Null Position Research Articles

Reflectivity null tuning in multi-layer phase-switched active radar absorbers

An ‘array’ polynomial approach is used to predict the positions of the reflectivity nulls for an ideal phase-switched screen (PSS) having many active layers. Several layer-switching schemes are examined and it is found that the resulting null position diagrams exhibit bifurcation behaviour similar to that found in many dynamical systems. The chaotic state is never achieved, however, since the ideal PSS is a linear system.

Prolonged Asymmetric Smooth-Pursuit Stimulation Leads to Downbeat Nystagmus in Healthy Human Subjects

Downbeat nystagmus (DBN) is a typical ocular motor sign in patients with lesions of the vestibulocerebellum. A vertical vestibular tone asymmetry, an upward shift of the eyes' null position for vertical gaze holding, or an imbalance of vertical smooth-pursuit signals have been proposed as mechanisms of DBN. The purpose of this study was to elaborate a possible link between an imbalance in the vertical smooth-pursuit system and DBN by relying on a healthy human model. Healthy subjects (n=6) were exposed to continuous asymmetric smooth-pursuit stimulation over 20 minutes. Prolonged asymmetric smooth-pursuit stimulation induced a drift lasting >5 minutes in the direction of the prior pursuit. Upward drift was faster than downward drift and showed eye-position dependence in accordance with Alexander's law, but no increase of drift velocity with lateral gaze. Upward drift violated Listing's law in three of four subjects tested. An experimentally induced vertical smooth-pursuit imbalance leads to DBN in healthy human subjects. Accordingly, because in patients with cerebellar disease upward smooth-pursuit eye movements are typically better preserved than downward, the resultant sustained imbalance of vertical smooth-pursuit input may play a major role in the generation of DBN.

Effects of extraocular muscle surgery on 15 patients with oculo-cutaneous albinism (OCA) and infantile nystagmus syndrome (INS)

The purpose of this report is to characterize the clinical and electrophysiological effects of extraocular muscle surgery in 15 patients with oculo-cutaneous albinism (OCA) and infantile nystagmus syndrome (INS). Our hypothesis is that surgery on the extraocular muscles of patients with OCA and INS changes their nystagmus and their visual function. Interventional, prospective, cohort, noncomparative case series. All 15 patients had surgery on all four virgin horizontal recti; three for strabismus alone, three for nystagmus alone, five for an eccentric gaze null zone alone, and four for an eccentric gaze null zone plus strabismus. All patients have been followed for at least six months. All 15 patients had the subjective outcome measure of pre- and postoperative binocular best optically corrected acuity (BBOCA). Objective outcome measures included anomalous head posture (AHP) in nine patients, eye movement recording measures of expanded nystagmus acuity function (NAFX) in 10 patients, null zone position (NUZP) and null zone width (NUZW) in 10 patients, and foveation time (FOV) in nine patients. The results are summarized as follows; BBOCA increased 0.1 LogMar or greater in 14 of 15 patients. In those operated on for an AHP with or without associated strabismus the AHP improved significantly (P < .01 for all). The NAFX, NUZP, NUZW, and FOV measured from eye movement recordings showed persistent, significant increases in all patients (P < .01 for all). This report adds to the evidence that surgery on the extraocular muscles in patients with INS has independent neurologic and visual results.

Neonatal Movement Disorders

After completing this article, readers should be able to: 1. Delineate the characteristics that differentiate hyperkinetic neonatal movement disorders from neonatal seizures. 2. Describe the primary characteristics of and treatment for hyperekplexia. 3. Explain the relationship between gastroesophageal reflux and movement disorders in neonates. 4. Describe the primary ocular movement disorders in neonates. Neonates often exhibit paroxysmal behaviors that are nonepileptic in nature, such as oral-buccal-lingual movements, pedaling, stepping, and rotatory arm movements. The electroencephalogram (EEG) is of particular value for distinguishing these nonepileptic behaviors from subtle seizures. EEG and video-EEG studies in such movement disorders do not exhibit seizure discharges during the events. This distinction is important because nonepileptic events do not require treatment with anticonvulsants. Although the absence of surface ictal activity does not exclude entirely the possibility of seizures arising from deeper structures, it generally is accepted that these disorders are not epileptic. For the purposes of this review, we include common nonepileptic paroxysmal (episodic) movements that occur from birth to early infancy (onset between birth and 3 months of age). Many of these movement disorders may persist during infancy or early childhood. We do not discuss more persistent motor problems, such as spasticity, hypotonia, or myotonia. Also, disorders that occur after 3 months of age are not included. We have classified the movement disorders into two major categories: 1) hyperkinetic movements and 2) dystonic or extrapyramidal type of movements. Because the pathophysiologic mechanisms underlying neonatal movement disorders often are unknown, they are categorized on the basis of phenomenology, rather than presumed cause. Some of these disorders may have overlapping symptomatology. A third category encompasses “ocular movement disorders.” ### Jitteriness of the Newborn Jitteriness may be due to many factors, including metabolic disturbance, hypoxic-ischemic encephalopathy, drug withdrawal, hypoglycemia, and hypocalcemia. Although jitteriness may be mistaken for neonatal seizures, a distinguishing feature of jitteriness is that the …

Origin of Orientation-Dependent Asymmetries in Vestibulo-Ocular Reflexes Evoked by Caloric Stimulation

A caloric stimulus evokes primarily a horizontal vestibulo-ocular reflex (VOR) when subjects are in a supine or prone orientation with the horizontal semicircular canal plane oriented vertically. In both monkeys and humans, the magnitude of VOR eye movements is greater in the supine than in the prone orientation, indicating that some factor or factors, other than the conventionally accepted convective stimulation of the horizontal canals, contributes to the generation of the VOR. We used long-duration caloric irrigations and mathematical models of canal-otolith interactions to investigate factors contributing to the prone/supine asymmetry. Binaural caloric irrigations were applied for 7.5 or 9.5 min with subjects in a null orientation with horizontal canals in the earth-horizontal plane (control trial), or with the subject's pitch orientation periodically changing between null, supine, and prone positions with each orientation held for 30 s (caloric step trial). The control trial responses identified a small response attributable to a direct thermal effect on vestibular afferent activity that accounted for only 15% of the observed prone/supine asymmetry. We show that the gravito-inertial force resolution hypothesis for sensory integration of canal and otolith information predicts that the central processing of canal and otolith information produces an internal estimate of motion that includes both a rotational motion component and a linear acceleration component. These components evoke a horizontal angular VOR and linear VOR, which combine additively in the supine orientation, but subtract in the prone orientation, thus accounting for the majority of the observed prone/supine asymmetry.

Null ellipsometer with phase modulation

A new null ellipsometer is described that uses photoelastic modulator (PEM). The phase modulation adds a good signal-to-noise ratio, high sensitivity, and linearity near null positions to the traditional high-precision nulling system. The ellipsometric angles Delta and psi are obtained by azimuth measurement of the analyzer and the polarizer-PEM system, for which the first and second harmonics of modulator frequency cross the zeros. We show that the null system is insensitive to ellipsometer misadjustment and component imperfections and modulator calibration is not needed. In addition, a fast ellipsometer mode for fine changes measurement of ellipsometric angles is proposed.

Beampattern control of a microphone array to minimize secondary source contamination.

A null-steering technique is adapted and applied to a linear delay-and-sum beamformer in order to measure the noise generated by one of the propellers of a 1/8 scale twin propeller aircraft model. The technique involves shading the linear array using a set of weights, which are calculated according to the locations onto which the nulls need to be steered (in this case onto the second propeller). The technique is based on an established microwave antenna theory, and uses a plane-wave, or far field formulation in order to represent the response of the array by an nth-order polynomial, where n is the number of array elements. The roots of this polynomial correspond to the minima of the array response, and so by an appropriate choice of roots, a polynomial can be generated, the coefficients of which are the weights needed to achieve the prespecified set of null positions. It is shown that, for the technique to work with actual data, the cross-spectral matrix must be conditioned before array shading is implemented. This ensures that the shading function is not distorted by the intrinsic element weighting which can occur as a result of the directional nature of aeroacoustic systems. A difference of 6 dB between measurements before and after null steering shows the technique to have been effective in eliminating the contribution from one of the propellers, thus providing a quantitative measure of the acoustic energy from the other.

Linear regression of eye velocity on eye position and head velocity suggests a common oculomotor neural integrator.

The oculomotor system produces eye-position signals during fixations and head movements by integrating velocity-coded saccadic and vestibular inputs. A previous analysis of nucleus prepositus hypoglossi (nph) lesions in monkeys found that the integration time constant for maintaining fixations decreased, while that for the vestibulo-ocular reflex (VOR) did not. On this basis, it was concluded that saccadic inputs are integrated by the nph, but that the vestibular inputs are integrated elsewhere. We re-analyze the data from which this conclusion was drawn by performing a linear regression of eye velocity on eye position and head velocity to derive the time constant and velocity bias of an imperfect oculomotor neural integrator. The velocity-position regression procedure reveals that the integration time constants for both VOR and saccades decrease in tandem with consecutive nph lesions, consistent with the hypothesis of a single common integrator. The previous evaluation of the integrator time constant relied upon fitting methods that are prone to error in the presence of velocity bias and saccades. The algorithm used to evaluate imperfect fixations in the dark did not account for the nonzero null position of the eyes associated with velocity bias. The phase-shift analysis used in evaluating the response to sinusoidal vestibular input neglects the effect of saccadic resets of eye position on intersaccadic eye velocity, resulting in gross underestimates of the imperfections in integration during VOR. The linear regression method presented here is valid for both fixation and low head velocity VOR data and is easy to implement.

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Best catch from NANOSNET.

A new differential diagnosis for spontaneous nystagmus: lateral canal cupulolithiasis.

Annals of the New York Academy of SciencesVolume 956, Issue 1 p. 579-580 A New Differential Diagnosis for Spontaneous Nystagmus Lateral Canal Cupulolithiasis ALEXANDRE R. BISDORFF, Corresponding Author ALEXANDRE R. BISDORFF Department of Neurology, Hôopital de la Ville, L-4240 Esch-sur-Alzette, LuxembourgAddress for correspondence: Alexandre Bisdorff, M.D., Department of Neurology, Hôpital de la Ville, Rue Emile Mayrisch, L-4240 Esch-sur-Alzette, Luxembourg. Voice: 352 5444491; fax: 352 547886; alexbis@pt.lu.Search for more papers by this authorDAMIEN DEBATISSE, DAMIEN DEBATISSE Department of Neurology, Hôopital de la Ville, L-4240 Esch-sur-Alzette, LuxembourgSearch for more papers by this author ALEXANDRE R. BISDORFF, Corresponding Author ALEXANDRE R. BISDORFF Department of Neurology, Hôopital de la Ville, L-4240 Esch-sur-Alzette, LuxembourgAddress for correspondence: Alexandre Bisdorff, M.D., Department of Neurology, Hôpital de la Ville, Rue Emile Mayrisch, L-4240 Esch-sur-Alzette, Luxembourg. Voice: 352 5444491; fax: 352 547886; alexbis@pt.lu.Search for more papers by this authorDAMIEN DEBATISSE, DAMIEN DEBATISSE Department of Neurology, Hôopital de la Ville, L-4240 Esch-sur-Alzette, LuxembourgSearch for more papers by this author First published: 24 January 2006 https://doi.org/10.1111/j.1749-6632.2002.tb02887.xCitations: 7Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume956, Issue1NEUROBIOLOGY OF EYE MOVEMENTS: FROM MOLECULES TO BEHAVIORApril 2002Pages 579-580 RelatedInformation

Two suboptimal algorithms for downlink beamforming in FDD DS-CDMA mobile radio

Two suboptimal algorithms are proposed for downlink beamforming in FDD DS-CDMA mobile radio by using uplink beamforming weights. One is a null-constrained method, which maintains the same null positions for both uplink and downlink beam patterns; the other is a frequency-calibrated method which constrains the same main beam positions for both patterns. We also evaluate the multicell downlink capacity of DS-CDMA systems using a per-user-per-weight beamforming scheme. Outer cell interference is modeled as an AWGN process whose variance is proportional to the average intracell total transmitted power. Computer simulations are given to compare the single cell and multicell capacities using different downlink beamforming weight generation algorithms.

Sensitivity study of the effects of manufacturing defects on the performance of a redundant hydraulic secondary flight control

Redundant flight control systems, designed in order to ensure the necessary safety level even in failure conditions, may behave improperly during normal operations, if the system architecture is unsuitable, when manufacturing defects are present. This work analyses the response of a secondary flight control hydraulic servo-mechanism, designed according to different redundancy architectures, when manufacturing defects are present. These defects may consist of non-symmetrical overlap of the passageways or incorrect null position with no input current (caused by an asymmetric geometry of the feedback spring or of the first-stage spring, resulting from an undesired magnetic field in the first-stage torque motor, etc.). These defects come into the usual quality standards of the components and must be accepted because they are not removable owing to the accuracy of the manufacturing processes. In the opinion of the authors this paper concerns a topic quite neglected, but important in the technical literature. To the best of their knowledge, no specific scientific work in this field is available, except for some industrial technical reports. Since the above-mentioned problems adversely effect the proper operation of the servo-mechanism, it is necessary to introduce some equalization devices able to improve substantially the improper behaviours. By means of an appropriate physical-mathematical model of a typical electro-hydraulic servo-mechanism, the work evaluates the effects of manufacturing defects in different cases and the effectiveness of the eventual compensation devices in limiting the improper behaviours.

Simulation of servo position error signal in perpendicular recording

We describe simulations using a medium model with random grain structure, to study the servo position error signal (PES) in a perpendicular recording system. Under various detection schemes, values of the PES null position jitter and the PES sensitivity variation at the null point are calculated. To evaluate the effect of medium noise on the PES detection, the values are compared to those of media with different average grain in-plane areas and grain size distributions. For both the PES null position jitter and the PES sensitivity variation at the null point, decreasing the average grain in-plane area or the grain size distribution yields smaller fluctuation. The servo PES has a strong dependence on the average-medium grain in-plane area. When PES of longitudinal recording systems is compared with that of perpendicular systems, perpendicular recording yields much better results because of a sharp write-field gradient at the track edge.

Reduction of congenital nystagmus amplitude with auditory biofeedback

Purpose: Treatment options for congenital nystagmus without null position are limited. The purpose of this study was to evaluate the role of auditory biofeedback in controlling congenital nystagmus. Methods: Ten patients with congenital nystagmus without null position underwent 6 sessions (twice a week for 3 weeks) of auditory biofeedback. Each half-hour session had simultaneous electronystagmographic recording done during the session. Results: The patients could reduce the nystagmus during the treatment sessions. Mean amplitude (degrees) of nystagmus was reduced from 6.28 ± 4.94 to 3.05 ± 2.48 (P = .028) and mean intensity (amplitude × frequency) was reduced from 33.37 ± 22.84 to 13.35 ± 7.99 (P = .0174), but the mean frequency change was not significant, from 5.8 ± 1.05 to 4.98 ± 1.35 (P = .148). The mean amplitude and mean intensity decreased by 51% and 60%, respectively. After completion of the session, although a subjective improvement was reported, the patient's binocular visual acuity on Snellen's charts and contrast sensitivity did not show any significant change. Also no sustained benefit was noted because the electronystagmographic recordings reverted to baseline after the auditory stimulus for biofeedback was discontinued. Conclusion: Simultaneous electronystagmographic recording shows significant reduction of nystagmus amplitude and intensity because of auditory biofeedback only during the treatment session. The beneficial effect does not persist after the auditory stimulus is discontinued. No objective effect on visual acuity and contrast sensitivity was noted after the therapy. (J AAPOS 2000;4:287-90)

Simulation of the effect of medium noise on servo position error signal detection in longitudinal recording

A medium model with random grain structure and desired grain size distribution is used to study the effect of medium noise on position error signal (PES) signal and sensitivity using 100 Gb/in/sup 2/ parameters in longitudinal media. Three PES detection schemes are compared: (1) average peak voltage; (2) integration of the absolute playback signal; and (3) maximization of the integration of the product of a phase-adjusted square wave with the playback signal. Scheme III is found to have the best output PES sensitivity for every case. The PES, PES sensitivity, and null point position of the RES signal along with their distributions are studied. It is found that the PES sensitivity has a nearly Gaussian distribution. In most cases, the null point position of the PES has a nonzero average and an asymmetric distribution. Media with different grain sizes and size distributions are investigated and compared. The grain size distribution has a small effect on the PES detection. Increasing the average grain size dramatically increases the standard deviation of the PES sensitivity while slightly decreasing the PES sensitivity. The simulation results also show that increasing the medium grain orientation ratio (OR) improves the PES sensitivity.

Error analysis of null ellipsometry with depolarization

Null ellipsometry is analyzed for components with depolarizations for unpolarized incident light. Serious imperfections include sample depolarization D < 0.2, misalignment (deltaC < 2 degrees) and off-quarter-wave retardance (deltatau < 5 degrees) of the compensator. The four-zone null positions are derived analytically to the second order of serious imperfections and are also simulated based on Mueller matrices with depolarization. Errors of all four-zone nulls increase with increasing D. Depolarizations of all components except the analyzer cause errors to the nulls. The errors associated with D always couple with the second order of deltaC and deltatau and are enhanced by csc(2) 2psi. These divergent errors limit the applicable region of null ellipsometry where the errors in psi and D are within 0.1 degrees, and the simulation agrees well with the analytic solutions.

Clinical and ocular motor analysis of congenital nystagmus in infancy

Purpose: The purpose of this study was to identify the clinical and ocular motility characteristics of congenital nystagmus and to establish the range of waveforms present in infancy. Background: The clinical condition of congenital nystagmus usually begins in infancy and may or may not be associated with visual sensory system abnormalities. Little is known about its specific waveforms in infancy or their relationship to the developing visual system. Methods: Forty-three infants with involuntary ocular oscillations typical of congenital nystagmus were included in this analysis. They were evaluated both clinically and with motility recordings. Eye movement analysis was performed off line from both chart recordings and computer analysis of digitized data. Variables analyzed included age, sex, vision, ocular abnormalities, head position, null-zone or neutral-zone characteristics, symmetry, conjugacy, waveforms, frequencies, foveation times, and responses to convergence and to monocular cover. Results: Patient ages ranged from 3 to 18 months (average, 9.2 months). Seventeen patients (40%) had abnormal vision, 3 had a positive family history of nystagmus, 11 had strabismus, 16 (37%) had a head posture, 26 (60%) had null and neutral positions, 14 (33%) had binocular asymmetry, and all were horizontally conjugate. Average binocular frequency was 2.8 Hz, and average monocular frequency was 4.6 Hz. The waveforms were both jerk and pendular; average foveation periods in patients with normal vision were more than twice as long as those in patients with abnormal vision. Conclusions: Common clinical characteristics and eye-movement waveforms of congenital nystagmus begin in infancy, and waveform analysis at this time helps with both diagnosis and visual status. (J AAPOS 1999;3:70-9)

Primary pressure standard for calibration in the medium vacuum range

Primary standards for gas pressure in the range from ∼0.1 to 10 Pa (1 to 100 mTorr) have received a significant amount of attention in recent years. The interest in the calibration capability in this vacuum range is, to some degree, driven by the increasingly stringent requirements for the semiconductor processing industry. We present a new standard that is based on calculating the gas pressure required to restore a diaphragm to a null position, while the diaphragm is tilted to various, measured inclinations. With this as a basis, the calculations are relatively simple; there are no gas property dependent effects; and the system design is simple, robust, and easily automated. A formal evaluation of uncertainty for the system being developed in this facility is reviewed and compared with other primary standards. For the medium vacuum range, it is anticipated that this standard will offer numerous practical advantages relative to liquid manometers, dead weight testers, volume expansion systems, and conductance-based systems. It is intended that this technique will enable more widespread capability for the calibration, and verification of vacuum gauging in a pressure range of critical interest to many processes.

Solar panel scattering effects on reflector antennaperformance

The effects of solar panel scattering on a Ku-Band reflector antenna performance are analysed. It is shown from computation and experiment that solar panel scattering interference causes mainlobe gain decrease and sidelobe gain increase on the sum patterns, as well as null position shift and null depth decrease on the difference patterns.

The construction of bounded game-theoretic controls for non-linear dynamical systems

The problem of transferring a non-linear dynamical system, subject to perturbations, to the null equilibrium position in a finite time by means of a bounded control is considered. Only the levels of uncontrollable perturbations are known, and are not assumed to be small. Sufficient conditions are obtained which ensure that the problem has a guaranteed solution for the given domain of initial conditions. An estimate of the guaranteed control time is obtained. The construction of the control can be reduced to the construction of game strategies for auxiliary linear game-theoretic problems. To estimate the “auxiliary noise” in the resulting linear system, the principle of “prescribing and subsequent confirmation” of noise levels is put forward. On the basis of this principle, these estimates are checked on the set of states of the auxiliary linear systems, where the control is also subsequently estimated. As a result, an iterative algorithm for solving the original non-linear problem is obtained. Within the framework of the method proposed a new solution of the game-theoretic problem of the reorientation of an asymmetric rigid body in the presence of noise is given.