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PubMed1999

Highly reproducible spatiotemporal patterns of mammalian embryonic movements at the developmental stage of the earliest spontaneous motility.

The principles underlying the variations in patterns of mammalian embryonic movements have not been established. In an attempt to clarify the mechanism that is responsible for the variations in motor patterns, we carried out a precise quantitative spatiotemporal analysis of movements in mouse embryos, using a transplacental perfusion method for the in vitro maintenance of live mammalian embryos. Episodes of spontaneous movements at the inception of motility, at embryonic day 12.5, occurred once every few minutes, lasted for several seconds and consisted of successive movements of body regions, the spatiotemporal patterns of which varied from episode to episode. By analysing and categorizing the patterns of these movements, we found that embryonic movements follow relatively few restricted patterns with respect to the order of the movements of body regions. A further analysis of episodes at high spatiotemporal resolution revealed that most of the episodes in a major category could be classified into two distinct subtypes. Each of these subtypes had its own highly reproducible spatiotemporal patterns of movement. Overall, these results show that early embryonic movements follow relatively few rather stereotyped patterns, and random local fluctuations have little effect on such movement patterns. The appearance of one pattern out of several rather stereotyped patterns may be the main cause of apparent variations in patterns of early embryonic movements. The stereotyped patterns may represent important orderly characteristics of spontaneous embryonic activities that may be involved in the development of orderly structures and functions in higher animals.

Animals↗
PubMed2006

A population coding account for systematic variation in saccadic dead time.

During movement programming, there is a point in time at which the movement system is committed to executing an action with certain parameters even though new information may render this action obsolete. For saccades programmed to a visual target this period is termed the dead time. Using a double-step paradigm, we examined potential variability in the dead time with variations in overall saccade latency and spatiotemporal configuration of two sequential targets. In experiment 1, we varied overall saccade latency by manipulating the presence or absence of a central fixation point. Despite a large and robust gap effect, decreasing the saccade latency in this way did not alter the dead time. In experiment 2, we varied the separation between the two targets. The dead time increased with separation up to a point and then leveled off. A stochastic accumulator model of the oculomotor decision mechanism accounts comprehensively for our findings. The model predicts a gap effect through changes in baseline activity without producing variations in the dead time. Variations in dead time with separation between the two target locations are a natural consequence of the population coding assumption in the model.

Action Potentials↗
PubMed2006

Spatiotemporal salient points for visual recognition of human actions.

This paper addresses the problem of human-action recognition by introducing a sparse representation of image sequences as a collection of spatiotemporal events that are localized at points that are salient both in space and time. The spatiotemporal salient points are detected by measuring the variations in the information content of pixel neighborhoods not only in space but also in time. An appropriate distance metric between two collections of spatiotemporal salient points is introduced, which is based on the chamfer distance and an iterative linear time-warping technique that deals with time expansion or time-compression issues. A classification scheme that is based on relevance vector machines and on the proposed distance measure is proposed. Results on real image sequences from a small database depicting people performing 19 aerobic exercises are presented.

Algorithms↗
PubMed2005

Domain decomposition for variational optical-flow computation.

We present an approach to parallel variational optical-flow computation by using an arbitrary partition of the image plane and iteratively solving related local variational problems associated with each subdomain. The approach is particularly suited for implementations on PC clusters because interprocess communication is minimized by restricting the exchange of data to a lower dimensional interface. Our mathematical formulation supports various generalizations to linear/nonlinear convex variational approaches, three-dimensional image sequences, spatiotemporal regularization, and unstructured geometries and triangulations. Results concerning the effects of interface preconditioning, as well as runtime and communication volume measurements on a PC cluster, are presented. Our approach provides a major step toward real-time two-dimensional image processing using off-the-shelf PC hardware and facilitates the efficient application of variational approaches to large-scale image processing problems.

Algorithms↗
PubMed2026

Effects of an actuated ankle exoskeleton on walking stability in healthy adults: a controlled laboratory study.

BACKGROUND: Ankle exoskeletons are widely used to reduce the metabolic cost of walking, yet their effects on walking stability during unperturbed gait remain insufficiently understood. Walking stability can be characterized using complementary measures that capture stride-to-stride variability, global temporal organization, and local dynamic stability. Understanding how walking with an actuated ankle exoskeleton system influences these different aspects of gait stability is essential for the safe design and control of wearable robotic devices. METHODS: Eighteen healthy adults walked on a treadmill at a constant speed (1.1&#xa0;m/s) with and without an actuated bilateral ankle exoskeleton in a randomized crossover design. Spatiotemporal variability was quantified using coefficients of variation (CoV) of stride length, step width, and stance ratio. Global gait stability was assessed using detrended fluctuation analysis of stride time. Local dynamic stability was evaluated using maximum Lyapunov exponent calculated for the trunk, hip, upper leg, lower leg, and foot. Paired-samples two-sided t-tests were used to compare conditions. RESULTS: Walking with the ankle exoskeleton resulted in increased stride-to-stride spatiotemporal variability, reflected by higher CoV values for stride length (p&#x2009;<&#x2009;0.001) and stance ratio (p&#x2009;=&#x2009;0.005), while mean stride length and step width remained unchanged. Mean stance ratio was reduced in the exoskeleton condition (p&#x2009;<&#x2009;0.001). Global gait stability did not differ between conditions, indicating preserved long-range temporal gait organization. Local dynamic stability increased at the lower leg (p&#x2009;<&#x2009;0.001) and foot (p&#x2009;=&#x2009;0.019) when walking with the exoskeleton. CONCLUSIONS: Walking with the actuated ankle exoskeleton alters gait control across multiple levels during steady walking. While stride-to-stride variability in stride length and stance ratio increased, global gait stability remained unchanged. Local dynamic stability was increased at the lower leg and foot, suggesting segment-specific effects of ankle-level assistance close to the assisted joint. However, these findings should be interpreted as the combined effect of wearing the exoskeleton and receiving active assistance, rather than the isolated effect of plantarflexion assistance. These&#xa0;results provide insight for the design and control of ankle exoskeletons with respect to stability-related effects during walking.

Humans↗
PubMed2004

The spatiotemporal precision of ganglion cell signals: a comparison of physiological and psychophysical performance with moving gratings.

Comparison of the spatiotemporal information delivered by ganglion cells with human psychophysical performance may give insight to how retinal information is utilized by cortical mechanisms, and constrain models of spatiotemporal processing. Ganglion cells' responses were measured with drifting gratings of various spatial and temporal frequencies and contrasts. The spatiotemporal precision of cell responses was estimated in terms of a noise measure and phase variation, and compared to human vernier performance. Noise and phase variation of magnocellular (MC) cells was least at low temporal frequencies, despite their transient responses. The patterns of spatiotemporal precision of MC cells resembled the patterns of human vernier thresholds while those of parvocellular cells did not, implying use of MC cells' signals in these tasks. The analysis further implied that cortical mechanisms must perform a sophisticated spatiotemporal analysis over local ganglion cell arrays.

Animals↗
PubMed2001

Spatiotemporal QRST cancellation techniques for analysis of atrial fibrillation.

A new method for QRST cancellation is presented for the analysis of atrial fibrillation in the surface electrocardiogram (ECG). The method is based on a spatiotemporal signal model which accounts for dynamic changes in QRS morphology caused, e.g., by variations in the electrical axis of the heart. Using simulated atrial fibrillation signals added to normal ECGs, the results show that the spatiotemporal method performs considerably better than does straightforward average beat subtraction (ABS). In comparison to the ABS method, the average QRST-related error was reduced to 58 percent. The results obtained from ECGs with atrial fibrillation agreed very well with those from simulated fibrillation signals.

Atrial Fibrillation↗
PubMed1990

Simulation of motion on the skin. III. Mechanisms used by rapidly adapting cutaneous mechanoreceptors in the primate hand for spatiotemporal resolution and two-point discrimination.

1. The contribution of rapidly adapting (RA) mechanoreceptors to two-point discrimination has been evaluated by examining their ability to resolve the spacing of grating patterns shifted across the skin. The experiments test two different neural coding mechanisms that have been proposed to underlie resolution of spatial detail on the hand: 1) a rate-intensity code in which the spacing of surface features is encoded by the average frequency of firing of individual sensory afferents, and 2) an isomorphic representation of shape in which variations in the firing patterns of individual afferents reflect the spatiotemporal profile of skin indentation. 2. To measure the spatial acuity of RA mechanoreceptors innervating the hands of macaque monkeys, we displayed pairs of horizontal bars spaced 1-13 mm apart on a computer-controlled OPTACON stimulator placed over glabrous skin. Two-point resolution was measured by simultaneously pulsing pairs of rows at rates of 100, 50 and 25 Hz; each pair was shifted in tandem across the hand to simulate lateral motion. Single-fiber recordings were made from physiologically identified RA afferents in anesthetized monkeys. 3. Receptive field diameter appears to be the critical determinant of spatial resolution of gaps between two bars. RAs fire continuously if bar spacing is less than the field diameter but do not summate inputs when both active rows are contained within the field. Response profiles evoked by two bars spaced less than 4.8 mm apart can be predicted from the single-bar profiles, assuming occlusion between overlapping inputs with the strongest member dominating axonal output. Two-thirds of the RAs tested discharge 1 spike/pulse as bar patterns cross the field, yielding a uniform spike train whose frequency reflects stimulus pulse rates but fails to indicate gaps between bars. An additional 17% fire 2 spikes/pulse when the bars contact or straddle the field center, but also fail to differentiate individual stripes spaced less than 3.6 mm apart. 4. Only 17% of RAs represent gaps narrower than the field diameter. These fibers show double-peaked response profiles to bar patterns spaced at least 2.4 mm apart, firing 2 spikes/pulse as first one, and then the second stripe crosses the field center. Timing between peaks corresponds to bar spacing. Responses are reduced in amplitude when adjacent bars straddle the field center, as occlusion between simultaneous inputs prevents summation of inputs from the two stimuli. Fifteen of 16 RAs failed to resolve bars spaced 1.2 mm apart, as double-spike responses were evoked only by the leading stripe.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed2005

Stabilization of a (3+1)-dimensional soliton in a Kerr medium by a rapidly oscillating dispersion coefficient.

Using the numerical solution of the nonlinear Schrödinger equation and a variational method it is shown that (3+1)-dimensional spatiotemporal optical solitons can be stabilized by a rapidly oscillating dispersion coefficient in a Kerr medium with cubic nonlinearity. This has immediate consequence in generating dispersion-managed robust optical soliton in communication as well as possible stabilized Bose-Einstein condensates in periodic optical-lattice potential via an effective-mass formulation. We also critically compare the present stabilization with that obtained by a rapid sinusoidal oscillation of the Kerr nonlinearity parameter.

Journal Article↗
PubMed2004

Dengue spatial and temporal patterns, French Guiana, 2001.

To study a 2001 dengue fever outbreak in Iracoubo, French Guiana, we recorded the location of all patients' homes and the date when symptoms were first observed. A geographic information system (GIS) was used to integrate the patient-related information. The Knox test, a classic space-time analysis technique, was used to detect spatiotemporal clustering. Analysis of the relative-risk (RR) variations when space and time distances vary, highlighted the maximum space and time extent of a dengue transmission focus. The results show that heterogeneity in the RR variations in space and time corresponds to known entomologic and epidemiologic factors, such as the mosquito feeding cycle and host-seeking behavior. This finding demonstrates the relevance and potential of the use of GIS and spatial statistics for elaborating a dengue fever surveillance strategy.

Dengue↗
PubMed1984

Effects of disturbed sleep on contingent negative variation.

To understand better the pathophysiological aspects of disturbed sleep, six healthy male students underwent a series of sleep recordings. On the experimental night, subjects' sleep was disturbed, and the effects on contingent negative variation (CNV) were studied. Mean CNV amplitude decreased in magnitude in the Fz as well as Cz areas; the decrease in Fz-CNV tended to be greater in those subjects whose sleep was most disturbed. The decrease in amplitude of CNV appears to be a nonspecific phenomenon that occurs after any kind of disturbed sleep. Contingent negative variation is composed of two or more components with different spatiotemporal distribution. Depending on the placement of electrodes, the effect that disturbed sleep has on CNV will or will not be observed. The decreases in CNV amplitude were noted in the Fz and Cz areas. Other than subjective discomforts, psychometric measurements did not change significantly. Our data indicate, from the topographical and constructional analysis of CNVs, the possible involvement of the frontal lobe function after disturbed sleep.

Adult↗
PubMed2000

Competing spatial and temporal instabilities in a globally coupled bistable semiconductor system near a codimension-two bifurcation

We study complex spatiotemporal dynamics in a globally coupled bistable reaction-diffusion model on a two-dimensional spatial domain. It is demonstrated that complex behavior appears near a codimension-two bifurcation point due to the competition of spatial and temporal instabilities. We derive sufficient conditions for the appearance of mixed spatiotemporal modes, and clarify the origin of a menagery of complex dynamics, such as periodic and chaotic oscillations of current filaments, low-dimensional spatiotemporal chaos including a Shil'nikov attractor, and periodic back-and-forth motion of current density fronts. Such dynamics is found in a wide range of domain sizes for square and rectangular domains. The type of dynamics is sensitive to small variations in the domain shape. We discuss and explain the differences between spatiotemporal dynamics on one-dimensional and two-dimensional domains.

Journal Article↗
PubMed1990

Spatiotemporal properties of stereoacuity.

This study investigated the effects of spatial and temporal luminance variations on stereoacuity across three experiments. In Experiment 1, the effect of counterphase flicker on performance was examined at five spatial frequencies. Flicker impaired performance at a high spatial frequency, whereas it improved performance at a low spatial frequency. In Experiment 2, the effect of sustained vs. transient temporal modulation on performance was investigated at four spatial frequencies. Relative to sustained modulation, transient modulation impaired performance at high spatial frequencies, but it affected performance little at low spatial frequencies. In Experiment 3, the effect of exposure duration on performance was examined at three spatial frequencies. Decreasing duration impaired performance at a high spatial frequency, but it affected performance less at low spatial frequencies. These results show that spatial and temporal variables interact to govern stereoacuity performance; stereoacuity is good only when tested with high spatial- and low temporal-frequencies.

Depth Perception↗
PubMed1987

Conduction velocity variations minimize conduction time differences among retinal ganglion cell axons.

The visual system is able to accurately represent the spatiotemporal relations among the elements of a changing visual scene as the image moves across the retinal surface. This precise spatiotemporal mapping occurs despite great variability in retinal position and conduction velocity even among retinal ganglion cells of the same physiological class-a variability that would seem to reduce the precision with which spatiotemporal information can be transmitted to central visual areas. There was a strong negative relation between the intraretinal and extraretinal conduction time for axons of individual ganglion cells of the X-cell class. The effect of this relation was to produce a nearly constant total transmission time between the soma of a retinal X cell and its central target site. Thus, the variation in the conduction velocities of retinal ganglion cell axons may ensure that, regardless of the constraints imposed by retinal topography, a precise spatiotemporal central representation of the retinal image is maintained.

Action Potentials↗
PubMed1981

Neural mechanisms of spatial tactile discrimination: neural patterns evoked by braille-like dot patterns in the monkey.

1. The experiments reported here were designed to investigate the responses of cutaneous mechanoreceptive afferents to spatially configured dot patterns scanned across the skin. Braille-like patterns were selected because the discrimination of Braille characters must depend on spatial patterning rather than some other facet of the afferent discharge. 2. A multifactorial experimental design was used in which each afferent fibre was studied using every combination of six dot patterns, two dot sizes, two dot spacings, two contact forces and two scanning velocities. Two other factors, scanning direction relative to the skin ridges and intermittent versus continuous scanning, were studied. 3. Beside the general question concerning the response properties of the mechanoreceptive afferents, three major questions were addressed here. (i) What is the critical spatial dimension at which neural spatial patterning breaks down and below which tactual discrimination must depend on facets of the afferent discharge other than spatial neural patterning? (ii) Which mechanoreceptive population sets this critical dimension? (iii) Why is tactual discrimination enhanced by lateral scanning? 4. The results presented here suggest that the critical dimension, below which spatial neural patterning breaks down, is of the order of 1.0 mm and that the slowly adapting (SA) afferent fibres are responsible for this limit. 5. At dimensions above approximately 1.0 mm the spatial contrast between peaks and troughs in the SA discharge is markedly enhanced during scanning. When the skin is stationary the discharge rates in the SA population drop rapidly to low levels. A second possible reason for enhanced tactual discrimination during scanning is related to the increased spatiotemporal information in a coherent pattern of neural activity moving across a discrete population of afferent fibres. 6. The effects of variations in conduction velocity are analysed and it is shown that they place serious constraints on the transmission of spatiotemporal information.

Animals↗
PubMed1990

Sensitivity for interaural time and intensity difference of auditory midbrain neurons in the grassfrog.

The sensitivity for interaural time (ITD) and intensity (IID) difference was investigated for single units in the auditory midbrain of the grassfrog. A temporally structured stimulus was used which was presented by means of a closed sound system. At best frequency (BF) the majority of units was selective for ITD as indicated by an asymmetrically (73%) or symmetrically (7%) shaped ITD-rate histogram. About 20% appeared to be nonselective. Units with a symmetrical rate histogram had BFs well above 0.9 kHz, whereas for the other categories no relationship with BF was observed. Most units had a selectivity for ITD which was rather independent from frequency and absolute intensity level. In 62% of the units interaural time difference could be traded by interaural intensity difference. In most cases this so-called time-intensity trading could be explained by the intensity-latency characteristics of auditory nerve fibres. About 20% was sensitive to IID only and 5% to ITD only. A binaural model is proposed which is based on the intensity-rate and intensity-latency characteristics of auditory nerve fibres, the linear summation of excitatory and inhibitory post synaptic potentials in second order neurons, and spatiotemporal integration at the level of third order neurons. By variation of only a small number of parameters, namely strengths and time constants of the connectivities, the range of experimentally observed response patterns could be reproduced.

Acoustic Stimulation↗
PubMed1991

Spatiotemporal representation of moving luminance edges in human vision.

The edges of straight bars in a square-wave luminance grating appear undulating to an observer when the retinal image of this pattern is in motion. The amplitude of the perceived undulations increases linearly with retinal image speed with an average slope of 30 +/- 4 ms. The period of the motion-induced bulges is 2.5 +/- 0.5 degree and shows no consistent variation with the retinal image velocity of the pattern. The close quantitative agreement between the spatiotemporal extent of this effect and recent estimates of the spatiotemporal parameters of human motion-sensitive mechanisms suggests the existence of motion-sensitive cells in the central nervous system that have a fixed time constant but change the shape and size of their retinal support with retinal image velocity.

Humans↗