IU Indianapolis ScholarWorks :: Browsing by Author "Ahn, Sungwoo"

Browsing by Author "Ahn, Sungwoo"

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Cortex – basal ganglia synchronization in Parkinson’s disease
(Office of the Vice Chancellor for Research, 2014-04-11) Zauber, S. Elizabeth; Ahn, Sungwoo; Worth, Robert M.; Witt, Thomas C.; Rubchinsky, Leonid L.
Increased synchrony in the beta band in cortico-basal ganglia circuits is well described in patients with PD. Less is known, however, about how these abnormal firing patterns are correlated across these brain regions. In this study we investigated how this intra-operative data recorded from STN correlates with scalp recorded EEG. Intraoperative single unit recordings and LFPs were obtained from STN and scalp EEG recordings were collected from four electrodes positioned over prefrontal and motor areas. We computed the STN spike-LFP (Local Filed Potential) phase synchrony over short temporal windows as it fluctuates in time. We also computed the EEG phase synchrony index time series for all 6 pairs of EEG electrodes. Next we explored cross-correlation between the two synchrony level time-series of the spike-LFP vs. EEG pairs. EEG synchrony was found to be correlated with spike-LFP synchrony. Correlation between surface EEG and STN was strongest for ipsilateral EEG and STN recordings. Spike-LFP synchronization is believed to characterize the input-output characteristics of STN dynamics and to be strongly relevant to the expression of motor symptoms. Our results indicate that non-invasive and relatively simple EEG recordings retain some information about synchronous dynamics in the subcortical regions, which can be access only in an invasive manner during functional neurosurgical procedures.
Dynamics of desynchronized episodes in intermittent synchronization
(Frontiers, 2014-06) Rubchinsky, Leonid L.; Ahn, Sungwoo; Park, Choongseok; Department of Mathematical Sciences, School of Science
Intermittent synchronization is observed in a variety of different experimental settings in physics and beyond and is an established research topic in nonlinear dynamics. When coupled oscillators exhibit relatively weak, intermittent synchrony, the trajectory in the phase space spends a substantial fraction of time away from a vicinity of a synchronized state. Thus to describe and understand the observed dynamics one may consider both synchronized episodes and desynchronized episodes (the episodes when oscillators are not synchronous). This mini-review discusses recent developments in this area. We explain how one can consider variation in synchrony on the very short time-scales, provided that there is some degree of overall synchrony. We show how to implement this approach in the case of intermittent phase locking, review several recent examples of the application of these ideas to experimental data and modeling systems, and discuss when and why these methods may be useful.
Dynamics of synchronized neural activity in prefrontal-hippocampal networks during behavioral sensitization
(Office of the Vice Chancellor for Research, 2013-04-05) Ahn, Sungwoo; Lapish, Christopher C.; Rubchinsky, Leonid L.
Neural synchrony exhibits temporal variability, therefore the temporal patterns of synchronization and desynchronization may have functional relevance. This study employs novel time-series analysis to explore how neural signals become transiently phase locked and unlocked during repeated injections of the psychostimulant, D-Amphetamine (AMPH). Short (but frequent) desynchronized events dominate synchronized dynamics in each of the animals we examined. After the first AMPH injection, only increases in the relative prevalence of short desynchronization episodes (but not in average synchrony strength) were significant. Throughout sensitization both strength and the fine temporal structure of synchrony (measured as relative prevalence of short desynchronizations) were similarly altered with AMPH injections, with each measure decreasing in the pre-injection epoch and increasing after injection. Decoupling between locomotor activity and synchrony was observed in AMPH, but not saline, animals. The increase in numerous short desynchronizations (as opposed to infrequent, but long desynchronizations) in AMPH treated animals may indicate that synchrony is easy to form yet easy to break. These data yield novel insight into how synchrony is dynamically altered in cortical networks by AMPH and identify neurophysiological changes that may be important to understand the behavioral pathologies of addiction.
Dysregulation of temporal dynamics of synchronous neural activity in adolescents on autism spectrum
(Wiley, 2020-01) Malaia, Evie A.; Ahn, Sungwoo; Rubchinsky, Leonid L.; Mathematical Sciences, School of Science
Autism spectrum disorder is increasingly understood to be based on atypical signal transfer among multiple interconnected networks in the brain. Relative temporal patterns of neural activity have been shown to underlie both the altered neurophysiology and the altered behaviors in a variety of neurogenic disorders. We assessed brain network dynamics variability in autism spectrum disorders (ASD) using measures of synchronization (phase-locking) strength, and timing of synchronization and desynchronization of neural activity (desynchronization ratio) across frequency bands of resting-state electroencephalography (EEG). Our analysis indicated that frontoparietal synchronization is higher in ASD but with more short periods of desynchronization. It also indicates that the relationship between the properties of neural synchronization and behavior is different in ASD and typically developing populations. Recent theoretical studies suggest that neural networks with a high desynchronization ratio have increased sensitivity to inputs. Our results point to the potential significance of this phenomenon to the autistic brain. This sensitivity may disrupt the production of an appropriate neural and behavioral responses to external stimuli. Cognitive processes dependent on the integration of activity from multiple networks maybe, as a result, particularly vulnerable to disruption.
Fine temporal structure of neural synchronization
(Office of the Vice Chancellor for Research, 2014-04-11) Ahn, Sungwoo; Rubchinsky, Leonid L.
While neural synchronization is widely observed in neuroscience, neural oscillations are rarely in perfect synchrony and go in and out of phase in time. Since this synchrony is not perfect, the same synchrony strength may be achieved with markedly different temporal patterns of activity (roughly speaking oscillations may go out of the phase-locked state for many short episodes or few long episodes). Provided that there is some average level of phase-locking is present, one can follow oscillations from cycle to cycle and to observe if the phase difference is close to the preferred phase lag or not. Here we study neural oscillations recorded by EEG in alpha and beta frequency bands in a large sample of healthy human subjects at rest and during the execution of a simple motor task. While the phase-locking strength depends on many factors, dynamics of synchrony has a very specific temporal pattern: synchronous states are interrupted by frequent, but short desynchronization episodes. The probability for a desynchronization episode to occur decreased with its duration. The modes and medians of distributions of desynchronization durations were always just one cycle of oscillations. Similar temporal patterning of synchrony in different brain areas in different states may suggest that i) this type of patterning is a generic phenomenon in the brain, ii) it may have some functional advantages for oscillating neural networks receiving, processing, and transmitting information, iii) it may be grounded in some general properties of neuronal networks calling for the development of appropriate nonlinear dynamical theory. To further investigate these conjectures we numerically studied a system of coupled simple neuronal models (of Morris-Lecar type) and showed that coupled neural oscillators exhibiting short desynchronizations require smaller values of synaptic connections between them of weaker common synaptic input to induce specified levels of synchrony strength than oscillators of the same frequency exhibiting more prolong desynchronizations. The results may suggests that whenever a (partially) synchronous cell assembly must be formed to facilitate some function, short desynchronization dynamics may allow for efficient formation and break-up of such an assembly.
Interaction of synchronized dynamics in cortical and subcortical circuits in Parkinson’s disease
(Office of the Vice Chancellor for Research, 2015-04-17) Ahn, Sungwoo; Zauber, S. Elizabeth; Worth, Robert M.; Witt, Thomas; Rubchinsky, Leonid L.
Parkinson’s disease pathophysiology is marked by increased oscillatory and synchronous activity in the beta frequency band in cortical and basal ganglia circuits. This study explores the functional connections between synchronized dynamics of cortical areas and dynamics of subcortical areas in Parkinson’s disease. We simultaneously recorded neuronal units (spikes) and local field potentials (LFP) from subthalamic nucleus (STN), and electroencephalograms (EEGs) from the scalp in parkinsonian patients and analyzed the correlation between the time-courses of the spike-LFP synchronization and inter-electrode EEG synchronization. We found the (noninvasively obtained) time-course of the synchrony strength between EEG electrodes and the (invasively obtained) time-course of the synchrony between spiking unit and LFP in STN to be weakly, but significantly correlated with each other. This correlation is largest for the bilateral motor EEG synchronization followed by bilateral frontal EEG synchronization. Our observations suggest that there may be multiple functional modes by which the cortical and basal ganglia circuits interact with each other in Parkinson’s disease: not only synchronization may be observed between some areas in cortex and the basal ganglia, but also synchronization within cortex and within basal ganglia may be related, suggesting potentially more global way of functional interaction. More coherent dynamics in one brain region may modulate or activate the dynamics of another brain region in a more powerful way causing correlations between changes in synchrony strength in both regions.
Mathematical model of subthalamic nucleus neuron: Characteristic activity patterns and bifurcation analysis
(AIP, 2021-11) Park, Choongseok; Rubchinsky, Leonid L.; Ahn, Sungwoo; Mathematical Sciences, School of Science
The subthalamic nucleus (STN) has an important role in the pathophysiology of the basal ganglia in Parkinson's disease. The ability of STN cells to generate bursting rhythms under either transient or sustained hyperpolarization may underlie the excessively synchronous beta rhythms observed in Parkinson's disease. In this study, we developed a conductance-based single compartment model of an STN neuron, which is able to generate characteristic activity patterns observed in experiments including hyperpolarization-induced bursts and post-inhibitory rebound bursts. This study focused on the role of three currents in rhythm generation: T-type calcium (CaT) current, L-type calcium (CaL) current, and hyperpolarization-activated cyclic nucleotide-gated (HCN) current. To investigate the effects of these currents in rhythm generation, we performed a bifurcation analysis using slow variables in these currents. Bifurcation analysis showed that the HCN current promotes single-spike activity patterns rather than bursting in agreement with experimental results. It also showed that the CaT current is necessary for characteristic bursting activity patterns. In particular, the CaT current enables STN neurons to generate these activity patterns under hyperpolarizing stimuli. The CaL current enriches and reinforces these characteristic activity patterns. In hyperpolarization-induced bursts or post-inhibitory rebound bursts, the CaL current allows STN neurons to generate long bursting patterns. Thus, the bifurcation analysis explained the synergistic interaction of the CaT and CaL currents, which enables STN neurons to respond to hyperpolarizing stimuli in a salient way. The results of this study implicate the importance of CaT and CaL currents in the pathophysiology of the basal ganglia in Parkinson's disease.
Neural synchronization: Average strength vs. temporal patterning
(Elsevier, 2018-04) Ahn, Sungwoo; Zauber, S. Elizabeth; Worth, Robert M.; Witt, Thomas; Rubchinsky, Leonid L.; Mathematical Sciences, School of Science
Potential Mechanisms and Functions of Intermittent Neural Synchronization
(Frontiers, 2017-05-30) Ahn, Sungwoo; Rubchinsky, Leonid L.; Mathematical Sciences, School of Science
Neural synchronization is believed to play an important role in different brain functions. Synchrony in cortical and subcortical circuits is frequently variable in time and not perfect. Few long intervals of desynchronized dynamics may be functionally different from many short desynchronized intervals although the average synchrony may be the same. Recent analysis of imperfect synchrony in different neural systems reported one common feature: neural oscillations may go out of synchrony frequently, but primarily for a short time interval. This study explores potential mechanisms and functional advantages of this short desynchronizations dynamics using computational neuroscience techniques. We show that short desynchronizations are exhibited in coupled neurons if their delayed rectifier potassium current has relatively large values of the voltage-dependent activation time-constant. The delayed activation of potassium current is associated with generation of quickly-rising action potential. This “spikiness” is a very general property of neurons. This may explain why very different neural systems exhibit short desynchronization dynamics. We also show how the distribution of desynchronization durations may be independent of the synchronization strength. Finally, we show that short desynchronization dynamics requires weaker synaptic input to reach a pre-set synchrony level. Thus, this dynamics allows for efficient regulation of synchrony and may promote efficient formation of synchronous assemblies.
Short desynchronization epochs in neural synchronization: detection, mechanisms, and functions
(2015-12-18) Rubchinsky, Leonid L; Ahn, Sungwoo; Department of Mathematical Sciences, School of Science

Browsing by Author "Ahn, Sungwoo"

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