Mechanistic role of alpha oscillations in a computational model of working memory.

Brain oscillations are believed to be involved in the different operations necessary to manipulate information during working memory tasks. We propose a mechanistic role for the observed inhibition effect of the alpha rhythm based on its interference with the theta rhythm. Using the Lisman-Idiart model for multi-item working memory, we show that the interaction between these two oscillations is capable of creating a long lasting destructive interference that prevents the cyclic reactivation of neuronal ensembles and, as a consequence, memory maintenance. Additionally, to ensure robustness we propose a modular version of the model and implement oscillations as traveling waves. Using this model, we show that the interactions between theta and gamma determine the allocation of multiple memories in distinct modules, while the interference between theta and alpha disrupts the maintenance of the information already stored in them. The effect of alpha in erasing or blocking storage is robust and seems fairly independent of frequency, as long as it stays within the alpha range. This model helps us to understand why the alpha and theta oscillations, which have close frequency bands, could have opposite roles in working memory.

How the Brain Represents Language and Answers Questions? Using an AI System to Understand the Underlying Neurobiological Mechanisms.

To understand the computations that underlie high-level cognitive processes we propose a framework of mechanisms that could in principle implement START, an AI program that answers questions using natural language. START organizes a sentence into a series of triplets, each containing three elements (subject, verb, object). We propose that the brain similarly defines triplets and then chunks the three elements into a spatial pattern. A complete sentence can be represented using up to 7 triplets in a working memory buffer organized by theta and gamma oscillations. This buffer can transfer information into long-term memory networks where a second chunking operation converts the serial triplets into a single spatial pattern in a network, with each triplet (with corresponding elements) represented in specialized subregions. The triplets that define a sentence become synaptically linked, thereby encoding the sentence in synaptic weights. When a question is posed, there is a search for the closest stored memory (having the greatest number of shared triplets). We have devised a search process that does not require that the question and the stored memory have the same number of triplets or have triplets in the same order. Once the most similar memory is recalled and undergoes 2-level dechunking, the sought for information can be obtained by element-by-element comparison of the key triplet in the question to the corresponding triplet in the retrieved memory. This search may require a reordering to align corresponding triplets, the use of pointers that link different triplets, or the use of semantic memory. Our framework uses 12 network processes; existing models can implement many of these, but in other cases we can only suggest neural implementations. Overall, our scheme provides the first view of how language-based question answering could be implemented by the brain.

Graph analysis of semantic word association among children, adults, and the elderly / Análise de grafos de associação semântica de palavras entre crianças, adultos e idosos

This study used graph analysis to investigate how age differences modify the structure of semantic word association networks of children and adults and if the networks present a small-world structure and a scale-free distribution which are typical of natural languages. Three age groups of Brazilian Portuguese speakers (children, adults and elderly people) participated in the experiment. Quantitative and qualitative measures suggested that adults and elderly speakers have similar network structures. Children's network showed fewer nodes, connections and clusters, and longer inter-node distances. All networks presented a small-world structure, but they did not show entirely scale-free distributions. These results suggest that from childhood to adulthood, there is an increase not only in the number of words semantically linked to a target but also an increase in the connectivity of the network.

Este estudo utilizou análise de grafos para investigar como a idade modifica a estrutura das redes de associações semânticas de palavras de crianças e adultos e se estas redes apresentam estrutura small-world e distribuição scale-free típicas de linguagens naturais. Participaram dos experimentos três grupos etários (crianças, adultos e idosos) falantes do Português Brasileiro. Medidas quantitativas e qualitativas sugeriram que adultos e idosos possuem redes semelhantes quanto a número de nós, conexões e agrupamentos. A rede das crianças expressou menor número de nós, conexões e agrupamentos e maiores distancias inter-nós. As redes apresentaram uma estrutura small-world, mas não uma distribuição scale-free completa. Os resultados sugerem que além do número de palavras semanticamente associadas aos alvos aumentar das crianças para os adultos, as redes se tornaram mais conectadas.

Graph analysis of semantic word association among children, adults, and the elderly / Análise de grafos de associação semântica de palavras entre crianças, adultos e idosos

This study used graph analysis to investigate how age differences modify the structure of semantic word association networks of children and adults and if the networks present a small-world structure and a scale-free distribution which are typical of natural languages. Three age groups of Brazilian Portuguese speakers (children, adults and elderly people) participated in the experiment. Quantitative and qualitative measures suggested that adults and elderly speakers have similar network structures. Children's network showed fewer nodes, connections and clusters, and longer inter-node distances. All networks presented a small-world structure, but they did not show entirely scale-free distributions. These results suggest that from childhood to adulthood, there is an increase not only in the number of words semantically linked to a target but also an increase in the connectivity of the network.(AU)

Este estudo utilizou análise de grafos para investigar como a idade modifica a estrutura das redes de associações semânticas de palavras de crianças e adultos e se estas redes apresentam estrutura small-world e distribuição scale-free típicas de linguagens naturais. Participaram dos experimentos três grupos etários (crianças, adultos e idosos) falantes do Português Brasileiro. Medidas quantitativas e qualitativas sugeriram que adultos e idosos possuem redes semelhantes quanto a número de nós, conexões e agrupamentos. A rede das crianças expressou menor número de nós, conexões e agrupamentos e maiores distancias inter-nós. As redes apresentaram uma estrutura small-world, mas não uma distribuição scale-free completa. Os resultados sugerem que além do número de palavras semanticamente associadas aos alvos aumentar das crianças para os adultos, as redes se tornaram mais conectadas.(AU)

Alternating predictive and short-term memory modes of entorhinal grid cells.

Several lines of evidence indicate that the entorhinal cortex has memory functions, but such functions have not been previously found in grid cells, a cell type that provides major input to the hippocampus. We examined the firing of grid cells as rats crossed (runs) through grid cell vertices. We found that on some runs, firing tended to occur mostly inbound as the rat approached a vertex center while on other runs firing occurred mainly outbound. These results suggest that cells have a predictive mode (inbound firing) in which they represent a position ahead of the animal and a short term memory (STM) mode (outbound firing) in which they represent positions just passed through. Analysis of cell pairs showed that when vertex crossings were less than 1 second apart, the two cells tended to have the same mode. This indicates that modes are a network property. The tendency to have the same mode disappeared if crossings were separated by 2-3 seconds, suggesting that modes alternate on the time scale of seconds. There was a small but statistically significant behavioral correlate of modes: velocity was slightly less in the STM mode. Both modes were organized by theta and gamma oscillations. The results suggest that the dual requirement for hippocampal storage and recall is met by rapidly alternating modes appropriate for predicting the future and storing the recent past.

A especificidade semântica como fator determinante na aquisição de verbos / The semantic specificity as a determining factor in the acquisition of verbs / La especificidad semántica como factor determinante en la adquisición de verbos

Miller e Fellbaum (1991) postulam que verbos genéricos são mais básicos e de ocorrência mais precoce do que específicos. O presente estudo verificou a especificidade como fator determinante na aquisição de verbos em crianças brasileiras. O corpus de 146 respostas de crianças entre 2:0 e 5:11 anos e 75 adultos universitários, emitidas para cada um dos 17 filmetes da Tarefa de Nomeação de Ações, foi analisado quanto à validade e especificidade dos verbos. O número de respostas válidas e de verbos específicos aumentou significativamente com a idade, confirmando que idade e gênero são fatores preditores do uso de verbos específicos. Os resultados sugerem que o desenvolvimento lexical das crianças brasileiras está associado a sua capacidade de categorização, já que utilizam inicialmente verbos genéricos para diferentes situações. Com a experiência e exposição ao insumo lingüístico, adquirem características semânticas mais complexas que possibilitam o uso de mais verbos específicos.(AU)

Miller and Fellbaum (1991) propose that generic verbs are more basic and of earlier occurrence than specific ones. This study verified the specificity as a determining factor in the acquisition of verbs in Brazilian children. The corpus of 146 answers of 2:0 to 5:11 year-old children and 75 undergraduate students, emitted to each of the 17 short films which compose the Actions Naming Task, was analyzed regarding the verbs validity and specificity. The number of valid answers and specific verbs increased significantly with age, confirming that age and gender are predicting factors of the use of specific verbs. The results suggest that the lexical development of Brazilian children is related to their categorization capacity, as they use firstly generic verbs for different situations. With experience and exposition to the linguistic input, they acquire more complex semantic characteristics which lead the usage of more specific verbs.(AU)