1,720,976 research outputs found
M1 Corticospinal Mirror Neurons and Their Role in Movement Suppression during Action Observation
No full textSummaryEvidence is accumulating that neurons in primary motor cortex (M1) respond during action observation [1, 2], a property first shown for mirror neurons in monkey premotor cortex [3]. We now show for the first time that the discharge of a major class of M1 output neuron, the pyramidal tract neuron (PTN), is modulated during observation of precision grip by a human experimenter. We recorded 132 PTNs in the hand area of two adult macaques, of which 65 (49%) showed mirror-like activity. Many (38 of 65) increased their discharge during observation (facilitation-type mirror neuron), but a substantial number (27 of 65) exhibited reduced discharge or stopped firing (suppression-type). Simultaneous recordings from arm, hand, and digit muscles confirmed the complete absence of detectable muscle activity during observation. We compared the discharge of the same population of neurons during active grasp by the monkeys. We found that facilitation neurons were only half as active for action observation as for action execution, and that suppression neurons reversed their activity pattern and were actually facilitated during execution. Thus, although many M1 output neurons are active during action observation, M1 direct input to spinal circuitry is either reduced or abolished and may not be sufficient to produce overt muscle activity
Large Identified Pyramidal Cells in Macaque Motor and Premotor Cortex Exhibit “Thin Spikes”: Implications for Cell Type Classification
No full textRecent studies have suggested that extracellular recordings of putative cortical interneurons have briefer spikes than those of pyramidal neurons, providing a means of identifying cortical cell types in recordings from awake monkeys. To test this, we investigated the spike duration of antidromically identified pyramidal tract neurons (PTNs) recorded from primary motor (M1) or ventral premotor cortex (area F5) in 4 awake macaque monkeys. M1 antidromic latencies (ADLs) were skewed toward short ADLs (151 PTNs; 0.5–5.5 ms, median 1.1 ms) and significantly different from that of F5 ADLs (54 PTNs; 1.0–6.9 ms, median 2.6 ms). The duration of PTN spikes, recorded with a high-pass filter of 300 Hz and measured from the negative trough to the positive peak of the spike waveform, ranged from 0.15 to 0.71 ms. Importantly, we found a positive linear correlation between ADL and spike duration in both M1 (R2= 0.40,p< 0.001) and F5 (R2= 0.57,p< 0.001). Thus PTNs with the shortest ADL (fastest axons) had the briefest spikes, and since PTN soma size is correlated with axon size and conduction velocity, it is likely that the largest pyramidal neurons (Betz cells in M1) have spikes with short durations (0.15–0.45 ms), which overlap heavily with those reported for putative interneurons in previous studies in non-primates. In summary, one class of physiologically identified cortical pyramidal neuron exhibits a wide variety of spike durations and the results suggest that spike duration alone may not be a reliable indicator of cell type.</jats:p
Effects of transcranial direct current stimulation over the human motor cortex on corticospinal and transcallosal excitability
No full textWeak transcranial direct current stimulation (tDCS) can induce long lasting changes in cortical excitability. In the present study we asked whether tDCS applied to the left primary motor cortex (M1) also produces aftereffects distant from the site of the stimulating electrodes. We therefore tested corticospinal excitability in the left and the right M1 and transcallosal excitability between the two cortices using transcranial magnetic stimulation (TMS) before and after applying tDCS. Eight healthy subjects received 10 min of anodal or cathodal tDCS (1 mA) to the left M1. We examined the amplitude of contralateral motor evoked potentials (MEPs) and the onset latency and duration of transcallosal inhibition with single pulse TMS. MEPs evoked from the tDCS stimulated (left) M1 were increased by 32% after anodal and decreased by 27% after cathodal tDCS, while transcallosal inhibition evoked from the left M1 remained unchanged. The effect on MEPs evoked from the left M1 lasted longer for cathodal than for anodal tDCS. MEPs evoked from the right M1 were unchanged whilst the duration of transcallosal inhibition evoked from the right M1 was shortened after cathodal tDCS and prolonged after anodal tDCS. The duration of transcallosal inhibition returned to control values before the effect on the MEPs from the left M1 had recovered. These findings are compatible with the idea that tDCS-induced aftereffects in the cortical motor system are limited to the stimulated hemisphere, and that tDCS not only affects corticospinal circuits involved in producing MEPs but also inhibitory interneurons mediating transcallosal inhibition from the contralateral hemisphere
Modulation of the Intracortical LFP during Action Execution and Observation
Get PDFThe activity of mirror neurons in macaque ventral premotor cortex (PMv) and primary motor cortex (M1) is modulated by the observation of another's movements. This modulation could underpin well documented changes in EEG/MEG activity indicating the existence of a mirror neuron system in humans. Because the local field potential (LFP) represents an important link between macaque single neuron and human noninvasive studies, we focused on mirror properties of intracortical LFPs recorded in the PMv and M1 hand regions in two macaques while they reached, grasped and held different objects, or observed the same actions performed by an experimenter. Upper limb EMGs were recorded to control for covert muscle activity during observation.The movement-related potential (MRP), investigated as intracortical low-frequency LFP activity (<9 Hz), was modulated in both M1 and PMv, not only during action execution but also during action observation. Moreover, the temporal LFP modulations during execution and observation were highly correlated in both cortical areas. Beta power in both PMv and M1 was clearly modulated in both conditions. Although the MRP was detected only during dynamic periods of the task (reach/grasp/release), beta decreased during dynamic and increased during static periods (hold).Comparison of LFPs for different grasps provided evidence for partially nonoverlapping networks being active during execution and observation, which might be related to different inputs to motor areas during these conditions. We found substantial information about grasp in the MRP corroborating its suitability for brain-machine interfaces, although information about grasp was generally low during action observation
Stimulus intensity and coil characteristics influence the efficacy of rTMS to suppress cortical excitability
No full textObjective: Low-frequency repetitive transcranial magnetic stimulation (rTMS) can reduce cortical excitability. Here we examined whether inhibitory after effects of low-frequency rTMS are influenced by stimulus intensity, the type of TMS coil and re-afferent sensory stimulation. Methods: In fifteen healthy volunteers, we applied 900 biphasic pulses of 1 Hz rTMS to the left primary motor cortex (M1) at an intensity that was 10% below or 15% above resting motor threshold. For rTMS, we used two different figure-of-eight shaped coils (Magstim or Medtronic coil) attached to the same stimulator. We recorded motor evoked potentials (MEPs) evoked with the same set-up used for rTMS (MEP-rTMS) before and twice after rTMS. Using a different TMS setup, we also applied monophasic pulses to the M1 in order to assess the effects of rTMS on corticospinal excitability, intracortical paired-pulse excitability and the duration of the cortical silent period (CSP). In a control experiment, the same measurements were performed after 15 min of 1 Hz repetitive electrical nerve stimulation (rENS) of the right ulnar nerve. Results: Analysis of variance revealed an interaction between intensity, coil and time of measurement (p < 0.035), indicating that the effect of 1 Hz rTMS on MEP-rTMS amplitude depended on the intensity and the type of coil used for rTMS. Suppression of corticospinal excitability was strongest after suprathreshold 1 Hz rTMS with the Medtronic coil (p < 0.01 for both post-rTMS measurements relative to pre-intervention baseline). Regardless of the type of coil, suprathreshold but not subthreshold rTMS transiently prolonged the CSP and attenuated paired-pulse facilitation. Suprathreshold 1 Hz rENS also induced a short-lasting inhibition of MEP-rTMS. Conclusions: Both the stimulation intensity and the type of TMS coil have an impact on the after effects of 1 Hz rTMS. Re-afferent feedback activation may at least in part account for the stronger suppression of corticospinal excitability by suprathreshold 1 Hz rTMS. Significance: These data should be considered when rTMS is used as a therapeutic means. (c) 2006 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.Medical Research Council [G9806600
Preconditioning with transcranial direct current stimulation sensitizes the motor cortex to rapid-rate transcranial magnetic stimulation and controls the direction of after-effects
No full textBackground: Rapid-rate repetitive transcranial magnetic stimulation (rTMS) can produce a lasting increase in cortical excitability in healthy subjects or induce beneficial effects inpatients with neuropsychriatric disorders; however, the conditioning effects of rTMS are often subtle and variable, limiting therapeutic applications. Here we show that magnitude and direction of after-effects induced by rapid-rate rTMS depend on the state of cortical excitability before stimulation and can be tuned by preconditioning with transcranial direct current stimulation (tDCS). Methods: Ten healthy volunteers received a 20-sec train of 5-Hz rTMS given at an intensity of individual active motor threshold to the left primary motor hand area. This interventional protocol was preconditioned by 10 min of anodal, cathodal, or sham tDCS. We used single-pulse TMS to assess corticospinal excitability at rest before, between, and after the two interventions. Results: The 5-Hz rTMS given after sham tDCS failed to produce any after-effect, whereas 5-Hz rTMS led to a marked shift in corticospinal excitability when given after effective tDCS. The direction of rTMS-induced plasticity critically depended on the polarity of tDCS conditioning. Conclusions: Preconditioning with tDCS enhances cortical plasticity induced by rapid-rate rTMS and can shape the direction of rTMS-induced after-effects
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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