324 research outputs found
Benefit sharing and community participation dynamics in forest management:a case study from Kankali community forestry, Nepal
A Normalization Model of Attentional Modulation of Single Unit Responses
Although many studies have shown that attention to a stimulus can enhance the responses of individual cortical sensory neurons, little is known about how attention accomplishes this change in response. Here, we propose that attention-based changes in neuronal responses depend on the same response normalization mechanism that adjusts sensory responses whenever multiple stimuli are present. We have implemented a model of attention that assumes that attention works only through this normalization mechanism, and show that it can replicate key effects of attention. The model successfully explains how attention changes the gain of responses to individual stimuli and also why modulation by attention is more robust and not a simple gain change when multiple stimuli are present inside a neuron's receptive field. Additionally, the model accounts well for physiological data that measure separately attentional modulation and sensory normalization of the responses of individual neurons in area MT in visual cortex. The proposal that attention works through a normalization mechanism sheds new light a broad range of observations on how attention alters the representation of sensory information in cerebral cortex
Exact diagonalization study of the Hubbard-parametrized four-spin ring exchange model on a square lattice
We have used exact numerical diagonalization to study the excitation spectrum
and the dynamic spin correlations in the next-next-nearest neighbor
Heisenberg antiferromagnet on the square lattice, with additional 4-spin ring
exchange from higher order terms in the Hubbard expansion. We have varied the
ratio between Hubbard model parameters, , to obtain different relative
strengths of the exchange parameters, while keeping electrons localized. The
Hubbard model parameters have been parametrized via an effective ring exchange
coupling, , which have been varied between 0 and 1.5. We find that
ring exchange induces a quantum phase transition from the ordered
Ne\`el state to a ordered state. This quantum critical point
is reduced by quantum fluctuations from its mean field value of to
a value of . At the quantum critical point, the dynamical correlation
function shows a pseudo-continuum at -values between the two competing
ordering vectors
Biased competition through variations in amplitude of γ-oscillations
Experiments in visual cortex have shown that the firing rate of a neuron in response to the simultaneous presentation of a preferred and non-preferred stimulus within the receptive field is intermediate between that for the two stimuli alone (stimulus competition). Attention directed to one of the stimuli drives the response towards the response induced by the attended stimulus alone (selective attention). This study shows that a simple feedforward model with fixed synaptic conductance values can reproduce these two phenomena using synchronization in the gamma-frequency range to increase the effective synaptic gain for the responses to the attended stimulus. The performance of the model is robust to changes in the parameter values. The model predicts that the phase locking between presynaptic input and output spikes increases with attention
Modelling fast forms of visual neural plasticity using a modified second-order motion energy model
The Adelson-Bergen motion energy sensor is well established as the leading model of low-level visual motion sensing in human vision. However, the standard model cannot predict adaptation effects in motion perception. A previous paper Pavan et al.(Journal of Vision 10:1-17, 2013) presented an extension to the model which uses a first-order RC gain-control circuit (leaky integrator) to implement adaptation effects which can span many seconds, and showed that the extended model's output is consistent with psychophysical data on the classic motion after-effect. Recent psychophysical research has reported adaptation over much shorter time periods, spanning just a few hundred milliseconds. The present paper further extends the sensor model to implement rapid adaptation, by adding a second-order RC circuit which causes the sensor to require a finite amount of time to react to a sudden change in stimulation. The output of the new sensor accounts accurately for psychophysical data on rapid forms of facilitation (rapid visual motion priming, rVMP) and suppression (rapid motion after-effect, rMAE). Changes in natural scene content occur over multiple time scales, and multi-stage leaky integrators of the kind proposed here offer a computational scheme for modelling adaptation over multiple time scales. © 2014 Springer Science+Business Media New York
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