Does Corticothalamic Feedback Control Cortical Velocity Tuning?

The thalamus is the major gate to the cortex and its contribution to cortical receptive field properties is well established. Cortical feedback to the thalamus is, in turn, the anatomically dominant input to relay cells, yet its influence on thalamic processing has been difficult to interpret. For an understanding of complex sensory processing, detailed concepts of the corticothalamic interplay need yet to be established. To study corticogeniculate processing in a model, we draw on various physiological and anatomical data concerning the intrinsic dynamics of geniculate relay neurons, the cortical influence on relay modes, lagged and nonlagged neurons, and the structure of visual cortical receptive fields. In extensive computer simulations we elaborate the novel hypothesis that the visual cortex controls via feedback the temporal response properties of geniculate relay cells in a way that alters the tuning of cortical cells for speed.Comment: 31 pages, 7 figure

Phase diagrams of soluble multi-spin glass models

We include p-spin interactions in a spherical version of a soluble mean-field spin-glass model proposed by van Hemmen. Due to the simplicity of the solutions, which do not require the use of the replica trick, we are able to carry out a detailed investigation of a number of special situations. For p larger or equal to 3, there appear first-order transitions between the paramagnetic and the ordered phases. In the presence of additional ferromagnetic interactions, we show that there is no stable mixed phase, with both ferromagnetic and spin-glass properties.Comment: To appear in Physica

Temporal Map Formation in the Barn Owl’s Brain

Barn owls provide an experimentally well-specified example of a temporal map, a neuronal representation of the outside world in the brain by means of time. Their laminar nucleus exhibits a place code of interaural time differences, a cue which is used to determine the azimuthal location of a sound stimulus, e.g., prey. We analyze a model of synaptic plasticity that explains the formation of such a representation in the young bird and show how in a large parameter regime a combination of local and nonlocal synaptic plasticity yields the temporal map as found experimentally. Our analysis includes the effect of nonlinearities as well as the influence of neuronal noise

Cellular and molecular aspects of articular cartilage resurfacing

The aim of this project is to find alternative ways of repairing articular cartilage defects in rabbits by implanting carbon fibre, collagen gel or hydrogel PC 97 plugs with associated chondrocytes. The results were assessed using histology, electron microscopy, biomechanical testing and immunocytochemistry using antibodies against collagen types I and II, chondroitins -4- and -6- sulphates and keratan sulphate. The antibodies were used as qualitative markers of extracellular matrix composition. Isolated chondrocytes, cultured in either a carbon fibre, collagen gel or hydrogel PC 97 plug, were transplanted into full-thickness defects of articular cartilage in the tibiae of mature rabbits. Grafts were examined 3, 6 and 12 months post-implantation using the techniques outlined above. The carbon fibre plugs with associated chondrocytes showed a cartilaginous matrix with incorporation of the carbon fibres at 3 weeks in vitro culture and 3 months postimplantation; after 6 and 12 months cartilage-like tissue was shown in all layers except the surface. After implantation of carbon fibre plugs without chondrocytes the repair tissue was fibro-cartilaginous. The stiffness of the carbon fibre implants at 6 and 12 months post-implantation showed values in the same range as normal rabbit articular cartilage of similar age (native cartilage). The collagen gel plugs with associated chondrocytes showed that after 3 weeks in vitro culture most of the chondrocyte-like cells were present at the surface of the gel, with more fibroblast-like cells within the gel. At 3, 6 and 12 months post-implantation, a variety of repair responses were observed, ranging from repair tissue resembling articular cartilage to fibrous-like graft tissue. Fibrocartilaginous repair tissue generated in the control joints was sparse, with little evidence of chondrogenesis. The stiffness of the collagen gel plugs was lower than in native cartilage. The hydrogel PC 97 plugs plus associated chondrocytes at 3 weeks in vitro culture were surrounded by a cartilage-like matrix. At 3 months post-implantation some areas of the matrix showed pericellular metachromatic staining, and the plugs were well incorporated into the bone, but not into the adjacent cartilage. The control plugs showed the presence of a growth response encapsulating the hydrogel PC 97. The hydrogel PC 97 plugs had a higher stiffness than in native cartilage

Reese Witherspoon: A True Role Model

A profile of Reese Witherspoon

Diabolical points in the magnetic spectrum of Fe_8 molecules

The magnetic molecule Fe_8 has been predicted and observed to have a rich pattern of degeneracies in its spectrum as an external magnetic field is varied. These degeneracies have now been recognized to be diabolical points. This paper analyzes the diabolicity and all essential properties of this system using elementary perturbation theory. A variety of arguments is gievn to suggest that an earlier semiclassical result for a subset of these points may be exactly true for arbitrary spinComment: uses europhys.sty package; 3 embedded ps figure

Spatiotemporal adaptation through corticothalamic loops: A hypothesis

The thalamus is the major gate to the cortex and its control over cortical responses is well established. Cortical feedback to the thalamus is, in turn, the anatomically dominant input to relay cells, yet its influence on thalamic processing has been difficult to interpret. For an understanding of complex sensory processing, detailed concepts of the corticothalamic interplay need yet to be established. Drawing on various physiological and anatomical data, we elaborate the novel hypothesis that the visual cortex controls the spatiotemporal structure of cortical receptive fields via feedback to the lateral geniculate nucleus. Furthermore, we present and analyze a model of corticogeniculate loops that implements this control, and exhibit its ability of object segmentation by statistical motion analysis in the visual field