Brain at work : time, sparseness and superposition principles

Abstract : Many studies explored mechanisms through which the brain encodes sensory inputs allowing a coherent behavior. The brain could identify stimuli via a hierarchical stream of activity leading to a cardinal neuron responsive to one particular object. The opportunity to record from numerous neurons offered investigators the capability of examining simultaneously the functioning of many cells. These approaches suggested encoding processes that are parallel rather than serial. Binding the many features of a stimulus may be accomplished through an induced synchronization of cell’s action potentials. These interpretations are supported by experimental data and offer many advantages but also several shortcomings. We argue for a coding mechanism based on a sparse synchronization paradigm. We show that synchronization of spikes is a fast and efficient mode to encode the representation of objects based on feature bindings. We introduce the view that sparse synchronization coding presents an interesting venue in probing brain encoding mechanisms as it allows the functional establishment of multilayered and time-conditioned neuronal networks or multislice networks. We propose a model based on integrate-and-fire spiking neurons

Adaptation-induced plasticity and spike-waveforms in cat visual cortex

Abstract : Orientation-selective neurons shift their preferred orientation after being adapted to a nonpreferred orientation. These shifts of the peaks of tuning curves may be in the attractive or repulsive direction in relation to the adapter orientation. In anaesthetized cats we recorded evoked electrical responses from the visual cortex in a conventional fashion. The recorded spikes in cortex may present two typical waveforms: regular spikes or fast spikes. However, there is no evidence whether the shapes of spikes are related to the attractive or repulsive shifts of orientation tuning curves of cells. Our results show that after adaptation the recorded cells with both attractive and repulsive shifts display one or the other shape of spike. However, the magnitude of shifts is systematically higher for regular spikes, which is attributed to putative pyramidal cells, while tuning curves for fast spikes have smaller magnitudes and are evoked by putative interneurons

Performance of three acoustical methods for localizing whales in the Saguenay - St. Lawrence Marine Park

Abstract : Three algorithms are explored to localize fin whale calls recorded from a large-aperture hydrophone array deployed in the Saguenay—St. Lawrence Marine Park. The methods have to cope with varying sound speed in space and time, errors in time differences of arrival (TDoA) measurements in a noisy environment, and often a limited number of hydrophones having recorded a particular event. The array was composed of 5 AURAL autonomous hydrophones with a total aperture of about 40 km, coupled with 2 hydrophones from a small-aperture cabled coastal array. The autonomous hydrophones clock drifts were estimated with a level of uncertainty from timed sources and the coastal array time reference. The calls were then localized by constant-speed hyperbolic fixing, variable-speed isodiachron Monte-Carlo simulations, and a ray-tracing propagation model. The Monte-Carlo simulations generate clouds of possible localizations from the uncertainty in hydrophone positions, TDoAs and the effective horizontal sound speeds along the different source-hydrophone paths. The ray-tracing model produces a fixed grid of TDoAs which can then be consulted to find the likeliest positions of the whales. Results from the different methods are compared and their relative advantages or limitations are discussed.Trois algorithmes sont explorés pour la localisation de vocalises de rorqual commun enregistrées par un réseau d'hydrophones à large ouverture déployé dans le Parc Marin du Saguenay-Saint-Laurent. Les méthodes doivent composer avec une vitesse du son variable dans l'espace et le temps, des erreurs dans les mesures des différences de temps d'arrivée (DTA) avec un environnement bruyant, et souvent un nombre limité d'hydrophones ayant capté un événement donné. Le réseau était composé de 5 hydrophones autonomes AURAL avec une ouverture totale d'environ 40 km, couplé avec 2 hydrophones d’un petit réseau côtier. La dérive des horloges des hydrophones autonomes a été évaluée avec un niveau d’incertitude à l'aide de sources aux temps connus ainsi que de la référence temporelle du réseau côtier. Les vocalises ont ensuite été localisées par la méthode à vitesse constante des hyperboles, par celle à vitesse variable des isodiachrones avec simulations de Monte-Carlo, et par un modèle de propagation de rayons. Les simulations de Monte-Carlo produisent des nuages de localisations possibles à partir des incertitudes sur les positions des hydrophones, sur les DTAs et sur les vitesses horizontales effectives du son le long des différentes trajectoires source-hydrophone. Le modèle de propagation des rayons produit une grille fixe de DTAs qui est ensuite consultée pour trouver les positions les plus probables des baleines. Les résultats des différentes méthodes sont comparés et leurs avantages ou limites relatives sont discutés