1,720,967 research outputs found
Linear summation of outputs in a balanced network modelof motor cortex
No full textGiven the nonlinearities of the the neural circuitry's elements, we would expect corticalcircuits to respond nonlinearly when activated. Surprisingly, when two points in the motorcortex are activated simultaneously, the EMG responses are the linear sum of the responsesevoked by each of the points activated separately. Additionally, the corticospinal transferfunction is close to linear, implying that the synaptic interactions in motor cortex must beeffectively linear. To account for this, here we develop a model of motor cortex composedof multiple interconnected points, each comprised of reciprocally connected excitatory andinhibitory neurons. We show how nonlinearities in neuronal transfer functions areeschewed by strong synaptic interactions within each point. Consequently, thesimultaneous activation of multiple points results in a linear summation of their respectiveoutputs. We also consider the effects of reduction of inhibition at a cortical point when oneor more surrounding points are active. The network response in this condition is linear overan approximately two to three fold decrease of inhibitory feedback strength. This resultsupports the idea that focal disinhibition allows linear coupling of motor cortical points togenerate movement related muscle activation patterns; albeit with a limitation on gaincontrol. The model also explains why neural activity does not spread as far out as the axonalconnectivity allows, whilst also explaining why distant cortical points can be, nonetheless,functionally coupled by focal disinhibition. Finally, we discuss the advantages that linearinteractions at the cortical level afford to motor command synthesis
Timing of cortical excitability changes during the reaction time of movements superimposed on tonic motor activity
No full textSeated subjects were instructed to react to an auditory cue by simultaneously contracting the tibialis anterior (TA) muscle of each ankle isometrically. Focal transcranial magnetic stimulation of the leg area of the motor cortex (MCx) was used to determine the time course of changes in motor-evoked potential amplitude (MEP) during the reaction time (RT). In one condition the voluntary contraction was superimposed on tonic EMG activity maintained at 10% of maximal voluntary contraction. In the other condition the voluntary contraction was made starting from rest. MEPs in the TA contralateral to the stimulation coil were evoked at various times during the RT in each condition. These were compared to the control MEPs evoked during tonic voluntary activity or with the subject at rest. The RT was measured trial by trial from the EMG activity of the TA ipsilateral to the magnetic stimulus, taking into account the nearly constant time difference between the two sides. The MEPs became far greater than control MEPs during the RT (mean = 332%, SD = 44 %, of control MEPs, P < 0.001) without any measurable change in the background level of EMG activity. The onset of this facilitation occurred on average 12.80 ms (SD = 7.55 ms) before the RT. There was no difference in the onset of facilitation between the two conditions. Because MEPs were facilitated without a change in the background EMG activity, it is concluded that this facilitation is specifically due to an increase of MCx excitability just before voluntary muscle activation. This conclusion is further reinforced by the observation that MEPs evoked by near-threshold anodal stimuli to the MCx were not facilitated during the RT, in contrast to those evoked by near-threshold transcranial magnetic stimulation. However, several observations in the present and previous studies indicate that MEP amplitude may be more sensitive to alpha-motoneuron activity than to motor cortical neuron activity, an idea that has important methodological implications
Neural mechanisms involved in the functional linking of motor cortical points
No full textWe sought to understand the basic neural processes involved in the functional linking of motor cortical points. We asked which of the two basic neural mechanisms, excitation or inhibition, is required to functionally link motor cortical points. In the ketamine-anaesthetized cat, a microstimulation electrode was positioned at a point (control point) that was identified by the following three characteristics of the EMG responses: the muscle(s) activated at threshold, any additional muscles recruited by supra-threshold stimulation, and their relative latency. A second distinct point (test point) producing activation of a muscle at a different joint was then identified. At this test cortical point the GABAA receptor antagonist bicuculline was ejected iontophoretically, while stimulating the control point near threshold. A combined response was elicited consisting of the response normally elicited at the control point plus that elicited at the test point. Thus, an artificial muscle synergy was produced following disinhibition of the test point. This was never the case when glutamate was ejected at the test point, even when supra-threshold stimuli were used at the control point. Therefore, simply increasing the excitability of a cortical point was not sufficient to release the muscle(s) represented at that point into a muscle synergy. Kynurenate, a broadly acting excitatory amino acid receptor antagonist, ejected at the bicuculline point reversed the effect of bicuculline. This shows that the release phenomenon was mediated synaptically and was not due to spread of the stimulating current. We suggest that release from inhibition may be one of the neural mechanisms involved in functionally linking motor cortical points. This functional linking may be part of the ensemble of motor cortical mechanisms involved in recruitment of muscle synergies
A Donders’ like law for arm movements: the signal not the noise
No full textExperiments were done to determine whether the starting position of the arm influences its final configuration (posture) when pointing to, or grasping, targets located within the common workspace of the arm. Subjects were asked to point to, or grasp, each of six targets from five, or seven, widely spaced starting positions. We found that the variability (standard deviation) of the arm's configuration, measured as the angle of inclination of the plane delimited by the arm and forearm, averaged about 4o for comfortable speed pointing movements and was only slightly higher for fast pointing movements. Comfortable speed reaches to grasp the targets were associated with slightly lower variability (3.5o) in final arm configuration. The average variability of repeated movements to a given target from a single start position (3.5o) was comparable to that of movements from different start positions to the same target (4.2o). A small difference in final arm inclination angle, averaged across all subjects and targets, of 3o was found between two pairs of starting positions. This small and possibly idiosyncratic effect is within the noise of final arm orientation variability for repeated movements (i.e. 3.5o). Thus, the variability of final posture is not for the most part due to different start positions, it is inherent to movement per se. Our results reconcile conflicting previous studies and are consistent with past works suggesting that a Donders’ like law is indeed largely upheld for unconstrained visually guided arm movements. In summary, considering movements within a typical work space, when the hand is moved voluntarily to a given spatial location the posture of the arm is nearly the same regardless of its starting position. Importantly, variability is inherent to the rule
Pointing to One's Moving Hand: Putative Internal Models Do Not Contribute to Proprioceptive Acuity
No full textWe can easily and without sight bring our fingertip to our nose, or swat a mosquito on our arm. These actions rely on proprioception, also known as kinesthesia, which classically has been attributed to processing of sensory inflow by the CNS. However, internal model theories of sensorimotor neuroscience propose that proprioceptive localization also involves a contribution from estimates of limb kinematics derived from motor commands. We tested this prediction in 19 subjects who moved the right index finger tip to touch the moving left index finger tip under three conditions: (1) vision allowed, active movement of the left hand (2) vision blocked, active movement of the left hand, and (3) vision blocked, passive movement of the left hand imposed by the experimenter. The target left index finger tip was moved in a wide range of directions by unrestricted movements of the arm. Mean errors in apposition of the right to the left index finger tips were small, averaging <2 cm between sensors fixed to the finger nails. Note that the average distance between the sensors was ~1.7 cm when the fingertips were brought together in “perfect” apposition under visual guidance. The 3D mean distance and variable distance errors were marginally lower by some 2 mm with eyes open compared to the eyes closed active condition. However, mean distance and variable distance errors did not differ between the active and passive conditions with eyes closed. Thus, proprioceptive localization of one's moving hand is very accurate, essentially as accurate as when vision is allowed. More importantly, our results demonstrate that hypothesized internal model derived estimates of arm kinematics do not contribute to localization accuracy beyond that provided by sensory signals, casting doubt on their existence
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Propriétés fonctionnelles et organisation du cortex moteur chez le chat
No full textTableau d’honneur de la Faculté des études supérieures et postdoctorales, 2007-2008.Le travail accompli au cours de mes études de doctorat permet de mieux comprendre l'organisation et le mode d'opération du cortex moteur. Les études rapportées dans cette thèse décrivent la relation entre l'activité du cortex moteur primaire et l'activité motrice qu'elle génère. Elles lèvent le voile sur certaines interactions intrinsèques du cortex moteur et apportent ainsi des éclaircissements importants sur son mode de fonctionnement. Cette thèse est d'abord constituée d'une introduction présentant l'organisation générale des structures impliquées dans le contrôle des mouvements. Elle comprend également quelques discussions sur des études antérieures pertinentes aux projets de recherches présentés par la suite. Ceux-ci constituent le corps de la thèse. Ils sont présentés sous forme de trois articles, rédigés en anglais. Le premier couvre plusieurs propriétés d'entrée-sortie du cortex moteur du chat sous anesthésie. Différentes conditions d'activation corticale sont analysées et mises en relation avec les réponses motrices afin d'évaluer les caractéristiques du contrôle corticomoteur. Le second article discute des interactions entre les sorties motrices de deux points du cortex moteur. Il y est démontré que l'activité musculaire et les mouvements évoqués par l'activation corticale simultanée de deux points corticaux se combinent de façon linéaire. Quant au troisième article, il traite d'un mécanisme de couplage fonctionnel entre les points corticaux impliquant la levée d'inhibition. Les résultats présentés prouvent qu'il est possible pour deux points corticaux contrôlant des muscles antagonistes d'interagir et de combiner leur sorties motrices. Le dernier chapitre de la thèse comprend d'abord la présentation de données anatomiques du cortex moteur permettant de mieux comprendre et d'évaluer les principaux résultats obtenus lors de mon doctorat. Il est complété par une discussion générale apportant une interprétation plus élaborée des résultats obtenus, tout en les plaçant dans un contexte plus large. Il s'en dégage une vue d'ensemble rassemblant toutes les observations rapportées dans cette thèse et résumant l'avancement scientifique qu'elle apporte
Variations on the Author
Full text link“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
Full text linkWe 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
- …
