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PubMed2005

The watercolor illusion and neon color spreading: a unified analysis of new cases and neural mechanisms.

Coloration and figural properties of neon color spreading and the watercolor illusion are studied using phenomenal and psychophysical observations. Coloration properties of both effects can be reduced to a common limiting condition, a nearby color transition called the two-dot limiting case, which clarifies their perceptual similarities and dissimilarities. The results are explained by the FACADE neural model of biological vision. The model proposes how local properties of color transitions activate spatial competition among nearby perceptual boundaries, with boundaries of lower-contrast edges weakened by competition more than boundaries of higher-contrast edges. This asymmetry induces spreading of more color across these boundaries than conversely. The model also predicts how depth and figure-ground effects are generated in these illusions.

Adaptation, Ocular↗
PubMed2002

Monte Carlo simulation studies of EEG and MEG localization accuracy.

Both electroencephalography (EEG) and magnetoencephalography (MEG) are currently used to localize brain activity. The accuracy of source localization depends on numerous factors, including the specific inverse approach and source model, fundamental differences in EEG and MEG data, and the accuracy of the volume conductor model of the head (i.e., the forward model). Using Monte Carlo simulations, this study removes the effect of forward model errors and theoretically compares the use of EEG alone, MEG alone, and combined EEG/MEG data sets for source localization. Here, we use a linear estimation inverse approach with a distributed source model and a realistic forward head model. We evaluated its accuracy using the crosstalk and point spread metrics. The crosstalk metric for a specified location on the cortex describes the amount of activity incorrectly localized onto that location from other locations. The point spread metric provides the complementary measure: for that same location, the point spread describes the mis-localization of activity from that specified location to other locations in the brain. We also propose and examine the utility of a "noise sensitivity normalized" inverse operator. Given our particular forward and inverse models, our results show that 1) surprisingly, EEG localization is more accurate than MEG localization for the same number of sensors averaged over many source locations and orientations; 2) as expected, combining EEG with MEG produces the best accuracy for the same total number of sensors; 3) the noise sensitivity normalized inverse operator improves the spatial resolution relative to the standard linear estimation operator; and 4) use of an a priori fMRI constraint universally reduces both crosstalk and point spread.

Algorithms↗
PubMed1990

Spatial distribution of neural activity evoked by electrical stimulation of the cochlea.

Activity in the central auditory system was mapped with 2-deoxyglucose (2-DG) autoradiography, using either pure tones or electrical stimulation of the normal cochlea. Electrical stimulation with both monopolar (distant reference electrode) and bipolar prostheses near threshold increased 2-DG uptake in auditory nuclei in a manner similar to that seen with a pure tone: increased 2-DG uptake was restricted to a small frequency region of brainstem and mid-brain auditory nuclei. The position of this area was related to the cochlear location of the prosthesis. At higher current amplitudes only the bipolar prosthesis retained spatial restriction of evoked neural activity, while stimulation through a monopolar prosthesis produced evoked activity in all frequency regions of auditory nuclei, and in non-auditory nuclei. Activation of non-auditory structures was consistent with spread of current through the brainstem, rather than activation of peripheral nerves. At all current amplitudes, a monopolar prosthesis evoked higher levels of 2-DG uptake than a bipolar prosthesis. The results suggest that while a bipolar prosthesis provides greater spatial restriction of evoked neural activity and a greater dynamic range, a monopolar prosthesis produces higher levels of evoked activity.

Acoustic Stimulation↗
PubMed2003

Functional imaging of the retinal layers by laser scattering: an approach for the study of Leão's spreading depression in intact tissue.

This paper presents a novel optical approach for the study of spreading depression in isolated retina. The method makes it possible to register the laser light scattered from each layer of the tissue, yielding a functional image of the retina during spreading depression. The tissue is kept intact, since histological cuts are not necessary. Measurements of other variables, such as extracellular potential, are also allowed by the described method. This is done simultaneously with the functional image in a high spatial resolution, with the positioning of the microelectrode tip being easily monitored. The information about temporal and spatial evolution of light was compacted in a single image. The image-processing technique used here enables the visualization of the light scattered by the inner plexiform layer (IPL), which is the most prominent scatter layer during spreading depression. The wavefront velocity and its increase as two wavefronts approach each other can then be determined, and it is also possible to observe the thickness variation of the tissue during the wave travel. The relationship between two peaks of light-scattering sequence during the phenomenon was studied at two wavelengths (632.8 and 543.5 nm). This relationship is shown to be dependent on the wavelength.

Animals↗
PubMed1967

The generation and spread of S-potentials in fish (Cyprinidae).

1. S-potentials from Luminosity-units in the excised eye of the tench (Tinca) were excited by white lights of various intensities and spatial distributions.2. When a small light spot of fixed size and intensity was presented at various distances from the recording electrode, the S-potential was found to suffer an exponential attenuation with distance (Fig. 3).3. A circular patch of light centred upon the electrode and varied in radius gave concordant results.4. The effects at the electrode site of a distant light flash was not due to light scattered to the electrode which acted upon the retina there; the light acted locally and it was the current that spread.5. With red and blue light stimuli superposed on the retina, it is the potentials not the lights that add.6. These experimental results seem to indicate that S-potentials can spread laterally across the retina within a thin layer at about the horizontal cell level.7. The layer is bounded by leaky membranes which appear to be electrogenic, no doubt as a result of transmitters released by light which change the permeability. Electric currents have no demonstrable effect.8. Two simple results were found. (a) The membrane contains an element whose conductance is proportional to the light intensity. (b) The S-potential is a hyperbolic function of light intensity.9. All these experimental results can be explained quantitatively in terms of the formal model of Fig. 8F. But the required change of membrane permeability with light has not yet been demonstrated.

Animals↗
PubMed1983

On spatial periodicity in the formation of cell adhesions to a substrate.

Interference reflection microscopy reveals a fairly regular array of marginal focal contacts in early states of cell spreading. A theoretical explanation of the onset of patchiness is given, based on the positively cooperative binding to the substrate of laterally diffusing receptor molecules.

Animals↗
PubMed1987

Three-dimensional representation of chemical gradients.

Perspective display techniques are applied to chemical and biochemical data sets. These represent spatially distributed gradients of reactive compounds that participate in pattern-formation processes due to reaction-diffusion or reaction-convection coupling. The patterns form in thin solution layers and are observed as chemical waves in the Belousov-Zhabotinskii reaction, as convection-induced stationary structures during oscillating glycolysis in yeast cytoplasm, and as the diffusive spreading of enzyme-catalyzed metabolic turnover in a substrate layer. The digital data are measured with a two-dimensional spectrophotometer based on a computerized video equipment with high spatial, temporal and intensity resolution. By application of three-dimensional procedures detailed structural properties of chemical and biochemical model systems will be presented yielding localization of reaction and transport events.

Journal Article↗
PubMed1992

The development of pericardial villi in the chick embryo.

The development of pericardial villi and their relation to the development of the cardiac surface was studied in chick embryos from the 3rd to 10th day of incubation by scanning electron microscopy. During the 3rd day of incubation (stage 14-17 HH) the coelomic epithelium covering the ventral wall of the sinus venosus forms villous protrusions. By the end of the 3rd day (stage 17 HH) these protrusions contact the dorsal wall of the heart, so that a secondary dorsal mesocardium is formed. This bridges the pericardial cavity between the ventral wall of the sinus venosus and the dorsal base of the ventricles. This sinu-ventricular mesocardium exists only temporarily, as on the 8th day of incubation it becomes a part of the cardiac wall due to fusion with the epicardium of the coronary sulcus. During the 4th and 5th day of incubation (stage 17-25 HH), the formation of the epicardium proceeds from the point of attachment of the sinu-ventricular mesocardium. Although these findings suggest that the epithelium of the villous protrusions spreads over the surface of the embryonic heart, one cannot exclude other hypotheses on epicardial origin. The impression of a spreading epicardium could also be created if epicardial cells were to delaminate from a local epithelium in a temporally and spatially organized pattern.

Animals↗
PubMed1993

The RF-cinematogram. A cross-correlation technique for mapping several visual receptive fields at once.

We present a spike-triggered averaging method capable of mapping the visual receptive fields of several neurons simultaneously. The stimulation is general and the mapping proceeds automatically without the need to match the stimulation to the cells' preference for position, orientation, direction, etc. The maps are spatiotemporal; receptive field (RF) structures are quantitatively determined in three dimensions: the two dimensions of visuotopic space, and time. The method presented is one of a family of "reverse correlation" or "spike-triggered averaging" techniques (DeBoer and Kuyper 1968) capable of revealing linear aspects of stimulus-response coupling. The formal relationship of these methods to stimulus-response cross-correlation is shown. The analysis is extended to provide some second-order axis-of-motion information ("direction marks"). The stimulus is a constantly illuminated, randomly jumping bright or dark spot, not an elongated bar. Spot diameters between one-third to 1 x RF width are effective. The method ascertains for each recorded action potential or "spike" the prior visual field position of the spot. The average or most probable spot positions define the receptive field spatially. Repeating the process for a succession of times prior to observed spikes defines the field temporally, presented here as a succession of spatial maps. We term this portrayal a receptive field cinematogram, RFc or ciné. The RFc reveals and economically portrays the spread of excitability and suppression across the receptive field, culminating in the generation of a spike. RFcs for LGN neurons and for simple cells recorded in cat cortical areas 17 and 18 are presented and interpreted in terms of classic ON/OFF regions. The availability of temporal information permits the separation of an excitatory exit response, generated when a moving bright spot leaves an OFF region, from an excitatory entrance response occurring when a bright spot enters an ON region, because these responses occur at different times (exit responses earlier). Spike emission remains coupled to (cross-correlated with) stimulus events over time periods as long as 96 ms, implying that some stimulus drive or afferent visual input is delayed by as much as 96 ms more than other input. This is a striking instance of temporal dispersion in the visual system. In some cells, said to be "spatiotemporally inseparable", the delay (latency) varies systematically across the visual field; i.e., the place for optimal stimulation varies with the time prior to spike emission. In these cells, the RFc shows receptive field structures which move across the visual field over trajectories equal to approximately twice the total conventional RF width.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Cell transformation induces a cytoplasmic Ca2+ oscillator in Madin-Darby canine kidney cells.

Alkaline stress transforms Madin-Darby canine kidney (MDCK) cells as indicated by loss of epithelial structure, multilayer cell growth and formation of foci. In the present study we report that transformed MDCK cells (MDCK-F cells) exhibit spontaneous and lasting oscillations of intracellular Ca2+ concentration ([Ca2+]i), which are absent in non-transformed cells. Oscillations, as revealed by Fura-2 video imaging, were due to the activity of an inositol 1,4,5-trisphosphate-(InsP3)-sensitive Ca2+ store since their frequency was dependent on bradykinin concentration and they were abolished by the phosphoinositidase C inhibitor U73122. Moreover, blockers of the cytoplasmic Ca(2+)-ATPase, thapsigargin and 2,5-di-(tetr-butyl)-1,4-benzohydroquinone inhibited oscillatory activity. In contrast, neither injection of ruthenium red, ryanodine nor caffeine had any effect on oscillations. Analysis of the spatial distribution of [Ca2+]i showed that Ca2+ transients originated from an initiation site constant for a given cell and spread through the cell as an advancing Ca2+ wave. Oscillations started in a random manner from single cells and spread over neighbouring cells, suggesting a kind of intercellular communication. We conclude that MDCK-F cells have acquired the ability for endogenous Ca2+ release through transformation. Oscillations are primarily due to the activity of an InsP3-sensitive cytosolic Ca2+ oscillator.

Animals↗
PubMed1985

Unilateral and bilateral cortical spreading depression interferes with radial maze performance in rats.

Seven rats with implanted cortical electrodes were trained in a 12-arm radial maze to asymptomatic performance (1.38 errors per trial). Single waves of cortical spreading depression (CSD) were elicited by application of cathodal current to the parieto-occipital cortex and monitored by suppression of callosal responses in the frontal cortex. Bilateral CSD elicited before the trial caused a small increase of error incidence in choices 1-6 but did not affect accuracy of choices 7-12. Bilateral CSD between choices 6 and 7 increased error incidence in the second half of the trial from 1.33 to 2.84, i.e. almost to the chance level of 3.0. Unilateral CSDs elicited before the first half of the trial in one hemisphere and in the same or in the contralateral hemisphere before the second half of the trial caused similar deterioration of performance in choices 7-12 (2.4 and 2.1 errors, respectively). It is concluded that the CSD effects have both proactive and retroactive components and that the working memory record is not lateralized at the cortical level.

Animals↗
PubMed1993

An improved mathematical description of the foveal visual point spread function with parameters for age, pupil size and pigmentation.

An analytical description of the point spread function (PSF) for human foveal vision, together with its closed form two-dimensional Fourier transform, the modulation transfer function (MTF), is proposed. It also possesses an analytical line spread function (LSF) counterpart. It fits to both PSF and MTF experimental data and holds over the full angular and spatial frequency domain. Variation in the PSF and MTF descriptions with pupil size, age and iris/skin pigmentation are accounted for by analytical expressions in the parameters.

Adult↗
PubMed2005

Persistence, spread and the drift paradox.

We derive conditions for persistence and spread of a population where individuals are either immobile or dispersing by advection and diffusion through a one-dimensional medium with a unidirectional flow. Reproduction occurs only in the stationary phase. Examples of such systems are found in rivers and streams, marine currents, and areas with prevalent wind direction. In streams, a long-standing question, dubbed 'the drift paradox', asks why aquatic insects faced with downstream drift are able to persist in upper stream reaches. For our two-phase model, persistence of the population is guaranteed if, at low population densities, the local growth rate of the stationary component of the population exceeds the rate of entry of individuals into the drift. Otherwise the persistence condition involves all the model parameters, and persistence requires a critical (minimum) domain size. We calculate the rate at which invasion fronts propagate up- and downstream, and show that persistence and ability to spread are closely connected: if the population cannot advance upstream against the flow, it also cannot persist on any finite spatial domain. By studying two limiting cases of our model, we show that residence in the immobile state always enhances population persistence. We use our findings to evaluate a number of mechanisms previously proposed in the ecological literature as resolutions of the drift paradox.

Models, Theoretical↗
PubMed1999

Three dimensional image restoration in fluorescence lifetime imaging microscopy.

A microscope set-up and numerical methods are described which enable the measurement and reconstruction of three-dimensional nanosecond fluorescence lifetime images in every voxel. The frequency domain fluorescence lifetime imaging microscope (FLIM) utilizes phase detection of high-frequency modulated light by homodyne mixing on a microchannel plate image intensifier. The output signal at the image intensifier's phosphor screen is integrated on a charge coupled device camera. A scanning stage is employed to obtain a series of phase-dependent intensity images at equally separated depths in a specimen. The Fourier transform of phase-dependent data gives three-dimensional (3D) images of the Fourier coefficients. These images are deblurred using an Iterative Constrained Tikhonov-Miller (ICTM) algorithm in conjunction with a measured point spread function. The 3D reconstruction of fluorescence lifetimes are calculated from the deblurred images of the Fourier coefficients. An improved spatial and temporal resolution of fluorescence lifetimes was obtained using this approach to the reconstruction of simulated 3D FLIM data. The technique was applied to restore 3D FLIM data of a live cell specimen expressing two green fluorescent protein fusion constructs having distinct fluorescence lifetimes which localized to separate cellular compartments.

Image Processing, Computer-Assisted↗
PubMed2002

Theory of electrochemical pattern formation.

The spatial coupling in electrochemical systems is mediated by ion migration under the influence of the electric field. Since field effects spread very rapidly, every point of an electrode can communicate with every other one practically instantaneously through migration coupling. Based on mathematical potential theory we present the derivation of a generally applicable reaction-migration equation, which describes the coupling via an integral over the whole electrode area. The corresponding coupling function depends only on the geometry of the electrode setup and has been computed for commonly used electrode shapes (such as ring, disk, ribbon or rectangle). The pattern formation observed in electrochemical systems in the bistable, excitable and oscillatory regime can be reproduced in computer simulations, and the types of patterns occurring under different geometries can be rationalized. (c) 2002 American Institute of Physics.

Journal Article↗
PubMed2006

Effects of flecainide and quinidine on arrhythmogenic properties of Scn5a+/Delta murine hearts modelling long QT syndrome 3.

Long QT3 (LQT3) syndrome is associated with incomplete Na+ channel inactivation, abnormal repolarization kinetics and prolonged cardiac action potential duration (APD). Electrophysiological effects of flecainide and quinidine were compared in Langendorff-perfused wild-type (WT), and genetically modified (Scn5a+/Delta) murine hearts modelling LQT3. Extra stimuli (S2) following trains of pacing stimuli (S1) applied to the right ventricular epicardium triggered ventricular tachycardia (VT) in 16 out of 28 untreated Scn5a+/Delta and zero out of 12 WT hearts. Paced electrogram fractionation analysis then demonstrated increased electrogram durations (EGD), expressed as EGD ratios, in arrhythmogenic Scn5a+/Delta hearts, and prolonged ventricular effective refractory periods in initially non-arrhythmogenic Scn5a+/Delta hearts. Nevertheless, comparisons of epicardial and endocardial monophasic action potential recordings demonstrated negative transmural repolarization gradients in both groups, giving DeltaAPD(90) values at 90% repolarization of -20.88 +/- 1.93 ms (n = 11) and -16.91 +/- 1.43 ms (n = 23), respectively. Flecainide prevented initiation of VT in 13 out of 16 arrhythmogenic Scn5a+/Delta hearts, reducing EGD ratio and restoring DeltaAPD90 to + 7.55 +/- 2.24 ms (n = 9) (P < 0.05). VT occurred in four out of eight non-arrhythmogenic Scn5a+/Delta hearts in the presence of quinidine, which increased EGD ratio but left DeltaAPD90 unchanged. In contrast (P < 0.05), WT hearts had positive DeltaAPD90 values (+ 11.72 +/- 2.17 ms) (n = 20). Flecainide then increased arrhythmic tendency and EGD ratio but conserved DeltaAPD90; reduced EGD ratios and unaltered DeltaAPD90 values accompanied the lower arrhythmogenicity associated with quinidine treatment. In addition to the changes in EGD ratio shown by WT hearts, these findings attribute arrhythmogenesis and its modification by flecainide and quinidine to alterations in DeltaAPD90 in Scn5a+/Delta hearts. This is consistent with a hypothesis in which incomplete Na+ channel inactivation in Scn5a+/Delta hearts generates functional substrates dependent on altered refractoriness that cause abnormalities in activation and conduction of subsequent cardiac impulses. Any spatial heterogeneities between the epicardial and endocardial layers would thus cause fragmentation of the activation wavefront and contribute to electrogram spreading.

Action Potentials↗
PubMed2004

Modification of spatial distribution of 2,4-dichlorophenoxyacetic acid degrader microhabitats during growth in soil columns.

Bacterial processes in soil, including biodegradation, require contact between bacteria and substrates. Knowledge of the three-dimensional spatial distribution of bacteria at the microscale is necessary to understand and predict such processes. Using a soil microsampling strategy combined with a mathematical spatial analysis, we studied the spatial distribution of 2,4-dichlorophenoxyacetic acid (2,4-D) degrader microhabitats as a function of 2,4-D degrader abundance. Soil columns that allowed natural flow were percolated with 2,4-D to increase the 2,4-D degrader abundance. Hundreds of soil microsamples (minimum diameter, 125 microm) were collected and transferred to culture medium to check for the presence of 2,4-D degraders. Spatial distributions of bacterial microhabitats were characterized by determining the average size of colonized soil patches and the average number of patches per gram of soil. The spatial distribution of 2,4-D degrader microhabitats was not affected by water flow, but there was an overall increase in colonized patch sizes after 2,4-D amendment; colonized microsamples were dispersed in the soil at low 2,4-D degrader densities and clustered in patches that were more than 0.5 mm in diameter at higher densities. During growth, spreading of 2,4-D degraders within the soil and an increase in 2,4-D degradation were observed. We hypothesized that spreading of the bacteria increased the probability of encounters with 2,4-D and resulted in better interception of the degradable substrate. This work showed that characterization of bacterial microscale spatial distribution is relevant to microbial ecology studies. It improved quantitative bacterial microhabitat description and suggested that sporadic movement of cells occurs. Furthermore, it offered perspectives for linking microbial function to the soil physicochemical environment.

2,4-Dichlorophenoxyacetic Acid↗
PubMed1983

Endomyocardial disease in South America--report on 23 cases in Venezuela.

Twenty-three cases of endomyocardial disease (ED) are presented, studied in Venezuela, a tropical country in northern South America. The diagnosis was confirmed in 18 cases by means of pathological studies, and in 5 cases by angiocardiography which showed the characteristic obliterative ventricular lesions. Eosinophilia was present in 35% of the patients. The most frequent clinical feature was heart failure associated with mitral regurgitation. Systemic embolism was the first clinical feature in 5 cases. In 2 cases, ED was associated with autoimmune haemolytic anaemia or vasculitis. Necropsy revealed a predominance of the left-sided (9/16 cases) and biventricular (6/16 cases) types. The pathological lesions were characterised by fibrous thickening of the endocardium at the apex and the ventricular inflow tracts extending to the myocardium and involving the atrioventricular valves. ED is frequently misdiagnosed as rheumatic valvular cardiopathy. The two-dimensional echocardiogram is a very useful procedure for determining the spatial anatomy of ED. The echo findings were closely correlated with ventriculographic and necropsy findings. Even though ED is widely spread around the world, it is most frequently found in tropical and subtropical countries in Africa, Asia and America, such as Venezuela and Brazil. This suggests that there are aetiological factors in these latitudes, about which little is known.

Adolescent↗