Interhemispheric transfer of plasticity in the cerebral cortex

Article Abstract:

In the nervous system, plasticity is the normal state of affairs. In response to everyday events, nerve tissue changes constantly in a process we call learning. However, plasticity also has another connotation, that of the ability to adapt in response to damage. For example, in some areas of the brain there is a somatosensory "map" of the body, where each region of the body is represented on the brain's surface in a rather distorted cartoon. One might expect that if a small nerve arriving from, say, a finger, is severed, then that area of the brain would cease to respond to stimuli. This does not happen; adjacent areas adapt to the changes, and areas of the brain which previously responded to neighboring stimuli expand their region of influence to include the affected region. How this happens in unclear. However, the specific signals controlling this phenomenon may be present in the brain. In most mammals, the two hemispheres of the brain are connected with a giant tract of white matter fibers called the corpus callosum. In experimental animals researchers demonstrated that the denervation of a small input leading to the somatosensory cortex leads not only to plasticity on the affected side of the brain, but on the other side as well. For example, the nerve from the finger of a flying fox connects to the finger spot of the somatosensory cortex on the opposite side of the fox's brain. (The nerve fibers cross sides before entering the cortex; consequently, the 'finger spot' on the same side does not respond.) As expected, when the right-sided nerve is cut, there is an expansion of receptive fields in the neighborhood of the finger spot on left side of the brain. However, the corresponding region on the right side of the brain, which has intact nerve inputs, nonetheless responds by increasing receptive field sizes as well. Continuing the example, these results suggest that signals arriving via the corpus callosum from the left side of the brain are capable of inducing plasticity in cellular responses on the right. (Consumer Summary produced by Reliance Medical Information, Inc.)

Neuroplasticity, Cerebral hemispheres

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Stereoscopic depth discrimination in the visual cortex: neurons ideally suited as disparity detectors

Article Abstract:

The physiological processes that allow us to see the world in three dimensions (stereoscopic depth discrimination) are unexplained, but are thought to occur in the visual cortex. In this brain region, neurons are classified as ''simple'' or ''complex,'' depending on the kinds of stimuli to which they respond. Thus, simple cells respond when stationary bars of light are flashed on or turned off, while complex cells respond to any stimulus within their receptive fields, regardless of its illumination (unless, of course, illumination falls below a detection threshold). Since complex cells are not thought to be sensitive to contrast information, they seem poorly suited to mediate stereoscopic depth discrimination. Results from fitting experimental results to predictions made by a model of three-dimensional vision suggest, however, that a subset of complex cells in the visual cortex could perform this function. Complex cells that receive input from four simple-cell subunits with specified relationships among themselves in terms of physiology and firing patterns, could detect differences in the images seen by the two eyes, leading to the perception of depth. Details of the model are elaborated upon, as are the response characteristics of the simple and complex cells whose behavior it explains. (Consumer Summary produced by Reliance Medical Information, Inc.)

Visual cortex, Binocular vision, Visual discrimination

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Analysis of mass transfer process in the pore free technique

Article Abstract:

The effect of the mass transfer process on the reliability of the Pore Free technique used in preventing air entrainment in pressure die casting was investigated. A model was developed to analyze the relationship between mass transfer mechanisms and pressure deviation. It was also used to evaluate the performance of the vacuum venting technique. Results indicate the presence of a dimensionless parameter that determines the usefulness of both techniques.

Research, Analysis, Mass transfer, Die-casting, Die casting (Process)

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