Analog Model for Gaze-Evoked Nystagmus

A relatively simple model of the fast eye movement (saccadic) system has been developed and simulated on an analog computer. The model is capable of producing normal saccadic eye movements and has several provisions which permit the simulation of clinical signs of gaze-evoked nystagmus. According to the model, gaze-evoked nystagmus may result from various abnormalities in the neuronal pool responsible for integration of the pulse of high frequency fiing which initiates each saccade. Simulation of these inferred neuronal deficits produced subtle differences in saccadic behavior as well as gaze-evoked nystagmus. These differences occurred both in the nystagmus-free range of gaze angles and after the nystagmus appeared. The subtlety of the deficits and the sparsity of studies of gaze-evoked nystagmus using modem oculographic recording techniques, probably explains why such saccadic behavior has not been previously noted. If one, or more than one, of these simulated mechanisms is, in fact, an accurate model of the actual physiological deficit, future studies of such patients should reveal the corresponding abnormal saccadic behavior.     

Predicting final eye position halfway through a saccade

When the visual environment is to be changed during a saccadic eye movement, it is useful to predict the final eye position before the eye comes to rest. We have built a microcomputer-based instrument to make such predictions. Two techniques were used: one based on the saccadic peak-velocity versus magnitude relationship, and the second based on peak-velocity occurring in the middle of the saccade. The second technique has been tailored to take advantage of the differences between temporal and nasal saccades. Depending on saccade duration, final eye position was predicted 4 to 60 ms before the end of the saccade.     

On the Identification and Analysis of Saccadic Eye Movements?A Quantitative Study of the Processing Procedures

A set of quantitative data about signal processing effects on the evaluation of amplitude, duration, and peak-velocity of single saccadic eye movements, as well as on the evaluation of the coefficients of amplitude-peak velocity and amplitude-duration characteristics of a family of saccades, is given. The following aspects of signal processing are theoretically discussed, experimentally supported, and summarized by normative graphs: 1) the analog low-pass filtering applied to the recorded eye movement; 2) its sampling and digitization; 3) the low-pass filtering applied to the digitized signal; 4) the algorithm used to compute the eye velocity; 5) the criteria adopted to determine the beginning and the ending points of the saccades; and 6) the laws used to fit the saccadic characteristics. According to the theoretical and experimental results obtained, some general operating rules and optimal setups for each of the most common experimental situations are proposed, with the aim of standardizing saccade analysis for both researchers and clinicians.     

Time-Optimal Control of Saccadic Eye Movements

A new theory describing the time-optimal control of saccadic eye movements is proposed based on Pontryagin's minimum principle and physiological considerations. The lateral and medial rectus muscle of each eye is assumed to be a parallel combination of an active state tension generator with a viscosity and elastic element, connected to a series elastic element. The eyeball is modeled as a sphere connected to a viscosity and elastic element. Each of these elements is assumed to be ideal and linear. The neuronal control strategy is shown to be a first-order time-optimal control signal. Under this condition, the active state tension for each muscle is a low-pass filtered pulse-step waveform. The magnitude of the agonist pulse is a maximum for saccades of all sizes and only the duration of the agonist pulse affects the size of the saccade. The antagonist muscle is completely inhibited during the period of maximum stimulation for the agonist muscle. Horizontal saccadic eye movements were recorded from infrared signals reflected from the anterior surface of the cornea and then digitized. Parameter estimates for the model were calculated by using a conjugate gradient search program which minimizes the integral of the absolute value of the squared error between the model and the data. The predictions of the model under a time-optimal controller are in good agreement with the data.     

Floating-Gate Circuits for Adaptation of Saccadic Eye Movement Accuracy

In this paper we describe an analog VLSI circuit, fabricatedusing a standard 2 µm, n-well, BiCMOS process,which utilizes floating-gate structures for non-volatile, on-chip,analog parameter storage. This circuit is designed to operatein the context of a hardware model of the primate oculomotorsystem and performs visually-guided, saccadic adaptation. Thechip contains a one-dimensional array of photoreceptors and floating-gatecircuits which are used to map retinal positions to motor outputcommands. The system's functionality is demonstrated by trainingthe chip with several different mapping functions using a supervised-learningtechnique.     

The linear homeomorphic saccadic eye movement model--a modification

The objective of this study was the modification of a linear homeomorphic horizontal saccadic eye movement model to a direct programming state-space representation through Laplace variable analysis about the operating point or initial eye position. The lateral and medial rectus muscle of each eye is modeled as a parallel combination of an active state tension generator with a viscosity and elastic element, connected to a series elastic element. The eyeball is modeled as a sphere connected to a viscosity and elastic element. Each of these elements is assumed to be ideal and linear.     

面向驾驶员状态监测的人眼凝视方向判别

基于眼球运动规律监测驾驶员状态是当前研究的热点,而获取人眼凝视持续时间、眼球扫视运动频率等眼球运动参数的基础和前提是实时判别人眼凝视方向。首先采用红外光源和红外滤光片的装置,通过CCD摄像头获取"暗瞳孔"图像;接着根据人脸特征粗定位人脸区域,并通过对人脸上半部分水平积分投影粗定位人眼区域;然后在定位出的人眼区域内,利用闪光点与周围瞳孔区域灰度值差异大的特征精确定位闪光点,并在闪光点周围提取暗瞳孔精确定位瞳孔质心;最后建立凝视方向判别模型,根据闪光点与瞳孔的相对位置判别出人眼的凝视方向。凝视区域判别实验结果表明,该算法对于获取眼球运动相关参数有较好的应用前景。     

Hot spot detection based on feature space representation of visual search

This paper presents a new framework for capturing intrinsic visual search behavior of different observers in image understanding by analysing saccadic eye movements in feature space. The method is based on the information theory for identifying salient image features based on which visual search is performed. We demonstrate how to obtain feature space fixation density functions that are normalized to the image content along the scan paths. This allows a reliable identification of salient image features that can be mapped back to spatial space for highlighting regions of interest and attention selection. A two-color conjunction search experiment has been implemented to illustrate the theoretical framework of the proposed method including feature selection, hot spot detection, and back-projection. The practical value of the method is demonstrated with computed tomography image of centrilobular emphysema, and we discuss how the proposed framework can be used as a basis for decision support in medical image understanding.     

Frequency limitations and optimal step size for the two-point central difference derivative algorithm with applications to human eye movement data

There are many algorithms for calculating derivatives. The two-point central difference algorithm is the simplest. Besides simplicity, the two most important characteristics of this algorithm are accuracy and frequency response. The frequency content of the data prescribes a lower limit on the sampling rate. The smoothness and accuracy of the data determine the optimal step size. We discuss the low-pass filter characteristics of this algorithm and derive the optimal step size for two types of human eye movement data. To calculate the velocity of fast (saccadic) eye movements, the algorithm should have a cutoff frequency of 74 Hz. For typical slow (smooth pursuit) eye movements, a step size of 25 or 50 ms is optimal.     

A microprocessor-based instrument for nystagmus analysis

This paper describes a microprocessor-based instrument for on-line recording and analysis of ocular nystagmus. A novel acceleration sensitive saccade detector is used to differentiate between slow and fast phases of nystagmus. Once saccades are located, they are removed and an estimate of cumulative slow phase position is obtained. Also available as outputs are slow phase velocity, saccadic frequency, and maxima and times of occurrence for each of these parameters. The advantages of such an insmunent include low cost coupled with a high level of sophistication. These factors would allow the instrument to be used in a physician's office by relatively naive personnel Software is written in STOIC, a new general purpose interactive language for microprocessors. An Intel 8080 microprocessor, 16K of memory, A/D and D/A converters, interrupting clock, and CRT terminal are used in the present configuration. The system provides semi-automatic calibration of eye movement monitoring and detection of excessive drift and noise artifacts. Flowcharts for the processing algorithm and examples of output are presented.     

Initial component control in disparity vergence: a model-basedstudy

The dual-mode theory for the control of disparity-vergence eye movements states that two components control the response to a step change in disparity. The initial component uses a motor preprogram to drive the eyes to an approximate final position. This initial component is followed by activation of a late component operating under visual feedback control that reduces residual disparity to within fusional limits. A quantitative model based on a pulse-step controller, similar to that postulated for saccadic eye movements, has been developed to represent the initial component. This model, an adaptation of one developed by D.S. Zee et al. (J. Neurophysiol., vol. 68, p. 1624-41, 1992), provides accurate simulations of Isolated initial component movements and is compatible with the known underlying neurophysiology and existing neurophysiological data. The model has been employed to investigate the difference in dynamics between convergent and divergent movements. Results indicate that the pulse-control component active in convergence is reduced or absent from the control signals of divergence movements. This suggests somewhat different control structures of convergence versus divergence, and is consistent with other directional asymmetries seen in horizontal vergence     

The use of visual search for knowledge gathering in image decision support

This paper presents a new method of knowledge gathering for decision support in image understanding based on information extracted from the dynamics of saccadic eye movements. The framework involves the construction of a generic image feature extraction library, from which the feature extractors that are most relevant to the visual assessment by domain experts are determined automatically through factor analysis. The dynamics of the visual search are analyzed by using the Markov model for providing training information to novices on how and where to look for image features. The validity of the framework has been evaluated in a clinical scenario whereby the pulmonary vascular distribution on Computed Tomography images was assessed by experienced radiologists as a potential indicator of heart failure. The performance of the system has been demonstrated by training four novices to follow the visual assessment behavior of two experienced observers. In all cases, the accuracy of the students improved from near random decision making (33%) to accuracies ranging from 50% to 68%.     

Effects of disorder on charge transport in semico++ nducting polymers

Polaron transport process in semiconducting polymers with disordered structure and morphology is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include diagonal disorder, off-diagonal disorder and an external electric field. It is found that the polaron transport mechanism is determined by the competition between the disorder and the electric field. For the case of pure diagonal disorder, the polaron transport undergoes a crossover from adiabatic drift to nonadiabatic hopping under a large scope of electric field (E < 2.0 mV/Å), whereas the field-assisted tunneling dominates charge transport for a higher electric field. For the combined disorder, it is demonstrated that the competition effects and the synergetic effects between the diagonal and off-diagonal disorder are equally important. The effects of diagonal disorder are negative in most cases, and the energetically easier pathways for charge transport are mainly opened by the off-diagonal disorder.     

A nystagmus strategy to linearize the vestibulo-ocular reflex

This paper addresses two important aspects of the vestibulo-ocular reflex (VOR). First, the linear range of ocular responses is much more extensive than expected from the characteristics of central pathways (CNS), and this is shown to result directly from early convergence of 'fast' and 'slow' premotor signals in the central processes, associated with significant and intermittent changes in functional connectivity (effective structural modulation). Second, the presence of such structural modulation implies that responses must be analyzed using transient analysis techniques, rather than previous steady state approaches, in order to properly evaluate reflex dynamics. Simulation results with a recent model of the VOR are used to illustrate the arguments. Relying on known inter-connections between saccadic burst circuits in the brainstem, and the ocular premotor areas of the vestibular nuclei, a viable strategy for the timing of nystagmus events is proposed. The strategy easily reproduces the characteristic changes in vestibular nystagmus with the amplitude of head velocities, and with the frequency of passive head oscillation.     

部分无序二维光子晶体透射特性研究

用时域有限差分法研究了光在部分无序二维光子晶体中的透射特性。结果表明：介质柱的位置和大小无序都对光子晶体的高频透射特性都有影响而对低频几乎无影响,随着无序度增大,禁带呈现拓宽的趋势。仅介质柱的位置无序变化时,每个带隙低频端边缘比高频端边缘对无序度变化更敏感。仅介质柱的大小无序变化时,每个带隙间的通带频率越高对无序度变化越敏感。介质柱的大小无序和位置无序度相同时,在高频段后者比前者作用更显著。     

Hole transport in p-diethylamino-benzaldehyde-diphenyl hydrazone

Hole mobilities have been measured in vapour-deposited films of p-diethylamino-benzaldehyde-diphenyl hydrazone (DEH). The results are described by a model based on hopping through a manifold of sites broadened by energetic and positional disorder. The application of the model leads to the conclusion that the width of the density of hopping states is influenced by dipolar disorder due to dipole moments of the DEH molecules. In comparing the results with those observed in other molecular solids, the degree of positional disorder is shown to be relatively small.     

A time-variant processing approach for the analysis of alpha and gamma MEG oscillations during flicker stimulus generated entrainment

Repetitive flicker stimulation (photic driving) offers the possibility to study the properties and coupling characteristics of stimulation-sensitive neuronal oscillators by means of the MEG/EEG analysis. With flicker frequencies in the region of the individual alpha band frequency, the dynamics of the entrainment process of the alpha oscillation, as well as the dynamics of the accompanying gamma oscillations and the coupling between the oscillations, are investigated by means of an appropriate combination of time-variant analysis methods. The Hilbert and the Gabor transformation reveal time-variant properties (frequency entrainment, phase locking, and n:m synchronization) of the entrainment process in the whole frequency range. Additionally, time-variant partial directed coherence is applied to identify ocular saccadic interferences and to study the directed information transfer between the recording sites of the simultaneously derived MEG/EEG data during the entrainment. The MEG data is the focus of this methodological study as the entrainment effects of the alpha oscillation are stronger in MEG than in the EEG. The occipital brain region (visual cortex) was mainly investigated and the dynamics of the alpha entrainment quantified. It can be shown that at the beginning of this entrainment, a transient, strongly phase-locked "40-Hz" gamma oscillation occurs.     

用转移矩阵法研究无序对二维光子晶体透射谱的影响

将转移矩阵法(Transfer Matrix Method)用于数值模拟无序情况下的二维光子晶体传输特性.充分利用Fortran语言中的RAND函数分别研究了晶格结构无序、圆柱半径无序、柱体形状无序、介电常数无序对光子晶体传输特性的影响.计算了多种因素无序情况下的二维正方晶格光子晶体的传输函数随频率的变化关系.计算结果表明弱无序时柱体形状影响最大,最小是圆柱半径无序.这说明在一定的无序范围里,可以认为光子晶体的传输特性能够被很好地保持.在强无序时存在共同的现象就是在低频即长波段出现光子局域化现象,无序会影响二维光子晶体的带隙宽度、位置,在强无序时会出现多个共振模.     

The role of alloying effects in the formation of electronic structure of unordered Group III nitride solid solutions

The effect of compositional and positional disorder on electronic properties of (Group III)-nitride solid solutions with the wurtzite structure was studied by the method of a model empirical pseudopotential using 32-atom supercells. The calculated values of the band-gap bowing parameter are found to be equal to 0.44, 2.72, and 4.16 for AlGaN, InGaN, InAlN, respectively. It is shown that the major contribution to the band-gap bowing parameter is made by the compositional disorder, whereas the bond-length relaxation reduces the effect of compositional disorder and the effects of the volume deformation.