Biomedical subjects

J D Enderle

Publications and source records attributed to J D Enderle.

At least 19 recordsLinked to original sources

PubMed1992

Selecting a stimulus signal for linear systems analysis of the vestibulo-ocular reflex.

We evaluated 3 types of stimulus signals for use in estimating the transfer function of the vestibulo-ocular reflex. We used individual sine-wave, sum-of-sine, and pseudorandom stimuli. Five normal human subjects were tested 5 times each using each of the 3 stimulus types. Frequency domain techniques were used to estimate the transfer function at 0.01, 0.03, and 0.05 Hz. The most consistent estimates were obtained using individual sine-wave stimuli. The pseudorandom signal yielded the most variable estimates. A sum-of-sine stimulus composed of 3 sine-wave signals provided estimates slightly more variable than the individual sine-wave stimuli, but much less variable than the pseudorandom stimulus. The redundency of the sine and sum-of-sine stimuli seems to be an advantage by providing stable estimates of the transfer function in the presence of noise.

Adult↗

PubMed1991

A comparison of static and dynamic characteristics between rectus eye muscle and linear muscle model predictions.

The characteristics of a muscle model are analyzed using rectus eye muscle parameter values and compared to rectus eye muscle data. The muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element, and length tension elastic element. Each of the elements is linear and their existence is supported with physiological evidence. The static and dynamic properties of the muscle model are compared to rectus eye muscle data. The length-tension characteristics of the model are in good agreement with the data within the operating region of the muscle. With the muscle model incorporated into a lever system to match the isotonic experiment paradigm, simulation results for this linear system yield a nonlinear force-velocity curve. Moreover, the family of force-velocity curves generated with different stimulus rates reported in the literature match the predictions of the model without parametric changes. The results of this paper are important in studies involving the oculomotor plant and oculomotor neural networks. Additionally, these results may be applicable to other muscles.

Isometric Contraction↗

PubMed1991

Relationships between manual reaction time and saccade latency in response to visual and auditory stimuli.

Manual reaction time (RT) responses were analyzed from seven human subjects. Responses were recorded using four kinds of target presentations: fixed visual target, moving visual target, fixed auditory target, and moving auditory target. Moving targets (moving in the horizontal plane) were presented at constant intensity and provided only a motion cue. Fixed targets "popped on" at the primary position and provided only an onset cue. RTs for the fixed and moving visual targets were 241.5 ms and 233.1 ms, respectively. The 8.4 ms (3.5%) advantage for the moving visual target over the fixed visual target was statistically significant, p less than 0.05. RT for the moving auditory target varied with target movement amplitude and ranged from 219 ms for 40 degree movements to 268 ms for 5 degree movements. For the fixed auditory target in the sagittal plane, average RT was 182.9 ms. Thus, sound-source motion detection was from 36 to 85 ms slower than sound onset detection, p less than 0.001. The RT results were compared to saccade latency measurements from an earlier study. Both RT and saccade latency showed the same dependency upon target movement amplitude. For small target displacements, saccade latencies for the moving auditory target were longer than for the moving visual target. The longer latencies for the moving auditory target are attributed to the increased processing time required to detect the sound-source motion.

Acoustic Stimulation↗

PubMed1991

Digitally calibrated eye movement measurement system.

A less expensive instrumentation alternative to record horizontal eye movements is presented in the form of a feasibility study. Currently, there are several techniques used to accurately measure eye position for research purposes. To implement these techniques, significant financial resources are required. The proposed technique costs less than $3000.00 and is based upon the limbus position technique. A linear charged coupled device (CCD) image array sensor and a biconvex lens are used to determine the position of the pupil. The self scanned CCD array is controlled by four analog clock signals generated from digital logic. The CCD video output signal is converted to a TTL compatible signal. The TTL signal is used to create a digital number based upon the limbus image formation upon the CCD pixels. This digital number is processed using standard signal acquisition techniques and is recorded on the hard drive disk of an IBM PC/AT compatible system. The C programming language is used to acquire the data and control calibration of the system. FORTRAN software is used to implement digital signal processing algorithms and estimate eye position.

Calibration↗

PubMed1991

Saccades simulated with rhesus monkey innervation data.

Extracellular single-unit data from the vicinity of the abducens nucleus from rhesus monkeys recorded during horizontal saccades are used as input to an updated oculomotor plant to simulate saccades to test the robustness of the model. Cells recorded from include: Long Lead Burst neurons, Medium Lead Burst neurons, and Burst Tonic neurons. Eye movement position data was collected using the magnetic coil technique. The oculomotor plant used in this study consists of the lateral and medial rectus muscle, and the eyeball. The muscles are modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element and length tension elastic element. The eyeball is modeled as a sphere, connected to two parallel viscoelastic elements, connected in series. Each of the elements is linear. All parameters and initial conditions are estimated or directly measured from physiological data. The active state tension generator waveforms are the low-passed filtered motoneural signals. The extracellular single-unit data is used to drive the agonist portion of a neural circuit, consisting of burst, tonic and pause cells. The antagonist portion of the neural activity is based on previously reported characteristics. Simulation results for eye position and higher derivatives are in good agreement with the eye position data and the data derived estimates of higher derivatives.

Animals↗

PubMed1991

Optimization of an adaptive nonlinear filter for the analysis of nystagmus.

An adaptive nonlinear digital filter has been designed for the analysis of an eye-movement signal called nystagmus. Nystagmus is a bi-phasic signal consisting of a sequence of tracking eye movements called "slow-phase" interspersed with brief, high-velocity refixation movements called "fast-phase." The objective of the analysis is to separate the nystagmus signal into its fast- and slow-phase components. Specifically, the goal is to produce an evenly sampled estimate of slow-phase velocity (SPV) and an estimate of the peak fast-phase velocity. Classically this has been done using pattern recognition methods that exploit the fact that the fast-phase is a relatively short duration, high-velocity movement compared to the slow-phase. Unfortunately, these velocity and duration differences do not reliably separate the slow- and fast-phases under all conditions, especially when the signal is noisy. We have designed and built an adaptive nonlinear digital filter that easily outperforms the more complex pattern recognition algorithms. This new filter, called an Adaptive Asymmetrically Trimmed-Mean (AATM) filter, works under the assumption that, on the average, the eyes spend more time in slow-phase than in fast-phase. Thus, in any given data segment, most of the data samples are slow-phase samples. By analyzing the amplitude distribution of the data samples in the segment we can determine which of these samples are slow-phase. We used computer generated nystagmus signals contaminated with 3 levels of noise to evaluate the filter. The filter parameters were then optimized using Monte Carlo procedures producing an extremely robust analysis method.

Electronystagmography↗

PubMed1991

The generation of horizontal off-center saccades.

Horizontal saccades are initiated in the lab by a display of nine LEDs spaced five degrees apart and centered with respect to primary position (looking straight ahead). From this setup, there are seventy two combinations of unique saccades. Temporal and spatial information from these recorded saccades are compared to saccades generated from a hypothetical model using the same experiment paradigm. The model uses two neural integrator feedback loops within the premotor circuitry. One is to control the duration of the high frequency burst of neural activity that caused the muscles to contract at saccade velocities and the other to produce the proper neural signal necessary to keep the eye in its new post-saccade position. Depending on the initial position of the eye, different temporal and spatial schemes are necessary to produce the same size saccade. These control strategies are realized by modifying known neural signals within the premotor circuitry.

Humans↗

PubMed1991

Computer modeling and functional evaluation of Venturi flow generators for use in ventilators.

A Venturi has been traditionally used for liquid and gas flow measurements based on Bernoulli and Venturi principles. Application of Venturi as a flow generator in a mechanical ventilator was limited to being a secondary source of flow or configured to compress flexible bags. The main reason for this level of utilization is the considerable flow deterioration of flow output under respiratory system load. A method was hence sought to probe into the possibility of using a Venturi device as the main flow generator with onload flow compensation. The Venturi was mathematically modeled to incorporate the three possible control variables which are controls of jet diameter, entrainment port size and the working pressure of the gas to be entrained. A set of experimental data were then quantitatively compared and evaluated with the results of computer simulation of the TUTSIM model of the system under equivalent conditions. The nature of the results can open up a range of different automatic control schemes and their practical implementation.

Computer Simulation↗

PubMed1991

Epidemic curve characteristics for the Reed-Frost model.

Some useful approximations are developed in this paper that describe large-scale epidemic phenomena for the deterministic and stochastic Reed-Frost model. These characteristics provide an a priori quantitative description of the epidemic curve for the deterministic case, such as a threshold requirement, the total size of the epidemic, and the degree of skewness in the epidemic curve. A Poisson distribution asymptotically describes the distribution of the total size of an epidemic when the relative removal rate is well below threshold for the Reed-Frost chain binomial model. These properties are established by an extension of the large-scale epidemic phenomena of the Kermack-McKendrick model.

Binomial Distribution↗

PubMed1991

A discrete-time communicable disease model with a stochastic contact rate for nonhomogeneous populations.

A discrete-time communicable disease model with a stochastic contact rate for nonhomogeneous populations is described which is capable of simulating the irregular incidence patterns seen in many communicable diseases. The epidemic curve for these contagious diseases is typically characterized by epidemics of varying intensity separated by variable time intervals. The compartmental model is an extension to the Reed-Frost theory with age-specific stochastic contact rates. The population is partitioned into age constant groups and is closed; the births flowing into the first age group equals the deaths from all age groups. The population in each age group is subdivided into four states that characterize the course of an infection: susceptible, incubation, infectious, and immune. The incubation and infectious states are divided further depending on the length of the time period and the statistics of the condition. The age specific contact rates are random variables with a seasonal variation and a population specified density function. An example is presented in which all parameters and distributions of the model are estimated for measles in the city of Baltimore, Maryland for the years 1900 through 1917. The computer calculated epidemic curves adequately describe the incidence pattern of the data with no significant differences noted.

Adolescent↗

PubMed1990

Design of an optimal ventilator.

When a patient is incapable of adequate ventilation by natural processes, mechanical assistance must be provided. Ventilation assures sufficient Oxygen delivery to the organs and tissues of the body and excessive amounts of Carbon Dioxide are not permitted to accumulate. Ventilators are employed continuously or intermittently to improve the ventilation of the lungs. The present work involves design of a ventilator that virtually eliminates any work done by the patients in their breathing process. Presently available ventilators approach this level of performance but they have a few reported shortcomings. These limitations include insufficiency of inspiratory airflow and rigidity of operating cycles. The new ventilator is built around an optimal controller and its performance is based on computational capability of microprocessors. Real time measurements of volume, flow rates and pressure are managed and processed by these devices for optimal determination of controlled parameters of the ensuing respiratory cycle. These measured parameters are used for estimating the lung characteristics such as lung compliance and airway resistance. An independent microprocessor system incorporating data acquisition and processing features with scope for a number of additional capabilities such as graphic output, database and communications functions can prove very useful in health care institutions.

Biomechanical Phenomena↗

PubMed1990

Temporal encoding for the control of saccades.

Pre-motor circuitry responsible for the spatial to temporal transformation in the saccadic eye movement system is modelled using gaussian random variables. Eighty percent of the time between stimulus onset and the end of a saccade is spent with the eyes stationary. This time span is when the brain converts the spatial orientation of the visual stimulus into a temporal code for controlling the duration of the high frequency burst of neural activity innervating the eye muscles. This system controls saccades accurately from all different initial positions. Therefore, a ten degree saccade can have durations varying two-fold depending on the eye's initial position in the orbit, and yet the brain centers receiving the visual input are innervated by the same group of neurons for all ten degree saccades. Multiple and double-step stimulus activity is also investigated. This hypothetical model includes neural pathways from the retina, longitudinal geniculate nucleus, superior colliculus, frontal eye fields, striate cortex and the cerebellum.

Brain↗

PubMed1990

Development of a non-linear smoothing filter for the processing of eye-movement signals.

The analysis of eye-movement (EM) signals poses problems for the designer of smoothing filters since many of the interesting types of EMs are bimodal. For example, optokinetic and/or vestibular stimulation results in an EM pattern called nystagmus consisting of alternating fast- and slow-phase components. Also, saccadic (refixation) EMs do not occur continuously, but are interspersed with periods of fixation. Conventional linear, low-pass filters (both finite impulse response (FIR) and infinite impulse response (IIR) types) smear the boundries between the fast- and slow-phases of nystagmus and the fixation and fast components of saccadic EMs. We have adapted a nonlinear smoothing filter (originally designed to optimize edge preservation in image processing applications) for the smoothing of EM signals. This filter is called a Predictive FIR-Median Hybrid (PFMH) filter. The PFMH filter operates on a moving window of data samples centered at the current point of interest. Several predictive FIR filters are applied to the "upper" and "lower" halves of the window and each are designed to predict the sample value at the center of the window. The median of these FIR filter outputs and the actual center data sample are taken as the PFMH filter output for each window position. By properly choosing the length and structure of the FIR subfilters, a PFMH filter can be designed to smooth a bimodal EM signal without blurring the boundries between the two signal components.

Electrooculography↗

PubMed1990

Additional developments in oculomotor plant modeling.

A new oculomotor plant is presented in this study using an updated third-order linear muscle model. The lateral and medial rectus muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element and length tension elastic element. The eyeball is modeled as a sphere, connected to two parallel viscoelastic elements, connected in series. Each of the elements is linear. The static and dynamic properties of the muscle model are in good agreement with rectus muscle data. The length-tension characteristics of the model match the data within the operating region of the muscle. Simulation results for the muscle model yield hyperbolic shaped force-velocity curves that match the data very well. All parameters and initial conditions are estimated or directly measured from physiological data. The oculomotor plant is derived through direct programming state-space representation by Laplace variable analysis about the operating point or initial eye position. The form of the oculomotor plant makes this representation even more ideal than previous models for use in the development of more sensitive tests of oculomotor pathology and in the description of normal oculomotor function.

Elasticity↗

PubMed1990

Temporal characteristics of saccadic eye movements induced by auditory stimuli.

Records of horizontal saccadic eye movements made in response to auditory stimuli in the absence of any target related visual stimuli were obtained from two normal human subjects. A band-limiting derivative filter was convolved with records of eye position to obtain estimates of eye velocity. Eye position and velocity records were analyzed off-line to determine the characteristics of the audio-ocular response (AOR). The latency of the AOR decreased with increasing target movement amplitude. The AOR also exhibited lower peak velocity and longer duration than previously reported visually induced saccades. The time at peak velocity increased as saccade amplitude increased until for eye movements with amplitudes greater than 15 degrees, time at peak velocity showed little further increase. Finally, the AOR displayed a high incidence of dynamic overshoot for abducting movements of the right eye and a low incidence of dynamic overshoot for adducting movements of the right eye. System parameters estimated using system identification techniques indicate that the pulse portion of the active state tension driving the agonist muscle is of lower magnitude for auditory saccades than for visual saccades.

Eye Movements↗

PubMed1990

Sensitivity analysis of human oculomotor muscle model.

Sensitivity analysis is a procedure for examining the importance of model parameters with respect to the input or output of the model. Presented is a sensitivity analysis of a recently updated fourth order linear homeomorphic oculomotor model. Each muscle is modeled as a viscoelastic parallel combination connected to an active state tension generator, viscosity element and length tension element parallel combination. The eyeball is modeled as a sphere with a moment of inertia which is connected to two voight elements in series. The sensitivity analysis revealed the parameters of the model obtained from mathematical analysis are acceptable to be used as nominal parameters. Consequently, the analytical parameters can vary considerably while allowing the model to perform as empirical data predicts.

Elasticity↗

PubMed1990

NDSU undergraduate design projects for the disabled.

Disabled persons of average means cannot afford custom made devices or adaptations to manufactured devices to meet their individual needs. Consequently, many disabled individuals have not been able to live as independently as society demands. At North Dakota State University, the need to give undergraduate electrical engineering students practical design experience has been fulfilled by requiring the students to design and build adaptive devices for the disabled. Consequently, the successful results of this effort have fulfilled the needs of two diverse human groups--student engineers and the disabled community. Presented is an effective approach used at North Dakota State University to manage senior design students.

Biomedical Engineering↗

PubMed1989

Stochastic variables responsible for observed saccadic variability.

A new stochastic local feedback model of the horizontal saccadic system based on time optimal neural control within the superior colliculus has been previously described. This model uses a premotor neural circuit composed of burst, tonic, and pause cells and innervates a fourth order linear homeomorphic muscle plant. A sequence of saccades recorded from human subjects shows great variability in peak velocity, final position, and amount and type of post-saccadic behavior. This variability is duplicated in a sequence of simulated saccades through the use of random variables within the neural circuitry. The random variables are agonist burst cell magnitude, antagonist post inhibitory rebound burst magnitude and timing, and to a lesser extent muscle saturation magnitude. These four random variables are shown to cause all the observed variability in human saccades, including: trajectory profile, velocity profile, dynamic overshoot, and glissadic overshoot and undershoot.

Eye Movements↗