Different types of signal coupling in the visual cortex related to neural mechanisms of associative Processing and perception

The hypothesis of object representation by synchronization in the visual cortex has been supported by our recent experiments in monkeys. They demonstrated local synchrony among /spl gamma/ activities (30-90 Hz) and their perceptual modulation, according to the rules of figure-ground segregation. However, /spl gamma/-synchrony in primary visual cortex is restricted to few mm, challenging the synchronization hypothesis for larger cortical object representations. The restriction is due to randomly changing phase relations among locally synchronized patches which, however, form continuous waves of /spl gamma/-activity, traveling across object representations. The phase continuity of these waves may support coding of object continuity. Interactions across still larger distances, measured among cortical areas in human data, involve amplitude envelopes of /spl gamma/ signals. Based on models with spiking neurons we discuss potentially underlying mechanisms. Most important for /spl gamma/ synchronization are local facilitatory connections with distance-dependent delays. They also explain the occurrence of /spl gamma/ waves and the restriction of /spl gamma/-synchrony. Fast local feedback inhibition generates /spl gamma/ oscillations and supports local synchrony, while slow shunting inhibitory feedback supports figure-ground segregation. Finally, dispersion in inter-areal far projections destroys coherence of /spl gamma/ signals, but preserves their amplitude modulations. In conclusion, we propose that the hypothesis of associative processing by /spl gamma/ synchronization be extended to more general forms of signal coupling.     

Lotka's law reconsidered: The evolution of publication and citation distributions in scientific fields

This paper reports early steps in research that seeks to clarify how publications of scientists interact dynamically with citations and reputation in shaping the evolution of scientific fields. We assume that Lotka's modified law holds for scientific fields. A primary approach to model publication productivity was published by Yablonsky. In contrast to Yablonsky's unfinished mathematical approach, our simulation approach is not predominantly driven by insight into the formal generation mechanisms of certain processes but more theory driven. It considers the evolution of publication and citation distributions over the histories of scientific fields using both simulated and real historical data.     

Theoretical study of third sound in a layered superfluid film

We present a calculation of the third sound speed c₃ of thin⁴He films adsorbed on H₂, Li, Mg, graphite and H₂-plated graphite. The results show a clear correlation of oscillations in c₃, with a period of one atomic layer, with the layering phenomena observed in adsorption isotherms. Comparison is made with recent experimental results for helium films on solid H₂ by Shirron and Mochel and by Brisson, Mester and Silvera. The theoretical analysis suggests explanations for disagreements between different interpretations of data. A standard way of determining the number of so-called inert layers is questioned. Other available experimental data on graphite and H₂-plated graphite are also discussed.     

Structure and expression of the mouse L23mrp gene downstream of the imprinted H19 gene: biallelic expression and lack of interaction with the H19 enhancers

Previous studies have identified populations of dopamine neurons in the midbrain that colocalize cholecystokinin some of which project to the nucleus accumbens and caudate-putamen. The contribution of dopamine-colocalized peptide to the total releasable pool of cholecystokinin in these brain regions was investigated using microdialysis. Dopamine, dihydroxyphenylacetic acid and cholecystokinin immunoreactive levels in dialysates of the posterior medial nucleus accumbens and medial caudate-putamen were determined following 6-hydroxydopamine lesions of the ventral tegmental area and substantia nigra or transection of the medial forebrain bundle. An 89-99% depletion in basal extracellular dihydroxyphenylacetic acid and an 87-99% decrease in veratridine-evoked extracellular dopamine levels was observed in the nucleus accumbens and caudate-putamen, 4 weeks after 6-hydroxydopamine lesion. No statistically significant difference was observed between lesioned and control animals in the basal or veratridine-evoked extracellular level of cholecystokinin immunoreactivity in either region. Similarly, transection of the medial forebrain bundle failed to significantly deplete the releasable pool of cholecystokinin immunoreactivity in the nucleus accumbens or caudate nucleus despite 89-99% depletions of dopamine and its metabolite. These data suggest that midbrain dopamine or non-dopaminergic cells are not the primary source of releasable cholecystokinin in the posterior medial nucleus accumbens and medial caudate-putamen measured by microdialysis.     

Prediction of reentrant wetting of3He-4He mixtures on cesium

We examine the effect of ³ He impurities on the wetting behavior of⁴He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of³He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves.     

Dispersion forces in wetting

tA simple, analytically tractable, model is used to obtain wetting phase diagrams for insulators on insulating and metallic substrates. The phase diagrams are consistent with the general rule that if the vapor phases of A and B are dilute and dispersion forces allow wetting of A on B then, wetting of B on A is not allowed.