Hongkeun Kim - Academia.edu (original) (raw)

Papers by Hongkeun Kim

Brain Research, 2018

Functional neuroimaging studies on episodic memory retrieval consistently indicated the activatio... more Functional neuroimaging studies on episodic memory retrieval consistently indicated the activation of the precuneus (PCU), mid-cingulate cortex (MCC), and lateral intraparietal sulcus (latIPS) regions. Although studies typically interpreted these activations in terms of memory retrieval processes, resting-state functional connectivity data indicate that these regions are part of the frontoparietal control network, suggesting a more general, cross-functional role. In this regard, this study proposes a novel hypothesis which suggests that the parietal control network plays a strong role in accommodating the co-occurrence of externally directed cognition (EDC) and internally directed cognition (IDC), which are typically antagonistic to each other. To evaluate how well this dual cognitive processes hypothesis can account for parietal activation patterns during memory tasks, this study provides a cross-function meta-analysis involving 3 different memory paradigms, namely, retrieval success (hit > correct rejection), repetition enhancement (repeated > novel), and subsequent forgetting (forgotten > remembered). Common to these paradigms is that the target condition may involve both EDC (stimulus processing and motor responding) and IDC (intentional remembering, involuntary awareness of previous encounter, or task-unrelated thoughts) strongly, whereas the reference condition may involve EDC to a greater extent, but IDC to a lesser extent. Thus, the dual cognitive processes hypothesis predicts that each of these paradigms will activate similar, overlapping PCU, MCC, and latIPS regions. The results were fully consistent with the prediction, supporting the dual cognitive processes hypothesis. Evidence from relevant prior studies suggests that the dual cognitive processes hypothesis may also apply to nonmemory domain tasks.

2013 13th International Conference on Control, Automation and Systems (ICCAS 2013), 2013

ABSTRACT The noise reduction disturbance observer is an unconventional construction of disturbanc... more ABSTRACT The noise reduction disturbance observer is an unconventional construction of disturbance observer which has the ability of attenuating the high frequency measurement noise as well as the low frequency input disturbance. While retaining the same capabilities, this paper explores a more concise and prevailing version of the noise reduction disturbance observer with a more relaxed almost necessary and sufficient condition for the closed-loop system to be internally stable without requiring the stability of the nominal plant. Moreover, the proposed version is reduced-order compared with the classical disturbance observers in the sense that the two low-pass filters, commonly set to be identical, are now distinct and, as a result, one of them can have order less than the relative degree of the nominal plant. All of which are the direct consequence of a simplified configuration, we additionally present a design method for the two distinct low-pass filters to guarantee the internal stability of the system.

IEEE Conference on Decision and Control and European Control Conference, 2011

ABSTRACT This paper addresses a fundamental property for a class of multi-agent systems, i.e., st... more ABSTRACT This paper addresses a fundamental property for a class of multi-agent systems, i.e., stabilizability of a group of single integrators, having external control inputs, under a fixed and weighted directed network topology. A necessary and sufficient condition for the stabilizability of the multi-agent system is presented. In particular, it is shown that the multi-agent system is stabilizable if and only if the external control inputs are applied to certain agents (e.g., root node of the communication network when the network is connected). The framework proposed here puts an emphasis on its ability in decentralized control; that is, each agent uses its own and its neighbors' state information as feedback, to stabilize the multi-agent system. Based on these results, the decentralized set-point control problem with formation is also addressed.

2008 American Control Conference, 2008

ABSTRACT The Hinfin-optimal control for nonlinear systems is hard to obtain because one must solv... more ABSTRACT The Hinfin-optimal control for nonlinear systems is hard to obtain because one must solve the Hamilton-Jacobi-Isaacs (HJI) equation. To overcome this problem, a nonlinear controller is proposed by Ezal, Pan, and Kokotovic. The controller guarantees local optimality and global inverse optimality, that is, it behaves as a linear optimal controller in the region where the linearized dynamics dominates, and is inverse optimal in the global sense. However, the system class under their consideration is single-input strict-feedback nonlinear systems which is somewhat restrictive. In this paper, we propose a nonlinear optimal controller for a class of multi-input nonlinear systems. Moreover, under the proposed controller, the closed-loop system is globally exponentially stable, whereas the controller proposed by Ezal et al. just guarantees global asymptotic stability.

Technical Digest. CLEO/Pacific Rim '99. Pacific Rim Conference on Lasers and Electro-Optics (Cat. No.99TH8464), 1999

ABSTRACT We performed the in-situ experimental study of nonlinear optical responses and dynamics ... more ABSTRACT We performed the in-situ experimental study of nonlinear optical responses and dynamics for crosslinked- and uncrosslinked-polymers by changing the poling and relaxation profiles. In the characterization both second harmonic generation and photoinduced birefringence were employed. Two newly synthesized polymer structures, P(MMA-Co-MMCN) (0.7:0.3) with chromophores (A) and P(MMA-Co-IPO) (0.5:0.5) without chromophore (B) were adopted as representative polymeric systems. When the dipoles in the two polymeric systems (A) and (B) were thermally crosslinked to the polymer main chain along the electric field, the rotational mobility of dipoles were found to be suppressed, resulting in the improvement of thermal stability. In order to understand the higher thermal stability of the crosslinked polymer system, we investigated the in-situ second harmonic generation response at various temperatures

IEEE Transactions on Automatic Control, 2000

ABSTRACT This technical note studies the output consensus problem for a class of heterogeneous un... more ABSTRACT This technical note studies the output consensus problem for a class of heterogeneous uncertain linear multi-agent systems. All the agents can be of any order (which might widely differ among the agents) and possess parametric uncertainties that range over an arbitrarily large compact set. The controller uses only the output information of the plant; moreover, the delivered information throughout the communication network is also restricted to the output of each agent. Based on the output regulation theory, it is shown that the output consensus is reached if the (state) consensus is achieved within the internal models among the agent's controllers (even though the plant's outputs, rather than the internal model's outputs, are communicated). The internal models can be designed and embedded into the controller, which provides considerable flexibility to designers in terms of the type of signals that are agreed on among the agents.

Automatica, 2010

Some control problems in practice are often formulated as a linear output regulation problem with... more Some control problems in practice are often formulated as a linear output regulation problem with time-varying exosystems. Although this problem has been studied recently, an explicit and constructive solution has been given only for minimum phase systems. This paper ...

Automatica, 2013

ABSTRACT This study addresses the problem of consensus of multi-agent systems, consisting of a se... more ABSTRACT This study addresses the problem of consensus of multi-agent systems, consisting of a set of identical MIMO LTI systems, under a time-varying network that has a well-defined average (with uniform convergence to the average). The information delivered through the communication network is the output of each system. First, it is shown that consensus is reached asymptotically by using a group of compensators if the network switches sufficiently fast and the compensators are designed such that the multi-agent system asymptotically achieves consensus for the average of the network. Further, we find a relation between the two agreements, one obtained from considering the switching network and the other obtained from replacing the network with its average. Then, for a class of minimum phase systems, we remove the fast switching condition by redesigning the compensators. Finally, the formation stabilization of unicycle-type mobile robots is dealt with as an application of the problem, and it is demonstrated via a computer simulation.

Abstract The paper addresses the design problem of a parallel feedforward compensator for a class... more Abstract The paper addresses the design problem of a parallel feedforward compensator for a class of SISO LTI systems, that renders the augmented system to be of minimum phase and have relative degree one. The plant can be of nonminimum phase and/or have high ...

Journal of Korean medical science, 1997

Thirty-three temporal lobe epilepsy (TLE) patients, 19 left TLE and 14 right TLE, underwent an in... more Thirty-three temporal lobe epilepsy (TLE) patients, 19 left TLE and 14 right TLE, underwent an intracarotid amobarbital procedure. For each patient, hemispheric memory laterality was determined by measuring the relative magnitude of recognition memory following left versus right hemisphere injection of sodium amobarbital. The patients were divided into early and late seizure onset groups, based on the median age (13 yrs) of seizure onset of the total sample. Early-onset left TLE was associated with a greater tendency toward right hemispheric representation of both verbal and visual memory compared with late-onset left TLE. Early-onset right TLE was associated with a greater tendency toward left hemispheric representation of visual, but not verbal, memory compared with late-onset right TLE. These findings indicate that interhemispheric plasticity for memory is greater in early than in late life, bidirectional, and at least partially material-specific.

Hippocampus, 2014

The encoding of sensory input is intertwined with external attention, whereas retrieval is intrin... more The encoding of sensory input is intertwined with external attention, whereas retrieval is intrinsically related to internal attention. This study proposes a model in which the encoding of sensory input involves mainly the anterior hippocampus and the external attention network, whereas retrieval, the posterior hippocampus and the internal attention network. This model is referred to as the HERNET (hippocampal encoding/retrieval and network) model. Functional neuroimaging studies have identified two intrinsic large-scale networks closely associated with external and internal attention, respectively. The dorsal attention network activates during any externally oriented mental activity, whereas the default mode network shows increased activity during internally oriented mental activity. Therefore, the HERNET model may predict the activation of the anterior hippocampus and the dorsal attention network during the encoding and activation of the posterior hippocampus and the default mode network during retrieval. To test this prediction, this study provides a meta-analysis of three memory-imaging paradigms: subsequent memory, laboratory-based recollection, and autobiographical memory retrieval. The meta-analysis included 167 individual studies and 2,856 participants. The results provide support for the HERNET model and suggest that the anterior-posterior gradient of encoding and retrieval includes amygdala regions. More broadly, humans continuously oscillate between external and internal attention and thus between encoding and retrieval processes. These oscillations may involve repetitive and spontaneous activity switching between the anterior hippocampus/dorsal attention network and the posterior hippocampus/default mode network. © 2014 Wiley Periodicals, Inc.

Duality and Unity of the Brain, 1987

Neuropsychologia, 1991

Sixty-three subjects (31 dextrals and 32 sinistrals) were given two visual half-field laterality ... more Sixty-three subjects (31 dextrals and 32 sinistrals) were given two visual half-field laterality tasks (recognition of words and faces). Subjects were classified as having typical or atypical hemispheric specialization on the basis of the relative magnitude of their asymmetry scores on the two laterality tasks. Those subjects whose word asymmetry scores were larger than their face asymmetry scores (asymmetry scores computed as RVF-LVF for both tasks) were classified into Group Typical and those subjects whose word asymmetry scores were smaller than their face asymmetry scores were classified into Group Atypical. For both dextrals and sinistrals, the proportion of subjects classified into Groups Typical and Atypical was consistent with estimates of the incidence of typical and atypical hemispheric asymmetry patterns based on sodium amytal testing. These results support the hypothesis that the relative magnitude of asymmetry scores on left and right hemisphere specialized tasks index individual differences in patterns of hemispheric specialization.

c , Daegu, Korea Background: The aim of this study was to identify factors predicting intellectua... more c , Daegu, Korea Background: The aim of this study was to identify factors predicting intellectual and memory changes following anterior temporal lobectomy (ATL) for mesial temporal lobe epilepsy (TLE). Methods: The sample consisted of 31 patients who underwent ATL for treatment of medically intractable TLE. All patients were administered intellectual and memory tests preoperatively and postoperatively. Results: All statistically significant intellectual and memory changes at 1-year follow-up were in the direction of improvement. Left vs. right ATL had significantly differential effects on verbal intelligence and verbal memory, reflecting greater decline (or less improvement) following a left ATL. A later onset age of seizures, an older age at surgery, and a higher presurgical cognitive performance predicted a greater cognitive decline following an ATL. Conclusions: At 1-year after ATL, most cognitive functions showed either no significant changes or significant changes in a favorab...

Journal of Neuroscience, 2007

Although memory confidence and accuracy tend to be positively correlated, people sometimes rememb... more Although memory confidence and accuracy tend to be positively correlated, people sometimes remember with high confidence events that never happened. How can confidence correlate with accuracy but apply also to illusory memories? One possible explanation is that high confidence in veridical versus illusory memories depends on different neural mechanisms. The present study investigated this possibility using functional magnetic resonance imaging and a modified version of the Deese-Roediger-McDermott false-memory paradigm. Participants read short lists of categorized words, and brain activity was measured while they performed a recognition test with confidence rating. The study yielded three main findings. First, compared with low-confidence responses, high-confidence responses were associated with medial temporal lobe (MTL) activity in the case of true recognition but with frontoparietal activity in the case of false recognition. Second, these regions showed significant confidence-by-veridicality interactions. Finally, only MTL regions showed greater activity for high-confidence true recognition than for high-confidence false recognition, and only frontoparietal regions showed greater activity for high-confidence false recognition than for high-confidence true recognition. These findings indicate that confidence in true recognition is mediated primarily by a recollection-related MTL mechanism, whereas confidence in false recognition reflects mainly a familiarity-related frontoparietal mechanism. This account is consistent with the fuzzy trace theory of false recognition. Correlation analyses revealed that MTL and frontoparietal regions play complementary roles during episodic retrieval. In sum, the present study shows that when one focuses exclusively on high-confidence responses, the neural correlates of true and false memory are clearly different.

Neuropsychology, 1994

Although hemispheric asymmetry among individuals is often treated as a categorical variable with ... more Although hemispheric asymmetry among individuals is often treated as a categorical variable with 3 values (i.e., left hemisphere dominance, right hemisphere dominance, and bilateral dominance), it is best viewed as a continuously distributed variable ranging from strong asymmetry in favor of the left hemisphere through nearly equal asymmetry to strong asymmetry in favor of the right hemisphere. The present study

Neuropsychology, 2004

This study investigated the lateralizing value of neuropsychological testing in presurgical evalu... more This study investigated the lateralizing value of neuropsychological testing in presurgical evaluation of mesial temporal lobe epilepsy (MTLE). This study differed from previous ones in that the cutoff values were determined to yield high positive predictive values (PPVs), multiple neuropsychological predictors were considered in combination, and patients with atypical language dominance or low intelligence were not excluded from the sample. The participants were 92 patients with MTLE (left, n ϭ 47; right, n ϭ 45) who showed good postoperative seizure control. With a stringent cutoff criterion, the multiple neuropsychological predictors considered in combination yielded a sensitivity of 15% and a PPV of 93%, and with a less stringent cutoff criterion, a sensitivity of 37% and a PPV of 83%.