Abstract not found

temporal relations requires the joint evaluation of responses from more than one neuron, only experiments that permit simultaneous measurements of responses 60528 Frankfurt from multiple units are considered. These include multi-Federal Republic of Germany electrode recordings from multiple individual cells, but also measurements of local field potentials (LFPs) and electroencephalographic (EEG) or magnetoencephalo-Most of our knowledge about the functional organization of neuronal systems is based on the analysis of the firing patterns of individual neurons that have been recorded one by one in succession. This approach permits as-sessment of event-related variations in discharge rate, but it precludes detection of any covariations in the amplitude or timing of distributed responses if these graphic (MEG) recordings. The signals of these latter

Based on a Hebbian adaptation process, the afferent and lateral connections in the RF-LISSOM model organize simultaneously and cooperatively, and form structures such as those observed in the primary visual cortex. The neurons in the model develop local receptive fields that are organized into orientation, ocular dominance, and size selectivity columns. At the same time, patterned lateral connections form between neurons that follow the receptive field organization. This structure is in a continuously-adapting dynamic equilibrium with the external and intrinsic input, and can account for reorganization of the adult cortex following retinal and cortical lesions. The same learning processes may be responsible for a number of low-level functional phenomena such as tilt aftereffects, and combined with the leaky integrator model of the spiking neuron, for segmentation and binding. The model can also be used to verify quantitatively the hypothesis that the visual cortex forms a sparse, redun...

Cells in the visual cortex are selective not only to ocular dominance and orientation of the input, but also to its size and spatial frequency. The simulations reported in this paper show how size selectivity could develop through Hebbian self-organization, and how receptive fields of different sizes could organize into columns like those for orientation and ocular dominance. The lateral connections in the network self-organize cooperatively and simultaneously with the receptive field sizes, and produce patterns of lateral connectivity that closely follow the receptive field organization. Together with our previous work on ocular dominance and orientation selectivity, these results suggest that a single Hebbian self-organizing process can give rise to all the major receptive field properties in the visual cortex, and also to structured patterns of lateral interactions, some of which have been verified experimentally and others predicted by the model. The model also suggests a functiona...

Large-scale neural networks are thought to be an essential substrate for the implementation of cognitive function by the brain. If so, then a thorough understanding of cognition is not possible without knowledge of how the large-scale neural networks of cognition (neurocognitive networks) operate. Of necessity, such understanding requires insight into structural, functional, and dynamical aspects of network operation, the intimate interweaving of which may be responsible for the intricacies of cognition. Knowledge of anatomical structure is basic to understanding how neurocognitive networks operate. Phylogenetically and ontogenetically determined patterns of synaptic connectivity form a structural network of brain areas, allowing communication between widely distributed collections of areas. The function of neurocognitive networks depends on selective activation of anatomically linked cortical and subcortical areas in a wide variety of configurations. Large-scale functional networks provide the cooperative processing which gives expression to cognitive function. The dynamics of neurocognitive network function relates to the evolving patterns of interacting brain areas that express cognitive function in real time. This article considers the proposition that a basic similarity of the structural, functional, and dynamical features of all neurocognitive networks in the brain causes them to function according to common operational principles. The formation of neural context through the coordinated mutual constraint of multiple interacting cortical areas, is considered as a guiding principle underlying all cognitive functions. Increasing knowledge of the operational principles of neurocognitive networks is likely to promote the advancement of cognitive theories, and to seed strategies for the enhancement of cognitive abilities.

We studied spatial integration at low contrasts by testing the detection thresholds of multi Gabor element displays, examining configuration parameters such as orientation uniformity, contour smoothness, continuity, spacing and relative phase. We find that detectability depends on stimulus geometry and is constrained by collinearity and proximity spatial relations. For a "coherent" configuration (e.g. smooth and continuous contour) thresholds decrease linearly with increased number of elements, on a log-log scale, as expected from probability summation (q = 4). However, for a "non-coherent" configuration (e.g. spacing ? 4 or jagged contour) thresholds are little affected by the number of patches. Similar behavior is observed for supra-threshold stimuli embedded in bandpass noise. These results suggest that contrast integration is primarily based on local mechanisms and constrained by contour properties. These local mechanism are possibly mediated by lateral interactions in the primary ...

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Abstract not found

Optical Imaging of intrinsic signals detects neural activation patterns by taking video images of the local activity-related changes in the light intensity reflected from neural tissue (intrinsic signals). At red light (605nm), these signals are mainly caused by local variations of the tissue absorption following deoxygenation of blood. In this work, we characterize the image generation process during Optical Imaging by Monte Carlo simulations of light propagation through a homogeneous model tissue equipped with a local absorber. Conventional video-imaging and Scanning Laser imaging are compared to each other. We find that, compared to video imaging, Scanning Laser techniques drastically increase both the contrast and the lateral resolution of optical recordings. Also, the maximum depth up to which the signals can be detected, is increased by roughly a factor of 2 using Scanning Laser Optical Imaging. Further, the radial profile of the diffuse reflectance pattern for each pixel is subj...

Experiments on multi-target visual detection show a weak improvement of thresholds with the number of targets, that corresponds to a fourth-root power law. It is shown that the data are inconsistent with probability summation models, but can be explained within a model of "physiological" integration based on excitatory lateral interactions in the visual cortex. The model captures a series of effects, confirmed by the experimental data reported, such as the absence of spatial and temporal uncertainty effects, temporal summation curves, and facilitation by a pedestal in 2AFC tasks. The summation exponents are shown to be dependent on the strength of the lateral interactions and on the distance and orientation relationship between the elements.