Kay A. Robbins

Propagating waves mediate information transfer in the motor cortex

High-frequency oscillations in the beta range (10–45 Hz) are most active in motor cortex during motor preparation and are postulated to reflect the steady postural state or global attentive state of the animal. By simultaneously recording multiple local field potential signals across the primary motor and dorsal premotor cortices of monkeys (Macaca mulatta) trained to perform an instructed-delay reaching task, we found that these oscillations propagated as waves across the surface of the motor cortex along dominant spatial axes characteristic of the local circuitry of the motor cortex. Moreover, we found that information about the visual target to be reached was encoded in terms of both latency and amplitude of evoked waves at a time when the field phase-locked with respect to the target onset. These findings suggest that high-frequency oscillations may subserve intra- and inter-cortical information transfer during movement preparation and execution.

An empirical evaluation of client-side server selection algorithms

Efficient server selection algorithms reduce retrieval time for objects replicated on different servers and are an important component of Internet cache architectures. This paper empirically evaluates six client-side server selection algorithms. The study compares two statistical algorithms, one using median bandwidth and the other median latency, a dynamic probe algorithm, two hybrid algorithms, and random selection. The server pool includes a topologically dispersed set of United States state government Web servers. Experiments were run on three clients in different cities and on different regional networks. The study examines the effects of time-of day, client resources, and server proximity. Differences in performance highlight the degree of algorithm adaptability and the effect that network upgrades can have on statistical estimators. Dynamic network probing performs as well or better than the statistical bandwidth algorithm and the two probe bandwidth hybrid algorithms. The statistical latency algorithm is clearly worse, but does outperform random selection.

The PREP pipeline: standardized preprocessing for large-scale EEG analysis.

The technology to collect brain imaging and physiological measures has become portable and ubiquitous, opening the possibility of large-scale analysis of real-world human imaging. By its nature, such data is large and complex, making automated processing essential. This paper shows how lack of attention to the very early stages of an EEG preprocessing pipeline can reduce the signal-to-noise ratio and introduce unwanted artifacts into the data, particularly for computations done in single precision. We demonstrate that ordinary average referencing improves the signal-to-noise ratio, but that noisy channels can contaminate the results. We also show that identification of noisy channels depends on the reference and examine the complex interaction of filtering, noisy channel identification, and referencing. We introduce a multi-stage robust referencing scheme to deal with the noisy channel-reference interaction. We propose a standardized early-stage EEG processing pipeline (PREP) and discuss the application of the pipeline to more than 600 EEG datasets. The pipeline includes an automatically generated report for each dataset processed. Users can download the PREP pipeline as a freely available MATLAB library from http://eegstudy.org/prepcode.

A viability analysis of cooperative proxy caching

Cooperating proxy caches are groups of HTTP proxy servers that share cached objects. This paper models the speedup in average user response time for proxy cooperation and derives expressions for the upper bound, a mesh organization, and a hierarchical organization. The expressions are parameterized using empirically determined hit rates and HTTP response times. Models account for overhead of different discovery mechanisms (ICP or metadata directory) and clarify the trade-off between discovery overhead and effectiveness. We find proxy cooperation to be only marginally viable when the sole criterion is average response time. By offering a choice of Web sites, however, proxy cooperation can potentially reduce the variability in response time, the number of pathologically long delays, and congestion near busy Web servers.

Detection and classification of subject-generated artifacts in EEG signals using autoregressive models.

We examine the problem of accurate detection and classification of artifacts in continuous EEG recordings. Manual identification of artifacts, by means of an expert or panel of experts, can be tedious, time-consuming and infeasible for large datasets. We use autoregressive (AR) models for feature extraction and characterization of EEG signals containing several kinds of subject-generated artifacts. AR model parameters are scale-invariant features that can be used to develop models of artifacts across a population. We use a support vector machine (SVM) classifier to discriminate among artifact conditions using the AR model parameters as features. Results indicate reliable classification among several different artifact conditions across subjects (approximately 94%). These results suggest that AR modeling can be a useful tool for discriminating among artifact signals both within and across individuals.

Deterministic chaos in laminar premixed flames : experimental classification of chaotic dynamics

Abstract Our previous experiments on the dynamics of laminar premixed flames on a circular porous plug burner have revealed periodic modes with different spatial and temporal characteristics. In this paper representative examples of nonperiodic modes will be presented, and it will be argued that these modes fall into two distinct classes. The method of data analysis used to make this identification will be explained and the experimental conditions that are conducive to observing the flame as a dynamical system will be described in detail. Our results suggest that the techniques of nonlinear dynamics are useful in classifying the periodic and chaotic modes of pulsating and cellular premixed flames but are probably not relevant to diffusion flames.

Experimental Observation of Ordered States of Cellular Flames

ABSTRACT Ordered states of cellular flames, consisting of concentric rings of approximately equally sized cells, are observed over a wide range of parameters in our experiments using heavy hydrocarbon-air mixtures on a circular porous plug burner at low pressure. These ordered states have been found with cell numbers ranging from five to thirty. At a critical value of the flow rate, which depends on the equivalence ratio, a transition to a disordered state is observed, in which the ring structure disappears; the cells change their shape, size and number; and they move about in an irregular manner.

Rotating and Modulated Rotating States of Cellular Flames

ABSTRACT Ordered states of cellular flames on circular porous plug burners consist of concentric rings of cells. At certain values of the flow rate and equivalence ratio a transition is made to a state in which entire rings of ceils rotate. The direction of rotation depends only on the initial conditions. Our observations of rotating cellular flames include a single rotating ring, an outer ring rotating about a single inner cell, a rotating inner ring surrounded by a fixed outer ring, and two corotating or counterrotaling concentric rings. A rotating ring of cells can also make a transition to a modulated rotating state in which the shapes of the cells and the speed of rotation periodically change. In another rotating mode, a single central cell takes the shape of a spiral which rotates inside a fixed outer ring of cells. The physical characteristics of these modes are described and comparisons with relevant theoretical studies are made.

Chaotic Dynamics Near The Extinction Limit of a Premixed Flame on a Porous Plug Burner

ABSTRACT Our previous experiments on the dynamics of premixed flames on circular porous plug burners have demonstrated a variety of periodic pulsating flames with different spatial and temporal characteristics. In the radial mode the circular flame front expands and contracts, periodically changing its radial extent. As the system parameters are adjusted near the extinction boundary, a sequence of transitions is observed. First, the radial mode loses its circular symmetry but remains periodic. Next, it undergoes a transition to a chaotic state. Finally, it makes a transition to extinction. All periodic pulsating modes are separated from the extinction boundary by a region of chaotic dynamics. The characteristics of chaotic flame dynamics beyond the extinction limit are discussed. The spatial and temporal characteristics of this chaotic mode are discussed and compared with the relevant theoretical studies.

Hopping Motion in Ordered States of Cellular Flames

ABSTRACT Our previous experiments have shown that cellular flames form ordered states consisting of concentric rings of cells. The numbers of cells in the inner and outer rings change independently in integer steps as the flow rate is increased. In this paper we report the observation of states characterized by a hopping motion in which cells abruptly change their angular position in the ring. This hopping proceeds sequentially to the other cells in the ring. The hopping states are typically observed in isobutane-air cellular flames at parameter values between those corresponding to two consecutive ordered states. The physical characteristics of these states are similar to those of modulated traveling waves found by Bayliss, Matkowsky and Riecke in numerical simulations of the full equations that describe the thermodiffusive instability. The similarities and differences between our experimental results and their theoretical predictions are discussed.