1,721,002 research outputs found
Action potential processing in a detailed Purkinje cell model reveals a critical role for axonal compartmentalization
No full textThe Purkinje cell (PC) is among the most complex neurons in the brain and plays a critical role for cerebellar functioning. PCs operate as fast pacemakers modulated by synaptic inputs but can switch from simple spikes to complex bursts and, in some conditions, show bistability. In contrast to original works emphasizing dendritic Ca-dependent mechanisms, recent experiments have supported a primary role for axonal Na-dependent processing, which could effectively regulate spike generation and transmission to deep cerebellar nuclei (DCN). In order to account for the numerous ionic mechanisms involved (at present including Nav1.6, Cav2.1, Cav3.1, Cav3.2, Cav3.3, Kv1.1, Kv1.5, Kv3.3, Kv3.4, Kv4.3, KCa1.1, KCa2.2, KCa3.1, Kir2.x, HCN1), we have elaborated a multicompartmental model incorporating available knowledge on localization and gating of PC ionic channels. The axon, including initial segment (AIS) and Ranvier nodes (RNs), proved critical to obtain appropriate pacemaking and firing frequency modulation. Simple spikes initiated in the AIS and protracted discharges were stabilized in the soma through Na-dependent mechanisms, while somato-dendritic Ca channels contributed to sustain pacemaking and to generate complex bursting at high discharge regimes. Bistability occurred only following Na and Ca channel down-regulation. In addition, specific properties in RNs K currents were required to limit spike transmission frequency along the axon. The model showed how organized electroresponsive functions could emerge from the molecular complexity of PCs and showed that the axon is fundamental to complement ionic channel compartmentalization enabling action potential processing and transmission of specific spike patterns to DCN
A realistic large-scale model of the cerebellum granular layer predicts circuit spatio-temporal filtering properties
No full textThe way the cerebellar granular layer transforms incoming mossy fiber signals into new spike patterns to be related to Purkinje cells is not yet clear. Here, a realistic computational model of the granular layer was developed and used to address four main functional hypotheses: center-surround organization, time-windowing, high-pass filtering in responses to spike bursts and coherent oscillations in response to diffuse random activity. The model network was activated using patterns inspired by those recorded in vivo. Burst stimulation of a small mossy fiber bundle resulted in granule cell bursts delimited in time (time windowing) and space (center-surround) by network inhibition. This burst-burst transmission showed marked frequency-dependence configuring a high-pass filter with cut-off frequency around 100 Hz. The contrast between center and surround properties was regulated by the excitatory-inhibitory balance. The stronger excitation made the center more responsive to 10-50 Hz input frequencies and enhanced the granule cell output (with spikes occurring earlier and with higher frequency and number) compared to the surround. Finally, over a certain level of mossy fiber background activity, the circuit generated coherent oscillations in the theta-frequency band. All these processes were fine-tuned by NMDA and GABA-A receptor activation and neurotransmitter vesicle cycling in the cerebellar glomeruli. This model shows that available knowledge on cellular mechanisms is sufficient to unify the main functional hypotheses on the cerebellum granular layer and suggests that this network can behave as an adaptable spatio-temporal filter coordinated by theta-frequency oscillation
Ensemble neuronal responses in a large-scale realistic model of the cerebellar cortex
No full textRealistic simulation of central networks remains a challenge due to the complexity of internal connectivity and cellular mechanisms involved. We have recently built a realistic model of the cerebellar granular layer..
High-Voltage-Activated Calcium Channels
No full textThe annual Computational Neuroscience Meeting (CNS) began in 1990 as a small workshop called Analysis and Modeling of Neural Systems. The goal of the workshop was to explore the boundary between neuroscience and computation. Riding on the success of several seminal papers, physicists had made "Neural Networks" fashionable, and soon the quantitative methods used in these abstract model networks started permeating the methods and ideas of experimental neuroscientists. Although experimental neurophysiological approaches provided many advances, it became increasingly evident that mathematical and computational techniques would be required to achieve a comprehensive and quantitative understanding of neural system function. “Computational Neuroscience” emerged to complement experimental neurophysiology. In 2002, the non-profit organization, Organization for Computational Neuroscience (OCNS) was formed. OCNS has now become the first professional society serving the global computational neuroscience community. OCNS as a society lives at the interface where experimental neuroscience meets theoretical, statistical and computer-simulation analyses, with the hope of turning large collections of experimental results into a principled understanding of nervous systems. It also supports the development of new engineering, computational and informatics techniques for data collection, analyses and management
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
Symbolic and non-symbolic numerical representations in old and young subjects. An exploratory EEG study
No full textNumeracy has significant implications throughout the lifecourse. Its age-induced
decline represents a vulnerability factor in elderly populations when it comes to solving everyday
tasks. However, the reasons behind age-induced decline in numeracy are still unclear. In this
study, we used a parity judgment task to study variations in number cognition between old and
young participants. Since numeracy is associated with both symbolic and non-symbolic
representations, three numerical formats were used: arabic digits (N), finger representations (F),
and dots (D). Fifty-three healthy subjects joined the study: 31 young (24.1±4.73 y.o.) and 22 old
(68.8±4.38 y.o.). EEG (64 channels) was recorded during the task. Mixed Effects Models
analysis showed that performance accuracy for D was significantly lower than for N and F (p =
.02). There were significant main effects for reaction time for Group (old, young), Condition (N,
F, D) and Gender. Notably, old adults responded slower than young adults (p < .001) and both
groups performed significantly faster in the task for N compared to F and D (p < .001).
Significant interaction effects showed that performance between males and females were
comparable for arabic digits (p = .14), with females showing significantly slower response for
non-symbolic stimuli than males (F, p = 0.04; D, p = .007). In sensor space, multivariate pattern
analysis shows that the neural correlates supporting the different number representations can be
classified as categorically separate, with an early dissociation (~200 ms) between N, F, and D
stimuli when compared to each other. This provides provisional evidence that symbolic and non-
symbolic representations recruit functionally distinct neuronal processes. Subjects in the old
group showed a slight reduction in decoding and an increase in its latency. Numerical cognition
involves a highly-integrated network, with a key role of the intraparietal sulcus and fusiform
gyrus. In source space, we found reduced activation levels and increased latency of response in
the primary cortical areas involved in numerical cognition in the old group. In the same group,
all stimuli also induced a wider cortical response with respect to young subjects. We hypothesize
that the reduced activity of the main cortical areas may contribute to the age-induced decay in
numeracy and that the recruitment of other cortical areas may serve as a compensatory
mechanism. The results of this exploratory study provide further evidence for the multifaceted
processes involved in numerical reasoning and its age-related decline, with a potentially distinct
neural network for symbolic and non-symbolic numerical representations
- …
