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Biomedical subjects

J M Nichols

Publications and source records attributed to J M Nichols.

At least 19 recordsLinked to original sources
PubMed2005

Detecting nonlinearity in structural systems using the transfer entropy.

The transfer entropy was recently proposed as a means of exploring coupling in dynamical systems. Transfer entropy is an information theoretic that quantifies the degree to which one dynamical process affects the transition probabilities (dynamics) of another. Here we demonstrate how this metric may be utilized to detect the presence of nonlinearity in a system. Using the method of surrogate data, the transfer entropy computed at various lag times are compared to values computed from linearized surrogates. The transfer entropy is shown to be a more sensitive indicator of nonlinearity than is the mutual information for both simulated and experimental data. This technique is particularly applicable to the field of structural health monitoring, where damage is often equated with the presence of a nonlinearity in an otherwise linear system.

Journal Article↗
PubMed2005

Inferences about information flow and dispersal for spatially extended population systems using time-series data.

This work explores an information-theoretic approach to drawing inferences about coupling of spatially extended ecological populations based solely on time-series of abundances. The efficacy of the approach, time-delayed mutual information, was explored using a spatially extended predator-prey model system in which populations at different patches were coupled via diffusive movement. The approach identified the relative magnitude and direction of information flow resulting from animal movement between populations, the change in information flow as a function of distance separating populations, and the diffusive nature of the information flow. In addition, when the diffusive movement was eliminated from the model, mutual information correctly provided no evidence of information flow, even when population synchrony was generated by a common environmental driving function. Thus, for this model system, time-delayed mutual information was useful in discriminating between the Moran effect and animal movement as causes of population synchrony, as well as in characterizing dispersal in terms of direction, relative speed and diffusive nature.

Computer Simulation↗
PubMed2005

Assessing spatial coupling in complex population dynamics using mutual prediction and continuity statistics.

A number of important questions in ecology involve the possibility of interactions or "coupling" among potential components of ecological systems. The basic question of whether two components are coupled (exhibit dynamical interdependence) is relevant to investigations of movement of animals over space, population regulation, food webs and trophic interactions, and is also useful in the design of monitoring programs. For example, in spatially extended systems, coupling among populations in different locations implies the existence of redundant information in the system and the possibility of exploiting this redundancy in the development of spatial sampling designs. One approach to the identification of coupling involves study of the purported mechanisms linking system components. Another approach is based on time series of two potential components of the same system and, in previous ecological work, has relied on linear cross-correlation analysis. Here we present two different attractor-based approaches, continuity and mutual prediction, for determining the degree to which two population time series (e.g., at different spatial locations) are coupled. Both approaches are demonstrated on a one-dimensional predator-prey model system exhibiting complex dynamics. Of particular interest is the spatial asymmetry introduced into the model as linearly declining resource for the prey over the domain of the spatial coordinate. Results from these approaches are then compared to the more standard cross-correlation analysis. In contrast to cross-correlation, both continuity and mutual prediction are clearly able to discern the asymmetry in the flow of information through this system.

Models, Theoretical↗
PubMed2004

Using chaotic interrogation and attractor nonlinear cross-prediction error to detect fastener preload loss in an aluminum frame.

Structural health monitoring is an important field concerned with assessing the current state (or "health") of a structural system or component with regard to its ability to perform its intended function appropriately. One approach to this problem is identifying appropriate features obtained from time series vibration responses of the structure that change as structural degradation occurs. In this work, we present a novel technique adapted from the nonlinear time series prediction community whereby the structure is excited by an applied chaotic waveform, and predictive maps built between structural response attractors are used as the feature space. The structural response is measured at several points on the structure, and pairs of attractors are used to predict each other. As the dynamics of the structure change due to damage, the prediction error rises. This approach is applied to detecting the preload loss in a bolted joint in an aluminum frame structure.

Journal Article↗
PubMed2003

Controlling system dimension: a class of real systems that obey the Kaplan-Yorke conjecture.

The Kaplan-Yorke conjecture suggests a simple relationship between the fractal dimension of a system and its Lyapunov spectrum. This relationship has important consequences in the broad field of nonlinear dynamics where dimension and Lyapunov exponents are frequently used descriptors of system dynamics. We develop an experimental system with controllable dimension by making use of the Kaplan-Yorke conjecture. A rectangular steel plate is driven with the output of a chaotic oscillator. We controlled the Lyapunov exponents of the driving and then computed the fractal dimension of the plate's response. The Kaplan-Yorke relationship predicted the system's dimension extremely well. This finding strongly suggests that other driven linear systems will behave similarly. The ability to control the dimension of a structure's vibrational response is important in the field of vibration-based structural health monitoring for the robust extraction of damage-sensitive features.

Models, Theoretical↗
PubMed2003

Use of chaotic excitation and attractor property analysis in structural health monitoring.

This work explores the utility of attractor-based approaches in the field of vibration-based structural health monitoring. The technique utilizes the unique properties of chaotic signals by driving the structure directly with the output of a chaotic oscillator. Using the Kaplan-Yorke conjecture, the Lyapunov exponents of the driving signal may be tuned to the dominant eigenvalues of the structure, thus controlling the dimension of the structural response. Data are collected at various stages of structural degradation and a simple nonlinear model, constructed from the undamaged data, is used to make predictions for the damaged response data. Prediction error is then introduced as a "feature" for classifying the magnitude of the damage. Results are presented for an experimental cantilevered beam instrumented with fiber-optic strain sensors.

Journal Article↗
PubMed2001

Fatigue of elbow flexor muscles in younger and older adults.

The aim of this study was to assess differences in the behavior of fatigue-related measures of neuromuscular function between younger (n = 10; 20-35 years) and older (n = 11; >65 years) healthy adults. Measures reflecting changes in voluntary activation, neuromuscular propagation, metabolite build-up, and excitation-contraction coupling processes were taken before, during, and after a sustained maximum elbow-flexion fatigue task. We found a greater role for a failure in voluntary activation (central fatigue) in about half of the older subjects compared with none of the younger subjects to explain the decrease in force-generating capacity with sustained activity. In contrast, similar behaviors in measures reflecting changes in peripheral mechanisms were noted for the two age groups. These results point to a potential shift in fatigue mechanisms with age, with a greater contribution of central fatigue in some older adults.

Adult↗
PubMed2001

Attractor reconstruction for non-linear systems: a methodological note.

Attractor reconstruction is an important step in the process of making predictions for non-linear time-series and in the computation of certain invariant quantities used to characterize the dynamics of such series. The utility of computed predictions and invariant quantities is dependent on the accuracy of attractor reconstruction, which in turn is determined by the methods used in the reconstruction process. This paper suggests methods by which the delay and embedding dimension may be selected for a typical delay coordinate reconstruction. A comparison is drawn between the use of the autocorrelation function and mutual information in quantifying the delay. In addition, a false nearest neighbor (FNN) approach is used in minimizing the number of delay vectors needed. Results highlight the need for an accurate reconstruction in the computation of the Lyapunov spectrum and in prediction algorithms.

Algorithms↗
PubMed1996

The effect of lorazepam on memory and event-related potentials in heavy and light social drinkers.

To investigate the effects of heavy social drinking on sober cognitive processing, event-related potentials were recorded from 13 heavy social drinkers and 13 light social drinkers in the presence and absence of a pharmacological challenge (i.e., lorazepam). Event-related potentials were elicited by a task requiring continuous recognition memory for visually presented words. The heavy social drinkers exhibited shorter P2 latencies than the light social drinkers and decreased P300 latencies to correctly identified "old" words (i.e., words presented previously in the task) versus "new" words. Lorazepam increased motor reaction time to correctly identified old words and produced a deficit in recognition memory only in the light social drinkers. Light social drinkers had an increased P300 latency and a larger P300 amplitude to new words in the lorazepam treatment. The differences in cognitive functioning evident between heavy and light social drinkers were reflected in event-related potential deviations and appear to indicate a tolerance in heavy social drinkers to the effects of lorazepam.

Adult↗
PubMed1996

The effect of heavy social drinking on recall and event-related potentials.

OBJECTIVE: It has been suggested that alcohol-related brain impairments progress on a continuum from subtle deficits in social drinkers through to end-state Korsakoff syndrome. The aim of the present study was to investigate whether heavy levels of social drinking have an adverse effect on sober cognitive functioning in young nonalcoholic adults. METHOD: Event-related potentials (ERPs) were recorded from heavy ( > 200 gm/week) and light ( < 20 gm/week) male social drinkers under the effects of a pharmacological challenge (lorazepam-ATIVAN) and a placebo. The heavy (n = 14) and light (n = 14) social drinkers completed a free-recall task with a rare word probability of .30. RESULTS: Analysis of the P300 component of the ERP to recalled and not-recalled words found that P300 amplitude in heavy social drinkers (HSDs) was reduced following placebo compared to the light social drinkers (LSDs). Lorazepam produced a distinctive pattern of anterograde memory deficits in both groups and reduced P300 amplitude to rare words in the LSDs compared to both the placebo treatment and the HSDs. CONCLUSIONS: The differences in central nervous system functioning evident between HSDs and LSDs were reflected in ERP deviations in both the presence and absence of lorazepam. Even though no definite statements can be made because of the small sample size, the results appear to indicate that HSDs have an impairment in their information-processing ability in the absence of an acute inhibitory agent (e.g., alcohol or a benzodiazepine) as well as a differential response to the depressant effects of lorazepam.

Adult↗
PubMed1993

A comparison of the effect of lorazepam on memory in heavy and low social drinkers.

The cognitive deficits, particularly memory impairment, observed in association with organic brain damage caused by chronic alcohol ingestion, are consistent with the profile of benzodiazepine-induced amnesia. This study examined the cognitive capabilities of a group of heavy social drinkers (n = 11) and a group of low social drinkers (n = 11) under the influence of a pharmacological challenge (lorazepam 2 mg) and a placebo treatment. Lorazepam impaired visual memory and verbal learning in both groups, but the effect of lorazepam was exacerbated in the heavy social drinkers for delayed recall of verbal material. Heavy social drinkers had lower verbal fluency scores and were less able to copy complex figures than low social drinkers whether or not the pharmacological challenge was present. Lorazepam induced deficits, in both groups, which confirmed to the classic profile of those observed in benzodiazepine-induced amnesia. The deficits, both in the absence and presence of lorazepam, shown by heavy social drinkers suggest that changes may have occurred in their brain functioning.

Adult↗
PubMed1989

Long-term deficits in water maze spatial conditional alternation performance following retrohippocampal lesions in rats.

The effects of large bilateral retrohippocampal lesions on long-term performance of conditional spatial alternation, incorporating a strong working memory component, were examined using a T-maze task motivated by swim-escape. The lesions, which included entorhinal cortex, subiculum, pre- and parasubiculum and invaded the molecular layer of the dentate gyrus, completely eliminated the previously acquired conditional alternation learning, and performance failed to recover with 40 days of testing. These findings support the contention that retrohippocampal structures are an important and necessary component of the neural circuitry mediating working memory.

Animals↗
PubMed1989

Genetic influences on brain growth restriction induced by development exposure to alcohol.

Genetic factors have been implicated as contributing to the considerable variation in the severity of alcohol-related birth defects in offspring of women who drink heavily during pregnancy. Two animal models of alcohol-related developmental effects incorporated different behavior genetic approaches to examine genetic influences on brain and body growth following alcohol exposure during development. The first, extensively developed in Sprague-Dawley rats, examined the effects of three doses of alcohol administered to two inbred rat strains (MR and M520) via artificial-rearing procedures during the early postnatal brain growth spurt. In both strains, alcohol produced a dose-dependent restriction of brain weight (but not body weight) on postnatal day 10, compared to artificially reared controls. The MR strain was more susceptible to cerebellar growth restriction than the M520 strain, an effect not attributable to strain differences in blood alcohol concentrations. In the second model, pregnant female Long-sleep and Short-sleep mice, selectively bred for differences in initial sensitivity to the hypnotic effects of acute alcohol administration, were intubated with ethanol from gestational days 7-18. Controls included either sucrose or maltose/dextrin intubation controls and non-intubated controls. The LS offspring showed growth deficits and brain weight reductions in adulthood, while the SS offspring were resistant to these detrimental effects of the prenatal alcohol exposure. Thus, differences in either maternal or fetal genotype may contribute to individual differences in the severity of the effects of alcohol exposure during development.

Animals↗
PubMed1977

Isolation of mitochondria and mitochondrial RNA from Crithidia fasciculata.

Two methods were used to isolate mitochondria from Crithidia fasciculata. In the first method, cells were weakened by exposure to hypotonic conditions and then disrupted by blending; mitochondria were subsequently isolated using disodium 3,5-diacetoamido-2,4,6-triiodobenzoate gradients. In the second, cells were treated with digitonin before disruption; mitochondria were purified by differential centrifugation. Both preparations were examined with the electron microscope and were also shown to possess several characteristic biochemical properties of mitochondria. Kinetoplast DNA was present in the mitochondria, uncontaminated by nuclear DNA. Analysis by polyacrylamide gel electrophoresis showed two RNA components of molecular weights of 0-47 X 10(6) and 0-22 X 10(6), in addition to cytoplasmic RNA contamination. Four mitochondrial components with sedimentation coefficients of 14-6S, 11-4S, 10-1S and 9-9S were identified on sucrose density gradients. Ethidium bromide abolished the incorporation of [5-3H]uridine into the presumed mitochondrial RNA.

Cell Fractionation↗
PubMedDate not supplied

P300 in heavy social drinkers: the effect of lorazepam.

The P300 component of the event-related potential (ERP) and reaction time (RT) were recorded during a simulated driving task using an oddball paradigm. ERPs and RTs were recorded from heavy social drinkers (n = 11) and low social drinkers (n = 11). A pharmacological challenge (lorazepam-ATIVAN) was administered to both groups in a double-blind procedure. In both groups, P300 amplitude was reduced and RT was increased by the presence of lorazepam; however, heavy social drinkers had longer latency P300 than low social drinkers regardless of the drug condition. The P300 amplitude results are consistent with reduced information processing being induced by lorazepam, or with reduced effectiveness of the eliciting stimuli. On the other hand, the P300 latency results suggest that P300 latency may reflect deficits in information processing induced by alcohol abuse or may have preceded the alcohol abuse. The P300 latency results are consistent with heavy social drinkers occupying an early point on the hypothesized continuum of alcohol-related brain damage.

Adult↗
PubMedDate not supplied

The effect of lorazepam on long-term verbal recall in heavy and light social drinkers.

Two experiments investigated long-term verbal memory performance in groups of 20-year-old heavy (HSDs) and light social drinkers (LSDs), in the presence and absence of a pharmacological challenge (lorazepam 2 mg). In Experiment 1 (n = 13), a verbal learning task was presented visually and it was found that lorazepam significantly impaired delayed verbal recall performance in both groups. Experiment 2 (n = 14) assessed the effect of presenting the verbal learning task in the auditory compared to the visual modality. Both groups' performance on the delayed trials of the visually presented task was reduced in the lorazepam treatment. However, in the auditory presented task, lorazepam reduced 30-min delayed recall performance in the HSDs but not in the LSDs. The differential effect of lorazepam on HSDs compared to LSDs on delayed recall performance when material is presented in the auditory modality may suggest that frequent heavy social drinking results in changes in CNS functioning.

Adult↗