Score-based diffusion models are a class of generative models whose dynamics is described by stochastic differential equations that map noise into data. While recent works have started to lay down a ...theoretical foundation for these models, a detailed understanding of the role of the diffusion time
is still lacking. Current best practice advocates for a large
to ensure that the forward dynamics brings the diffusion sufficiently close to a known and simple noise distribution; however, a smaller value of
should be preferred for a better approximation of the score-matching objective and higher computational efficiency. Starting from a variational interpretation of diffusion models, in this work we quantify this trade-off and suggest a new method to improve quality and efficiency of both training and sampling, by adopting smaller diffusion times. Indeed, we show how an auxiliary model can be used to bridge the gap between the ideal and the simulated forward dynamics, followed by a standard reverse diffusion process. Empirical results support our analysis; for image data, our method is competitive with regard to the state of the art, according to standard sample quality metrics and log-likelihood.
Predicting the performance of various infrastructure design options in complex federated infrastructures with computing sites distributed over a wide area network that support a plethora of users and ...workflows, such as the Worldwide LHC Computing Grid (WLCG), is not trivial. Due to the complexity and size of these infrastructures, it is not feasible to deploy experimental test-beds at large scales merely for the purpose of comparing and evaluating alternate designs. An alternative is to study the behaviours of these systems using simulation. This approach has been used successfully in the past to identify efficient and practical infrastructure designs for High Energy Physics (HEP). A prominent example is the Monarc simulation framework, which was used to study the initial structure of the WLCG. New simulation capabilities are needed to simulate large-scale heterogeneous computing systems with complex networks, data access and caching patterns. A modern tool to simulate HEP workloads that execute on distributed computing infrastructures based on the SimGrid and WRENCH simulation frameworks is outlined. Studies of its accuracy and scalability are presented using HEP as a case-study. Hypothetical adjustments to prevailing computing architectures in HEP are studied providing insights into the dynamics of a part of the WLCG and candidates for improvements.
Differential diagnoses between vegetative and minimally conscious states (VS and MCS, respectively) are frequently incorrect. Hence, further research is necessary to improve the diagnostic accuracy ...at the bedside. The main neuropathological feature of VS is the diffuse damage of cortical and subcortical connections. Starting with this premise, we used electroencephalography (EEG) recordings to evaluate the cortical reactivity and effective connectivity during transcranial magnetic stimulation (TMS) in chronic VS or MCS patients. Moreover, the TMS-EEG data were compared with the results from standard somatosensory-evoked potentials (SEPs) and event-related potentials (ERPs). Thirteen patients with chronic consciousness disorders were examined at their bedsides. A group of healthy volunteers served as the control group. The amplitudes (reactivity) and scalp distributions (connectivity) of the cortical potentials evoked by TMS (TEPs) of the primary motor cortex were measured. Short-latency median nerve SEPs and auditory ERPs were also recorded. Reproducible TEPs were present in all control subjects in both the ipsilateral and the contralateral hemispheres relative to the site of the TMS. The amplitudes of the ipsilateral and contralateral TEPs were reduced in four of the five MCS patients, and the TEPs were bilaterally absent in one MCS patient. Among the VS patients, five did not manifest ipsilateral or contralateral TEPs, and three of the patients exhibited only ipsilateral TEPs with reduced amplitudes. The SEPs were altered in five VS and two MCS patients but did not correlate with the clinical diagnosis. The ERPs were impaired in all patients and did not correlate with the clinical diagnosis. These TEP results suggest that cortical reactivity and connectivity are severely impaired in all VS patients, whereas in most MCS patients, the TEPs are preserved but with abnormal features. Therefore, TEPs may add valuable information to the current clinical and neurophysiological assessment of chronic consciousness disorders.
Transcranial direct current stimulation (tDCS), a non-invasive brain stimulation technique able to transiently modulate brain activity, is surging as one of the most promising therapeutic solutions ...in many neurological and psychiatric disorders. However, profound limitations exist in current placebo (sham) protocols that limit single- and double-blinding, especially in non-naïve subjects.
To ensure better blinding and strengthen reliability of tDCS studies and trials, we tested a new optimization algorithm aimed at creating an “active” sham tDCS condition (ActiSham hereafter) capable of inducing the same scalp sensations perceived during real stimulation while preventing currents from reaching the cortex and cause changes in brain excitability.
A novel model-based multielectrode technique — optimizing the location and currents of a set of small electrodes placed on the scalp — was used to control the relative amount of current delivered transcranially in real and placebo multichannel tDCS conditions. The presence, intensity and localization of scalp sensations during tDCS was evaluated by means of a specifically designed questionnaire administered to the participants. We compared blinding ratings by directly addressing subjects’ ability to discriminate across conditions for both traditional (Bifocal-tDCS and Sham, using sponge electrodes) and our novel multifocal approach (both real Multifocal-tDCS and ActiSham). Changes in corticospinal excitability were monitored based on Motor Evoked Potentials (MEPs) recorded via concurrent Transcranial Magnetic Stimulation (TMS) and electromyography (EMG).
Participants perceived Multifocal-tDCS and ActiSham similarly in terms of both localization and intensity of scalp sensations, whereas traditional Bifocal stimulation was rated as more painful and annoying compared to its Sham counterpart. Additionally, differences in scalp localization were reported for active/sham Bifocal-tDCS, with Sham tDCS inducing more widespread itching and burning sensations. As for MEPs amplitude, a main effect of stimulation was found when comparing Bifocal-Sham and ActiSham (F(1,13) = 6.67, p = .023), with higher MEPs amplitudes after the application of Bifocal-Sham.
Compared to traditional Bifocal-tDCS, ActiSham offers better participants’ blinding by inducing very similar scalp sensations to those of real Multifocal tDCS both in terms of intensity and localization, while not affecting corticospinal excitability.
•Canonical On/Off solutions for Sham tDCS do not allow proper blinding.•We tested an “Active-Sham” multi-electrode stimulation based on controlled current shunting.•Comparable scalp sensations were reported for Active-Sham and real Multifocal-tDCS.•A significant difference was observed between Bifocal Sham and Active-Sham conditions.•Active-Sham could improve blinding for both research and clinical trials.
We applied transcranial alternating current stimulation (tACS) to the primary motor cortex (M1) at different frequencies during an index-thumb pinch-grip observation task. To estimate changes in the ...corticospinal output, we used the size of motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS) of M1 using an online MRI-guided simultaneous TMS-tACS approach. The results of the beta-tACS confirm a non-selective increase in corticospinal excitability in subjects at rest; an increase was observed for both of the tested hand muscles, the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM). However, during action observation of the pinch-grip movement, the increase of corticospinal excitability was only observed for the prime mover FDI muscle and took place during alpha-tACS, while gamma-tACS affected both the FDI and control muscle (ADM) responses. These phenomena likely reflect the hypothesis that the mu and gamma rhythms specifically index the downstream modulation of primary sensorimotor areas by engaging mirror neuron activity. The current neuromodulation approach confirms that tACS can be used to induce neurophysiologically detectable state-dependent enhancement effects, even in complex motor-cognitive tasks.
•The field of NIBS is expanding and adequate training for all NIBS practitioners is needed.•Training should be matched to the responsibilities of Technicians, Clinicians, and Scientists.•We define ...competencies and propose curricula for TMS and tES organized in Core knowledge, Safety/ethics, Basic Skills, and Advanced Skills.
As the field of noninvasive brain stimulation (NIBS) expands, there is a growing need for comprehensive guidelines on training practitioners in the safe and effective administration of NIBS techniques in their various research and clinical applications. This article provides recommendations on the structure and content of this training. Three different types of practitioners are considered (Technicians, Clinicians, and Scientists), to attempt to cover the range of education and responsibilities of practitioners in NIBS from the laboratory to the clinic. Basic or core competencies and more advanced knowledge and skills are discussed, and recommendations offered regarding didactic and practical curricular components. We encourage individual licensing and governing bodies to implement these guidelines.
Cross‐sectional data suggest videogaming as promoting modifications in perceptual and cognitive skills of players, as well as inducing structural brain changes. However, whether such changes are both ...possible after a systematic gaming exposure, and last beyond the training period, is not known. Here, we originally quantified immediate and long‐lasting cognitive and morphometric impact of a systematic gaming experience on a first‐person shooter (FPS) game. Thirty‐five healthy participants, assigned to a videogaming and a control group, underwent a cognitive assessment and structural magnetic resonance imaging at baseline (T0), immediately post‐gaming (T1) and after 3 months (T2). Enhancements of cognitive performance were found on perceptual and attentional measures at both T1 and T2. Morphometric analysis revealed immediate structural changes involving bilateral medial and posterior thalamic nuclei, as well as bilateral superior temporal gyrus, right precentral gyrus, and left middle occipital gyrus. Notably, significant changes in pulvinar volume were still present at T2, while a voxel‐wise regression analysis also linked baseline pulvinar volume and individual changes in gaming performance. Present findings extend over the notion that videogame playing might impact cognitive and brain functioning in a beneficial way, originally showing long‐term brain structural changes even months after gaming practice. The involvement of posterior thalamic structures highlights a potential link between FPS games and thalamo‐cortical networks related to attention mechanisms and multisensory integration processing.
We investigated the impact of systematic, repeated gaming exposure to a first‐person shooter (FPS) action videogame in a sample of healthy subjects. Both cognitive and morphometric changes were found after training, including a significant long‐lasting change in posterior thalamus gray matter 3 months after exposure. The same thalamic region was identified as the best predictor of changes in gaming performance. Intensive practice with a FPS action videogame induces long‐lasting structural brain changes.
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