Reduced premovement positivity during the stimulus‐response interval precedes errors: Using single‐trial and regression ERPs to understand performance deficits in ADHD

Brain mechanisms linked to incorrect response selections made under time pressure during cognitive task performance are poorly understood, particularly in adolescents with attention‐deficit hyperactivity disorder (ADHD). Using subject‐specific multimodal imaging (electroencephalogram, magnetic resonance imaging, behavior) during flanker task performance by a sample of 94 human adolescents (mean age = 15.5 years, 50% female) with varying degrees of ADHD symptomatology, we examined the degree to which amplitude features of source‐resolved event‐related potentials (ERPs) from brain‐independent component processes within a critical (but often ignored) period in the action selection process, the stimulus‐response interval, were associated with motor response errors (across trials) and error rates (across individuals). Response errors were typically preceded by two smaller peaks in both trial‐level and trial‐averaged ERP projections from posterior medial frontal cortex (pMFC): a frontocentral P3 peaking about 390 ms after stimulus onset, and a premovement positivity (PMP) peaking about 110 ms before the motor response. Separating overlapping stimulus‐locked and response‐locked ERP contributions using a “regression ERP” approach showed that trial errors and participant error rates were primarily associated with smaller PMP, and not with frontocentral P3. Moreover, smaller PMP mediated the association between larger numbers of errors and ADHD symptoms, suggesting the possible value of using PMP as an intervention target to remediate performance deficits in ADHD.     

Non‐water‐suppressed short‐echo‐time magnetic resonance spectroscopic imaging using a concentric ring k‐space trajectory

Water‐suppressed MRS acquisition techniques have been the standard MRS approach used in research and for clinical scanning to date. The acquisition of a non‐water‐suppressed MRS spectrum is used for artefact correction, reconstruction of phased‐array coil data and metabolite quantification. Here, a two‐scan metabolite‐cycling magnetic resonance spectroscopic imaging (MRSI) scheme that does not use water suppression is demonstrated and evaluated. Specifically, the feasibility of acquiring and quantifying short‐echo (T_E = 14 ms), two‐dimensional stimulated echo acquisition mode (STEAM) MRSI spectra in the motor cortex is demonstrated on a 3 T MRI system. The increase in measurement time from the metabolite‐cycling is counterbalanced by a time‐efficient concentric ring k‐space trajectory. To validate the technique, water‐suppressed MRSI acquisitions were also performed for comparison. The proposed non‐water‐suppressed metabolite‐cycling MRSI technique was tested for detection and correction of resonance frequency drifts due to subject motion and/or hardware instability, and the feasibility of high‐resolution metabolic mapping over a whole brain slice was assessed. Our results show that the metabolite spectra and estimated concentrations are in agreement between non‐water‐suppressed and water‐suppressed techniques. The achieved spectral quality, signal‐to‐noise ratio (SNR) > 20 and linewidth <7 Hz allowed reliable metabolic mapping of five major brain metabolites in the motor cortex with an in‐plane resolution of 10 × 10 mm² in 8 min and with a Cramér‐Rao lower bound of less than 20% using LCModel analysis. In addition, the high SNR of the water peak of the non‐water‐suppressed technique enabled voxel‐wise single‐scan frequency, phase and eddy current correction. These findings demonstrate that our non‐water‐suppressed metabolite‐cycling MRSI technique can perform robustly on 3 T MRI systems and within a clinically feasible acquisition time.     

Task-dependent modulation of excitatory and inhibitory functions within the human primary motor cortex

We evaluated motor evoked potentials (MEPs) and duration of the cortical silent period (CSP) from the right first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) of the left motor cortex in ten healthy subjects performing different manual tasks. They abducted the index finger alone, pressed a strain gauge with the thumb and index finger in a pincer grip, and squeezed a 4-cm brass cylinder with all digits in a power grip. The level of FDI EMG activity across tasks was kept constant by providing subjects with acoustic-visual feedback of their muscle activity. The TMS elicited larger amplitude FDI MEPs during pincer and power grip than during the index finger abduction task, and larger amplitude MEPs during pincer gripping than during power gripping. The CSP was shorter during pincer and power grip than during the index finger abduction task and shorter during power gripping than during pincer gripping. These results suggest excitatory and inhibitory task-dependent changes in the motor cortex. Complex manual tasks (pincer and power gripping) elicit greater motor cortical excitation than a simple task (index finger abduction) presumably because they activate multiple synergistic muscles thus facilitating corticomotoneurons. The finger abduction task probably yielded greater motor cortical inhibition than the pincer and power tasks because muscles uninvolved in the task activated the cortical inhibitory circuit. Increased cortical excitatory and inhibitory functions during precision tasks (pincer gripping) probably explain why MEPs have larger amplitudes and CSPs have longer durations during pincer gripping than during power gripping. Electronic Publication     

Facilitation of cortically evoked potentials with motor imagery during post-exercise depression of corticospinal excitability

This study examined whether muscle fatigue alters the facilitatory effect of motor imagery on corticospinal excitability. We aimed to determine if post-exercise depression of potentials evoked magnetically from the motor cortex is associated with alterations in internally generated movement plans. In experiment 1, motor-evoked potentials (MEPs) were recorded from two right hand and two right forearm muscles, at rest and during motor imagery of a maximal handgrip contraction, in eight neurologically normal subjects, before and after a 2-min maximal voluntary handgrip contraction. Resting MEP amplitude was facilitated by motor imagery in three of the four muscles, but consistently only in two. Motor imagery also reduced the trial-to-trial variability of resting MEPs. Following the exercise, resting MEP amplitude was depressed reliably in only one muscle engaged in the task, although two other muscles exhibited some depression. Motor imagery MEPs were smaller after exercise, but the degree of facilitation compared to the rest MEP was unchanged. In experiment 2, TMS intensity was increased after exercise-induced MEP depression so that the MEP amplitude matched the pre-exercise baseline. The amplitude of the MEP facilitated with motor imagery was not altered by MEP depression, nor was it increased when the TMS intensity was increased. These results suggest, at least with a simple motor task, that while post-exercise depression reduces corticospinal excitability, it does not appear to significantly affect the strength of the input to the motor cortex from those areas of the brain responsible for the storage and generation of internal representations of movement.     

Linking visual gamma to task‐related brain networks—a simultaneous EEG‐fMRI study

There is a growing interest in human gamma‐band oscillatory activity due to its direct link to neuronal populations, its associations with many cognitive processes, and its positive relationship with fMRI BOLD signal. Visual gamma has been successfully detected using concurrent EEG‐fMRI recordings and linked to activity in the visual cortex using voxel‐wise regression analysis. As gamma‐band oscillations reflect predominantly feedforward projections between brain regions, its inclusion in functional connectivity analysis is highly recommended; however, very few studies have investigated this line of research. In the current study, we aimed to explore this gap by asking which fMRI brain network is related to gamma activity induced by the color discrimination task. Advanced denoising strategies and multitaper spectral decomposition were applied to EEG data to detect gamma oscillations, and group independent component analysis was performed on fMRI data to identify task‐related neural networks. Despite using only trials without motor response (50% of the trials), the two neural measures were successfully coupled. One of the six task‐related networks, the occipito‐parietal network, exhibited significant trial‐by‐trial covariations with gamma oscillations. In addition to the expected extrastriate visual cortex, the network encompasses extensive brain activations in the precuneus, bilateral intraparietal, and anterior insular cortices. We argue that the visual cortex is the source of gamma, whereas the remaining brain regions exhibit feedforward and feedback connections related to this oscillatory activity. Our findings provide evidence for the electrophysiological basis of the connectivity revealed by BOLD signal and impart novel insights into the neural mechanism of color discrimination.     

Unintentional covert motor activations predict behavioral effects: Multilevel modeling of trial‐level electrophysiological motor activations

The present experiment measured an EEG indicator of motor cortex activation, the lateralized readiness potential (LRP), while participants performed a speeded category classification task. The LRP data showed that visually masked words triggered covert motor activations. These prime‐induced motor activations preceded motor activations by subsequent (to‐be‐classified) visible target words. Multilevel statistical analyses of trial‐level effects, applied here for the first time with electrophysiological data, revealed that accuracy and latency of classifying target words was affected by both (a) covert motor activations caused by visually masked primes and (b) spontaneous fluctuations in covert motor activations. Spontaneous covert motor fluctuations were unobserved with standard subject‐level (multi‐trial) analyses of grand‐averaged LRPs, highlighting the utility of multilevel modeling of trial‐level effects.     

Rhythmic movement in Parkinson’s disease: effects of visual feedback and medication state

The aim of this study was to determine how and whether changes in the primary motor cortex (M1) are affected by dual motor task. We further investigated how dual motor task is dependent on task properties measured using transcranial magnetic stimulation (TMS). TMS delivered to left M1 during the dual motor task and motor-evoked potential (MEP) were simultaneously evoked in the right FDI, thenar, FCR and ECR muscles. In experiment 1, subjects were asked to simultaneously walk on a treadmill and perform finger prehension. The gait conditions were employed 30, 50 and 80% of maximum walking speed (gait 30%, gait 50% and gait 80%). Conditions for finger prehension while following the visual tracking task varied with force outputs of 5 and 25% of maximum voluntary contraction (MVC). In experiment 2, the subjects were asked to perform optimal walking synchronized with the finger prehension task with an optimal walking rhythm (2-Hz dual motor task), as well as optimal walking desynchronized with the finger prehension task (0.7-Hz dual motor task). In experiment 1, MEPs were markedly decreased under the gait 50% condition compared with those under the gait 30 and 80% conditions at 5% MVC. In experiment 2, MEPs were markedly decreased with the 2-Hz dual motor task compared with those with the 0.7-Hz dual motor task. Our results suggest that the excitability changes in M1 during the dual motor task were dependent on changes in the gait speed, precision of prehension task and temporal movement.     

Human cortical activities during Go/NoGo tasks with opposite motor control paradigms

Abstract. The purpose of this study was to investigate the cortical activities during two types of Go/NoGo task with different movement instructions (Push-Go and Release-Go) using transcranial magnetic stimulation (TMS) and event-related potential (ERP) recordings. In the Push-Go condition, ten subjects were instructed either to push a button with their right index finger as fast as possible after a Go signal or not to push it after a NoGo signal. In the Release-Go condition, they were asked beforehand to continually depress a button by pushing, and instructed either to release it as fast as possible after a Go signal or not to release it after a NoGo signal. TMS was applied to the left primary motor cortex at 20--300 ms after each signal. In the Push-Go condition trials, the amplitude of the motor evoked potentials (MEPs) recorded from the right first dorsal interosseous muscle significantly increased at 120--300 ms after the Go signals and decreased at 160--200 ms after the NoGo signals. In contrast, the MEP amplitudes recorded during the Release-Go condition trials significantly decreased at 160--300 ms after the Go signals and significantly increased at 160--180 and 220--300 ms after the NoGo signals. On the other hand, the ERPs recorded in the frontocentral cortex after each signal for five of the subjects were identical in both the Push-Go and Release-Go condition trials. These results indicated that, while the cortical activities related to the Go/NoGo decision were similar in the two task conditions, the corticospinal excitability was modulated so as to suppress or facilitate the required Go responses depending on the given movement instructions. This suggests that the Go/NoGo decision may be separate from the subsequent motor program.     

Corticospinal Facilitation Studied During Voluntary Contraction of Human Abdominal Muscles

Transcranial magnetic stimulation (TMS) of the human motor cortex was used to study facilitation of motor-evoked potentials (MEPs) in the rectus abdominis (RA) muscle, a trunk flexor, during voluntary activation. MEPs could be produced in the relaxed RA muscles of all six normal subjects studied. The MEPs had short latencies (18-22 ms) which are consistent with other studies suggesting a fast corticospinal input to the trunk muscles. Marked facilitation was observed in the MEPs when subjects were asked to produce graded levels of voluntary contractions. The two tasks used to produce voluntary contractions were a forced expiration during a breath-holding task (FEBH) and bilateral trunk flexion (BTF). Maximal voluntary EMG activity during the BTF task produced around 4.2 times more integrated EMG than during the FEBH task. Similarly the MEP amplitude at MVC was 2.3 times greater during BTF than FEBH. The pattern of MEP facilitation with increasing voluntary EMG was not linear and a maximal MEP amplitude was observed at a level of voluntary contraction around 30 % MVC in both tasks. There were some subtle differences in the pattern of facilitation in the two tasks. When TMS was applied to the right cortex only, MEPs were seen in both left and right RA muscles suggesting some ipsilateral corticospinal innervation. The latency of the right (ipsilateral) response was approximately 2 ms longer than the left. Comparison with studies in hand and leg muscles suggests that the facilitation pattern in RA may reflect a substantial degree of corticospinal innervation. Experimental Physiology (2001) 86.1, 131-136.     

The role of non‐rapid eye movement slow‐wave activity in prefrontal metabolism across young and middle‐aged adults

Electroencephalographic slow‐wave activity (0.5–4 Hz) during non‐rapid eye movement (NREM) sleep is a marker for cortical reorganization, particularly within the prefrontal cortex. Greater slow wave activity during sleep may promote greater waking prefrontal metabolic rate and, in turn, executive function. However, this process may be affected by age. Here we examined whether greater NREM slow wave activity was associated with higher prefrontal metabolism during wakefulness and whether this relationship interacted with age. Fifty‐two participants aged 25–61 years were enrolled into studies that included polysomnography and a ¹⁸[F]‐fluoro‐deoxy‐glucose positron emission tomography scan during wakefulness. Absolute and relative measures of NREM slow wave activity were assessed. Semiquantitative and relative measures of cerebral metabolism were collected to assess whole brain and regional metabolism, focusing on two regions of interest: the dorsolateral prefrontal cortex and the orbitofrontal cortex. Greater relative slow wave activity was associated with greater dorsolateral prefrontal metabolism. Age and slow wave activity interacted significantly in predicting semiquantitative whole brain metabolism and outside regions of interest in the posterior cingulate, middle temporal gyrus and the medial frontal gyrus, such that greater slow‐wave activity was associated with lower metabolism in the younger participants and greater metabolism in the older participants. These results suggest that slow‐wave activity is associated with cerebral metabolism during wakefulness across the adult lifespan within regions important for executive function.     

Time course of determination of movement direction in the reaction time task in humans

The primary motor cortex produces motor commands that include encoding the direction of movement. Excitability of the motor cortex in the reaction time (RT) task can be assessed using transcranial magnetic stimulation (TMS). To elucidate the timing of the increase in cortical excitability and of the determination of movement direction before movement onset, we asked six right-handed, healthy subjects to either abduct or extend their right thumb after a go-signal indicated the appropriate direction. Between the go-signal and movement onset, single TMS pulses were delivered to the contralateral motor cortex. We recorded the direction of the TMS-induced thumb movement and the amplitude of motor-evoked potentials (MEPs) from the abductor pollicis brevis and extensor pollicis brevis muscles. Facilitation of MEPs from the prime mover, as early as 200 ms before the end of the reaction time, preceded facilitation of MEPs from the nonprime mover, and both preceded measurable directional change. Compared with a control condition in which no voluntary movement was required, the direction of the TMS-induced thumb movement started to change in the direction of the intended movement as early as 90 ms before the end of the RT, and maximum changes were seen shortly before the end of reaction time. Movement acceleration also increased with maxima shortly before the end of the RT. We conclude that in concentric movements a change of the movement direction encoded in the primary motor cortex occurs in the 200 ms prior to movement onset, which is as early as increased excitability itself can be detected.     

An enhanced level of motor cortical excitability during the control of human standing

Aim: The study examined the role of the motor cortex in the control of human standing. Methods: Subjects (n = 15) stood quietly with or without body support. The supported standing condition enabled subjects to stand with a reduced amount of postural sway. Peripheral electrical stimulation, transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (TES) was applied to elicit a soleus (SOL) H‐reflex, or motor‐evoked potentials (MEPs) in the SOL and the tibialis anterior (TA). Trials were grouped based on the standing condition (i.e. supported vs. normal) as well as sway direction (i.e. forward and backward) while subjects were standing normally. Results: During normal when compared to supported standing, the SOL H‐reflex was depressed (?11 ± 4%), while the TMS‐evoked MEPs from the SOL and TA were facilitated (35 ± 11% for the SOL, 51 ± 15% for the TA). TES‐evoked SOL and TA MEPs were, however, not different between the normal and supported standing conditions. The data based on sway direction indicated that the SOL H‐reflex, as well as the SOL TMS‐ and TES‐evoked MEPs were all greater during forward when compared to backward sway. In contrast, the TMS‐ and TES‐evoked MEPs from the TA were smaller when swaying forward as compared to backward. Conclusions: The results indicated the presence of an enhanced cortical excitability because of the need to control for postural sway during normal standing. The increased cortical excitability was, however, unlikely to be involved in an on‐going control of postural sway, suggesting that postural sway is controlled at the spinal and/or subcortical levels.     

Symmetric corticospinal excitability and representation of vastus lateralis muscle in right‐handed healthy subjects

The purpose of this study was to determine the size and location of the representations of the anterior thigh muscles on the human motor cortex in the dominant and non‐dominant hemispheres. Motor‐evoked potentials (MEPs) induced by transcranial magnetic stimulation were recorded from the right and left vastus lateralis (rVL, lVL) muscles. A total of ten right‐handed healthy volunteers participated in the study. In a single session experiment, we investigated VL muscle corticospinal excitability (motor threshold, MEP size, short interval intracortical inhibition, intracortical facilitation) and cortical representation (map area, volume, and location) in the dominant and non‐dominant hemispheres. The motor threshold, MEPs, and intracortical excitability did not differ significantly between the hemispheres (P > 0.05). Furthermore, no difference between sides was found in the location of VL motor representation (mediolateral and anteroposterior axis) or in map area and volume (P > 0.05). Vastus lateralis muscle corticospinal excitability and cortical map were symmetrical in right‐handed subjects. Future studies on patients with unilateral lower extremity injuries could examine side‐to‐side plastic reorganization in corticomotor output and map location in both hemispheres. Clin. Anat. 27:1053–1057, 2014. © 2014 Wiley Periodicals, Inc.     

The time-course of preparatory spinal and cortico-spinal inhibition: an H-reflex and transcranial magnetic stimulation study in man

 In a previous study where reaction-time methods were combined with transcranial magnetic stimulation (TMS) of the motor cortex, cortico-spinal excitability was shown to reflect time preparation. Provided that subjects can accurately estimate time, the amplitude of motor-evoked potentials (MEPs) diminish progressively during the interval separating the warning signal from the response signal (i.e., the foreperiod). On the other hand, several experiments have demonstrated that the amplitude of the Hoffman (H) reflex elicited in prime movers diminishes during the foreperiod of reaction-time tasks. The aim of the present study was to compare the time course of the respective decrements of H-reflex and MEP amplitude during a constant 500-ms foreperiod. The subjects (n=8) participated in two experimental sessions. In one session, H-reflexes were induced in a tonically activated, responding hand muscle, the flexor pollicis brevis, at different times during the foreperiod of a visual-choice reaction-time task. In the other session, motor potentials were evoked in the same muscle by TMS of the motor cortex delivered in the same behavioral conditions and at the same times as in the first session. The results show that both H-reflexes and MEPs diminish in amplitude during the foreperiod, which replicates and extends previous findings. Interestingly, the time constants of the two decrements differed. There was a facilitatory effect of both electrical and magnetic stimulations on the subject’s performance: reaction time was shorter for the trials during which a stimulation was delivered than for the no-stimulation trials. This facilitation was maximal when the stimulations were delivered simultaneously with the warning signal and vanished progressively with stimulation time. Received: 6 November 1997 / Accepted: 2 June 1998     

Spatial and Temporal Characteristics of Set-Related Inhibitory and Excitatory Inputs from the Dorsal Premotor Cortex to the Ipsilateral Motor Cortex Assessed by Dual-Coil Transcranial Magnetic Stimulation

The capacity to produce movements only at appropriate times is fundamental in successful behavior and requires a fine interplay between motor inhibition and facilitation. Evidence in humans indicates that the dorsal premotor cortex (PMCd) is involved in such preparatory and inhibitory processes, but how PMCd modulates motor output in humans is still unclear. We investigated this issue in healthy human volunteers, using a variant of the dual-coil transcranial magnetic stimulation (TMS) technique that allows testing the short-latency effects of conditioning TMS to the left PMCd on test TMS applied to the ipsilateral orofacial primary motor cortex (M1). Participants performed a delayed cued simple reaction time task. They were asked to produce a lip movement cued by an imperative GO-signal presented after a predictable SET-period, during which TMS was applied at different intervals. Results showed that the area of motor evoked potentials (MEPs) to test TMS was modulated by conditioning TMS. A transient inhibition cortico-bulbar excitability by PMCd stimulation was observed around the middle of the SET-period. Conversely, a ramping excitatory effect of PMCd stimulation appeared towards the end of the SET-period, as the time of the predicted GO-signal approached. The time-course of PMCd–M1 activity scaled to the varying SET-period duration. Our data indicate that inhibition and excitation of motor output during a delayed reaction time task are two distinct neural phenomena. They both originate in PMCd and are conveyed via cortico–cortical connections to the ipsilateral M1, where they are integrated to produce harmonic fluctuations of motor output.     

Density‐weighted concentric rings k‐space trajectory for 1H magnetic resonance spectroscopic imaging at 7 T

It has been shown that density‐weighted (DW) k‐space sampling with spiral and conventional phase encoding trajectories reduces spatial side lobes in magnetic resonance spectroscopic imaging (MRSI). In this study, we propose a new concentric ring trajectory (CRT) for DW‐MRSI that samples k‐space with a density that is proportional to a spatial, isotropic Hanning window. The properties of two different DW‐CRTs were compared against a radially equidistant (RE) CRT and an echo‐planar spectroscopic imaging (EPSI) trajectory in simulations, phantoms and in vivo experiments. These experiments, conducted at 7 T with a fixed nominal voxel size and matched acquisition times, revealed that the two DW‐CRT designs improved the shape of the spatial response function by suppressing side lobes, also resulting in improved signal‐to‐noise ratio (SNR). High‐quality spectra were acquired for all trajectories from a specific region of interest in the motor cortex with an in‐plane resolution of 7.5 × 7.5 mm² in 8 min 3 s. Due to hardware limitations, high‐spatial‐resolution spectra with an in‐plane resolution of 5 × 5 mm² and an acquisition time of 12 min 48 s were acquired only for the RE and one of the DW‐CRT trajectories and not for EPSI. For all phantom and in vivo experiments, DW‐CRTs resulted in the highest SNR. The achieved in vivo spectral quality of the DW‐CRT method allowed for reliable metabolic mapping of eight metabolites including N‐acetylaspartylglutamate, γ‐aminobutyric acid and glutathione with Cramér‐Rao lower bounds below 50%, using an LCModel analysis. Finally, high‐quality metabolic mapping of a whole brain slice using DW‐CRT was achieved with a high in‐plane resolution of 5 × 5 mm² in a healthy subject. These findings demonstrate that our DW‐CRT MRSI technique can perform robustly on MRI systems and within a clinically feasible acquisition time.     

Verbal working memory‐related neural network communication in schizophrenia

Impaired working memory (WM) in schizophrenia is associated with reduced hemodynamic and electromagnetic activity and altered network connectivity within and between memory‐associated neural networks. The present study sought to determine whether schizophrenia involves disruption of a frontal‐parietal network normally supporting WM and/or involvement of another brain network. Nineteen schizophrenia patients (SZ) and 19 healthy comparison subjects (HC) participated in a cued visual‐verbal Sternberg task while dense‐array EEG was recorded. A pair of item arrays each consisting of 2–4 consonants was presented bilaterally for 200 ms with a prior cue signaling the hemifield of the task‐relevant WM set. A central probe letter 2,000 ms later prompted a choice reaction time decision about match/mismatch with the target WM set. Group and WM load effects on time domain and time‐frequency domain 11–15 Hz alpha power were assessed for the cue‐to‐probe time window, and posterior 11–15 Hz alpha power and frontal 4–8 Hz theta power were assessed during the retention period. Directional connectivity was estimated via Granger causality, evaluating group differences in communication. SZ showed slower responding, lower accuracy, smaller overall time‐domain alpha power increase, and less load‐dependent alpha power increase. Midline frontal theta power increases did not vary by group or load. Network communication in SZ was characterized by temporal‐to‐posterior information flow, in contrast to bidirectional temporal‐posterior communication in HC. Results indicate aberrant WM network activity supporting WM in SZ that might facilitate normal load‐dependent and only marginally less accurate task performance, despite generally slower responding.     

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.     

Motor evoked potential depression following repetitive central motor initiation

Prior reports have described a transient and focal decline in transcranial magnetic stimulation (TMS)-induced motor evoked potential (MEP) amplitude following fatiguing motor tasks. However, the neurophysiological causes of this change in MEP amplitude are unknown. The aim of this study was to determine whether post-task depression of MEPs is associated with repetitive central motor initiation. We hypothesized that MEP depression is related to repeated central initiation of motor commands in task-related cortex independent of motor fatigue. Twenty healthy adults had MEPs measured from the dominant first dorsal interosseous (FDI) muscle before and after six different tasks: rest (no activity), contralateral fatiguing hand-grip, ipsilateral fatiguing hand-grip, contralateral finger tapping, ipsilateral finger tapping, and imagined hand-grip (motor imagery). Changes in MEPs from baseline were assessed for each task immediately following the task and at 2-min intervals until MEPs returned to a stable baseline. Measures of subjective effort and FDI maximum voluntary contractions (MVC) were also recorded following each task. A statistically significant drop in MEP amplitude was noted only with contralateral finger tapping and imagined grip. Changes in MEP amplitude did not correlate with subjective fatigue or effort. There was no significant change in FDI MVCs following hand-grip or finger-tapping tasks. This study extends our knowledge of the observed decline in MEP amplitude following certain tasks. Our results suggest that central initiation of motor programs may induce a change in MEP amplitude, even in the absence of objective fatigue.     

Attention to a threat‐related feature does not interfere with concurrent attentive feature selection

Visual features associated with a task and those that predict noxious events both prompt selectively heightened visuocortical responses. Conflicting views exist regarding how the competition between a task‐related and a threat‐related feature is resolved when they co‐occur in time and space. Utilizing aversive classical Pavlovian conditioning, we investigated the visuocortical representation of two simultaneously presented, fully overlapping visual stimuli. Isoluminant red and green random dot kinematogram (RDK) stimuli were flickered at distinct tagging frequencies (8.57 Hz, 12 Hz) to elicit distinguishable steady‐state visual evoked potentials (ssVEPs). Occasional coherent motion events prompted a motor response (task) or predicted a noxious noise (threat). These events occurred either in the green (task cue), the red (threat cue), or in both RDKs simultaneously. In the initial habituation phase, participants responded to coherent motion of the green RDK with a key press, but no loud noise was presented at any time. Here, selective amplification was seen for the task‐relevant (green) RDK, and interference was observed when both RDKs simultaneously showed coherent motion. Upon pairing the threat cue with the noxious noise in the subsequent acquisition phase, the threat cue‐evoked ssVEP (red RDK) was also amplified, but this amplification did not interact with amplification of the task cue or alter the behavioral or visuocortical interference effect observed during simultaneous coherent motion. Although competing feature conjunctions resulted in interference in the visual cortex, the acquisition of a bias toward an individual threat‐related feature did not result in additional cost effects.