Biomedical subjects

G M Edelman

Publications and source records attributed to G M Edelman.

383 records · Page 22Linked to original sources

PubMedDate not supplied

Spatial signaling in the development and function of neural connections.

In the vertebrate central nervous system, afferent axons find their appropriate target structures under the influence of local environmental cues. In many target regions, appropriate patterns of activity in the afferents are also required to establish normal mappings between the source cells and the target region. Specific mappings arise in these targets because temporal contiguity in firing is somehow transformed into spatial contiguity of synaptic contacts. In this article, we propose a theory that utilizes the covariance of a transient diffusive signal produced at active synapses with the firing of presynaptic axon terminals to account for many of these activity-dependent features of vertebrate neuroanatomy. Computer simulations of the growth of axons within a three-dimensional volume of neural tissue reveal the generality of the proposed mechanism in accounting for the self-organization of a broad range of diverse neuroanatomical structures, including those in the cerebral cortex. The proposed mechanism is consistent with detailed anatomical and physiological data, and direct experimental tests of predictions of the theory are suggested.

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PubMedDate not supplied

Reentry and the problem of integrating multiple cortical areas: simulation of dynamic integration in the visual system.

Studies of the cerebral cortex, particularly those of the visual system, demonstrate the existence of multiple, anatomically segregated and functionally specialized cortical areas. There is no evidence that these areas, which are linked by a network of reciprocal connections, are coordinated by a higher-order center. The visual image that we perceive, however, seems to be unified and coherent. In this article, we address the problem of integration posed by these observations. In an extension of our previous work, we develop a dynamic model of reentry. Reentry is a process of parallel and recursive signaling along ordered anatomical connections that achieves integration by giving rise to constructive and correlative properties within and among maps. We present and test a computer model simulating nine functionally segregated visual areas organized into three streams for form, color, and motion. The model receives visual input consisting of camera images of objects of different shapes and colors. We show the specialized response properties of the areas in the three streams. A computational strategy involving a phase variable is introduced to represent explicitly the dynamics of short-term temporal correlations among thousands of units distributed across different areas. We then illustrate constructive and correlative consequences of reentry within a system of reciprocal intra- and interareal connections by two examples taken from psychophysics: generation of form from motion and motion capture. The model solves the so-called "binding problem" through short-term correlations, which serve to link similar object features within a simulated cortical area and to bind multiple attributes of one or more objects across several areas, including a nontopographic one. Integration emerges from cooperative effects within and among the specialized areas. These effects lead to a simple output, a simulated foveation response, that is used as a basis for conditioning. Reward is mediated by the activation of a saliency system that is modeled on diffuse projection systems in the brain. As a result, the visual cortical model carries out foveation responses to input stimuli that require the dynamic conjunction and discrimination of form, color, and location for successful performance.

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PubMedDate not supplied

A model of color vision based on cortical reentry.

It is known that the perceived color of an object depends on the context in which it is viewed, its reflectance properties and the spectral distribution of the illuminating light. What is not known, however, is how the visual system functions so that color percepts depend upon the integration of local and contextual cues. While phenomenological theories of color vision exist, robust neurally based theories consistent with psychophysical observations are sparse. In the present study we develop such a theory and establish its self-consistency by computer simulations of cerebral cortical areas involved in color perception. The simulations test the hypothesis that long-range reciprocal connections within and between cortical areas mediate a dynamic process of reentry that integrates contextual cues into the color percept. When stimuli similar to those used in psychophysical testing of contextual influence were used, firing patterns consistent with psychophysical data on color constancy and color induction in humans were observed. Selective disruption of reciprocal inter- or intra-areal connections reduced the correspondence between the model's responses and the psychophysical data. The findings are consistent with the proposal that reentrant interactions within and between cortical areas provide a major basis for the context-sensitive aspects of color vision.

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PubMedDate not supplied

Neural dynamics in a model of the thalamocortical system. I. Layers, loops and the emergence of fast synchronous rhythms.

A large-scale computer model was constructed to gain insight into the structural basis for the generation of fast synchronous rhythms (20-60 Hz) in the thalamocortical system. The model consisted of 65,000 spiking neurons organized topographically to represent sectors of a primary and secondary area of mammalian visual cortex, and two associated regions of the dorsal thalamus and the thalamic reticular nucleus. Cortical neurons, both excitatory and inhibitory, were organized in supragranular layers, infraganular layers and layer IV. Reciprocal intra- and interlaminar, interareal, thalamocortical, corticothalamic and thalamoreticular connections were set up based on known anatomical constraints. Simulations of neuronal responses to visual input revealed sporadic epochs of synchronous oscillations involving all levels of the model, similar to the fast rhythms recorded in vivo. By systematically modifying physiological and structural parameters in the model, specific network properties were found to play a major role in the generation of this rhythmic activity. For example, fast synchronous rhythms could be sustained autonomously by lateral and interlaminar interactions within and among local cortical circuits. In addition, these oscillations were propagated to the thalamus and amplified by corticothalamocortical loops, including the thalamic reticular complex. Finally, synchronous oscillations were differentially affected by lesioning forward and backward interareal connections.

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PubMedDate not supplied

Neural dynamics in a model of the thalamocortical system. II. The role of neural synchrony tested through perturbations of spike timing.

Activity in the mammalian thalamocortical system is often accompanied by a synchronous discharge of cortical and thalamic neurons. Although many functions have been attributed to such synchronous firing, it is not known whether or how synchrony of firing per se affects thalamocortical operations. Direct experimental tests of the consequences of neuronal synchronization in vivo are hard to carry out, whereas theoretical studies based on single-neuron models cannot reveal the effects of synchrony at the system level. To overcome these limitations, we have used a perturbational approach to test the causal efficacy of synchrony per se in large-scale simulations of the thalamocortical system. The test consists of selectively disrupting firing synchrony by 'jittering' the timing of action potentials in the simulations and determining whether firing rates are modified by this perturbation. The simulations are based in detail on the known anatomy and physiology of the thalamocortical-visual system of the cat, and have been shown in a companion paper to produce episodes of fast synchronous activity at multiple levels. By carrying out the perturbation analysis, we established that neurons can have long membrane time constants (8-16 ms) and balanced synaptic activations, and yet function collectively in such a way that synchrony within a time window of 4 ms significantly affects the rates and selectivity of the responses to visual stimuli. The simulations also revealed a complex interplay, at the network level, between synchrony of firing and rate of firing. The dynamic consequences of firing synchrony were most evident when spike jittering was applied to specific polysynaptic loops involving corticocortical and corticothalamic connections. These results support the view that firing synchrony within thalamocortical and corticocortical loops plays a causal role in the cooperative and competitive neural interactions that produce pattern-selective responses in the cortex.

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