Design principles of columnar organization in visual cortex

Visual space is represented by cortical cells in an orderly manner. Only little variation in the cell behavior is found with changing depth below the cortical surface, that is, all cells in a column with axis perpendicular to the cortical plane have approximately the same properties (Hubel and Wiesel 1962, 1963, 1968). Therefore, the multiple features of the visual space (e.g., position in visual space, preferred orientation, and orientation tuning strength) are mapped on a two-dimensional space, the cortical plane. Such a dimension reduction leads to complex maps (Durbin and Mitchison 1990) that so far have evaded an intuitive understanding. Analyzing optical imaging data (Blasdel 1992a, b; Blasdel and Salama 1986; Grinvald et al. 1986) using a theoretical approach we will show that the most salient features of these maps can be understood from a few basic design principles: local correlation, modularity, isotropy, and homogeneity. These principles can be defined in a mathematically exact sense in the Fourier domain by a rather simple annulus-like spectral structure. Many of the models that have been developed to explain the mapping of the preferred orientations (Cooper et al. 1979; Legendy 1978; Linsker 1986a, b; Miller 1992; Nass and Cooper 1975; Obermayer et al. 1990, 1992; Soodak 1987; Swindale 1982, 1985, 1992; von der Malsburg 1973; von der Malsburg and Cowan 1982) are quite successful in generating maps that are close to experimental maps. We suggest that this success is due to these principles, which are common properties of the models and of biological maps

Generation of Direction Selectivity by Isotropic Intracortical Connections

To what extent do the mechanisms generating different receptive field properties of neurons depend on each other? We investigated this question theoretically within the context of orientation and direction tuning of simple cells in the mammalian visual cortex. In our model a cortical cell of the "simple" type receives its orientation tuning by afferent convergence of aligned receptive fields of the lateral geniculate nucleus (Hubel and Wiesel 1962). We sharpen this orientation bias by postulating a special type of radially symmetric long-range lateral inhibition called circular inhibition. Surprisingly, this isotropic mechanism leads to the emergence of a strong bias for the direction of motion of a bar. We show that this directional anisotropy is neither caused by the probabilistic nature of the connections nor is it a consequence of the specific columnar structure chosen but that it is an inherent feature of the architecture of visual cortex

The Relocation of Russian Industry 1987-1993

This study is about the effects reforms have had on industry's dispersion in Russia. We use data on regional production by industry reaching from 1987 to 1993 to consider to what extent the division of labor among regions has changed during this time period. In particular we address three issues on an industry by industry basis: Have there been significant changes in the localization of industries and have the changes been associated with concentration or diversification? Are there significant changes in the relative productivity's of industries in regions? Is there a connection between productivity changes and changes in localization?Russia; industry; relocation; productivity; reform

On The Road to Euro: How Synchronized Is Estonia with the Euro zone?

While the currency board served Estonia well during transition in the 1990s, it has limited its ability to counter the impact of the global financial crisis and heightened the currency risks. The euro adoption has thus become a top policy priority again. However, this paper finds that even after almost two decades of hard peg with the core of the euro zone shocks affecting Estonia are relatively weakly synchronized with those of the zone, contributing to large output volatility. Nevertheless, the case for euro adoption by Estonia holds, since the costs of the loss of independent monetary policy were paid, and – as the global financial crisis demonstrated – the currency board is no substitute for the common currency. To reduce future output volatility, Estonia should move to counter-cyclical fiscal policies, maintain labor and product market flexibility, and adopt policies stimulating rise in the knowledge and high-tech content of its production.shock synchronization; structural VAR; euro adoption; financial crisis; Estonia

Ungleichgewichte im Eurogebiet: Eine Geschichte in zwei Ländern

Ungleichgewichte in einem Währungsgebiet entstehen durch Kapitalströme, die nicht durch eine ent-sprechende Steigerung der Kapazität Einkommen zu erzielen begleitet werden. Dadurch entsteht eine Überschuldung in den „Defizitländern“, während in den „Überschussländern“ Forderungen abge-schrieben werden müssen. In beiden Fällen kommt es zu einer Verringerung zukünftigen Wirtschafts-wachstums und einer Verzerrung der Struktur der Wirtschaft, weil sich kreditfinanziertes Wachstum üb-licherweise auf einige wenige Wirtschaftsbereiche, insbesondere Immobilien und Bauwirtschaft be-schränkt. Eine Besonderheit des Eurogebietes besteht nun darin, dass nur die Geldpolitik nach einheit-lichen Kriterien durchgeführt wird, während die Fiskalpolitik sowie die Überwachung des Finanzsektors nach wie vor in der Hand der einzelnen Mitgliedsstaaten liegt. Anhand von zwei Länderstudien wird argumentiert, wie ein Überschussland (Deutschland) zum Abbau von Ungleichgewichten beitragen kann und welches Ausmaß an Flexibilität notwendig ist, um aus einem „Boom/Bust“-Zyklus wieder herauszukommen (Estland).

Cortical column design: a link between the maps of preferred orientation and orientation tuning strength?

We demonstrate that the map of the preferred orientations and the corresponding map of the orientation tuning strengths as measured with optical imaging are not independent, but that band-pass filtering of the preferred orientation map at each location yields a good approximation of the orientation tuning strength. Band-pass filtering is performed by convolving the map of orientation preference with its own autocorrelation function. We suggest an interpretation of the autocorrelation function of the preferred orientations as synaptic coupling function, i.e., synaptic strength as a function of intracortical distance between cortical cells. In developmental models it has been shown previously that a “Mexican hat”-shaped synaptic coupling function (with a shape similar to that of the autocorrelation function) can produce a realistical-looking pattern of preferred orientations. Since optical imaging performs surface averaging, we discuss the possibility that the connection between the two maps is a measurement artifact of optical imaging. Whether this is the case can only be decided by combining electrode penetrations with optical imaging techniques for which we suggest experiments. We present a model for the generation of both maps from a single computational concept. The model is based on inverse Fourier transform of rather simple two-dimensional annulus-shaped spectra which will produce a column structure very similar to real data. Thus, our approach shows that the complex appearance of cortical orientation columns has a rather simple description in the Fourier domain. Our theoretical analysis explains why singularities in the cortex do not have vorticities other than ±1/2, a result which corresponds to recent experimental findings. This study combines the results from several modeling approaches with recently available optical imaging data to construct a model of both aspects (angle and strength) of the cortical orientation column system. This could alter ideas about cortical development if the link between the two maps can be established as a physiological result

Multiple chaotic central pattern generators with learning for legged locomotion and malfunction compensation

An originally chaotic system can be controlled into various periodic dynamics. When it is implemented into a legged robot's locomotion control as a central pattern generator (CPG), sophisticated gait patterns arise so that the robot can perform various walking behaviors. However, such a single chaotic CPG controller has difficulties dealing with leg malfunction. Specifically, in the scenarios presented here, its movement permanently deviates from the desired trajectory. To address this problem, we extend the single chaotic CPG to multiple CPGs with learning. The learning mechanism is based on a simulated annealing algorithm. In a normal situation, the CPGs synchronize and their dynamics are identical. With leg malfunction or disability, the CPGs lose synchronization leading to independent dynamics. In this case, the learning mechanism is applied to automatically adjust the remaining legs' oscillation frequencies so that the robot adapts its locomotion to deal with the malfunction. As a consequence, the trajectory produced by the multiple chaotic CPGs resembles the original trajectory far better than the one produced by only a single CPG. The performance of the system is evaluated first in a physical simulation of a quadruped as well as a hexapod robot and finally in a real six-legged walking machine called AMOSII. The experimental results presented here reveal that using multiple CPGs with learning is an effective approach for adaptive locomotion generation where, for instance, different body parts have to perform independent movements for malfunction compensation.Comment: 48 pages, 16 figures, Information Sciences 201