Экспериментальное исследование динамики фрикционных процессов при трении латуни Л63 с ультрамелкозернистой структурой

В работе рассматривается экспериментальное исследование динамики фрикционных процессов при трении объемных ультрамелкозернистых материалов. В качестве модельных образцов использовалась латунь Л63 с крупнокристаллической и ультрамелкозернистой структурой, сформированной методом равноканального углового прессования. В процессе сухого трения скольжения осуществлялась регистрация сигналов виброускорений и акустической эмиссии. Анализ коротких сигналов с применением преобразований Фурье позволил установить характерные параметры сигналов, полученных при трении крупнокристаллических и ультрамелкозернистых материалов

The data acquisition system of the CHORUS experiment

In the years 1994-1998 the CHORUS Collaboration has recorded data in the CERN WA95 experiment. Here we describe the data acquisition system that has been used, featuring concurrent hierarchical state machines, a remote operating system, a buffer manager, a dispatcher, a control panel and a supervisor

Transcranial Magnetic Stimulation Intensities in Cognitive Paradigms

BACKGROUND: Transcranial magnetic stimulation (TMS) has become an important experimental tool for exploring the brain's functional anatomy. As TMS interferes with neural activity, the hypothetical function of the stimulated area can thus be tested. One unresolved methodological issue in TMS experiments is the question of how to adequately calibrate stimulation intensities. The motor threshold (MT) is often taken as a reference for individually adapted stimulation intensities in TMS experiments, even if they do not involve the motor system. The aim of the present study was to evaluate whether it is reasonable to adjust stimulation intensities in each subject to the individual MT if prefrontal regions are stimulated prior to the performance of a cognitive paradigm. METHODS AND FINDINGS: Repetitive TMS (rTMS) was applied prior to a working memory task, either at the 'fixed' intensity of 40% maximum stimulator output (MSO), or individually adapted at 90% of the subject's MT. Stimulation was applied to a target region in the left posterior middle frontal gyrus (pMFG), as indicated by a functional magnetic resonance imaging (fMRI) localizer acquired beforehand, or to a control site (vertex). Results show that MT predicted the effect size after stimulating subjects with the fixed intensity (i.e., subjects with a low MT showed a greater behavioral effect). Nevertheless, the individual adaptation of intensities did not lead to stable effects. CONCLUSION: Therefore, we suggest assessing MT and account for it as a measure for general cortical TMS susceptibility, even if TMS is applied outside the motor domain

The cognitive neuroscience of prehension: recent developments

Prehension, the capacity to reach and grasp, is the key behavior that allows humans to change their environment. It continues to serve as a remarkable experimental test case for probing the cognitive architecture of goal-oriented action. This review focuses on recent experimental evidence that enhances or modifies how we might conceptualize the neural substrates of prehension. Emphasis is placed on studies that consider how precision grasps are selected and transformed into motor commands. Then, the mechanisms that extract action relevant information from vision and touch are considered. These include consideration of how parallel perceptual networks within parietal cortex, along with the ventral stream, are connected and share information to achieve common motor goals. On-line control of grasping action is discussed within a state estimation framework. The review ends with a consideration about how prehension fits within larger action repertoires that solve more complex goals and the possible cortical architectures needed to organize these actions

TMS stimulation over the inferior parietal cortex disrupts prospective sense of agency

Sense of agency refers to the feeling of controlling an external event through one's own action. On one influential view, sense of agency is inferred after an action, by "retrospectively" comparing actual effects of actions against their intended effects. However, it has been recently shown that earlier processes, linked to action selection, may also contribute to sense of agency, in advance of the action itself, and independently of action effects. The inferior parietal cortex (IPC) may underpin this "prospective" contribution to agency, by monitoring signals relating to fluency of action selection in dorsolateral prefrontal cortex (DLPFC). Here, we combined transcranial stimulation (TMS) with subliminal priming of action selection to investigate the causal role of these regions in the prospective coding of agency. In a first experiment, we showed that TMS over left IPC at the time of action selection disrupts perceived control over subsequent effects of action. In a second experiment, we exploited the temporal specificity of single-pulse TMS to pinpoint the exact timing of IPC contribution to sense of agency. We replicated the reduction in perceived control at the point of action selection, while observing no effect of TMS-induced disruption of IPC at the time of action outcomes

Excitability of human motor and visual cortex before, during and after hyperventilation

In humans, hyperventilation (HV) has various effects on systemic physiology and, in particular, on neuronal excitability and synaptic transmission. However, it is far from clear how the effects of HV are mediated at the cortical level. In this study we investigated the effects of HV-induced hypocapnia on primary motor (M1) and visual cortex (V1) excitability. We used 1) motor threshold (MT) and phosphene threshold (PT) and 2) stimulus-response (S-R) curves (i.e., recruitment curves) as measures of excitability. In the motor cortex, we additionally investigated 3) the intrinsic inhibitory and facilitatory neuronal circuits using a short-interval paired-pulse paradigm. Measurements were performed before, during, and after 10 min of HV (resulting in a minimum end-tidal Pco(2) of 15 Torr). HV significantly increased motor-evoked potential (MEP) amplitudes, particularly at lower transcranial magnetic stimulation (TMS) intensities. Paired-pulse stimulation indicated that HV decreases intracortical inhibition (ICI) without changing intracortical facilitation. The results suggestthat low Pco(2) levels modulate, in particular, the intrinsic neuronal circuits of ICI, which are largely mediated by neurons containing gamma-aminobutyric acid. Modulation of MT probably resulted from alterations of Na(+) channel conductances. A significant decrease of PT, together with higher intensity of phosphenes at low stimulus intensities, furthermore suggested that HV acts on the excitability of M1 and V1 in a comparable fashion. This finding implies that HV also affects other brain structures besides the corticospinal motor system. The further exploration of these physiological mechanisms may contribute to the understanding of the various HV-related clinical phenomenona

Task-dependent modulation of functional connectivity between hand motor cortices and neuronal networks underlying language and music: a transcranial magnetic stimulation study in humans

Although language functions are, in general, attributed to the left hemisphere, it is still a matter of debate to what extent the cognitive functions underlying the processing of music are lateralized in the human brain. To investigate hemispheric specialization we evaluated the effect of different overt musical and linguistic tasks on the excitability of both left and right hand motor cortices using transcranial magnetic stimulation (TMS). Task-dependent changes of the size of the TMS-elicited motor evoked potentials were recorded in 12 right-handed, musically naive subjects during and after overt speech, singing and humming, i.e. the production of melody without word articulation. The articulation of meaningless syllables served as control condition. We found reciprocal lateralized effects of overt speech and musical tasks on motor cortex excitability. During overt speech, the corticospinal projection of the left (i.e. dominant) hemisphere to the right hand was facilitated. In contrast, excitability of the right motor cortex increased during both overt singing and humming, whereas no effect was observed on the left hemisphere. Although the traditional concept of hemispheric lateralization of music has been challenged by recent neuroimaging studies, our findings demonstrate that right-hemisphere preponderance of music is nevertheless present. We discuss our results in terms of the recent concepts on evolution of language and gesture, which hypothesize that cerebral networks mediating hand movement and those subserving language processing are functionally linked. TMS may constitute a useful tool to further investigate the relationship between cortical representations of motor functions, music and language using comparative approaches

Transcranial magnetic stimulation and the challenge of coil placement: A comparison of conventional and stereotaxic neuronavigational strategies

The combination of transcranial magnetic stimulation (TMS) with functional neuroimaging has expanded the potential of TMS for human brain mapping. The precise and reliable positioning of the TMS coil is not a simple task, however. Modern frameless stereotaxic systems allow investigators to base navigation either on the subject's structural magnetic resonance imaging (MRI), functional MRI data, or the use of functional neuroimaging data from the literature, so-called "probabilistic approach." The latter assumes consistency across individuals in the location of task-related "activations" in standardized stereotaxic space. Conventional nonstereotaxic localization of brain areas is also a common method for defining the coil position. Our aim was to evaluate the accuracy of five different localization strategies in one single study. The left primary motor cortex (left M1-Hand) was used as target region. Three approaches were based on real-time frameless stereotaxy using information based on either anatomical or functional MRI. The remaining two strategies relied either on standard cranial landmarks (i.e., the International 10-20 EEG system) or a standardized function-guided procedure (i.e., the spatial relationship between the left and right M1-Hand). The results were compared to a TMS-based mapping of the primary motor cortex; center of gravity of motor-evoked potentials (MEP-CoG) was calculated for each subject (n = 10). Our findings suggest that highest precision can be achieved with fMRI-guided stimulation, which was accurate within the range of millimeters. Very consistent results were also obtained with the "probabilistic" approach. In view of these findings, we discuss the methods and special characteristics of each localization strategy

The role of the anterior intraparietal sulcus in crossmodal processing of object features in humans: An rTMS study

Investigations in macaques and humans have shown that the anterior intraparietal sulcus (IPS) has an important function in the integration of information from tactile and visual object manipulation. The goal of this study was to investigate the special functional role of the anterior IPS in visuo-tactile matching in humans. We used the "virtual-lesion" technique of repetitive transcranial magnetic stimulation (rTMS) to test the functional relevance of anterior IPS for visuo-tactile crossmodal matching. Two crossmodal (visual encoding and tactile recognition and vice versa) and two unimodal delayed matching-to-sample tests with geometrical patterns were performed by 12 healthy subjects. We determined error rates before and after focal low-frequency rTMS applied over the left anterior IPS, right anterior IPS and vertex. During the manipulation of objects with the right hand, rTMS over the left anterior IPS induced a significant deterioration for visual encoding and tactile recognition, but not for tactile encoding and visual recognition. For the visual and tactile unimodal conditions, no significant alterations in task performance were found. rTMS application over right IPS when manipulating objects with the left hand did not affect crossmodal task performance. In conclusion, we have demonstrated an essential functional role of the left anterior IPS for visuo-tactile matching when manipulating objects with the right hand. However, we found no clear evidence for left IPS involvement in tactile encoding and visual recognition. The differential effect of rTMS on tactile and visual encoding and recognition are not consistently explained by previous concepts of visuo-tactile integration