Integrated Calendar

In recent years, experiments detecting the electrical firing patterns in slices of in vitro brain tissue have been analyzed to suggest the presence of scale invariance and possibly criticality in the brain. Much of the work done however has been limited in two ways: 1) the data collected is from local field potentials that do not represent the firing of individual neurons; 2) the analysis has been primarily limited to histograms. In our work we examine data based on the firing of individual neurons (spike data), and greatly extend the analysis by considering shape collapse and exponents. Our results strongly suggest that the brain operates near a tuned critical point of a highly distinctive universality class.

We study the geometry and mechanics that drive the opening of Bauhinia seeds pods. The pod valve wall consists of two fibrous layers oriented at with respect to the pod axis. Upon drying, each of the layers shrinks uniaxially, perpendicularly to the fibers orientation.
This active deformation turns the valve into an incompatible sheet with reference saddle-like curvature tensor and a flat (Euclidean) reference metric. These two intrinsic properties are incompatible. The shape is, therefore, selected by a stretching-bending competition.
Strips cut from the valve tissue and from synthetic model material adopt various helical configurations. We provide analytical expressions for these configurations in the bending and stretching dominated regimes and show how plants use these mechanical principles using different tissue architectures.
Finally, we point to geometrical and mechanical equivalence between elastic strips with negative reference curvature and self assembled macromolecules made of twisted elements.
Based on this equivalence we provide explanation and quantitative predictions for shape transitions that have been observed in self assembled macromolecules.

It has taken about 30 years for the notion of attractor dynamics to get the attention of the experimental neuroscience community. Now that some are beginning to investigate the more sophisticated idea of continuous attractors, where marginal stability can be used for cognitive operations such as path integration or the prediction of the consequences of one's own actions, it is time to tell the truth about continuous attractors. I will discuss a quantitative approach to the smoothness of the spatial maps that can be established in the CA3 hippocampal network, and suggest that in the space of memories, we may jump more often than slide.

We employed a solid state dewetting technique to produce an array of faceted single crystalline Au particles of sub-micrometer dimensions on the sapphire substrate. The faceted single crystal particles exhibited a profound size and shape stability, even after prolonged anneals in air at the temperatures close to the melting point of gold. The microparticles were tested in compression employing the depth-sensing indentation instruments equipped with the sharp “cube corner” and flat diamond tips. The nanoindentation tests performed with cube corner indenter revealed that plastic deformation compliance of the particles increases with decreasing particles size. Gold thin films of comparable thickness exhibited much higher resistance to plastic deformation than the particles. On the contrary, during the nanoindentation tests performed with the flat diamond punch, small particles exhibited higher yield strength than their large counterparts (smaller is stronger). To understand these differences in the indentation behavior of the microparticles and thin films, we performed atomistic molecular dynamic simulations of the indentation process. The simulations showed that in the case of cube corner indenter the dislocations are nucleated at the interface between the indenter and the particles/films, while in the case of flat punch the nucleation occurs at the corners of the upper particle facet. The dislocations in the particles were short-lived and did not form complex dislocation structures before annihilating at the free surfaces. In the thin film the dislocations accumulated around and beneath the indenter, resulting in complex, sessile dislocation structures contributing to film hardening. We proposed a stress-gradient dislocation nucleation model relating the indentation size effect to stress gradients in the particle along the slip plane.