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Results for “spatial localization”

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At least 1,711 records · Page 95Linked to original sources
PubMed2001

Functional and mutational analysis of p19, a DNA transfer protein with muramidase activity.

Protein P19 encoded by the conjugative resistance plasmid R1 has been identified as being one member of a large family of muramidases encoded by bacteriophages and by type III and type IV secretion systems. We carried out a mutational analysis to investigate the function of protein P19 and used in vivo complementation assays to test those of several P19 mutants. The results indicated that conserved residues present in the presumed catalytic center of P19 are absolutely essential for its function in conjugation of plasmid R1 and infection by the RNA phage R17. Overexpression of protein P19 in an early growth phase resulted in a massive lysis of Escherichia coli cells in liquid culture, as indicated by a rapid and distinct decrease in cell culture densities after induction. Change of the proposed catalytic glutamate at position 44 to glutamine completely abolished this effect. P19-induced cell lysis was directly shown by transmission and scanning electron microscopy. Typically, P19-overexpressing cells showed bulges protruding from the cell surfaces. Our interpretation is that these protrusions arose from a localized and spatially confined disruption of the bacterial cell wall. To our knowledge such an effect has not previously been documented for any member of the lytic transglycosylase family. From the data presented here, we conclude that protein P19 possesses the proposed localized peptidoglycan-hydrolyzing activity. This activity would be a prerequisite for efficient penetration of the cell envelope by the DNA translocation complex encoded by the conjugative plasmid.

Amino Acid Sequence↗
PubMed1998

Behavioral studies of sound localization in the cat.

Using the magnetic search coil technique to measure eye and ear movements, we trained cats by operant conditioning to look in the direction of light and sound sources with their heads fixed. Cats were able to localize noise bursts, single clicks, or click trains presented from sources located on the horizontal and vertical meridians within their oculomotor range. Saccades to auditory targets were less accurate and more variable than saccades to visual targets at the same spatial positions. Localization accuracy of single clicks was diminished compared with the long-duration stimuli presented from the same sources. Control experiments with novel auditory targets, never associated with visual targets, demonstrated that the cats localized the sound sources using acoustic cues and not from memory. The role of spectral features imposed by the pinna for vertical sound localization was shown by the breakdown in localization of narrow-band (one-sixth of an octave) noise bursts presented from sources along the midsagittal plane. In addition, we show that cats experience summing localization, an illusion associated with the precedence effect. Pairs of clicks presented from speakers at (+/-18 degrees,0 degrees ) with interclick delays of +/-300 microsec were perceived by the cat as originating from phantom sources extending from the midline to approximately +/-10 degrees.

Acoustic Stimulation↗
PubMed1986

Speculations on the role of frequency in sound localization.

This presentation reviews behavioral and physiological evidence suggesting that the perceived power spectrum of a sound, as modified by the external ears, provides important cues for horizontal and vertical sound localization. This spatial information is mostly likely encoded by the relative levels of excitation among the tonotopically organized hair cells of the cochlea. This suggests that the preservation of the cochlear frequency map within the central auditory system contributes an anatomical substrate for the localization of sound.

Animals↗
PubMed2005

Local infrared microspectroscopy with subwavelength spatial resolution with an atomic force microscope tip used as a photothermal sensor.

We describe a new method of infrared microspectroscopy. It is intended for performing chemical mapping of various objects with subwavelength lateral resolution by using the infrared vibrational signature characterizing different molecular species. We use the photothermal expansion effect, detected by an atomic force microscope tip, probing the local transient deformation induced by an infrared pulsed laser tuned at a sample absorbing wavelength. We show that this new tool opens the way for measuring and identifying spectroscopic contrasts not accessible by far-field or near-field optical methods and with a subwavelength lateral resolution.

Escherichia coli↗
PubMed1995

[The analysis of the spatial aggregation of health events using risk surface density estimation methods].

Methods for the analysis of the spatial aggregation of health events has received growing attention under the pressure of public opinion concern and as tools for the identification of potential risk sources, for monitoring relevant geographical areas and, finally, for public health decisions. The development of statistical methods for the detection and localization of spatial clusters has mainly concerned individual data. This paper is aimed at describing one of the methods proposed for the identification of clusters in the case of information at individual level and at presenting its extension to grouped data. This method, the surface density estimation method using the Kernel approach, offers remarkable advantages in terms of simplicity of implementation and flexibility, this latter being an extremely important characteristic in the case of exploratory analyses. For exemplification purposes, the density estimation method has been applied to individual data concerning the spatial distribution of cerebral tumors in Campi Bisenzio (FI) and to the distribution of gastric cancer mortality in the municipalities around Arezzo and Pesaro.

Brain Neoplasms↗
PubMed1987

Spatial frequency and selective attention to local and global information.

Probe methods were used to investigate whether the distribution of attention to the local or the global structure of a stimulus affects the detectability of different spatial frequencies. Four experiments are reported in which the detectability of threshold probe gratings of different spatial frequencies was measured while subjects analyzed either the local or the global information from a display. A relative shift in the detectability of low and high frequencies was observed. Low frequencies were facilitated during global processing and/or high frequencies were facilitated during local processing.

Adult↗
PubMed2002

Molecular profiling and cellular localization of connexin isoforms in the rat ciliary epithelium.

The functionally distinct epithelial layers of the ciliary body act as a syncitium to produce the aqueous humour. Ultrastructural studies have shown that the pigmented (PE) and non-pigmented (NPE) cell layers of the ciliary epithelium are connected by gap junctions. However the molecular composition of gap junctions both between and within the two cell layers has not been comprehensively studied. To address this issue the authors have performed an extensive molecular screening of connexin (Cx) expression patterns in ciliary epithelium of the rat. Initially, mRNA was extracted from rat ciliary bodies, reverse-transcribed, and subjected to two rounds of PCR using primer sets designed against each of the 14 Cx isoforms known to be expressed in the rat. This initial screening protocol amplified eight candidate Cx isoforms (Cxs 26, 31, 33, 37, 40, 43, 45 and 46). The Cx isoforms identified in this initial screen were then first assigned to the ciliary epithelium itself (Cxs 26, 31, 40 and 43) or structures outside the epithelium (Cxs 37, 40, and 45) using immunohistochemistry performed on ciliary body whole mounts. No convincing evidence for either Cx 33 or 46 labelling was found in the ciliary body. Then the four Cx isoforms localized to the epithelium were further localized to specific membrane domains within the epithelial cell layers by performing high resolution imaging of the antibody labeling patterns obtained in cryosections. This enabled Cx26 and 31 to be specifically localized to spatially different gap junctions between NPE cells. Cx31 labeled gap junctions associated with an extensive network of membrane interdigitations found between NPE cells at their basal surfaces. In contrast Cx26 labeling in NPE cells was restricted to the basolateral membranes of adjacent NPE cells. Cx40 and Cx43 were both localized to the PE-NPE interface where they formed discrete homomeric/homotypic gap junction plaques. No convincing evidence was found for antibody labeling between PE cells. Thus it appears that intercellular communication, both within the NPE layer and between the PE and NPE cell layers, is mediated by gap junction channels that have distinctive permeability properties. In particular the results raise the possibility that the permeability of PE-NPE gap junctions can be modulated by changing the Cx43 : Cx40 expression ratio. Whether such a change in Cx expression ratios occurs and what effect it has on aqueous humour production and composition remains to be determined.

Animals↗
PubMed2000

Saccadic localization of occluded targets.

Saccadic eye movements are able to localize spatially-extended targets, including patterns of random dots and simple shapes, with a high degree of precision [McGowan, Kowler, Sharma & Chubb (1998). Vision Research, 38, 895-909; Melcher & Kowler (1999). Vision Research, 39, 2929-2946]. This paper investigates the representations of object shape that guide saccades. We studied saccadic localization of partially-occluded triangles (two or three vertices removed) to find out whether saccades have access to a representation of the full shape, despite the missing portions. Targets were configured so that they could be seen either as triangles, which were partially occluded by polygons, or as fragments in front of the same polygons. Subjects tried to saccade to the inferred full triangle and a discrimination paradigm was used to evaluate their success. Occlusion cues were ineffective in that saccades directed to the occluded triangles landed near the center of the visible fragment, even when it was configured as a triangle behind occluders. Removing the occluders and leaving only three segments of the triangle (vertices removed) helped somewhat, but performance never resembled that achieved with either a full triangle or a 3-dot configuration. We conclude that the saccadic system is insensitive to at least some cues that can be used to infer the shape of objects. For occluded targets, the representation used by saccades may be closer to the configuration of the retinal image.

Form Perception↗
PubMed2004

Localization of human Mcm10 is spatially and temporally regulated during the S phase.

Mcm10 (Dna43) is an essential protein for the initiation of DNA replication in Saccharomyces cerevisiae. Recently, we identified a human Mcm10 homolog and found that it is regulated by proteolysis and phosphorylation in a cell cycle-dependent manner and that it binds chromatin exclusively during the S phase of the cell cycle. However, the precise roles that Mcm10 plays are still unknown. To study the localization dynamics of human Mcm10, we established HeLa cell lines expressing green fluorescent protein (GFP)-tagged Mcm10. From early to mid-S phase, GFP-Mcm10 appeared in discrete nuclear foci. In early S phase, several hundred foci appeared throughout the nucleus. In mid-S phase, the foci appeared at the nuclear periphery and nucleolar regions. In the late S and G phases, GFP-Mcm10 was localized to nucleoli. Although (2)the distributions of GFP-Mcm10 during the S phase resembled those of replication foci, GFP-Mcm10 foci did not colocalize with sites of DNA synthesis in most cases. Furthermore, the transition of GFP-Mcm10 distribution patterns preceded changes in replication foci patterns or proliferating cell nuclear antigen foci patterns by 30-60 min. These results suggest that human Mcm10 is temporarily recruited to the replication sites 30-60 min before they replicate and that it dissociates from chromatin after the activation of the prereplication complex.

Binding Sites↗
PubMed1997

Localization of brain electrical activity via linearly constrained minimum variance spatial filtering.

A spatial filtering method for localizing sources of brain electrical activity from surface recordings is described and analyzed. The spatial filters are implemented as a weighted sum of the data recorded at different sites. The weights are chosen to minimize the filter output power subject to a linear constraint. The linear constraint forces the filter to pass brain electrical activity from a specified location, while the power minimization attenuates activity originating at other locations. The estimated output power as a function of location is normalized by the estimated noise power as a function of location to obtain a neural activity index map. Locations of source activity correspond to maxima in the neural activity index map. The method does not require any prior assumptions about the number of active sources of their geometry because it exploits the spatial covariance of the source electrical activity. This paper presents a development and analysis of the method and explores its sensitivity to deviations between actual and assumed data models. The effect on the algorithm of covariance matrix estimation, correlation between sources, and choice of reference is discussed. Simulated and measured data is used to illustrate the efficacy of the approach.

Algorithms↗
PubMed2003

Spatial tuning to virtual sounds in the inferior colliculus of the guinea pig.

How do neurons in the inferior colliculus (IC) encode the spatial location of sound? We have addressed this question using a virtual auditory environment. For this purpose, the individual head-related transfer functions (HRTFs) of 18 guinea pigs were measured under free-field conditions for 122 locations covering the upper hemisphere. From 257 neurons, 94% responded to the short (50-ms) white noise stimulus at 70 dB sound pressure level (SPL). Out of these neurons, 80% were spatially tuned with a receptive field that is smaller than a hemifield (at 70 dB). The remainder responded omnidirectionally or showed fractured receptive fields. The majority of the neurons preferred directions in the contralateral hemisphere. However, preference for front or rear positions and high elevations occurred frequently. For stimulation at 70 dB SPL, the average diameter of the receptive fields, based on half-maximal response, was less than a quarter of the upper hemisphere. Neurons that preferred frontal directions responded weakly or showed no response to posterior directions and vice versa. Hence, front/back discrimination is present at the single-neuron level in the IC. When nonindividual HRTFs were used to create the stimuli, the spatial receptive fields of most neurons became larger, split into several parts, changed position, or the response became omnidirectional. Variation of absolute sound intensity had little effect on the preferred directions of the neurons over a range of 20 to 40 dB above threshold. With increasing intensity, most receptive fields remained constant or expanded. Furthermore, we tested the influence of binaural decorrelation and stimulus bandwidth on spatial tuning. The vast majority of neurons with a low characteristic frequency (<2.5 kHz) lost spatial tuning under stimulation with binaurally uncorrelated noise, whereas high-frequency units were mostly unaffected. Most neurons that showed spatial tuning under broadband stimulation (white noise and 1 octave wide noise) turned omnidirectional when stimulated with 1/3 octave wide noise.

Acoustic Stimulation↗
PubMed2001

Asymmetric performance in the cocktail party effect: implications for the design of spatial audio displays.

An experiment was conducted to determine the extent to which hemispheric specialization is manifested in the performance of tasks in which listeners are required to attend to one of several simultaneously spoken speech communications. Speech intelligibility and response time were measured under factorial combinations of the number of simultaneous talkers, the target talker hemifield, and the spatial arrangement of talkers. Intelligibility was found to be mediated by all of the independent variables. Results are discussed in terms of the design of adaptive spatial audio interfaces for speech communications. Actual or potential applications of this research include the design of adaptive spatial audio interfaces for speech communications.

Adolescent↗
PubMed1986

The role of spatial complexity in the perception of speech and pure tones in dichotic listening.

Ear advantages for CV syllables were determined for 28 right-handed individuals in a target monitoring dichotic task. In addition, ear dominance for dichotically presented tones was determined when the frequency difference of the two tones was small compared to the center frequency and when the frequency difference of the tones was larger. On all three tasks, subjects provided subjective separability ratings as measures of the spatial complexity of the dichotic stimuli. The results indicated a robust right ear advantage (REA) for the CV syllables and a left ear dominance on the two tone tasks, with a significant shift toward right ear dominance when the frequency difference of the tones was large. Although separability ratings for the group data indicated an increase in the perceived spatial separation of the components of the tone complex across the two tone tasks, the separability judgment ratings and the ear dominance scores were not correlated for either tone task. A significant correlation, however, was evidenced between the laterality measure for speech and the judgment of separability, indicating that a REA of increased magnitude is associated with more clearly localized and spatially separate speech sounds. Finally, the dominance scores on the two tone tasks were uncorrelated with the laterality measures of the speech task, whereas the scores on the tone tasks were highly correlated. The results suggest that spatial complexity does play a role in the emergence of the REA for speech. However, the failure to find a relationship between speech and nonspeech tasks suggest that all perceptual asymmetries observed with dichotic stimuli cannot be accounted for by a single theoretical explanation.

Acoustic Stimulation↗
PubMed2003

Cavity solitons in two-level lasers with dense amplifying medium.

Local-field effects are known to induce bistability in dense optical media. We examine theoretically whether this property is preserved in broad-area cavities, and show that bistability between the homogeneous lasing and nonlasing states of the system persists provided a Fourier filtering technique is used to prevent off-axis emission. The resulting bistability gives rise to spatial light localization in the form of cavity solitons, which exhibit a particularly large degree of plasticity as a function of the characteristics of the addressing beam. This is the simplest laser able to sustain cavity solitons.

Journal Article↗
PubMed1995

Spatial and sub-cellular localization of the membrane cytoskeleton-associated protein alpha-adducin in the rat brain.

Studies on the identification and characterization of constituents of rat brain synaptic junctions have lead to the isolation of cDNA clones encoding segments of alpha-adducin. These and other studies suggest that adducin, a protein involved in promoting the assembly of actin and spectrin filaments at the plasma membrane, may play a role in dynamic assembly-disassembly processes underlying synaptic plasticity. In order to verify that brain alpha-adducin is indeed a constituent of synaptic structures, we have generated monoclonal antibodies against epitopes in the C-terminal region of alpha-adducin and have determined its spatial and sub-cellular distribution in postnatal day-30 rat brain. Alpha-adducin is found to be highly enriched in regions with high synapse densities of the hippocampus, corpus striatum, cerebral cortex and cerebellum. Immuno-electron microscopic analysis of peroxidase stained sections of the hippocampus and the cerebellum revealed that alpha-adducin is localized at distinct sub-cellular structures. In the CA1 and CA3 regions of the hippocampus alpha-adducin immunoreactivity is found in a distinct subset of dendrites and dendritic spines. In the molecular layer of the cerebellum, a distinct fraction of pre-synaptic terminals of parallel fiber terminals is labeled. In both cases the majority of synaptic structures does not contain adducin. Significant immunoreactivity is also detected in processes of glial cells both in the hippocampus and the cerebellum.

Animals↗
PubMed2006

The role of spatial phase in texture segmentation and contour integration.

It has been recently argued that the visual system possesses just two phase "detector" mechanisms, namely, +cosine and -cosine (P. C. Huang, F. A. Kingdom, & R. F. Hess, 2006). This suggests rather limited access to the rich distribution of receptive field phase that neurophysiologists tell us are represented in the different response profiles of striate simple cells. Whereas that study has suggested that striate receptive field phase is not directly available to perception for the detection/discrimination of localized stimuli, here we investigate whether such information might be used in more integrative striate or extrastriate functions such as texture segregation or contour integration. Specifically, given that simple cells have different local absolute phase response profiles, we ask whether a network of simple cells with similar phase preferences interact in such a way as to extract textures, contours, or both based on phase alone. Two novel texture segmentation experiments and one contour integration experiment were carried out with the intention of providing an answer to the question of how useful is local absolute spatial phase for texture segmentation and contour integration. The results support the possibility of two phase mechanisms (+/-cosine) for global texture segmentation, as well as for contour integration, when the elements that make up a given contour are orthogonal to contour paths.

Form Perception↗
PubMed2005

Local gene density predicts the spatial position of genetic loci in the interphase nucleus.

Specific chromosomal translocations are hallmarks of many human leukemias. The basis for these translocation events is poorly understood, but it has been assumed that spatial positioning of genes in the nucleus of hematopoietic cells is a contributing factor. Analysis of the nuclear 3D position of the gene MLL, frequently involved in chromosomal translocations and five of its translocation partners (AF4, AF6, AF9, ENL and ELL), and two control loci revealed a characteristic radial distribution pattern in all hematopoietic cells studied. Genes in areas of high local gene density were found positioned towards the nuclear center, whereas genes in regions of low gene density were detected closer to the nuclear periphery. The gene density within a 2 Mbp window was found to be a better predictor for the relative positioning of a genomic locus within the cell nucleus than the gene density of entire chromosomes. Analysis of the position of MLL, AF4, AF6 and AF9 in cell lines carrying chromosomal translocations involving these genes revealed that the position of the normal genes was different from that of the fusion genes, and this was again consistent with the changes in local gene density within a 2 Mbp window. Thus, alterations in gene density directly at translocation junctions could explain the change in the position of affected genes in leukemia cells.

Animals↗
PubMed1988

Visual masking and visual integration across saccadic eye movements.

The visual world appears unified, stable, and continuous despite rapid changes in eye position. How this is accomplished has puzzled psychologists for over a century. One possibility is that visual information from successive eye fixations is fused in memory according to environmental or spatiotopic coordinates. Evidence supporting this hypothesis was provided by Davidson, Fox, and Dick (1973). They presented a letter array in one fixation and a mask at one letter position in a subsequent fixation and found that the mask inhibited report of the letter that shared its retinal coordinates but appeared to occupy the same position as the letter that shared its spatial coordinates. This suggests the existence of a retinotopic visual persistence at which transsaccadic masking occurs and a spatiotopic visual persistence at which transsaccadic integration, or fusion, occurs. Using a similar procedure, we found retinotopic masking and retinotopic integration: The mask interfered with the letter that shared its retinal coordinates, but also appeared to cover that letter. In another experiment, instead of a mask we presented a bar marker over one letter position, and subjects reported the letter that appeared underneath the bar; subjects usually reported the letter with the same retinal coordinates as the bar, again suggesting retinotopic rather than spatiotopic integration across saccades. In Experiment 3 a bar marker was again presented over one letter position, but in addition a visual landmark was presented after the saccade so that subjects could localize the bar's spatial position; subjects still reported that the letter that shared the bar's retinal coordinates appeared to be under it, but they were also able to accurately specify the bar's spatial position. This ability could have been based on retinal information (the visual landmark) present in the second fixation only, however, rather than spatiotopic visual persistence. Because such a visual landmark was present in the Davidson et al. (1973) experiments, we conclude that their findings can be explained solely in retinotopic terms and provide no convincing evidence for spatiotopic visual persistence. But the exposure parameters that Davidson et al. (1973) and we used were biased in favor of retinotopic, rather than spatiotopic, coding: The stimuli were presented very briefly just before a saccadic eye movement, and subjects are poor at spatially localizing stimuli under these conditions. Thus, in Experiment 4 we presented the letter array about 200 ms before the saccade; then, subjects reported that the letter with the same spatial coordinates as the bar appeared under it.(ABSTRACT TRUNCATED AT 400 WORDS)

Attention↗