differences in early and delayed hemodynamic responses were found between the C3 and MEP-CoG position. These results indicate that using TMS for NIRS optodes positioning on the motor cortex does not result in higher hemodynamic response amplitudes. This could be explained if NIRS and TMS assess slightly different functions.     

Altered activation and connectivity of the supplementary motor cortex at motor initiation in Parkinson’s disease patients with freezing

ObjectiveMotor initiation failure is a key feature of freezing of gait (FOG) due to Parkinson’s disease (PD). The supplementary motor cortex (SMC) plays a central role in its pathophysiology. We aimed at investigating SMC activation, connectivity and plasticity with regard to motor initiation in FOG.MethodsTwelve patients with FOG and eleven without FOG underwent a multimodal electrophysiological evaluation of SMC functioning including the Bereitschaftspotential and movement-related desynchronisation of cortical beta oscillations. SMC plasticity was modulated by intermittent theta burst stimulation (iTBS) and its impact on gait initiation was assessed by a three-dimensional gait analysis.ResultsPrior to volitional movements the Bereitschaftspotential was smaller and beta power was less strongly attenuated over the SMC in patients with FOG compared to those without. Pre-motor coherence between the SMC and the primary motor cortex in the beta frequency range was also stronger in patients with FOG. iTBS resulted in a relative deterioration of gait initiation.ConclusionsReduced activation of the SMC along with increased SMC connectivity in the beta frequency range hinder a flexible shift of the motor set as it is required for gait initiation.SignificanceAltered SMC functioning plays an important role for motor initiation failure in PD-related FOG.     

Plasticity of motor threshold and motor-evoked potential amplitude – A model of intrinsic and synaptic plasticity in human motor cortex?

BackgroundNeuronal plasticity is the physiological correlate of learning and memory. In animal experiments, synaptic (i.e. long-term potentiation (LTP) and depression (LTD)) and intrinsic plasticity are distinguished. In human motor cortex, cortical plasticity can be demonstrated using transcranial magnetic stimulation (TMS). Changes in motor-evoked potential (MEP) amplitudes most likely represent synaptic plasticity and are thus termed LTP-like and LTD-like plasticity.Objective/hypothesisWe investigated the role of changes of motor threshold and their relation to changes of MEP amplitudes.MethodsWe induced plasticity by paired associative stimulation (PAS) with 25 ms or 10 ms inter-stimulus interval or by motor practice (MP) in 64 healthy subjects aged 18–31 years (median 24.0).ResultsWe observed changes of MEP amplitudes and motor threshold after PAS[25], PAS[10] and MP. In all three protocols, long-term individual changes in MEP amplitude were inversely correlated to changes in motor threshold (PAS[25]: P = .003, n = 36; PAS[10]: P = .038, n = 19; MP: P = .041, n = 19).ConclusionWe conclude that changes of MEP amplitudes and MT represent two indices of motor cortex plasticity. Whereas increases and decreases in MEP amplitude are assumed to represent LTP-like or LTD-like synaptic plasticity of motor cortex output neurons, changes of MT may be considered as a correlate of intrinsic plasticity.     

Are the after-effects of low-frequency rTMS on motor cortex excitability due to changes in the efficacy of cortical synapses?

OBJECTIVES: To investigate the mechanisms responsible for suppressing the amplitude of electromyogram (EMG) responses to a standard transcranial magnetic stimulus (TMS) after prior conditioning of the motor cortex with repetitive subthreshold TMS (rTMS) at a frequency of 1 Hz. METHODS: EMG responses from the first dorsal interosseous, abductor pollicis brevis and flexor carpi radialis (FCR) muscles were recorded after suprathreshold TMS of the motor cortex. In some experiments, H-reflexes were also obtained in the FCR. The amplitude of these responses was compared before and after applying from 150 to 1500 rTMS pulses to motor cortex at an intensity of 95% resting motor threshold through the same figure-of-8 coil. RESULTS: When tested with subjects relaxed, rTMS conditioning reduced the amplitude of motor evoked potentials (MEPs) to approximately 60% of pre-conditioning values for 2-10 min after the end of the conditioning train, depending on the number of pulses in the train. There was more suppression with 1500 rTMS pulses than with 150 pulses. There was no effect on H-reflexes. There was no effect on MEPs if the test stimuli were given during active contraction of the target muscle. CONCLUSIONS: The findings confirm previous observations that low-frequency, low-intensity rTMS to motor cortex can produce transient depression of MEP excitability. Since there was no effect on spinal H-reflexes, this is consistent with the idea that some of the suppression occurs because of an effect on the motor cortex itself. The lack of any conditioning effect on MEPs evoked in actively contracting muscle is not readily consistent with the idea that rTMS depresses transmission in synaptic connections to pyramidal cells activated by the test TMS pulse. An alternative explanation is that rTMS reduces the excitability of cortical neurones in relaxed subjects, so that responses to a given input are smaller than before conditioning. Voluntary contraction normalises excitability levels so that the effect is no longer seen.