Sonya Bahar

Blood volume and hemoglobin oxygenation response following electrical stimulation of human cortex

Our understanding of perfusion-based human brain mapping techniques relies on a detailed knowledge of the relationship between neuronal activity and cerebrovascular hemodynamics. We performed optical imaging of intrinsic signals at wavelengths sensitive to total hemoglobin (Hbt; which correlate with cerebral blood volume (CBV)) and deoxygenated hemoglobin (Hbr) directly in humans during neurosurgical operations and investigated the optical signals associated with bipolar cortical stimulation at a range of amplitudes. Cortical stimulation elicited a rapid focal increase in Hbr (initial dip) in all subjects. An equally rapid increase in Hbt (<200 ms), with a slightly higher signal-to-noise ratio, was also highly localized for <2 s in spite of the non-columnar nature of the stimulus, after which the signal spread to adjacent gyri. A later decrease in Hbr (>3 s), which is relevant to the blood oxygen level dependent (BOLD) signal, was poorly localized. Increasing the stimulus amplitude elicited a linear increase in the area of the optical signal for Hbt and the initial dip but not the late decrease in Hbr, and a nonlinear increase in optical signal amplitude with a plateau effect for initial dip, Hbt and late decrease in Hbr.

Temporal Dependence in Uncoupling of Blood Volume and Oxygenation during Interictal Epileptiform Events in Rat Neocortex

We investigated the dynamic spatiotemporal relationships of cerebral blood volume (CBV), deoxygenated hemoglobin (Hbr), and light scatter (LS) associated with interictal epileptiform events with multiwavelength optical recording of intrinsic signals and simultaneous field potential recording. Interictal spikes (IISs) were induced with iontophoresis of bicuculline methiodide in rat neocortex. Intrinsic signal changes appeared as early as 100 msec after the IIS at all wavelengths and could be appreciated after only a single IIS. Initially, the largest signal arose from a focal increase in deoxygenation, which lasted for ∼2 sec, consistent with an “initial dip.” An equally early focal increase in CBV had a smaller amplitude than the Hbr signal until >2 sec after the IIS, when its amplitude surpassed that of the Hbr signal but also spread to a larger, less focal area. The most spatially restricted and smallest amplitude signal was produced by LS. A later hyperoxygenation, or increase in blood oxygenation level-dependent signal, was often seen in the draining veins but inconsistently seen in the IIS focus. An inverted optical signal was recorded at all wavelengths from multiple regions in the surrounding cortex within 100 msec of the IIS. We therefore conclude that the IIS induces a rapid increase in metabolic demand, which cannot be met by a rapid, initially focal but small increase in CBV that results in a prolonged increase in Hbr (epileptic dip in oxygenated hemoglobin). The inverted optical signal in the surround arises from a decrease in CBV and a decrease in Hbr, likely resulting from a combination of shunting of CBV to the focus and decreased metabolic demand resulting from decreased neuronal activity, consistent with “surround inhibition.”

BURST-ENHANCED SYNCHRONIZATION IN AN ARRAY OF NOISY COUPLED NEURONS

Stochastic phase synchronization is studied in an array of coupled neurons that are capable of exhibiting diverse bursting behaviors as a system parameter is tuned. It is found that synchronization occurs for lower coupling constants when the neurons are initially in a bursting state. Neurons not initially in a bursting state begin to exhibit bursting behavior concurrently with the onset of synchronization. The system passes through various synchronization regimes as the coupling constant is increased, with the number of spikes per burst increasing in each window of synchronization. In addition, spatial organization, measured by the distribution of local phase differences, is observed during the synchronization windows.

Imaging cortical electrical stimulation in vivo : fast intrinsic optical signal versus voltage-sensitive dyes

We applied high-temporal-resolution optical imaging utilizing both the fast intrinsic optical signal (fIOS) and voltage-sensitive dyes (VSDs) to observe the spatiotemporal characteristics of rat somatosensory cortex during electrical stimulation. We find that changes in both the fIOS and VSD signals occur rapidly (<30 ms) after the stimulus is applied, suggesting that both membrane depolarization and transmembrane ion movement occur shortly after the stimulus, preceding the more gradual physiological changes in oxygen consumption revealed by the slower component of the intrinsic optical signal. We find that the VSD signal spreads through a much larger area of cortex than the fIOS.

Stochastic resonance and the evolution of Daphnia foraging strategy

Search strategies are currently of great interest, with reports on foraging ranging from albatrosses and spider monkeys to microzooplankton. Here, we investigate the role of noise in optimizing search strategies. We focus on the zooplankton Daphnia, which move in successive sequences consisting of a hop, a pause and a turn through an angle. Recent experiments have shown that their turning angle distributions (TADs) and underlying noise intensities are similar across species and age groups, suggesting an evolutionary origin of this internal noise. We explore this hypothesis further with a digital simulation (EVO) based solely on the three central Darwinian themes: inheritability, variability and survivability. Separate simulations utilizing stochastic resonance (SR) indicate that foraging success, and hence fitness, is maximized at an optimum TAD noise intensity, which is represented by the distributions characteristic width, sigma. In both the EVO and SR simulations, foraging success is the criterion, and the results are the predicted characteristic widths of the TADs that maximize success. Our results are twofold: (1) the evolving characteristic widths achieve stasis after many generations; (2) as a hop length parameter is changed, variations in the evolved widths generated by EVO parallel those predicted by SR. These findings provide support for the hypotheses that (1) sigma is an evolved quantity and that (2) SR plays a role in evolution.

Effect of cognitive load on eye-target synchronization during smooth pursuit eye movement

In mild traumatic brain injury (mTBI), the fiber tracts that connect the frontal cortex with the cerebellum may suffer shear damage, leading to attention deficits and performance variability. This damage also disrupts the enhancement of eye-target synchronization that can be affected by cognitive load when subjects are tested using a concurrent eye-tracking test and word-recall test. We investigated the effect of cognitive load on eye-target synchronization in normal and mTBI patients using the nonlinear dynamical technique of stochastic phase synchronization. Results demonstrate that eye-target synchronization was negatively affected by cognitive load in mTBI subjects. In contrast, eye-target synchronization improved under intermediate cognitive load in young (≤40years old) normal subjects.

Chaotic orbits and bifurcation from a fixed point generated by an iterated function system

Abstract In the past, iterated function systems have been used [M. F. Barnsley and S. Demko, Iterated function systems and the global construction of fractals, Proc. R. Soc. Lond. A 399 , 243–275 (1985); M. F. Barnsley, V. Ervin, D. Hardin and J. Lancaster, Solution of an inverse problem for fractals and other sets, Proc. Natl. Acad. Sci. USA 83 , 1975–1977 (1986); M. F. Barnsley, Fractals Everywhere . Academic Press Inc., New York (1988)] for the generation of fractal images. It has also been shown by Berger [M. A. Berger, Random affine iterated function systems: curve generation and wavelets, SIAM Rev . 34 , 361–385 (1992)] and Massopust [P. R. Massopust, Smooth interpolating curves and surfaces generated by iterated function systems, J. Analysis and Applications (Z. Anal. Anwendungen) 12 , 201–210 (1993)] that curves and wavelets may be generated by iterated function systems. In the following paper we introduce an iterated function system (IFS) which exhibits bifurcation from a fixed point, and an IFS which generates a closed curve which undergoes period doubling, exhibits a period-3 orbit, and generates a chaotic attractor as parameters are varied.

Synchronization analysis of voltage-sensitive dye imaging during focal seizures in the rat neocortex

Seizures are often assumed to result from an excess of synchronized neural activity. However, various recent studies have suggested that this is not necessarily the case. We investigate synchronization during focal neocortical seizures induced by injection of 4-aminopyridine (4AP) in the rat neocortex in vivo. Neocortical activity is monitored by field potential recording and by the fluorescence of the voltage-sensitive dye RH-1691. After removal of artifacts, the voltage-sensitive dye (VSD) signal is analyzed using the nonlinear dynamics-based technique of stochastic phase synchronization in order to determine the degree of synchronization within the neocortex during the development and spread of each seizure event. Results show a large, statistically significant increase in synchronization during seizure activity. Synchrony is typically greater between closer pixel pairs during a seizure event; the entire seizure region is synchronized almost exactly in phase. This study represents, to our knowledge, the first application of synchronization analysis methods to mammalian VSD imaging in vivo. Our observations indicate a clear increase in synchronization in this model of focal neocortical seizures across a large area of the neocortex; a sharp increase in synchronization during seizure events was observed in all 37 seizures imaged. The results are consistent with a recent computational study which simulates the effect of 4AP in a neocortical neuron model.

Physical constraints and the evolution of different foraging strategies in aquatic space

Optimal balance between energy usage and feeding is crucial to an animals fitness, and thus is a driving force in the evolution of species. Animals show varieties of different foraging strategies, each adapted to particular ecological and physical constraints. For example, while both the paddlefish and the zooplankton Daphnia filter-feed on patchy food sources, they differ greatly in size and in their methods of propulsion, affecting the degrees of influence of the viscous and inertial forces on these swimmers. To examine the effects of these physical constraints on the evolution of foraging strategies, we modified a recent evolution simulation designed for Daphnia by adding an energy penalty proportional to each turning angle used by modelled foraging agents. This modification accounts for the loss of linear momentum of larger swimmers as they change directions, directly contrasting with Daphnia, which, as smaller, rowing organisms, never build an appreciable momentum since they are halted by the surrounding medium after each forward stroke. While realistic random-walk-like trajectories were predicted by the simulation for Daphnia, distinct circling trajectories and nonzero peaks in the turning angle distributions were predicted for larger species, such as paddlefish. These results are strikingly similar to experimental data also reported here: the circling patterns and bimodal turning angle distributions observed in juvenile paddlefish.

The Nonlinear Dynamics of the crayfish mechanoreceptor System

We review here the nonlinear dynamical properties of the crayfish mechanoreceptor system from the hydrodynamically sensitive hairs on the tailfan through the caudal photoreceptor neurons embedded in the 6th ganglion. Emphasis is on the extraction of low dimensional behavior from the random processes (noise) that dominate this neural system. We begin with stochastic resonance in the sensory root afferents and continue with a discussion of the photoreceptor oscillator and its instabilities. Stochastic synchronization, rectification and the generation of second harmonic responses in the photoreceptors are finally discussed.