A mechanism for upper airway stability during slow wave sleep

STUDY OBJECTIVES

The severity of obstructive sleep apnea is diminished (sometimes markedly) during slow wave sleep (SWS). We sought to understand why SWS stabilizes the upper airway. Increased single motor unit (SMU) activity of the major upper airway dilating muscle (genioglossus) should improve upper airway stability. Therefore, we hypothesized that genioglossus SMUs would increase their activity during SWS in comparison with Stage N2 sleep.

DESIGN

The activity of genioglossus SMUs was studied on both sides of the transition between Stage N2 sleep and SWS.

SETTING

Sleep laboratory.

PARTICIPANTS

Twenty-nine subjects (age 38 ± 13 yr, 17 males) were studied.

INTERVENTION

SWS.

MEASUREMENT AND RESULTS

Subjects slept overnight with fine-wire electrodes in their genioglossus muscles and with full polysomnographic and end tidal carbon dioxide monitors. Fifteen inspiratory phasic (IP) and 11 inspiratory tonic (IT) units were identified from seven subjects and these units exhibited significantly increased inspiratory discharge frequencies during SWS compared with Stage N2 sleep. The peak discharge frequency of the inspiratory units (IP and IT) was 22.7 ± 4.1 Hz in SWS versus 20.3 ± 4.5 Hz in Stage N2 (P < 0.001). The IP units also fired for a longer duration (expressed as a percentage of inspiratory time) during SWS (104.6 ± 39.5 %TI) versus Stage N2 sleep (82.6 ± 39.5 %TI, P < 0.001). The IT units fired faster during expiration in SWS (14.2 ± 1.8 Hz) versus Stage N2 sleep (12.6 ± 3.1 Hz, P = 0.035). There was minimal recruitment or derecruitment of units between SWS and Stage N2 sleep.

CONCLUSION

Increased genioglossus SMU activity likely makes the airway more stable and resistant to collapse throughout the respiratory cycle during SWS.

Psychosocial problems and recruitment of incentive neurocircuitry: exploring individual differences in healthy adolescents

Maturational differences in brain responsiveness to rewards have been implicated in the increased rates of injury and death in adolescents from behavior-related causes. However, much of this morbidity is related to drug intoxication or other externalizing behaviors, and may be concentrated in a subset of adolescents who are at psychosocial or neurobiological risk. To examine whether individual differences in psychosocial and behavioral symptomatology relate to activation of motivational neurocircuitry, we scanned 26 psychiatrically healthy adolescents using fMRI as they performed a monetary incentive delay task. Overall Problem Density on the Drug Use Screening Inventory (DUSI-OPD) correlated positively with activation of ventral mesofrontal cortex (mFC) during anticipation of responding for rewards (vs responding for no incentive). In addition, DUSI-OPD correlated positively with right ventral striatum recruitment during anticipation of responding to win rewards (vs responding for no incentive or to avoid losses of identical magnitudes). Finally, a psychophysiological interaction (PPI) analysis indicated that increased connectivity between nucleus accumbens and portions of anterior cingulate and mFC as a function of reward prospects also correlated with DUSI-OPD. These findings extend previous reports demonstrating that in adolescents, individual differences in reactivity of motivational neurocircuitry relate to different facets of impulsivity or externalizing behaviors.

How does age affect leg muscle activity/coactivity during uphill and downhill walking?

Walking uphill and downhill can be challenging for community-dwelling old adults. We investigated the effects of age on leg muscle activity amplitudes and timing during level, uphill, and downhill walking. We hypothesized that old adults would exhibit smaller increases in ankle extensor muscle activities and greater increases in hip extensor muscle activities compared to young adults during uphill vs. level walking. We also hypothesized that, compared to level walking, antagonist leg muscle coactivation would be disproportionately greater in old vs. young adults during downhill walking. Ten old (72±5yrs) and ten young (25±4yrs) subjects walked at 1.25m/s on a treadmill at seven grades (0°, ±3°, ±6°, ±9°). We quantified the stance phase electromyographic activities of the gluteus maximus (GMAX), biceps femoris (BF), rectus femoris (RF), vastus medialis (VM), medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA). Old adults exhibited smaller increases in MG activity with steeper uphill grade than young adults (e.g., +136% vs. +174% at 9°). A disproportionate recruitment of hip muscles led to GMAX activity approaching the maximum isometric capacity of these active old adults at steep uphill grades (e.g., old vs. young, 73% MVC vs. 33% MVC at +9°). Neither uphill nor downhill walking affected the greater coactivation of antagonist muscles in old vs. young adults. We conclude that the disproportionate recruitment of hip muscles with advanced age may have critical implications for maintaining independent mobility in old adults, particularly at steeper uphill grades.

Neuropeptide Y (NPY) in the extended amygdala is recruited during the transition to alcohol dependence

Neuropeptide Y (NPY) is abundant in the extended amygdala, a conceptual macrostructure in the basal forebrain important for regulation of negative affective states. NPY has been attributed a central role in anxiety-like behavior, fear, nociception, and reward in rodents. Deletion of the NPY gene in mice produces a high-anxiety high-alcohol-drinking phenotype. NPY infused into the brains of rats selectively bred to consume high quantities of alcohol suppresses alcohol drinking by those animals, an effect that is mediated by central amygdala (CeA). Likewise, alcohol-preferring rats exhibit basal NPY deficits in CeA. NPY infused into the brains of alcohol-dependent rats blocks excessive alcohol drinking by those animals, an effect that also has been localized to the CeA. NPY in CeA may rescue dependence-induced increases in anxiety and alcohol drinking via inhibition of downstream effector regions that receive GABAergic inputs from CeA. It is hypothesized here that NPY modulates anxiety-like behavior via Y2R regulation of NPY release, whereas NPY modulation of alcohol-drinking behavior in alcohol-dependent animals occurs via Y2R regulation of GABA release.

Emotion effects on attention, amygdala activation, and functional connectivity in schizophrenia

Emotional abnormalities are a critical clinical feature of schizophrenia (SCZ), but complete understanding of their underlying neuropathology is lacking. Numerous studies have examined amygdala activation in response to affective stimuli in SCZ, but no consensus has emerged. However, behavioral studies examining 'in-the-moment' processing of affect have suggested intact emotional processing in SCZ. To examine which aspects of emotional processing may be impaired in SCZ, we combined behavior and neuroimaging to investigate effects of aversive stimuli during minimal cognitive engagement, at the level of behavior, amygdala recruitment, and its whole-brain task-based functional connectivity (tb-fcMRI) because impairments may manifest when examining across-region functional integration. Twenty-eight patients and 24 matched controls underwent rapid event-related fMRI at 3 T while performing a simple perceptual decision task with negative or neutral distraction. We examined perceptual decision slowing, amygdala activation, and whole-brain amygdala tb-fcMRI, while ensuring group signal-to-noise profile matching. Following scanning, subjects rated all images for experienced arousal and valence. No significant group differences emerged for negative vs neutral reaction time, emotional ratings across groups, or amygdala activation. However, even in the absence of behavioral or activation differences, SCZ subjects demonstrated significantly weaker amygdala-prefrontal cortical coupling, specifically during negative distraction. Whereas in-the-moment perception, behavioral response, and amygdala recruitment to negative stimuli during minimal cognitive load seem to be intact, there is evidence of aberrant amygdala-prefrontal integration in SCZ subjects. Such abnormalities may prove critical for understanding disturbances in patients' ability to use affective cues when guiding higher level cognitive processes needed in social interactions.

A simulation-based analysis of motor unit number index (MUNIX) technique using motoneuron pool and surface electromyogram models

Motor unit number index (MUNIX) measurement has recently achieved increasing attention as a tool to evaluate the progression of motoneuron diseases. In our current study, the sensitivity of the MUNIX technique to changes in motoneuron and muscle properties was explored by a simulation approach utilizing variations on published motoneuron pool and surface electromyogram (EMG) models. Our simulation results indicate that, when keeping motoneuron pool and muscle parameters unchanged and varying the input motor unit numbers to the model, then MUNIX estimates can appropriately characterize changes in motor unit numbers. Such MUNIX estimates are not sensitive to different motor unit recruitment and rate coding strategies used in the model. Furthermore, alterations in motor unit control properties do not have a significant effect on the MUNIX estimates. Neither adjustment of the motor unit recruitment range nor reduction of the motor unit firing rates jeopardizes the MUNIX estimates. The MUNIX estimates closely correlate with the maximum M-wave amplitude. However, if we reduce the amplitude of each motor unit action potential rather than simply reduce motor unit number, then MUNIX estimates substantially underestimate the motor unit numbers in the muscle. These findings suggest that the current MUNIX definition is most suitable for motoneuron diseases that demonstrate secondary evidence of muscle fiber reinnervation. In this regard, when MUNIX is applied, it is of much importance to examine a parallel measurement of motor unit size index (MUSIX), defined as the ratio of the maximum M-wave amplitude to the MUNIX. However, there are potential limitations in the application of the MUNIX methods in atrophied muscle, where it is unclear whether the atrophy is accompanied by loss of motor units or loss of muscle fiber size.

Shoulder muscles recruitment during a power backward giant swing on high bar: a wavelet-EMG-analysis

This study aimed at determining the upper limb muscles coordination during a power backward giant swing (PBGS) and the recruitment pattern of motor units (MU) of co-activated muscles. The wavelet transformation (WT) was applied to the surface electromyographic (EMG) signal of eight shoulder muscles. Total gymnast's body energy and wavelet synergies extracted from the WT-EMG by using a non-negative matrix factorization were analyzed as a function of the body position angle of the gymnast. A cross-correlation analysis of the EMG patterns allowed determining two main groups of co-activated muscles. Two wavelet synergies representing the main spectral features (82% of the variance accounted for) discriminated the recruitment of MU. Although no task-group of MU was found among the muscles, it appeared that a higher proportion of fast MU was recruited within the muscles of the first group during the upper part of the PBGS. The last increase of total body energy before bar release was induced by the recruitment of the muscles of the second group but did not necessitate the recruitment of a higher proportion of fast MU. Such muscle coordination agreed with previous simulations of elements on high bar as well as the findings related to the recruitment of MU.

Influence of proprioceptive feedback on the firing rate and recruitment of motoneurons

We investigated the relationships of the firing rate and maximal recruitment threshold of motoneurons recorded during isometric contraction with the number of spindles in individual muscles. At force levels above 10% of maximal voluntary contraction, the firing rate was inversely related to the number of spindles in a muscle, with the slope of the relationship increasing with force. The maximal recruitment threshold of motor units increased linearly with the number of spindles in the muscle. Thus, muscles with a greater number of spindles had lower firing rates and a greater maximal recruitment threshold. These findings may be explained by a mechanical interaction between muscle fibres and adjacent spindles. During low-level (0% to 10%) voluntary contractions, muscle fibres of recruited motor units produce force twitches that activate nearby spindles to respond with an immediate excitatory feedback that reaches maximal level. As the force increases further, the twitches overlap and tend towards tetanization, the muscle fibres shorten, the spindles slacken, their excitatory firings decrease, and the net excitation to the homonymous motoneurons decreases. Motoneurons of muscles with greater number of spindles receive a greater decrease in excitation which reduces their firing rates, increases their maximal recruitment threshold, and changes the motoneuron recruitment distribution.

Aging and the neural correlates of source memory: over-recruitment and functional reorganization

Behavioral evidence suggests that memory for context (i.e., source memory) is more vulnerable to age-related decline than item memory. It is not clear, however, whether this pattern reflects a specific age-related deficit in context memory or a more general effect of task difficulty. In the present study, we used event-related functional magnetic resonance imaging (fMRI) with healthy younger and older adults to dissociate the effects of age, task (item vs. source memory), and task difficulty (1 vs. 2 study presentations) on patterns of blood oxygen level-dependent (BOLD) signal changes during memory retrieval. Behavioral performance was similar in both age groups, but was sensitive to task and difficulty (item > source; easy > difficult). Data-driven multivariate analyses revealed age differences consistent with age-related overrecruitment of frontoparietal regions during difficult task conditions, and age-related functional reorganization in bilateral frontal and right-lateralized posterior regions that were sensitive to difficulty in younger adults, but to task (i.e., context demand) in older adults. These findings support the hypothesis of a specific context memory deficit in older adults.

Muscular response to physiologic tensile stretch of the caprine c5/6 facet joint capsule: dynamic recruitment thresholds and latencies

This study examined the cervical muscle response to physiologic, high-rate (100 mm/s) tensile facet joint capsule (FJC) stretch. Six in-vivo caprine C5/6 FJC preparations were subjected to an incremental tensile loading paradigm. EMG activity was recorded from the right trapezius (TR) and multifidus (MF) muscle groups at the C5 and C6 levels; and from the sternomastoid (SM) and longus colli (LC) muscle groups bilaterally at the C5/6 level; during FJC stretch. Capsule load during the displacement applications was recorded via a miniature load cell, and 3D capsule strains (based on stereoimaging of an array of markers on the capsule surface) were reconstructed using finite element methods. EMG traces from each muscle were examined for onset of muscular activity. Capsule strains and loads at the time of EMG onset were recorded for each muscle, as was the time from the onset of FJC stretch to the onset of muscle activity. All muscles were responsive to physiologic high-rate FJC stretch. The deep muscles (MF and LC) were recruited at significantly smaller capsule loads and onset latencies than the superficial muscles (TR and SM). MF activation strain was significantly smaller than LC and TR activation strains. These data were also compared to previously published low-rate data. MF was the first muscle group to be recruited regardless of the activation criterion under consideration (i.e. strain, load, or latency) or the rate of FJC stretch. LC recruitment occurred significantly sooner under high-rate vs. low-rate FJC stretch. The results of this study provide further evidence of extensive ligamento-muscular reflex pathways between the FJC and the cervical musculature, which are responsive to both low-rate and high-rate FJC stretch. These data add to our knowledge of the dynamic response of paraspinal muscles relative to facet joint motion and provide a unique contribution to enhance the precision of computer-simulated impacts.

Cross-modal versus within-modal recall: differences in behavioral and brain responses

Although human experience is multisensory in nature, previous research has focused predominantly on memory for unisensory as opposed to multisensory information. In this work, we sought to investigate behavioral and neural differences between the cued recall of cross-modal audiovisual associations versus within-modal visual or auditory associations. Participants were presented with cue-target associations comprised of pairs of nonsense objects, pairs of nonsense sounds, objects paired with sounds, and sounds paired with objects. Subsequently, they were required to recall the modality of the target given the cue while behavioral accuracy, reaction time, and blood oxygenation level dependent (BOLD) activation were measured. Successful within-modal recall was associated with modality-specific reactivation in primary perceptual regions, and was more accurate than cross-modal retrieval. When auditory targets were correctly or incorrectly recalled using a cross-modal visual cue, there was re-activation in auditory association cortex, and recall of information from cross-modal associations activated the hippocampus to a greater degree than within-modal associations. Findings support theories that propose an overlap between regions active during perception and memory, and show that behavioral and neural differences exist between within- and cross-modal associations. Overall the current study highlights the importance of the role of multisensory information in memory.

Neural plasticity and neurorehabilitation: teaching the new brain old tricks

Following brain injury or disease there are widespread biochemical, anatomical and physiological changes that result in what might be considered a new, very different brain. This adapted brain is forced to reacquire behaviors lost as a result of the injury or disease and relies on neural plasticity within the residual neural circuits. The same fundamental neural and behavioral signals driving plasticity during learning in the intact brain are engaged during relearning in the damaged/diseased brain. The field of neurorehabilitation is now beginning to capitalize on this body of work to develop neurobiologically informed therapies focused on key behavioral and neural signals driving neural plasticity. Further, how neural plasticity may act to drive different neural strategies underlying functional improvement after brain injury is being revealed. The understanding of the relationship between these different neural strategies, mechanisms of neural plasticity, and changes in behavior may facilitate the development of novel, more effective rehabilitation interventions for treating brain injury and disease.

LEARNING OUTCOMES

Readers will be able to: (a) define neural plasticity, (b) understand how learning in the intact and damaged brain can drive neural plasticity, (c) identify the three basic neural strategies mediating functional improvement, and (d) understand how adjuvant therapies have the potential to upregulate plasticity and enhance functional recovery.

Phenomenology of consciousness from apprehension to judgment

We explore two different moments of human cognition, namely apprehension (A), whereby a coherent perception emerges by recruitment of large neuron groups and judgment (B), whereby memory retrieval of different (A) units coded in a suitable language and comparison of them leads to the formulation of a judgment. The first one has a duration around 1 sec (from 0.5 to 3 sec), it appears as an a-temporal present and its neural correlate is a wide synchronization in the EEG gamma band. It may be described as an interpretation of sensorial stimuli in terms of some stored algorithm, via a Bayes procedure. The second one entails the comparison of two apprehensions acquired at different times, coded in a given language, and retrieved by memory. It lasts around 3 sec and requires self-consciousness, as the judging agent must be well aware that he/she is the same one who faces the two coded apprehensions under scrutiny in order to extract a mutual relation. At variance with (A), (B) does not presuppose an algorithm, but it rather builds a new behavioural model by an inverse Bayes procedure. It will be shown how this build up of a novel model is related to creativity and free will.

The intensity and effects of strength training in the elderly

BACKGROUND

The elderly need strength training more and more as they grow older to stay mobile for their everyday activities. The goal of training is to reduce the loss of muscle mass and the resulting loss of motor function. The dose-response relationship of training intensity to training effect has not yet been fully elucidated.

METHODS

PubMed was selectively searched for articles that appeared in the past 5 years about the effects and dose-response relationship of strength training in the elderly.

RESULTS

Strength training in the elderly (>60 years) increases muscle strength by increasing muscle mass, and by improving the recruitment of motor units, and increasing their firing rate. Muscle mass can be increased through training at an intensity corresponding to 60% to 85% of the individual maximum voluntary strength. Improving the rate of force development requires training at a higher intensity (above 85%), in the elderly just as in younger persons. It is now recommended that healthy old people should train 3 or 4 times weekly for the best results; persons with poor performance at the outset can achieve improvement even with less frequent training. Side effects are rare.

CONCLUSION

Progressive strength training in the elderly is efficient, even with higher intensities, to reduce sarcopenia, and to retain motor function.

Patterns of brain reorganization subsequent to left fusiform damage: fMRI evidence from visual processing of words and pseudowords, faces and objects

Little is known about the neural reorganization that takes place subsequent to lesions that affect orthographic processing (reading and/or spelling). We report on an fMRI investigation of an individual with a left mid-fusiform resection that affected both reading and spelling (Tsapkini & Rapp, 2010). To investigate possible patterns of functional reorganization, we compared the behavioral and neural activation patterns of this individual with those of a group of control participants for the tasks of silent reading of words and pseudowords and the passive viewing of faces and objects, all tasks that typically recruit the inferior temporal lobes. This comparison was carried out with methods that included a novel application of Mahalanobis distance statistics, and revealed: (1) normal behavioral and neural responses for face and object processing, (2) evidence of neural reorganization bilaterally in the posterior fusiform that supported normal performance in pseudoword reading and which contributed to word reading (3) evidence of abnormal recruitment of the bilateral anterior temporal lobes indicating compensatory (albeit insufficient) recruitment of mechanisms for circumventing the word reading deficit.

Discharge characteristics of biceps brachii motor units at recruitment when older adults sustained an isometric contraction

The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference (P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences (P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.

Orofacial thermal thresholds: time-dependent variability and influence of spatial summation and test site

AIM

To investigate time-dependent variability and influence of test site and stimulation area size on intraoral cold detection, warmth detection, and heat pain thresholds.

METHODS

Thirty healthy volunteers (15 women and 15 men) participated. Six extra- and intraoral sites were examined, and cold detection, warmth detection, and heat pain thresholds were measured. Time variability and influence of spatial summation were also studied at one site-the tip of the tongue-three times over a 6-week period. One-way ANOVA for repeated measures and paired sample t test compared mean values and SD within and between sites for all thresholds.

RESULTS

Several between-site differences were significant (P < .05). Lowest intraoral thresholds for all stimuli were measured at the tongue site, and at the tongue, thresholds for warmth detection and heat pain, but not cold detection, decreased with increasing size of stimulation area (P < .05). Overall, thresholds at the tongue site varied nonsignificantly over time (P > .05).

CONCLUSION

Test site affects orofacial thermal thresholds substantially, whereas time variability and spatial summation on the tongue appear to be modest.

Characteristics and organization of discharge properties in rat hindlimb motoneurons

The discharge properties of hindlimb motoneurons in ketamine-xylazine anesthetized rats were measured to assess contributions of persistent intrinsic currents to these characteristics and to determine their distribution in motoneuron pools. Most motoneurons (30/37) responded to ramp current injections with adapting patterns of discharge and the frequency-current (f-I) relations of nearly all motoneurons included a steep subprimary range of discharge. Despite the prevalence of adapting f-I relations, responses included indications that persistent inward currents (PICs) were activated, including increased membrane noise and prepotentials before discharge, as well as counterclockwise hysteresis and secondary ranges in f-I relations. Examination of spike thresholds and afterhyperpolarization (AHP) trajectories during repetitive discharge revealed systematic changes in threshold and trajectory within the subprimary, primary, and secondary f-I ranges. These changes in the primary and secondary ranges were qualitatively similar to those described previously for cat motoneurons. Within the subprimary range, AHP trajectories often included shallow approaches to threshold following recruitment and slope of the AHP ramp consistently increased until the subprimary range was reached. We suggest that PICs activated near recruitment contributed to these slope changes and formation of the subprimary range. Discharge characteristics were strongly correlated with motoneuron size, using input conductance as an indicator of size. Discharge adaptation, recruitment current, and frequency increased with input conductance, whereas both subprimary and primary f-I gains decreased. These results are discussed with respect to potential mechanisms and their functional implications.

Relationship between firing rate and recruitment threshold of motoneurons in voluntary isometric contractions

We used surface EMG signal decomposition technology to study the control properties of numerous simultaneously active motor units. Six healthy human subjects of comparable age (21 +/- 0.63 yr) and physical fitness were recruited to perform isometric contractions of the vastus lateralis (VL), first dorsal interosseous (FDI), and tibialis anterior (TA) muscles at the 20, 50, 80, and 100% maximum voluntary contraction force levels. EMG signals were collected with a five-pin surface array sensor that provided four channels of data. They were decomposed into the constituent action potentials with a new decomposition algorithm. The firings of a total of 1,273 motor unit action potential trains, 20-30 per contraction, were obtained. The recruitment thresholds and mean firing rates of the motor units were calculated, and mathematical equations were derived. The results describe a hierarchical inverse relationship between the recruitment thresholds and the firing rates, including the first and second derivatives, i.e., the velocity and the acceleration of the firing rates. This relationship describes an "operating point" for the motoneuron pool that remains consistent at all force levels and is modulated by the excitation. This relationship differs only slightly between subjects and more distinctly across muscles. These results support the "onion skin" property that suggests a basic control scheme encoded in the physical properties of motoneurons that responds consistently to a "common drive" to the motoneuron pool.

Diaphragm motor unit recruitment in rats

We hypothesized that considerable force reserve exists for the diaphragm muscle (DIAm) to generate transdiaphragmatic pressures (Pdi) necessary to sustain ventilation. In rats, we measured Pdi and DIAm EMG activity during different ventilatory (eupnea and hypoxia (10% O(2))-hypercapnia (5% CO(2))) and non-ventilatory (airway occlusion and sneezing induced by intranasal capsaicin) behaviors. Compared to maximum Pdi (Pdi(max) generated by bilateral phrenic nerve stimulation), the Pdi generated during eupnea (21+/-2%) and hypoxia-hypercapnia (28+/-4%) were significantly less (p<0.0001) than that generated during airway occlusion (63+/-4%) and sneezing (94+/-5%). The Pdi generated during spontaneous sighs was 62+/-5% of Pdi(max). Relative DIAm EMG activity (root mean square [RMS] amplitude) paralleled the changes in Pdi during different ventilatory and non-ventilatory behaviors (r(2)=0.78; p<0.0001). These results support our hypothesis of a considerable force reserve for the DIAm to accomplish ventilatory behaviors. A model for DIAm motor unit recruitment predicted that ventilatory behaviors would require activation of only fatigue resistant units.

Control of wrist position and muscle relaxation by shifting spatial frames of reference for motoneuronal recruitment: possible involvement of corticospinal pathways

It has previously been established that muscles become active in response to deviations from a threshold (referent) position of the body or its segments, and that intentional motor actions result from central shifts in the referent position. We tested the hypothesis that corticospinal pathways are involved in threshold position control during intentional changes in the wrist position in humans. Subjects moved the wrist from an initial extended to a final flexed position (and vice versa). Passive wrist muscle forces were compensated with a torque motor such that wrist muscle activity was equalized at the two positions. It appeared that motoneuronal excitability tested by brief muscle stretches was also similar at these positions. Responses to mechanical perturbations before and after movement showed that the wrist threshold position was reset when voluntary changes in the joint angle were made. Although the excitability of motoneurons was similar at the two positions, the same transcranial magnetic stimulus (TMS) elicited a wrist extensor jerk in the extension position and a flexor jerk in the flexion position. Extensor motor-evoked potentials (MEPs) elicited by TMS at the wrist extension position were substantially bigger compared to those at the flexion position and vice versa for flexor MEPs. MEPs were substantially reduced when subjects fully relaxed wrist muscles and the wrist was held passively in each position. Results suggest that the corticospinal pathway, possibly with other descending pathways, participates in threshold position control, a process that pre-determines the spatial frame of reference in which the neuromuscular periphery is constrained to work. This control strategy would underlie not only intentional changes in the joint position, but also muscle relaxation. The notion that the motor cortex may control motor actions by shifting spatial frames of reference opens a new avenue in the analysis and understanding of brain function.

Share or compete? Load-dependent recruitment of prefrontal cortex during dual-task performance

Dual-task performance requires flexible attention allocation to two or more streams of information. Dorsolateral prefrontal cortex (DLPFC) is considered important for executive function, and recent modeling work proposes that attention control may arise from selective activation and inhibition of different processing units within this region. Here, we used a tone discrimination task and a visual letter memory task to examine whether this type of competition could be measurable using a neuroimaging technique, the event-related optical signal, with high spatial and temporal resolution. Left and right DLPFC structures were differentially affected by task priority and load, with the left middle frontal gyrus (MFG) being preferentially recruited by the visual memory task, whereas the two tasks competed for recruitment, in a spatially segregated manner, in right MFG. The data provide support for a competition view of dual-task processing.

Vastus lateralis surface and single motor unit electromyography during shortening, lengthening and isometric contractions corrected for mode-dependent differences in force-generating capacity

AIM

Knee extensor neuromuscular activity, rectified surface electromyography (rsEMG) and single motor unit EMG was investigated during isometric (60 degrees knee angle), shortening and lengthening contractions (50-70 degrees, 10 degrees s(-1)) corrected for force-velocity-related differences in force-generating capacity. However, during dynamic contractions additional factors such as shortening-induced force losses and lengthening-induced force gains may also affect force capacity and thereby neuromuscular activity. Therefore, even after correction for force-velocity-related differences in force capacity we expected neuromuscular activity to be higher and lower during shortening and lengthening, respectively, compared to isometric contractions.

METHODS

rsEMG of the three superficial muscle heads was obtained in a first session [10 and 50% maximal voluntary contraction (MVC)] and additionally EMG of (46) vastus lateralis motor units was recorded during a second session (4-76% MVC). Using superimposed electrical stimulation, force-generating capacity for shortening and lengthening contractions was found to be 0.96 and 1.16 times isometric (Iso) force capacity respectively. Therefore, neuromuscular activity during submaximal shortening and lengthening was compared with isometric contractions of respectively 1.04Iso (=1/0.96) and 0.86Iso (=1/1.16). rsEMG and discharge rates were normalized to isometric values.

RESULTS

rsEMG behaviour was similar (P > 0.05) during both sessions. Shortening rsEMG (1.30 +/- 0.11) and discharge rate (1.22 +/- 0.13) were higher (P < 0.05) than 1.04Iso values (1.05 +/- 0.05 and 1.03 +/- 0.04 respectively), but lengthening rsEMG (1.05 +/- 0.12) and discharge rate (0.90 +/- 0.08) were not lower (P > 0.05) than 0.86Iso values (0.76 +/- 0.04 and 0.91 +/- 0.07 respectively).

CONCLUSION

When force-velocity-related differences in force capacity were taken into account, neuromuscular activity was not lower during lengthening but was still higher during shortening compared with isometric contractions.

Dynamics of primate oculomotor plant revealed by effects of abducens microstimulation

Despite their importance for deciphering oculomotor commands, the mechanics of the extraocular muscles and orbital tissues (oculomotor plant) are poorly understood. In particular, the significance of plant nonlinearities is uncertain. Here primate plant dynamics were investigated by measuring the eye movements produced by stimulating the abducens nucleus with brief pulse trains of varying frequency. Statistical analysis of these movements indicated that the effects of stimulation lasted about 40 ms after the final pulse, after which the eye returned passively toward its position before stimulation. Behavior during the passive phase could be approximated by a linear plant model, corresponding to Voigt elements in series, with properties independent of initial eye position. In contrast, behavior during the stimulation phase revealed a sigmoidal relation between stimulation frequency and estimated steady-state tetanic tension, together with a frequency-dependent rate of tension increase, that appeared very similar to the nonlinearities previously found for isometric-force production in primate lateral rectus muscle. These results suggest that the dynamics of the oculomotor plant have an approximately linear component related to steady-state viscoelasticity and a nonlinear component related to changes in muscle activation. The latter may in part account for the nonlinear relations observed between eye-movement parameters and single-unit firing patterns in the abducens nucleus. These findings point to the importance of recruitment as a simplifying factor for motor control with nonlinear plants.

Changes in recruitment of transversus abdominis correlate with disability in people with chronic low back pain

OBJECTIVES

Although motor control exercises have been shown to be effective in the management of low back pain (LBP) the mechanism of action is unclear. The current study investigated the relationship between the ability to recruit transversus abdominis and clinical outcomes of participants in a clinical trial.

METHODS

Ultrasonography was used to assess the ability to recruit transversus abdominis in a nested design: a sample of 34 participants with chronic LBP was recruited from participants in a randomised controlled trial comparing the efficacy of motor control exercise, general exercise and spinal manipulative therapy. Perceived recovery, function, disability and pain were also assessed.

RESULTS

Participants with chronic LBP receiving motor control exercise had a greater improvement in recruitment of transversus abdominis (7.8%) than participants receiving general exercise (4.9% reduction) or spinal manipulative therapy (3.7% reduction). The effect of motor control exercise on pain reduction was greater in participants who had a poor ability to recruit transversus abdominis at baseline. There was a significant, moderate correlation between improved recruitment of transversus abdominis and a reduction in disability (r = -0.35; 95% CI 0.02 to 0.62).

CONCLUSION

These data provide some support for the hypothesised mechanism of action of motor control exercise and suggest that the treatment may be more effective in those with a poor ability to recruit transversus abdominis.

Dissociation of the neural networks recruited during a haptic object-recognition task: complementary results with a tensorial independent component analysis

BACKGROUND AND PURPOSE

The cerebral and cerebellar networks involved in bimanual object recognition were assessed by blood oxygen level-dependent functional MR imaging by using multivariate model-free analysis, because conventional univariate model-based analysis, such as the general linear model (GLM), does not allow investigation of resting, background, and transiently task-related brain activities.

MATERIALS AND METHODS

Data from 14 healthy right-handed volunteers, scanned while successively performing bilateral finger movements and a bimanual tactile-tactile matching discrimination task were analyzed by using tensor-independent component analysis (TICA), which computes statistically independent spatiotemporal processes (P > .7) thought to reflect specific and distinct anatomofunctional neural networks. These results were compared with the network obtained in a previous study by using the same paradigm based on GLM to evaluate the advantages of TICA.

RESULTS

TICA characterized and distinguished the following: 1) resting-state networks such as the default-mode networks, 2) networks transiently synchronized with the beginning and end of the task, such as temporo-pericentral and temporo-pericentral-occipital networks, and 3) task-related networks such as cerebello-fronto-parietal, cerebello-prefrontocingulo-insular, and cerebello-parietal networks.

CONCLUSION

Bimanual tactile-tactile matching discrimination specifically recruits a complex neural network, which can be dissociated into 3 distinct but cooperative neural subnetworks related to sensorimotor function, salience detection, executive control, and, possibly, sensory expectation. This tripartite network involved in bimanual object recognition could not be demonstrated by GLM. Moreover, TICA allowed monitoring of the temporal succession of the networks recruited during the resting phase, audition of the "go" and "stop" signals, and the tactile discrimination task.

Motor unit number estimation in the assessment of performance and function in motor neuron disease

Motor unit number estimation (MUNE) is a unique electrophysiologic test used to estimate the number of surviving motor units in a muscle or group of muscles. It is used most frequently to monitor lower motor neuron loss in amyotrophic lateral sclerosis and spinal muscle atrophy. Of particular interest is its use as an endpoint measure in clinical trials for these diseases. This article describes the principles of MUNE and the factors that need to be considered, and reviews several techniques that have been used in clinical trials and in monitoring progression. It then reviews experience with MUNE in clinical trials for amyotrophic lateral sclerosis and spinal muscle atrophy and discusses how MUNE correlates with measures of function.

Timing of neuromuscular activation of the quadriceps and hamstrings prior to landing in high school male athletes, female athletes, and female non-athletes

There is a discrepancy between males and females in regards to lower extremity injury rates, particularly at the knee [Agel, J., Arendt, E.A., Bershadsky, B., 2005. Anterior cruciate ligament injury in National Collegiate Athletic Association basketball and soccer: a 13-year review. American Journal of Sports Medicine 33, (4) 524-530]. Gender differences in neuromuscular recruitment characteristics of the muscles that stabilize the knee are often implicated as a factor in this discrepancy. There is considerable research in the area of gender differences in regards to neuromuscular characteristics of the lower extremity in response to perturbation; however, most studies have been performed on the adult population only. Additionally, there is no consensus as to the gender differences that have been demonstrated. The purpose of this study was to compare muscular preactivation of selected lower extremity muscles (vastus medialis, rectus femoris, and medial/lateral hamstrings) in adolescent female basketball athletes, male basketball athletes, and female non-athletes in response to a drop landing. Subjects in the female non-athlete group recruited rectus femoris significantly slower than both the female athlete and male athlete groups (619.9=588.5>200.1ms prior to ground contact). The female non-athlete group also demonstrated a significantly slower vastus medialis compared to the female athlete group (127.1 vs 408.1ms), but not significantly slower than the male athlete group (127.1 vs 275.7ms). There were no differences between female athletes and male athletes for time to initial contraction of any muscle groups. No differences were found among the groups for medial or lateral hamstring activation. This study demonstrates that physical conditioning due to basketball participation appears to affect neuromuscular recruitment in adolescents and reveals a necessity to find alternate methods of training the hamstrings for improved neuromuscular capabilities to prevent injury.

Division of motor units into fast and slow of the basis of profile of 20 Hz unfused tetanus

In the medial gastrocnemius muscle of intact rats, division of motor units (MUs) into slow (S) or fast (F) types is typically based on presence of a sag phenomenon in 40 Hz unfused tetanic contraction. MUs with sag are classified as F, while those without sag as S. However, in rats one month after spinal cord injury this phenomenon almost completely disappears and cannot be used as a basis for MUs differentiation, whereas the twitch contraction time increased significantly. Analysis of myosin heavy chain (MHC) isoform composition confirmed transformational changes of muscle fibres after spinal cord transection and indicated unchanged proportion of type I MHC isoforms, disappearance of type IIa MHC isoforms, and increase of type IIb MHC isoforms. We proposed an additional method for division of MUs into types when standard criteria are not applicable. It was observed that relative effectiveness of force summation during 20 Hz tetanus, described as a ratio of the force of the last contraction of this tetanus to the force of the first contraction, did not change after spinal cord injury. This ratio for S MUs both in intact and spinal rats exceeded 2.0, whereas for F units was lower than 2.0. Calculations of this ratio made for better fused tetani, evoked by 30 Hz or 40 Hz stimulation, showed overlapping values. We conclude that this 20 Hz tetanus index appears to be an alternative method useful for division of motor units into S and F types.

Patterns of leg muscle recruitment vary between novice and highly trained cyclists

This study compared patterns of leg muscle recruitment and coactivation, and the relationship between muscle recruitment, coactivation and cadence, in novice and highly trained cyclists. Electromyographic (EMG) activity of tibialis anterior (TA), tibialis posterior (TP), peroneus longus (PL), gastrocnemius lateralis (GL) and soleus (SOL) was recorded using intramuscular fine-wire electrodes. Four experimental conditions of varying cadence were investigated. Differences were evident between novice and highly trained cyclists in the recruitment of all muscles. Novice cyclists were characterized by greater individual variance, greater population variance, more extensive and more variable muscle coactivation, and greater EMG amplitude in periods between primary EMG bursts. Peak EMG amplitude increased linearly with cadence and was not different at individual preferred cadence in either novice or highly trained cyclists. However, EMG amplitude in periods between primary EMG bursts, as well as the duration of primary EMG bursts, increased with increasing cadence in novice cyclists but were not influenced by cadence in highly trained cyclists. Our findings suggest that muscle recruitment is highly skilled in highly trained cyclists and less refined in novice cyclists. More skilled muscle recruitment in highly trained cyclists is likely a result of neuromuscular adaptations due to repeated performance of the cycling movement in training and competition.

Pulse charge and not waveform affects M-wave properties during progressive motor unit activation

The aim of this study was to investigate changes in experimentally recorded M-waves with progressive motor unit (MU) activation induced by transcutaneous electrical stimulation with different pulse waveforms. In 10 subjects, surface electromyographic signals were detected with a linear electrode array during electrically elicited contractions of the biceps brachii muscle. Three different monophasic waveforms of 304-micros duration were applied to the stimulation electrode on the main muscle motor point: triangular, square, and sinusoidal. For each waveform, increasing stimulation current intensities were applied in 10 s (frequency: 20 Hz). It was found that: (a) the degree of MU activation, as indicated by M-wave average rectified value, was a function of the injected charge and not of the stimulation waveform, and (b) MUs tended to be recruited in order of increasing conduction velocity with increasing charge of transcutaneous stimulation. Moreover, the subjects reported lower discomfort during the contractions elicited by the triangular waveform with respect to the others. Since subject tolerance to the stimulation protocol must be considered as important as MU recruitment in determining the effectiveness of neuromuscular electrical stimulation (NMES), we suggest that both charge and waveform of the stimulation pulses should be considered relevant parameters for optimizing NMES protocols.

Atypical recruitment of medial prefrontal cortex in autism spectrum disorders: an fMRI study of two executive function tasks

Recent studies have suggested an uneven profile of executive dysfunction in autism spectrum disorders (ASD). For example, some authors have reported deficits on newly developed tests of executive function sensitive to rostral prefrontal function, despite spared, or even superior, performance on other tests. We investigated the performance of a group of high-functioning participants with ASD (N=15) and an age- and IQ-matched control group (N=18) on two executive function tests, whilst undergoing functional magnetic resonance imaging (fMRI). Behaviourally, there were no significant differences between the two groups. In a classical test of executive function (random response generation), BOLD signal differed between the groups in the cerebellum but not in the frontal lobes. However, on a new test of executive function (selection between stimulus-oriented and stimulus-independent thought), the ASD group exhibited significantly greater signal-change in medial rostral prefrontal cortex (especially Brodmann Area 10) in the comparison of stimulus-oriented versus stimulus-independent attention. In addition, the new test (but not the classical test) provided evidence for abnormal functional organisation of medial prefrontal cortex in ASD. These results underline the heterogeneity of different tests of executive function, and suggest that executive functioning in ASD is associated with task-specific functional change.

Reinstating the ability of the motor cortex to modulate cutaneomuscular reflexes in hemicerebellectomized rats

The pathways passing through the cerebellum calibrate cutaneomuscular responses. Indeed, the enhancement of cutaneomuscular responses associated with subthreshold high-frequency trains of stimulation applied on motor cortex following a period of peripheral repetitive stimulation (PRS) is prevented by hemicerebellectomy. We analysed the effects of low-frequency repetitive stimulation of motor cortex (LFRSM1) on interhemispheric inhibition (IHI) and on the modulation of cutaneomuscular reflexes in rats with left hemicerebellar ablation. IHI was assessed by paired-pulse method with a conditioning stimulus (CS) to M1 followed by a test stimulus (TS) to the opposite M1. LFRSM1 reduced IHI. Combination of LFRSM1 with PRS increased significantly the magnitudes of cutaneomuscular responses evoked ipsilaterally to the hemicerebellar ablation. The increase of the intensity of cutaneomuscular responses was correlated with the reduction of IHI. Excitability of anterior horn motoneurons pool, assessed by F-wave, remained unchanged. Conjunction of LFRSM1 with PRS can be used to restore the ability of the motor cortex to modulate the intensity of cutaneomuscular responses in case of extensive unilateral cerebellar lesion. This study underlines for the first time the potential role of callosal pathways in the deficits of corticomotor tuning of cutaneomuscular responses contralaterally to acute extensive cerebellar lesion.

A novel electromyographic signal simulator for muscle contraction studies

Mathematical simulation has been widely used in biomedical and biological sciences. In the case of the surface electromyographic (SEMG) activity, some models have been proposed aiming to study muscle contraction strategies that are used during different tasks and conditions. Most of SEMG simulators are based on energy modulation of a Gaussian noise. This work proposes a novel simulator in which the user-defined parameters are associated with the motor units (MUs) recruitment and their firing rate. Comparison between the mean spectrum of real SEMG signals collected in isometric contraction of the muscle biceps brachii and the mean spectrum obtained from simulated SEMG signals showed a good agreement, pointing the proposed simulator seems to be capable to generate consistent electromyographic signals in time and frequency domains and that can be used in many studies, in particular in the evaluation of automatic methods aimed to detect muscular contraction.

Functional re-recruitment of dysfunctional brain areas predicts language recovery in chronic aphasia

Functional recovery in response to a brain lesion, such as a stroke, can even occur years after the incident and may be accelerated by effective rehabilitation strategies. In eleven chronic aphasia patients, we administered a short-term intensive language training to improve language functions and to induce cortical reorganization under rigorously controlled conditions. Overt naming performance was assessed during functional magnetic resonance imaging (fMRI) prior to and immediately after the language training. Regions of interest (ROIs) for statistical analyses were constituted by areas with individually determined abnormally high densities of slow wave generators (identified by magnetoencephalography prior to the language intervention) that clustered mainly in left perilesional areas. Three additional individually defined regions served to control for the specificity of the results for the selected respective target region: the homologue area of the individual patient's lesion, the mirror image of the delta ROI in the right hemisphere and left hemispheric regions that did not produce a significant amount of slow wave activity. Treatment-induced changes of fMRI brain activation were highly correlated with improved naming of the trained pictures, but selectively within the pre-training dysfunctional perilesional brain areas. Our results suggest that remodeling of cortical functions is possible even years after a stroke. The behavioral gain seems to be mediated by brain regions that had been partially deprived from input after the initial stroke. We therefore provide first time direct evidence for the importance of treatment-induced functional reintegration of perilesional areas in a heterogeneous sample of chronic aphasia patients.

Functional deficit in the medial prefrontal cortex during a language comprehension task in patients with schizophrenia

OBJECTIVE

We and others have observed that patients with schizophrenia commonly presented a reduced left recruitment in language semantic brain regions. However, most studies include patients with leftward and rightward lateralizations for language. We investigated whether a cohort comprised purely of patients with typical lateralization (leftward) presented a reduced left recruitment in semantic regions during a language comprehension task. The goal was to reduce the inter-subject variability and thus improve the resolution for studying functional abnormalities in the language network.

METHODS

Twenty-three patients with schizophrenia (DSM-IV) were matched with healthy subjects in age, sex, level of education and handedness. All patients exhibited leftward lateralization for language. Functional MRI was performed as subjects listened to a story comprising characters and social interactions. Functional MRI signal variations were analyzed individually and compared among groups.

RESULTS

Although no differences were observed in the recruitment of the semantic language network, patients with schizophrenia presented significantly lower signal variations compared to controls in the medial part of the left superior frontal gyrus (MF1) (x=-6, y=58, z=20; Z(score)=5.6; p<0.001 uncorrected). This region corresponded to the Theory of Mind (ToM) network. Only 5 of the 23 patients (21.7%) and 21 of the 23 (91.3%) control subjects demonstrated a positive signal variation in this area.

CONCLUSIONS

A left functional deficit was observed in a core region of the ToM network in patients with schizophrenia and typical lateralizations for language. This functional defect could represent a neural basis for impaired social interaction and communication in patients with schizophrenia.

Theory of Mind disruption and recruitment of the right hemisphere during narrative comprehension in autism

The intersection of Theory of Mind (ToM) processing and complex narrative comprehension in high functioning autism was examined by comparing cortical activation during the reading of passages that required inferences based on either intentions, emotional states, or physical causality. Right hemisphere activation was substantially greater for all sentences in the autism group than in a matched control group suggesting decreased LH capacity in autism resulting in a spillover of processing to RH homologs. Moreover, the ToM network was disrupted. The autism group showed similar activation for all inference types in the right temporo-parietal component of the ToM network whereas the control participants selectively activated this network only when appropriate. The autism group had lower functional connectivity within the ToM network and also between the ToM and a left hemisphere language network. Furthermore, the within-network functional connectivity in autism was correlated with the size of the anterior portion of the corpus callosum.

Muscle characteristics of persons with fibromyalgia syndrome

PURPOSE

To explore agonist and antagonist muscle characteristics, depression and physical functioning of patients with Fibromyalgia Syndrome (FMS).

METHOD

Individuals with FMS (N=34) and chronic pain (N=36) were recruited from a chronic pain program. Normal controls (N=37) were recruited from the general community.

MEASURES

Demographic data (age, gender, marital status, ethnicity, and employment status), self-report physical and psychological functioning, current level of pain, level of depression, and agonist and antagonist muscle activity (SCM, Biceps/triceps, Forearm flexor/extensor, Lower leg tibialis anterior/gastrocnemius) were measured with sEMG.

RESULTS

There was a significant difference in upper arm muscle contra-activation between the FMS patients and normal controls but not between the non-FMS chronic pain patients and normal controls. When compared to normal controls, the FMS group reported more depression and a higher level of physical functioning problems, but it was not significantly different from the mean score for the non-FMS chronic pain group. Problems with physical functioning and depression did not predict strength of contra-activation in the upper arm muscle.

CONCLUSION

These findings suggest presence of unusual muscle activity occurring in the bicep muscle of FMS and non-FMS chronic pain patients, which establishes a possible link between muscle dysfunction, mood and pain.

Anticipatory postural adjustments modify the movement-related potentials of upper extremity voluntary movement

To elucidate the effect on movement-related potentials (MRPs) of anticipatory postural adjustments (APAs) accompanied by voluntary focal movement, we examined the MRPs of shoulder flexion movement under standing and sitting postural conditions in 12 normal subjects. MRPs were evaluated based on three components: readiness potential (RP), motor potential (MP), and movement-monitoring potential. APAs were observed in the activities of postural muscles including the biceps femoris and erector spinae muscles only under standing conditions. The amplitudes of the three MRP components were larger under standing conditions than under sitting conditions for all recorded electrode positions, and the RP and MP amplitudes at the vertex position, which lies over the supplementary motor area (SMA), showed a prominent increase under standing conditions with the highest statistical significance. These results suggest that a recruited neural process of the cortical area including the SMA may be necessary to generate voluntary movement accompanied by APA.

Can stroke localisation be used to map out the neural network for yawning behaviour?

What are the neuroanatomical structures involved in repetitive yawning during stroke?

Temporal modulations of agonist and antagonist muscle activities accompanying improved performance of ballistic movements

Although many studies have examined performance improvements of ballistic movement through practice, it is still unclear how performance advances while maintaining maximum velocity, and how the accompanying triphasic electromyographic (EMG) activity is modified. The present study focused on the changes in triphasic EMG activity, i.e., the first agonist burst (AG1), the second agonist burst (AG2), and the antagonist burst (ANT), that accompanied decreases in movement time and error. Twelve healthy volunteers performed 100 ballistic wrist flexion movements in ten 10-trial sessions under the instruction to "maintain maximum velocity throughout the experiment and to stop the limb at the target as fast and accurately as possible". Kinematic parameters (position and velocity) and triphasic EMG activities from the agonist (flexor carpi radialis) and antagonist (extensor carpi radialis) muscles were recorded. Comparison of the results obtained from the first and the last 10 trials, revealed that movement time, movement error, and variability of amplitudes reduced with practice, and that maximum velocity and time to maximum velocity remained constant. EMG activities showed that AG1 and AG2 durations were reduced, whereas ANT duration did not change. Additionally, ANT and AG2 latencies were reduced. Integrated EMG of AG1 was significantly reduced as well. Analysis of the alpha angle (an index of the rate of recruitment of the motoneurons) showed that there was no change in either AG1 or AG2. Correlation analysis of alpha angles between these two bursts further revealed that the close relationship of AG1 and AG2 was kept constant through practice. These findings led to the conclusion that performance improvement in ballistic movement is mainly due to the temporal modulations of agonist and antagonist muscle activities when maximum velocity is kept constant. Presumably, a specific strategy is consistently applied during practice.

Proactive and Reactive Mechanisms Play a Role in Stepping on Inverting Surfaces During Gait

Ankle inversions have been studied extensively during standing conditions. However, inversion traumas occur during more dynamic conditions, like walking. Therefore in this study sudden ankle inversions were elicited in 12 healthy subjects who stepped on a trap door while walking on a treadmill. First, 10 control trials (0° of rotation) were presented. Then, 20 stimulus (25° of rotation) and control trials were presented randomly. EMG recordings were made of six lower leg muscles. All muscles showed a short-latency response (SLR) of about 40 ms and a late-latency response (LLR) of about 90 ms. The peroneal muscles showed the largest amplitudes in both responses. The functionally more important, larger, and more consistent LLR response was too late to resist the induced stretch during the inversion. The functional relevance of this response must lie after the inversion. During the first trial a widespread “startle-like” coactivation of the LLR was observed. The last trials showed only a recruitment of the peroneal muscles and, to a lesser extent, the gastrocnemius lateralis, which is seen as a switch from reactive control to more proactive adaptive strategies. These proactive strategies were investigated separately by comparing trials in which inversion was expected (but did not occur) with those in which subjects knew that no inversion would occur. Anticipation of a possible inversion was expressed in decreased tibialis anterior activity before initial contact, consistent with a more cautious and stable foot placement. Furthermore, immediately after landing, the peroneal muscles were activated to counteract the upcoming stretch.

Does the Recruitment of Excitation and Inhibition in the Motor Cortex Differ?

The level of excitability within the motor cortex can be described as a balance between excitation and inhibition, but it is unknown how well both processes correlate. To address this question, the authors measured motor cortical excitability and inhibition in healthy human subjects, comparing the recruitment of motor evoked potentials (MEPs) and the duration of the cortical silent period (CSP) after transcranial magnetic stimulation (TMS). Single-pulse "focal" TMS was applied at intensities varying between 90% and 200% of motor thresholds to the right motor cortex of 15 healthy volunteers. The peak-to peak size of MEP responses and the duration of the CSP were measured in small hand muscles. Stimulus-response (S-R) curves were constructed by plotting the MEP size and CSP duration against stimulus intensities. The absolute duration of CSP and the size MEPs correlated significantly and to a similar extent with stimulus intensity (r = 0.60 and 0.53, respectively). The slope of the MEP-S-R was steeper compared with CSP-S-R, particularly at low stimulation intensities. CSP duration saturated earlier and CSP-S-Rs were shifted upwards at a given stimulus intensity compared with MEP-S-Rs. The findings suggest that recruitment of inhibition and excitation within the sensorimotor cortex correlate. However, inhibitory effects are recruited at lower intensities and saturate earlier than excitation.

Muscle Synergies Characterizing Human Postural Responses

Postural control is a natural behavior that requires the spatial and temporal coordination of multiple muscles. Complex muscle activation patterns characterizing postural responses suggest the need for independent muscle control. However, our previous work shows that postural responses in cats can be robustly reproduced by the activation of a few muscle synergies. We now investigate whether a similar neural strategy is used for human postural control. We hypothesized that a few muscle synergies could account for the intertrial variability in automatic postural responses from different perturbation directions, as well as different postural strategies. Postural responses to multidirectional support-surface translations in 16 muscles of the lower back and leg were analyzed in nine healthy subjects. Six or fewer muscle synergies were required to reproduce the postural responses of each subject. The composition and temporal activation of several muscle synergies identified across all subjects were consistent with the previously identified “ankle” and “hip” strategies in human postural responses. Moreover, intertrial variability in muscle activation patterns was successfully reproduced by modulating the activity of the various muscle synergies. This suggests that trial-to-trial variations in the activation of individual muscles are correlated and, moreover, represent variations in the amplitude of descending neural commands that activate individual muscle synergies. Finally, composition and temporal activation of most of the muscle synergies were similar across subjects. These results suggest that muscle synergies represent a general neural strategy underlying muscle coordination in postural tasks.

Selective abnormal modulation of hippocampal activity during memory formation in first-episode psychosis

CONTEXT

Memory is one of the cognitive functions most affected in schizophrenia, with deficits observed from the first episode of psychosis (FEP). Previous studies have indicated that some memory processes may be more affected than others.

OBJECTIVE

To examine the neural correlates of 3 specific memory processes in FEP by means of functional magnetic resonance imaging (fMRI).

DESIGN

Case-control study.

SETTING

Prevention and Early Intervention Program for Psychoses of the Douglas Hospital and Montreal Neurological Institute, McGill University. Subjects Twenty-six patients with FEP and 20 healthy controls.

MAIN OUTCOME MEASURES

Behavioral performance and regional brain activity measured during memory encoding by fMRI. Our fMRI design included 3 within-subject contrasts (associative vs item-oriented encoding, encoding of arbitrary vs semantically related image pairs, and successful vs unsuccessful memory encoding) that were then used for group conjunctions and between-group analyses.

RESULTS

Patients with FEP showed normal activation of several brain regions, including the prefrontal cortex, hippocampus, and parahippocampal cortex, during successful memory encoding and associative encoding. In contrast, the hippocampus and surrounding medial temporal areas showed reduced activity during the encoding of arbitrary pairs. This selective dysfunction reflected by abnormal brain activation during encoding was accompanied by a greater deficit for subsequent recognition of arbitrary pairs relative to the semantically related pairs.

CONCLUSIONS

This study demonstrated that, in the same group of patients with FEP, the hippocampus could show either normal or abnormal modulation of activation depending on the specific cognitive process that was examined. The normal modulation of hippocampal activation observed during successful memory encoding in FEP argues against a general inability to recruit this region. Instead, the dysfunction was specifically linked to semantic relatedness. This selective deficit seems to affect memory performance in FEP and denotes an important representational problem that may confer greater vulnerability to psychotic disorders and would thus be interesting to examine in high-risk populations.

Pedophilia is linked to reduced activation in hypothalamus and lateral prefrontal cortex during visual erotic stimulation

BACKGROUND

Although pedophilia is of high public concern, little is known about underlying neural mechanisms. Although pedophilic patients are sexually attracted to prepubescent children, they show no sexual interest toward adults. This study aimed to investigate the neural correlates of deficits of sexual and emotional arousal in pedophiles.

METHODS

Thirteen pedophilic patients and 14 healthy control subjects were tested for differential neural activity during visual stimulation with emotional and erotic pictures with functional magnetic resonance imaging.

RESULTS

Regions showing differential activations during the erotic condition comprised the hypothalamus, the periaqueductal gray, and dorsolateral prefrontal cortex, the latter correlating with a clinical measure. Alterations of emotional processing concerned the amygdala-hippocampus and dorsomedial prefrontal cortex.

CONCLUSIONS

Hypothesized regions relevant for processing of erotic stimuli in healthy individuals showed reduced activations during visual erotic stimulation in pedophilic patients. This suggests an impaired recruitment of key structures that might contribute to an altered sexual interest of these patients toward adults.