1,721,546 research outputs found
Come funziona la memoria : le basi neurali della capacità di ricordare
No full textCome funziona la memoria? Come riusciamo a trattenere, dal flusso di sensazioni, eventi ed esperienze in cui siamo immersi, quelle informazioni e quelle capacità che vanno a formare il nostro corredo individuale? In che cosa consistono le tracce mnestiche? Per andare oltre al tipo di risposte che può dare la semplice introspezione, o anche l'analisi scientifica ma puramente psicologica, bisogna entrare dentro il cervello umano medio, e capire i meccanismi che permettono di depositarvi memorie
Localized activity profiles and storage capacity of rate-based autoassociative networks
No full textWe study analytically the effect of metrically structured connectivity on the behavior of autoassociative networks. We focus on three simple rate-based model neurons: threshold-linear, binary or smoothly saturating units. For a connectivity which is short range enough the threshold-linear network shows localized retrieval states. The saturating and binary models also exhibit spatially modulated retrieval states if the highest activity level that they can achieve is above the maximum activity of the units in the stored patterns. In the zero quenched noise limit, we derive an analytical formula for the critical value of the connectivity width below which one observes spatially non-uniform retrieval states. Localization reduces storage capacity, but only by a factor of 2~3. The approach that we present here is generic in the sense that there are no specific assumptions on the single unit input-output function nor on the exact connectivity structure
Characterization of the variability of glutamatergic synaptic responses to presynaptic trains in rat hippocampal pyramidal neurons
No full textExcitatory postsynaptic currents from CA3 hippocampal neurons, elicited by trains of presynaptic action potentials either in mossy fibres or associative commissural fibres, have been analysed, by using a quantal analysis approach, in order to characterize their variability and the correlation among successive responses. As quantal parameters may change during the train according to the previous release events, correlation within consecutive EPSCs is expected. We tested simple hypotheses on how quantal parameters p and N may change on the basis of correlation detection in EPSCs. The statistical significance of these tests has been evaluated. The tests showed that, although simple binomial distributions can give a good description of synaptic responses at the level of single spikes, only stochastic chains can always account for correlations observed within the train. A systematic model fitting procedure has been developed and applied to extract information on the dynamics of synaptic transmission. As an application of this novel type of analysis, a measure of transmitted information to be associated with synaptic variability, a quantity that allows an estimate of the capability of the synapse to transmit reliable information in time, is proposed. We showed that this transmitted information depends on short-term plasticity and that the change in the type of short-term plasticity from facilitating to depressing obtained by increasing the extracellular calcium concentration results in a change of the related transmitted information
Computational constraints on compositional interpretation: Refocusing the debate on language universals
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Semantic Cognition: Distributed, but then Attractive
No full textThe parallel distributed processing (PDP) perspective brings forward the important point that all semantic phenomena are based on analog underlying mechanisms, involving the weighted summation of multiple inputs by individual neurons. It falls short of indicating, however, how the essentially discrete nature of semantic processing may emerge at the cognitive level. Bridging this gap probably requires attractor networks
Some properties and problems of accretion disks about Kerr naked singularities
Full text linkDisk accretion onto a Kerr naked singularity is investigated by examining the stable circular equatorial orbits of a test particle in a Kerr background. It is assumed that there is a mechanism (e.g., viscous stress) at large radii that disperses energy and transfers angular momentum outward and mass inward, while maintaining the accretion flow in quasi-geodesic circular orbits. The results obtained show that: (1) the radius and energy of the last stable orbit increase as the ratio of the angular-momentum density and mass (a/M) of the naked singularity increases; (2) for a/M less than the square root of (32/27) there is a region where the energy becomes negative, suggesting that more energy than the mass-energy of a test particle can be extracted; (3) pathologies related to the definition of positive and negative energy states exist for a/M between unity and the square root of (32/27); and (4) the entire mass-energy of a test particle can be extracted for a/M equal to the square root of (32/27)
The Upward Bias in Measures of Information Derived from Limited Data Samples
No full textExtracting information measures from limited experimental samples, such as those normally available when using data recorded in vivo from mammalian cortical neurons, is known to be plagued by a systematic error, which tends to bias the estimate upward. We calculate here the average of the bias, under certain conditions, as an asymptotic expansion in the inverse of the size of the data sample. The result agrees with numerical simulations, and is applicable, as an additive correction term, to measurements obtained under such conditions. Moreover, we discuss the implications for measurements obtained through other usual procedures
Disappearance of spurious states in analog associative memories
No full textWe show that symmetric n-mixture states, when they exist, are almost never stable in autoassociative networks with threshold-linear units. Only with a binary coding scheme we could find a limited region of the parameter space in which either 2-mixtures or 3-mixtures are stable attractors of the dynamics
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