Showing papers in "Experimental Brain Research in 2008"

A computational neuroanatomy for motor control

Abstract: The study of patients to infer normal brain function has a long tradition in neurology and psychology. More recently, the motor system has been subject to quantitative and computational characterization. The pur- pose of this review is to argue that the lesion approach and theoretical motor control can mutually inform each other. Specifically, one may identify distinct motor control pro- cesses from computational models and map them onto specific deficits in patients. Here we review some of the impairments in motor control, motor learning and higher- order motor control in patients with lesions of the corti- cospinal tract, the cerebellum, parietal cortex, the basal ganglia, and the medial temporal lobe. We attempt to explain some of these impairments in terms of computa- tional ideas such as state estimation, optimization, prediction, cost, and reward. We suggest that a function of the cerebellum is system identification: to build internal models that predict sensory outcome of motor commands and correct motor commands through internal feedback. A function of the parietal cortex is state estimation: to inte- grate the predicted proprioceptive and visual outcomes with sensory feedback to form a belief about how the commands affected the states of the body and the envi- ronment. A function of basal ganglia is related to optimal control: learning costs and rewards associated with sensory states and estimating the ''cost-to-go'' during execution of a motor task. Finally, functions of the primary and the premotor cortices are related to implementing the optimal control policy by transforming beliefs about proprioceptive and visual states, respectively, into motor commands.

Interindividual variability and age-dependency of motor cortical plasticity induced by paired associative stimulation.

Abstract: Paired associative stimulation (PAS) can increase motor cortical excitability, possibly by long-term potentiation (LTP)-like mechanisms. As the capability of the cortex for plasticity decreases with age, we were interested here in testing interindividual variability and age-dependency of the PAS effect. Motor-evoked potentials (MEPs) were recorded from the resting right abductor pollicis brevis muscle before and for 30 min after PAS in 27 healthy subjects (22–71 years of age). PAS consisted of 225 pairs (rate, 0.25 Hz) of right median nerve stimulation followed at an interval equaling the individual N20-latency of the median nerve somatosensory-evoked cortical potential plus 2 ms by transcranial magnetic stimulation of the hand area of left primary motor cortex (PASN20+2). The PASN20+2-induced changes in MEP amplitude (ratio post PAS/pre PAS) were highly variable (1.00 ± 0.07, range 0.36–1.68). Fourteen subjects showed the expected LTP-like MEP increase (responders) while 13 subjects showed a long-term depression (LTD)-like MEP decrease (non-responders). Responders had a significantly lower resting motor threshold (RMT) and minimum stimulus intensity to elicit MEPs of 1 mV (MEP1 mV) than non-responders. RMT and MEP1 mV correlated significantly negatively with the PASN20+2 effect. The absolute PASN20+2 effect size irrespective of its direction decreased with age (r = −0.57, P = 0.002), i.e., LTP-like and LTD-like plasticity were large in young subjects but substantially smaller in elderly subjects. In conclusion, measures of motor cortical excitability (RMT, MEP1 mV) and age determine direction and magnitude of PAS effects in individual subjects.

The statistics of natural hand movements

Abstract: Humans constantly use their hands to interact with the environment and they engage spontaneously in a wide variety of manual activities during everyday life. In contrast, laboratory-based studies of hand function have used a limited range of predefined tasks. The natural movements made by the hand during everyday life have thus received little attention. Here, we developed a portable recording device that can be worn by subjects to track movements of their right hand as they go about their daily routine outside of a laboratory setting. We analyse the kinematic data using various statistical methods. Principal component analysis of the joint angular velocities showed that the first two components were highly conserved across subjects, explained 60% of the variance and were qualitatively similar to those reported in previous studies of reach-to-grasp movements. To examine the independence of the digits, we developed a measure based on the degree to which the movements of each digit could be linearly predicted from the movements of the other four digits. Our independence measure was highly correlated with results from previous studies of the hand, including the estimated size of the digit representations in primary motor cortex and other laboratory measures of digit individuation. Specifically, the thumb was found to be the most independent of the digits and the index finger was the most independent of the fingers. These results support and extend laboratory-based studies of the human hand.

Children with cerebral palsy exhibit greater and more regular postural sway than typically developing children

Abstract: Following recent advances in the analysis of centre-of-pressure (COP) recordings, we examined the structure of COP trajectories in ten children (nine in the analyses) with cerebral palsy (CP) and nine typically developing (TD) children while standing quietly with eyes open (EO) and eyes closed (EC) and with concurrent visual COP feedback (FB). In particular, we quantified COP trajectories in terms of both the amount and regularity of sway. We hypothesised that: (1) compared to TD children, CP children exhibit a greater amount of sway and more regular sway and (2) concurrent visual feedback (creating an external functional context for postural control, inducing a more external focus of attention) decreases both the amount of sway and sway regularity in TD and CP children alike, while closing the eyes has opposite effects. The data were largely in agreement with both hypotheses. Compared to TD children, the amount of sway tended to be larger in CP children, while sway was more regular. Furthermore, the presence of concurrent visual feedback resulted in less regular sway compared to the EO and EC conditions. This effect was less pronounced in the CP group where posturograms were most regular in the EO condition rather than in the EC condition, as in the control group. Nonetheless, we concluded that CP children might benefit from therapies involving postural tasks with an external functional context for postural control.

Alterations of the visual pathways in congenital blindness.

Abstract: We used whole brain MRI voxel-based morphometry (VBM) to study the anatomical organization of the visual system in congenitally blind (CB) adults. Eleven CB without a history of visual perception were compared with 21 age- and sex-matched normal-sighted controls (NS). CB showed significant atrophy of the geniculo-striate system, encompassing the optic nerves, the optic chiasm, the optic radiations and the primary visual cortex (BA17). The volume decrease in BA17 reached 25% in both hemispheres. The pulvinar and its projections to the associative visual areas were also dramatically altered, BA18/19 and the middle temporal cortex (MT) showing volume reductions of up to 20%. Additional significant white matter alterations were observed in the inferior longitudinal tract and in the posterior part of the corpus callosum, which links the visual areas of both hemispheres. Our data indicate that the afferent projections to the visual cortex in CB are largely atrophied. Despite the massive volume reductions in the occipital lobes, there is compelling evidence from the literature (reviewed in Noppeney 2007; Ptito and Kupers 2005) that blind subjects activate their visual cortex when performing tasks that involve somatosensory or auditory inputs, suggesting a reorganization of the neural pathways that transmit sensory information to the visual cortex.

Mapping causal interregional influences with concurrent TMS–fMRI

Abstract: Transcranial magnetic stimulation (TMS) produces a direct causal effect on brain activity that can now be studied by new approaches that simultaneously combine TMS with neuroimaging methods, such as functional magnetic resonance imaging (fMRI). In this review we highlight recent concurrent TMS-fMRI studies that illustrate how this novel combined technique may provide unique insights into causal interactions among brain regions in humans. We show how fMRI can detect the spatial topography of local and remote TMS effects and how these may vary with psychological factors such as task-state. Concurrent TMS-fMRI may furthermore reveal how the brain adapts to so-called virtual lesions induced by TMS, and the distributed activity changes that may underlie the behavioural consequences often observed during cortical stimulation with TMS. We argue that combining TMS with neuroimaging techniques allows a further step in understanding the physiological underpinnings of TMS, as well as the neural correlated of TMS-evoked consequences on perception and behaviour. This can provide powerful new insights about causal interactions among brain regions in both health and disease that may ultimately lead to developing more efficient protocols for basic research and therapeutic TMS applications.

Complexity of human postural control in young and older adults during prolonged standing

Abstract: Aging is known to have a degrading influence on many structures and functions of the human sensorimotor system. The present work assessed aging-related changes in postural sway using fractal and complexity measures of the center of pressure (COP) dynamics with the hypothesis that complexity and fractality decreases in the older individuals. Older subjects (68 ± 4 years) and young adult subjects (28 ± 7 years) performed a quiet stance task (60 s) and a prolonged standing task (30 min) where subjects were allowed to move freely. Long-range correlations (fractality) of the data were estimated by the detrended fluctuation analysis (DFA); changes in entropy were estimated by the multi-scale entropy (MSE) measure. The DFA results showed that the fractal dimension was lower for the older subjects in comparison to the young adults but the fractal dimensions of both groups were not different from a 1/f noise, for time intervals between 10 and 600 s. The MSE analysis performed with the typically applied adjustment to the criterion distance showed a higher degree of complexity in the older subjects, which is inconsistent with the hypothesis that complexity in the human physiological system decreases with aging. The same MSE analysis performed without adjustment showed no differences between the groups. Taken all results together, the decrease in total postural sway and long-range correlations in older individuals are signs of an adaptation process reflecting the diminishing ability to generate adequate responses on a longer time scale.

Vestibular thresholds for yaw rotation about an earth-vertical axis as a function of frequency

Abstract: Perceptual direction detection thresholds for yaw rotation about an earth-vertical axis were measured at seven frequencies (0.05, 0.1, 0.2, 0.5, 1, 2, and 5 Hz) in seven subjects in the dark. Motion stimuli consisted of single cycles of sinusoidal acceleration and were generated by a motion platform. An adaptive two-alternative cate- gorical forced-choice procedure was used. The subjects had to indicate by button presses whether they perceived yaw rotation to the left or to the right. Thresholds were mea- sured using a 3-down, 1-up staircase paradigm. Mean yaw rotation velocity thresholds were 2.8 deg s -1 for 0.05 Hz, 2.5 deg s -1 for 0.1 Hz, 1.7 deg s -1 for 0.2 Hz, 0.7 deg s -1 for 0.5 Hz, 0.6 deg s -1 for 1 Hz, 0.4 deg s -1 for 2 Hz, and 0.6 deg s -1 for 5 Hz. The results show that motion thresholds increase at 0.2 Hz and below and plateau at 0.5 Hz and above. Increasing velocity thresholds at lower frequencies qualitatively mimic the high-pass characteris- tics of the semicircular canals, since the increase at 0.2 Hz and below would be consistent with decreased gain/sensi- tivity observed in the VOR at lower frequencies. In fact, the measured dynamics are consistent with a high pass filter having a threshold plateau of 0.71 deg s -1 and a cut- off frequency of 0.23 Hz, which corresponds to a time constant of approximately 0.70 s. These findings provide no evidence for an influence of velocity storage on per- ceptual yaw rotation thresholds.

Effects of catastrophizing on pain perception and pain modulation.

Abstract: The multidimensional experience of pain is thought to be partially influenced by the pain modulation system as well as by individual psychological components. Recent studies demonstrated possible common neural network mediating both domains. The present study examined the relationships between pain perception, pain modulation, and catastrophizing in healthy subjects. Forty-eight participants (29 females and 19 males) completed the pain catastrophizing scale (PCS) and underwent psychophysical tests in order to evaluate the modulation of pain, using the diffuse noxious inhibitory control (DNIC) paradigm. Contact heat pain (47.0°C applied for 1 min), which was used as the “test” stimulation, was applied before and after a physical effort that induces pain (repeated squeezing of a hand grip device), which was used as a “conditioning” stimulus. Numeric pain scale intensities (NPS, 0–10) were evaluated four times during each of two separate consecutive runs of heat stimulation. Results showed a significant positive correlation of PCS with heat pain (r = 0.48, p < 0.0005) and with muscle pain (r = 0.31, p = 0.03). In addition, significant negative correlations were found between PCS and DNIC effect (r = −0.34, p = 0.02). Moreover, once catastrophizing was entered into the regression analysis, the previously significant effect of gender was no longer found. In conclusion, individuals with high catastrophizing levels demonstrated higher pain intensities and lower effects of DNIC indicating that catastrophizing might have a significant impact on pain perception via an association with pain modulation.

Effects of rhythmic sensory stimulation (auditory, visual) on gait in Parkinson’s disease patients

Abstract: This study has focused on how sensory stimulation affects gait in Parkinson’s disease (PD). The kinematic parameters of gait [cadence, step amplitude, velocity, coefficient of variation of stride time (CVstride-time), and the coefficient of variation of the step amplitude (CVstep-amplitude)] were analysed in 25 PD patients and 10 control subjects. Step amplitude, velocity and CVstride-time were altered in the patients with PD. However, when kinematic parameters were analysed as a function of disease severity, none of the parameters differed between early PD (I–II Hoehn and Yahr) and the controls. Nevertheless, more severely affected PD patients (III–IV Hoehn and Yahr) walked with a reduced step amplitude, lower velocity, higher CVstride-time, and higher CVstep-amplitude than the controls. The administration of auditory stimulation at a frequency matching the preferred walking cadence led to a decrease in the CVstride-time in PDIII–IV patients, and to an increase in step amplitude in PDIII–IV and controls. Visual stimulation at the same frequency did not modify any of the altered kinematic parameters in PDIII–IV patients. When different stimulation frequencies were utilised, auditory stimulation significantly changed some of the altered walking parameters in Parkinson patients. Frequencies matching preferred walking cadence or above this, up to the fastest walking, were those that seem to interact most effectively with the abnormal kinematic parameters in PDIII–IV patients. Visual stimulation negatively modulated cadence in PDIII–IV in the frequency range used. Sensory stimulation facilitates gait in PD. Studies using sensory stimulation as a tool to facilitate walking should take into account the grade of disability of the patients.

Coordination of the eyes and head during visual orienting.

Abstract: Changing the direction of the line of sight is essential for the visual exploration of our environment. When the head does not move, re-orientation of the visual axis is accomplished with high velocity, conjugate movements of the eyes known as saccades. Our understanding of the neural mechanisms that control saccadic eye movements has advanced rapidly as specific hypotheses have been developed, evaluated and sometimes rejected on the basis of new observations. Constraints on new hypotheses and new tests of existing models have often arisen from the careful assessment of behavioral observations. The definition of the set of features (or rules) of saccadic eye movements was critical in the development of hypotheses of their neural control. When the head is free to move, changes in the direction of the line of sight can involve simultaneous saccadic eye movements and movements of the head. When the head moves in conjunction with the eyes to accomplish these shifts in gaze direction, the rules that helped define head-restrained saccadic eye movements are altered. For example, the slope relationship between duration and amplitude for saccadic eye movements is reversed (the slope is negative) during gaze shifts of similar amplitude initiated with the eyes in different orbital positions. Modifications to the hypotheses developed in head-restrained subjects may be needed to account for these new observations. This review briefly recounts features of head-restrained saccadic eye movements, and then describes some of the characteristics of coordinated eye-head movements that have led to development of new hypotheses describing the mechanisms of gaze shift control.

Changes in the degree of motor variability associated with experimental and chronic neck-shoulder pain during a standardised repetitive arm movement.

Abstract: The aim of the present study was to investigate the effect of experimental and chronic neck-shoulder pain on the magnitude of cycle-to-cycle variability of task timing, kinematics and muscle activation during repetitive arm movement performed for 3 or 5 min. In an experimental part, acute muscle pain was induced in healthy subjects by intramuscular injection of hypertonic saline in trapezius (n = 10) and infraspinatus (n = 10) muscles. In a clinical part, workers with (n = 12) and without (n = 6) chronic neck-shoulder pain were compared. Cycle-to-cycle standard deviations of task duration, arm and trunk movement in 3D and surface electromyographic (EMG) root mean square activity were computed to assess the degree of variability. The variability in task timing increased in presence of both experimental and chronic pain (P < 0.05) compared with non-painful conditions. Experimental pain increased the variability of the starting position of the arm (P < 0.05), the arm range of motion (P < 0.01), the arm and trunk movement area (P < 0.01) and the acceleration of the arm (P < 0.01). In the chronic pain condition, the variability of arm and trunk acceleration (P < 0.01) and EMG activity (P < 0.05) was decreased compared with healthy controls. These results indicate that pain alters the magnitude of motor variability, and that the transition from acute to chronic pain is accompanied by changes in motor patterns. Experimental pain likely resulted in a quest for a motor solution reducing nociceptive influx, while chronic pain was characterised by a diminished motor flexibility.

An object for an action, the same object for other actions: effects on hand shaping

Abstract: Objects can be grasped in several ways due to their physical properties, the context surrounding the object, and the goal of the grasping agent. The aim of the present study was to investigate whether the prior-to-contact grasping kinematics of the same object vary as a result of different goals of the person grasping it. Subjects were requested to reach toward and grasp a bottle filled with water, and then complete one of the following tasks: (1) Grasp it without performing any subsequent action; (2) Lift and throw it; (3) Pour the water into a container; (4) Place it accurately on a target area; (5) Pass it to another person. We measured the angular excursions at both metacarpal-phalangeal (mcp) and proximal interphalangeal (pip) joints of all digits, and abduction angles of adjacent digit pairs by means of resistive sensors embedded in a glove. The results showed that the presence and the nature of the task to be performed following grasping affect the positioning of the fingers during the reaching phase. We contend that a one-to-one association between a sensory stimulus and a motor response does not capture all the aspects involved in grasping. The theoretical approach within which we frame our discussion considers internal models of anticipatory control which may provide a suitable explanation of our results.

The effect of age on task-related modulation of interhemispheric balance.

Abstract: Normal aging is associated with less lateralised task-related activation of the primary motor cortices. It has been hypothesized, but not tested, that this phenomenon is mediated transcallosaly. We have used Transcranial Magnetic Stimulation to look for age-related changes in interhemispheric inhibition (IHI). Thirty healthy individuals (aged 19–78 years) were studied using a paired-pulse protocol at rest and during a low-strength isometric contraction with the right hand. The IHI targeting the right motor cortex was assessed at two intervals, 10 ms (IHI10) and 40 ms (IHI40). The corticospinal excitability of the left hemisphere was assessed by means of input–output curves constructed during voluntary construction. Age was not correlated with IHI10 or IHI40 at rest. During muscle contraction IHI tended to increase at both intervals. However, this increase in IHI during the active condition (changeIHI) was less evident with advancing age for the 40 ms interval (r = 0.444, P = 0.02); in fact a degree of disinhibition was often present. There was no correlation between age and changeIHI10. Age was negatively correlated with the area under the recruitment curve (r = −0.585, P = 0.001) and the size of the maximum MEP collected (r = −0.485, P = 0.007). ChangeIHI and measures of corticospinal excitability were not intercorrelated. In conclusion, task-related increases in interhemispheric inhibition seem to diminish with advancing age. This phenomenon is specific for long-latency IHI and may underlie the age-related bihemispheric activation seen in functional imaging studies. The mechanism underlying changes in IHI with advancing age and the association with changes in corticospinal excitability need further investigation.

Visual information from the lower visual field is important for walking across multi-surface terrain.

Abstract: Visual information concerning characteristics of the environment is critical for safe navigation. The purpose of this study was to determine the importance of vision from the lower visual field for negotiating multi-surface terrain. Ten healthy young adults and ten healthy older adults walked across a walkway where the middle portion consisted of solid, rock, slippery, compliant, tilt, and irregular surfaces (i.e. multi-surface terrain). Participants performed the walking trials with and without special glasses that blocked the lower visual field. Head pitch angle along with step parameters were measured. Young and older adults demonstrated increased mean and maximum head pitch angle downward when the lower visual field was blocked suggesting the importance of vision from this area when stepping on multi-surface terrain. In addition, young and older adults altered their gait pattern by reducing gait speed and step length when the lower visual field was blocked. These results suggest that information from the lower visual field is normally used when walking across multi-surface terrain. The results have implications for those individuals who wear multi-focal glasses and who use them while walking in complex environments, which may challenge balance.

Interaction between startle and voluntary reactions in humans.

Abstract: The startle reaction (SR) is usually understood as an involuntary reaction to an unexpected sensory input. However, there is evidence that the mechanisms involved in the SR contribute also to the execution of actions that we consider voluntary. We need to exert a fine control of the motor output, which usually involves inhibition of the reflex reaction but it may also imply letting the SR manifest, shaped in accordance with our willed actions. The latter occurs mainly with relatively simple ballistic movements but it has also been shown to occur in complex movements. In this review, we describe arguments published in the literature in favour of the contribution of subcortical motor tracts involved in the generation of the SR to the execution of voluntary movements. Some of these studies have brought insight on the mechanisms accounting for motor preparation and execution of voluntary movements.

Properties of attentional selection during the preparation of sequential saccades.

Abstract: We examined the allocation of attention during the preparation of sequences of saccades in a dual task paradigm. As a primary task, participants performed a sequence of two or three saccades to targets arranged on a circular array. The secondary task was a two-alternative discrimination in which a critical discrimination stimulus (digital “E” or “3”) was presented among distractors either at one of the saccade goals or at any other position. The findings show that discrimination performance is enhanced at all the saccade target locations of the planned sequence, while it is close to chance level at the positions that are not relevant for the saccade sequence. An analysis of the discrimination performance at the intermediate locations indicates that saccade target selection involves spatially distinct, non-contiguous foci of attention. Further, our findings demonstrate that the movement-relevant locations are selected in parallel rather than serially in time. We conclude that during the preparation of a saccade sequence––well before the actual execution of the eye movement––attention is allocated in parallel to each of the individual movement targets.

Task prioritization in aging: effects of sensory information on concurrent posture and memory performance.

Abstract: In older adults, cognitive resources play a key role in maintaining postural stability In the present study, we evaluated whether increasing postural instability using sway referencing induces changes in resource allocation in dual-task performance leading older adults to prioritize the more age-salient posture task over a cognitive task Young and older adults participated in the study which comprised two sessions In the first session, three posture tasks (stable, sway reference visual, sway reference somatosensory) and a working memory task (n-back) were examined In the second session, single- and dual-task performance of posture and memory were assessed Postural stability improved with session Participants were more unstable in the sway reference conditions, and pronounced age differences were observed in the somatosensory sway reference condition In dual-task performance on the stable surface, older adults showed an almost 40% increase in instability compared to single-task However, in the sway reference somatosensory condition, stability was the same in single- and dual-task performance, whereas pronounced (15%) costs emerged for cognition These results show that during dual-tasking while standing on a stable surface, older adults have the flexibility to allow an increase in instability to accommodate cognitive task performance However, when instability increases by means of compromising somatosensory information, levels of postural control are kept similar in single- and dual-task, by utilizing resources otherwise allocated to the cognitive task This evidence emphasizes the flexible nature of resource allocation, developed over the life-span to compensate for age-related decline in sensorimotor and cognitive processing

High- and low-frequency repetitive transcranial magnetic stimulation differentially activates c-Fos and zif268 protein expression in the rat brain.

Abstract: Repetitive transcranial magnetic stimulation (rTMS) has been shown to alter cortical excitability depending on the stimulus-frequency used, with high frequency (5 Hz and higher) increasing it but low frequency (usually 1 Hz or lower) reducing it. To determine the efficiency of different rTMS protocols in inducing cortical network activity, we tested the acute effect of one low-frequency rTMS protocol (1 Hz) and two different high-frequency protocols (10 Hz and intermittent theta-burst stimulation, iTBS) on the expression of the two immediate early gene (IEG) proteins c-Fos and zif268 in the rat brain. The cortical expression of both IEGs was specifically changed in an rTMS-dependent manner. One and 10 Hz rTMS enhanced c-Fos protein expression in all cortical areas tested, while iTBS was effective only in limbic cortices. Zif268 expression was increased in almost all cortical areas after iTBS, while 10 Hz rTMS was effective only in the primary motor and sensory cortices. One Hertz rTMS had no effect on cortical zif268 expression. Furthermore, sham-rTMS had no effect on zif268 expression but increased c-Fos in limbic cortices. This is the first study demonstrating that cortical zif268 and c-Fos expression can be specifically modulated by acute rTMS depending on the pattern of stimulation applied.

Short and long duration transcranial direct current stimulation (tDCS) over the human hand motor area.

Abstract: The aim of the present paper is to study effects of short and long duration transcranial direct current stimulation (tDCS) on the human motor cortex. In eight normal volunteers, motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) were recorded from the right first dorsal interosseous muscle, and tDCS was given with electrodes over the left primary motor cortex (M1) and the contralateral orbit. We performed two experiments: one for short duration tDCS (100 ms, 1, 3 or 5 mA) and the other for long duration tDCS (10 min, 1 mA). The stimulus onset asynchrony (SOA) between the onset of tDCS and TMS were 1–7 and 10–120 ms for the former experiment. In the latter experiment, TMS was given 0–20 min after the end of 10 min tDCS. We evaluated the effect of tDCS on the motor cortex by comparing MEPs conditioned by tDCS with control MEPs. Cathodal short duration tDCS significantly reduced the size of responses to motor cortical stimulation at SOAs of 1–7 ms when the intensity was equal to or greater than 3 mA. Anodal short duration tDCS significantly increased MEPs when the intensity was 3 mA, but the enhancement did not occur when using 5 mA conditioning stimulus. Moreover, both anodal and cathodal short duration tDCS decreased responses to TMS significantly at SOAs of 20–50 ms and enhanced them at an SOA of 90 ms. Long duration cathodal tDCS decreased MEPs at 0 and 5 min after the offset of tDCS and anodal long duration tDCS increased them at 1 and 15 min. We conclude that the effect at SOAs less than 10 ms is mainly caused by acute changes in resting membrane potential induced by tDCS. The effect at SOAs of 20–100 ms is considered to be a nonspecific effect of a startle-like response produced by activation of skin sensation at the scalp. The effect provoked by long duration tDCS may be short-term potentiation or depression like effects.

Tactile feedback and timing accuracy in piano performance

Abstract: Sequential actions such as playing a piano or tapping in synchrony to an external signal put high cognitive and motor demands on producers, including the generation of precise timing at a wide variety of rates. Tactile information from the fingertips has been shown to contribute to the control of timing in finger tapping tasks. We addressed the hypothesis that reduction of timing errors is related to tactile afferent information in pianists’ finger movements during performance. Twelve pianists performed melodies at four rates in a synchronization-continuation paradigm. The pianists’ finger motion trajectories toward the piano keys, recorded with a motion capture system, contained different types and amounts of kinematic landmarks at different performance rates. One landmark, a finger–key (FK) landmark, can occur when the finger makes initial contact with the key surface and changes its acceleration abruptly. Overall, there were more FK landmarks in the pianists’ keystrokes, as the performance rate increased. The pianists were divided into two groups: those with low percentages of FK in the medium rates that increased with increasing performance rate and those with persistently high FK percentages. Low-FK pianists showed a positive relationship between increased tactile feedback from the current keystroke and increased temporal accuracy in the upcoming keystroke. These findings suggest that sensory information available at finger–key contact enhances the timing accuracy of finger movements in piano performance.

Audiovisual temporal adaptation of speech: temporal order versus simultaneity judgments.

Abstract: The temporal perception of simple auditory and visual stimuli can be modulated by exposure to asynchronous audiovisual speech. For instance, research using the temporal order judgment (TOJ) task has shown that exposure to temporally misaligned audiovisual speech signals can induce temporal adaptation that will influence the TOJs of other (simpler) audiovisual events (Navarra et al. (2005) Cognit Brain Res 25:499–507). Given that TOJ and simultaneity judgment (SJ) tasks appear to reflect different underlying mechanisms, we investigated whether adaptation to asynchronous speech inputs would also influence SJ task performance. Participants judged whether a light flash and a noise burst, presented at varying stimulus onset asynchronies, were simultaneous or not, or else they discriminated which of the two sensory events appeared to have occurred first. While performing these tasks, participants monitored a continuous speech stream for target words that were either presented in synchrony, or with the audio channel lagging 300 ms behind the video channel. We found that the sensitivity of participant’s TOJ and SJ responses was reduced when the background speech stream was desynchronized. A significant modulation of the point of subjective simultaneity (PSS) was also observed in the SJ task but, interestingly, not in the TOJ task, thus supporting previous claims that TOJ and SJ tasks may tap somewhat different aspects of temporal perception.

The effect of exposure to asynchronous audio, visual, and tactile stimulus combinations on the perception of simultaneity.

Abstract: Information about an event takes different amounts of time to be processed depending on which sensory system the event activates. However, despite the variations in processing time for lights and sounds, the point of subjective simultaneity (PSS) for briefly presented audio/visual stimuli is usually close to true simultaneity. Here we confirm that the simultaneity constancy mechanism that achieves this for audio/visual stimulus pairs is adaptable, and extend the investigation to other multimodal combinations. We measured the PSS and just noticeable differences (JNDs) for temporal order judgements for three stimulus combinations (sound/light, sound/touch, and light/touch) before and after repeated exposure to each one of these pairs presented with a 100 ms asynchrony (i.e., nine adapt-test combinations). Only the perception of simultaneity of the sound/light pair was affected by our exposure regime: the PSS shifted after exposure to either a temporally staggered sound/light or light/touch pair, and the JND decreased following exposure to a sound/touch pair. No changes were found in the PSSs or JNDs of sound/touch or light/touch pairs following exposure to any of the three time-staggered combinations. Participants’ reaction times (RT) to the three stimuli were also tested before and after each adaptation exposure. In general, exposure did not affect attention or processing time; the only change in RTs (of the 9 tested) was an increased RT for light following exposure to a sound/light pair with light leading. We suggest that the neural correlates of multisensory sound/light processing are resynchronised by a separate, more flexible simultaneity constancy mechanism than the light/touch or the sound/touch simultaneity processing systems.

Interactions between auditory and somatosensory feedback for voice F0 control

Abstract: Previous studies have demonstrated the importance of both kinesthetic and auditory feedback for control of voice fundamental frequency (F0). In the present study, a possible interaction between auditory feedback and kinesthetic feedback for control of voice F0 was tested by administering local anesthetic to the vocal folds in the presence of perturbations in voice pitch feedback. Responses to pitch-shifted voice feedback were larger when the vocal fold mucosa was anesthetized than during normal kinesthesia. A mathematical model incorporating a linear combination of kinesthesia and pitch feedback simulated the main aspects of our experimental results. This model indicates that a feasible explanation for the increase in response magnitude with vocal fold anesthesia is that the vocal motor system uses both pitch and kinesthesia to stabilize voice F0 shortly after a perturbation of voice pitch feedback has been perceived.

The effects of neuromuscular fatigue on task performance during repetitive goal-directed movements.

Abstract: Proper movement timing is essential to the successful execution of many motor tasks and may be adversely affected by muscle fatigue. This study quantified how muscle fatigue affected task performance during a repetitive upper extremity task. A total of 14 healthy young adults pushed a low load back and forth along a low-friction horizontal track in time with a metronome until volitional exhaustion. Kinematic, force, and electromyography (EMG) data were measured continuously throughout the task. The first and last 3.5 min were analyzed to represent "early" and "late" fatigue. Means and standard deviations of movement distance, speed, and timing errors were computed. We also decomposed variations in movement distance and speed into deviations that directly affected achieving the task goal and those that did not, by identifying the goal equivalent manifold (GEM) of all valid solutions to this task. Detrended fluctuation analysis was used to quantify the temporal persistence in each time series. Principle components analysis provided a direct measure of alignment with the GEM. Median power frequencies of the EMG significantly decreased in six of the nine muscles tested indicating that subjects did fatigue. However, there were no differences in the means or variability of movement distance, speed, or timing errors. Thus, subjects maintained overall performance despite fatigue. Subjects applied slightly higher peak handle forces when they were fatigued (P = 0.032). Muscle fatigue caused significant reductions in the temporal persistence of movement speed (P = 0.037) and timing errors (P = 0.046), indicating that subjects corrected errors more quickly when fatigued. Mean deviations and variability perpendicular to the GEM were much smaller than variability along the GEM (P < 0.001). Deviations perpendicular to the GEM were also corrected much more rapidly than those along the GEM (P < 0.001). Subjects aligned themselves very closely (<+/-7 degrees ), but not exactly (P < 0.001), with the GEM. These measures were not significantly affected by muscle fatigue. Overall, these results indicated that subjects altered their biomechanical movement patterns in response to muscle fatigue, but did so in a way that specifically preserved the goal relevant features of task performance.

Post-activation depression of soleus stretch reflexes in healthy and spastic humans.

Abstract: Reduced depression of transmitter release from Ia afferents following previous activation (post-activation depression) has been suggested to be involved in the pathophysiology of spasticity However, the effect of this mechanism on the myotatic reflex and its possible contribution to increased reflex excitability in spastic participants has not been tested To investigate these effects, we examined post-activation depression in Soleus H-reflex responses and in mechanically evoked Soleus stretch reflex responses Stretch reflex responses were evoked with consecutive dorsiflexion perturbations delivered at different intervals The magnitude of the stretch reflex and ankle torque response was assessed as a function of the time between perturbations Soleus stretch reflexes were evoked with constant velocity (175°/s) and amplitude (6°) plantar flexion perturbations Soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa The stretch reflex and H-reflex responses of 30 spastic participants (with multiple sclerosis or spinal cord injury) were compared with those of 15 healthy participants In the healthy participants, the magnitude of the soleus stretch reflex and H-reflex decreased as the interval between the stimulus/perturbation was decreased Similarly, the stretch-evoked torque decreased In the spastic participants, the post-activation depression of both reflexes and the stretch-evoked torque was significantly smaller than in healthy participants These findings demonstrate that post-activation depression is an important factor in the evaluation of stretch reflex excitability and muscle stiffness in spasticity, and they strengthen the hypothesis that reduced post-activation depression plays a role in the pathophysiology of spasticity

Reduced postural sway during quiet standing by light touch is due to finger tactile feedback but not mechanical support

Abstract: It is well known that a light and voluntary touch with a fingertip on a fixed surface improves postural stability during quiet standing. To determine whether the effect of the light touch is due to the tactile sensory input, as opposed to mechanical support, we investigated the light touch effect on postural stability during quiet standing with and without somatosensory input from the fingertip. Seven young subjects maintained quiet standing on a force platform with (LT) and without (NT) lightly touching a fixed surface, and with (TIS) and without (CON) the application of tourniquet ischemia, which removed the tactile sensation from the fingertip. The mean velocity of centre of pressure (CoP) was calculated to assess the postural sway in each condition. The mean velocity of CoP was significantly smaller in the LT condition compared to the NT condition only under the CON condition, whereas the light touch effect was not significant under the TIS condition. We found that the reduction of the horizontal ground reaction force due to the light touch was about 20%, which was approximately equivalent to the reduction of mean velocity of CoP in the LT condition compared to the NT condition. Since the fingertip contact force was relatively large compared to the horizontal ground reaction force, one could say that the light touch effect might be due to the mechanical support provided by the contact itself. However, we demonstrated experimentally that light touch effects were diminished due to loss of finger tactile feedback induced by the tourniquet ischemia, but not due to the mechanical support provided by the light touch. One possible reason is the lack of feedback information in controlling posture, and the other is the altered control of the arm induced by the loss of tactile feedback.

Cortical excitability changes following grasping exercise augmented with electrical stimulation

Abstract: Rehabilitation with augmented electrical stimulation can enhance functional recovery after stroke, and cortical plasticity may play a role in this process. The purpose of this study was to compare the effects of three training paradigms on cortical excitability in healthy subjects. Cortical excitability was evaluated by analysing the input-output relationship between transcranial magnetic stimulation intensity and motor evoked potentials (MEPs) from the flexor muscles of the fingers. The study was performed with 25 healthy volunteers who underwent 20-min simulated therapy sessions of: (1) functional electrical stimulation (FES) of the finger flexors and extensors, (2) voluntary movement (VOL) with sensory stimulation, and (3) therapeutic FES (TFES) where the electrical stimulation augmented voluntary activation. TFES training produced a significant increase in MEP magnitude throughout the stimulation range, suggesting an increase in cortical excitability. In contrast, neither the FES nor voluntary movement alone had such an effect. These results suggest that the combination of voluntary effort and FES has greater potential to induce plasticity in the motor cortex and that TFES might be a more effective approach in rehabilitation after stroke than FES or repetitive voluntary training alone.

The effects of low- and high-frequency repetitive TMS on the input/output properties of the human corticospinal pathway

Abstract: The objective of this study was to characterize the effects of various parameters (notably the frequency and intensity) of repetitive transcranial magnetic stimulation (rTMS) applied over the primary motor (M1) and premotor (PMC) cortices on the excitability of the first dorsalis interosseus (FDI) corticospinal pathway. To this end, we applied a comprehensive input–output analysis after fitting the experimental results to a sigmoidal function. Twenty-six healthy subjects participated in the experiments. Repetitive TMS was applied either over M1 or PMC at 1 Hz (LF) for 30 min (1,800 pulses) or at 20 Hz (HF) for 20 min (1,600 pulses). In the HF condition, the TMS intensity was set to 90% (HF90) of the FDI’s resting motor threshold (RMT). In the LF condition, the TMS intensity was set to either 90% (LF90) or 115% (LF115) of the RMT. The FDI input/output (I/O) curve was measured on both sides of the body before rTMS (the Pre session) and then during two Post sessions. For each subject, the I/O curves (i.e., the integral of the FDI motor-evoked potential (MEP) vs. stimulus intensity) were fitted using a Boltzmann sigmoidal function. The graph’s maximum slope, S50 and plateau value were then compared between Pre and Post sessions. LF115 over M1 increased the slope of the FDI I/O curve but did not change the S50 and plateau value. This also suggested an increase in the RMT. HF90 led to a more complex effect, with an increase in the slope and a decrease in the S50 and plateau value. We did not see a cross effect on the homologous FDI corticospinal pathway, and only PMC LF90 had an effect on ipsilateral corticospinal excitability. Our results suggest that rTMS may exert a more complex influence on cortical network excitability than is usually reported (i.e. simple inhibitory or facilitatory effects). Analysis of the fitted stimulus response curve indicates a dichotomous influence of both low- and high-frequency rTMS on M1 cortical excitability; this may reflect intermingled effects on excitatory and inhibitory cortical networks.

Ipsilateral versus contralateral cortical motor projections to a shoulder adductor in chronic hemiparetic stroke: implications for the expression of arm synergies

Abstract: An increase in ipsilateral descending motor pathway activity has been reported following hemiparetic stroke. In axial muscles, increased ipsilateral cortical activity has been correlated with good recovery whereas in distal arm muscles it is correlated with poor recovery. Currently, little is known about the control of proximal upper limb muscles following stroke. This muscle group is less impaired than the distal arm muscles following stroke, yet contributes to the abnormal motor coordination patterns associated with movements of the arm which can severely impair reaching ability. This study used transcranial magnetic stimulation (TMS) to evaluate the presence and magnitude of ipsilateral and contralateral projections to the pectoralis major (PMJ) muscle in stroke survivors. A laterality index (LI) was used to investigate the relationship between ipsilateral and contralateral projections and strength, clinical impairment level, and the degree of abnormal coordination expressed in the arm. The ipsilateral and contralateral hemispheres were stimulated using 90% TMS intensity while the subject generated shoulder adduction torques in both arms. Motor evoked potentials (MEPs) were measured in the paretic and non-paretic PMJ. The secondary torque at the elbow was measured during maximal adduction as an indicator of the degree of extensor synergy. Ipsilateral MEPs were most common in stroke survivors with moderate to severe motor deficits. The LI was correlated with clinical impairment level (P = 0.05) and the degree of extension synergy expressed in the arm (P = 0.03). The LI was not correlated with strength. These results suggest that increased excitability of ipsilateral pathways projecting to the proximal upper arm may contribute to the expression of the extension synergy following stroke. These findings are discussed in relation to a possible unmasking or upregulation of oligosynaptic cortico-bulbospinal pathways following stroke.