Dynamics of the disparity vergence fusion sustain component

The stereotypical vergence response to a step stimulus consists of two dynamic components: a high velocity fusion initiating component followed by a slower component that may mediate sustained fusion.  The initial component has been well-studied and is thought to be controlled by an open-loop mechanism. Less is known about the slow, or fusion sustaining component except that it must be feedback controlled to achieve the positional precision of sustained fusion.  Given the delays in disparity vergence control, a feedback control system is likely to exhibit oscillatory behavior.  Vergence responses to 4 deg step changes in target position were recorded in eight subjects. The slow component of each response was isolated manually using interactive graphics and the frequency spectrum determined.  The frequency spectra of all isolated slow vergence movements showed a large low frequency peak between 1.0 and 2.0 Hz and one or more higher frequency components.  The higher frequency components were found to be harmonics of the low frequency oscillation.  A feedback model of the slow component was developed consisting of a time delay, an integral/derivative controller and an oculomotor plant based on Robinson’s model.  Model simulations showed that a direction dependent asymmetry in the derivative element was primarily responsible for the higher frequency harmonic components. Simulations also showed that the base frequencies are primarily dependent on the time delay in the feedback control system. The fact that oscillatory behavior was found in all subjects provides strong support that the slow, fusion sustaining component is mediated by a feedback system

Error Correction in Vergence Eye Movements: Evidence Supporting Hering’s Law

In pure symmetrical vergence eye movements, a fusion initiating component quickly brings the eyes close to the desired position. A small error usually remains after this response which must be corrected to attain the small final vergence error (i.e., fixation disparity). Error correction will usually involve both version and version movements so possible mechanisms include: small saccades, smooth pursuit, symmetrical vergence, or some combination. Alternatively, an asymmetrical vergence or uniocular slow eye movement could be used to achieve the highly precise final position. Saccade-free late fusion sustaining components during the steady state to a symmetrical vergence step stimulus are analyzed using independent component analysis. Results suggest that fine correction is most likely the product of closely coordinated version and vergence components

Assessment of Dual-Mode and Switched-Channel Models with Experimental Vergence Responses

Controversy exists in the literature regarding the basic neural control structure that mediates convergence responses. This study constructed and simulated two models, the switched-channel feedback model and the dual-mode model consisting of preprogrammed with feedback control. Models were constructed and compared to experimental data. The stimuli consisted of 2 deg and 4 deg vergence steps. Both closed- and open-loop settings were utilized. After parameter adjustment, both models could accurately simulate step responses from subjects having a range of response dynamics. The model with a preprogrammed element required less parameter modification when stimulus amplitude changed. Both models could accurately simulate some attributes of vergence; however, neither model could represent the modifications commonly observed within the transient portion of the vergence response

Vergence fusion sustaining oscillations

Introduction:  Previous studies have shown that the slow, or fusion sustaining, component of disparity vergence contains oscillatory behavior.  Given the delays in disparity vergence control, a feedback control system would be expected to exhibit oscillations following the initial transient period.  This study extends the examination of this behavior to a wider range of frequencies and a larger number of subjects.  Methods:  Disparity vergence responses to symmetrical 4.0 deg step changes in target position were recorded in 15 subjects. Approximately three seconds of the late component of each response were isolated using interactive graphics and the frequency spectrum calculated.  Peaks in these spectra associated with oscillatory behavior were identified and examined.  Results: All subjects exhibited oscillatory behavior with primary frequencies ranging between 0.45 and 0.6 Hz; much lower than those identified in the earlier study.  All responses showed significant higher frequency components.  These higher frequency components were related in both frequency and amplitude with the primary frequency indicating that they are harmonics probably generated by nonlinearities in the neural control processes. A correlation was found across subjects between the amplitude of the primary frequency and the maximum velocity of the fusion initialing component probably due the gain of shared neural pathways. Conclusion:  Low frequency oscillatory behavior was found in all subjects adding support that the slow, or fusion sustaining, component is mediated by a feedback control. Data have clinical implications in that dysfunction in feedback control may manifest as additional vergence error which may be reflected in the frequency spectrum

Correction of Saccade-Induced Midline Errors in Responses to Pure Disparity Vergence Stimuli.

Purely symmetrical vergence stimuli aligned along the midline (cyclopean axis) require only a pure vergence response. Yet, in most responses saccades are observed and these saccades must either produce an error in the desired midline response or correct an error produced by asymmetry in the vergence response. A previous study (Semmlow, et al. 2008) has shown that the first saccade to appear in a response to a pure vergence stimulus usually increased the deviation from the midline, although all subjects (N = 12) had some responses where the initial saccade corrected a vergence induced midline error. This study focuses on those responses where the initial saccade produces an increased midline deviation and the resultant compensation that ultimately brings the eyes to the correct binocular position. This correction is accomplished by a higher level compensatory mechanism that uses offsetting asymmetrical vergence and/or corrective saccades. While responses consist of a mixture of the two compensatory mechanisms, the dominant mechanism is subject-dependent. Since fixation errors are quite small (minutes of arc), some feedback controlled physiological process involving smooth eye movements, and possibly saccades, must move the eyes to reduce binocular error to fixation disparity levels

Model validation for a noninvasive arterial stenosis detection problem

Copyright @ 2013 American Institute of Mathematical SciencesA current thrust in medical research is the development of a non-invasive method for detection, localization, and characterization of an arterial stenosis (a blockage or partial blockage in an artery). A method has been proposed to detect shear waves in the chest cavity which have been generated by disturbances in the blood flow resulting from a stenosis. In order to develop this methodology further, we use both one-dimensional pressure and shear wave experimental data from novel acoustic phantoms to validate corresponding viscoelastic mathematical models, which were developed in a concept paper [8] and refined herein. We estimate model parameters which give a good fit (in a sense to be precisely defined) to the experimental data, and use asymptotic error theory to provide confidence intervals for parameter estimates. Finally, since a robust error model is necessary for accurate parameter estimates and confidence analysis, we include a comparison of absolute and relative models for measurement error.The National Institute of Allergy and Infectious Diseases, the Air Force Office of Scientific Research, the Deopartment of Education and the Engineering and Physical Sciences Research Council (EPSRC)

Signals and systems for bioengineers : a MATLAB-based introduction / John Semmlow.

Rev. ed. of.: Circuits, signals, and systems for bioengineers / John Semmlow. c2005.Includes bibliographical references and index.Book fair2012xii, 591 p. :Signals and Systems for Bioengineers, Second Edition, is the only textbook that relates important electrical engineering concepts to biomedical engineering and biological studies. It explains in detail the basic engineering concepts that underlie biomedical systems, medical devices, biocontrol, and biosignal analysis. It is perfect for the one-semester bioengineering course usually offered in conjunction with a laboratory on signals and measurements which presents the fundamentals of systems and signal analysis. The target course occupies a pivotal position in the bioengineering curriculum and will play a critical role in the future development of bioengineering students. This book provides increased coverage of time-domain signal analysis as well as biomeasurement, using examples in ultrasound and electrophysiology. It also presents new applications in biocontrol, with examples from physiological systems modeling such as the respiratory system. It contains double the number of Matlab and non-Matlab exercises to provide ample practice solving problems - by hand and with computational tools. More biomedical figures are found throughout the book. For instructors using this text in their course, an accompanying website (www.elsevierdirect.com, in Semmlow page) includes support materials such as MATLAB data and functions needed to solve the problems, a few helpful routines, and all of the MATLAB examples. Intended readers include biomedical engineering students, practicing medical technicians, mechanical engineers, and electrical engineers

Biosignal and medical image processing. Third edition

Boca Raton, F