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At least 217 records · Page 12Linked to original sources
PubMed1994

Localizing the spatial and temporal electromagnetic fields on the axis of a lossy cylinder.

Given a specified form, both in time and space, for the electromagnetic fields on the axis of a lossy cylinder, we determine, analytically, the required azimuthally symmetric source distribution on the surface of the cylinder that generates such internal fields. We then show that this source is equivalent to an array with finite number of cylindrical slots in a metal encasing that are impulsed by specified voltages at a finite sequence of discrete times. A confirming forward calculation exhibits excellent agreement between the specified field form and that generated by the array of cylindrical slots. Potential applications are to hyperthermic cancer therapy, bioelectromagnetics and nondestructive testing.

Electromagnetic Fields↗
PubMed1992

[Effect of various light sources on visual functions in the age aspect. I. Conscious reaction time and the manual spatial visual localization].

The authors present the results of investigations performed in 28 right-handed, normally seeing persons divided into 2 groups: 15 of them were in productive age and 13 in older age. The authors compared the dynamics of the behaviour of the visual functions in dependence on the age of the subjects and the stay in the glow and sodium light. No adverse influence of the sodium light on the hand localization could be detected, instead it seems that it negatively influenced the selective reaction in older productive persons in the sense of an increased number of faults.

Adult↗
PubMed1997

Spatial analysis of the distribution of LaCrosse encephalitis in Illinois, using a geographic information system and local and global spatial statistics.

The spatial and temporal distribution of LaCrosse encephalitis cases in Illinois was analyzed using a geographic information system (GIS) and spatial statistics. Case data were obtained from the Illinois Department of Public Health and mapped on the county, town, and address level. Human cases were concentrated in and around the city of Peoria in central Illinois. Local spatial statistics were used to identify hot spots where cases appear to be concentrated in the Peoria region. Several small towns surrounding the city of Peoria appeared as foci where cases were most common. Second-order spatial analysis of the case distribution was conducted on the address level. Cases were clustered within a range of 3.0 km in the city of Peoria. Since most cases appear to be associated with residential (peridomestic) exposure, and since several cases have been reported from neighboring addresses, transmission may be concentrated around specific sites (hardwood ravines, tire piles). The GIS and spatial analysis may be useful in identifying and targeting for intervention potential sites of enzootic transmission.

Adolescent↗
PubMed1977

Spatial frequency and light-spread descriptions of visual acuity and hyperacuity.

Resolution (visual acuity) and differential spatial localization (hyperacuity) targets were selected to allow rigorous psychophysical measurements as well as ready expression of both their spatial frequency spectrum and their retinal image light distribution. Thresholds were about 1 arc min for acuity and 4-6 arc sec for hyperacuity. As is consistent with the reciprocal relationship between the space and spatial frequency domains, the small locally restricted spatial differences between just distinguishable patterns are represented in the frequency domain by equally small differences, which are distributed over the entire spatial frequency spectrum. While they occur in many test situations, phase variations of spatial frequency components are not necessary for achieving optimum acuity and hyperacuity.

Female↗
PubMed1982

Spatial organization of neurochemically classified interneurons of the goldfish retina-I. Local patterns.

Certain interneurons in the goldfish retina are uniquely specifiable by their abilities to take up exogenously supplied [3H]gamma-aminobutyric acid (GABA), [3H]glycine, [3H]dopamine of [3H]serotonin al low micromolar concentrations. Each ligand labels one or two unique populations of interneurons yielding six cell types characterized by soma location, soma size, level and form of dendritic arborization, and local spatial patterning. This report summarizes these qualitative features and provides quantitative evidence on the size and spatial distributions of : (1) GABA-ergic horizontal cells, (2) GABA-ergic amacrine cells, (3) glycinergic amacrine cells; (4) glycinergic interplexiform cells; (5) dopaminergic interplexiform cells; and (6) indoleaminergic amacrine cells.

Animals↗
PubMed1972

The caudate nucleus egocentric localization system.

It is hypothesized that the motor and spatial functions ascribed to the caudate nucleus are actually different aspects of a single system for spatial localization. Within this system movement programs are the code for spatial locations, i.e. the position of any point in space is defined by the head/eye movement necessary to bring that point into focus. This is an egocentric system; points in space are defined solely in terms of their distance and direction from the observer. The experimental finding that caudate lesions in rats impair their performance in an egocentric localization task, but not in another, equally difficult spatial task, demonstrates a caudate role in this system. Since, in an egocentric system, positions in space are defined relative to the observer, every movement of the observer must be accompanied by a compensatory updating of the internal representation of any given point. Studies with Huntington's Chorea patients are consistent with a caudate involvement in this feature of the system also. Electrophysiological experiments with cats indicate that vestibular information, which can be used in the compensatory updating process, is available to the caudate. It is suggested that this system is used by normal animals in delayed response performance.

Animals↗
PubMed1997

Retention of local information in generation of subjective contours.

Temporal integration characteristics of subjective contour perception was investigated, using the sequential presentation of two pairs of disks with a sector removed. In the first experiment, by matching the contrast of a "real" stimulus, the perceived contrast of the subjective contours was measured as a function of the stimulus onset asynchrony (SOA) between the two pairs of the disks. With increasing SOA, the perceived contrast decreased gradually and levelled off at the SOA of ca 372 msec (1 SD = 119 msec). In the second experiment, the perceived contrast of the inducing disks was measured as a function of SOA using the matching method. The time limit of the additive effect for the contrast perception of the inducing disks was much shorter than that for subjective contours; the critical SOA was ca 65 msec (1 SD = 36 msec). The remarkable difference of the integration time was explained by a hierarchical process; the local spatial filtering, the retention of local information, and the completion of gaps by multiplicative or AND operation.

Contrast Sensitivity↗
PubMed1994

Localization of element clusters by the human visual system.

A spatial localization task was used to determine the accuracy of localization of random element clusters about their centroid. The random element clusters were generated within a circular region and the observers' task was to decide whether the cluster lay to the right or left of a reference line, defined by two vertically separated reference elements. Both the reference elements and the cluster elements themselves were comprised of spatially narrowband stimuli (gabor patches), and were presented at a constant suprathreshold level. The thresholds for localization of the element cluster were measured with varying element number, and for a number of different sizes of circular region and element. The relationship between localization threshold and element number was not monotonic. Thresholds were found to rise with increasing element number up to about six elements within a region, and to then fall with further increase in element number. Asymptotic thresholds at high element number were indistinguishable from those obtained for filled circles of the same size. The results do not conform to any one of a number of models based on a centroid analysis alone. By manipulating the spatial and orientational properties of the elements comprising the cluster to be localized it was determined that the more central mechanism underlying localization receive input from different spatial and orientation filters.

Contrast Sensitivity↗
PubMed1986

Altered excitability of goldfish Mauthner cell following axotomy. II. Localization and ionic basis.

The ionic basis and spatial localizations of spike generation were examined in normal and axotomized goldfish Mauthner (M-) cells using intra- and extracellular recordings and pharmacological manipulation of ionic conductances, including localized iontophoretic drug applications. Tetrodotoxin (TTX) abolished both the initial segment (IS) spike in normal cells and the larger, two-component action potential in axotomized cells, whereas calcium (Ca2+) blockers did not. Thus, sodium (Na+) appears to be the major inward current carrier in both cases. A shoulder or plateau following the fast-rising Na+-dependent action potential was unmasked in both normal and axotomized M-cells by intracellular injections of tetraethylammonium (TEA), either alone or in conjunction with 4-aminopyridine (4-AP) or cesium (Cs+). This plateau potential was abolished by superfusing with saline containing the Ca2 antagonists, Co2+, Mn2+, or Cd2+. However, barium (Ba2+), which normally substitutes for Ca2+ and also blocks K+ conductances, did not produce a plateau spike, and no action potentials could be evoked in the presence of TTX. Simultaneous extra- and intracellular recordings from the soma and lateral dendrite revealed that both the full-sized axotomized spike and its individual labile components were always maximal at the soma. These data support the earlier suggestion that the axotomy-induced electrogenicity is primarily localized to that region. Iontophoretic application of TTX inside the axon cap, a distinctive neuropil surrounding the initial segment and the axon hillock and circumscribed by a glial border, and at various positions along the lateral dendrite confirmed the Na+-dependency of the action potentials recorded in normal and axotomized cells and further demonstrated that the soma generates the additional spike component in the latter. The results suggest that axotomy causes a persistent change in voltage-gated Na+ channel distribution in the M-cell, with Na+ channels appearing or becoming more numerous in the soma while becoming less concentrated in the initial segment-axon hillock. Possible related shifts in other voltage-dependent conductances are also discussed. Finally, these are the first detailed studies of the ionic basis of axotomy-induced electrogenicity in a vertebrate neuron, central or peripheral, and the similarity to the results obtained with invertebrate neurons suggests common mechanisms underlying the axon reaction.

Action Potentials↗
PubMed1992

[Errors of monocular localization in strabismic amblyopia. Two-dimensional distortion].

Spatial distortions, i.e. spatial localization errors, and precision of localization were measured monocularly in the central visual field of 7 normal observers, 17 strabismic amblyopes and 2 anisometropic amblyopes. The task of the subjects was to construct circles of 2 degrees, 4 degrees and 6 degrees radius around a fixation point, using the dominant and the amblyopic eye in turn. Normal observers set distances on the vertical meridian smaller than distances on the horizontal meridian. Anisometropic amblyopes showed localization errors and variances similar to those of normal observers. The amblyopic eyes of strabismic observers with a large angle strabismus and deep amblyopia showed significant, individually different localization errors correlating with the depth of amblyopia. Strabismics with microstrabismus exhibited parallels between monocular localization and dichoptic retinal correspondence.

Amblyopia↗
PubMed1996

Geometric determinants of the place fields of hippocampal neurons.

The human hippocampus has been implicated in memory, in particular episodic or declarative memory. In rats, hippocampal lesions cause selective spatial deficits, and hippocampal complex spike cells (place cells) exhibit spatially localized firing, suggesting a role in spatial memory, although broader functions have also been suggested. Here we report the identification of the environmental features controlling the location and shape of the receptive fields (place fields) of the place cells. This was done by recording from the same cell in four rectangular boxes that differed solely in the length of one or both sides. Most of our results are explained by a model in which the place field is formed by the summation of gaussian tuning curves, each oriented perpendicular to a box wall and peaked at a fixed distance from it.

Animals↗
PubMed1996

Localizing quantal currents along frog neuromuscular junctions.

1. We spatially localized the origins of quantal currents by recording simultaneously with two intracellular electrodes and employing the prediction of the one-dimensional cable equations that the time integrals of the resulting voltage changes fall off exponentially with distance. 2. Miniature endplate potentials (MEPPs) were more frequent near the centre of the endplate. In contrast to some work using other methods, we did not find MEPPs originating at the margins of the endplate to be strikingly smaller. 3. Spontaneous MEPPs and uniquantal endplate potentials (EPPs) were released over the same length of endplate and with the same relative probabilities at different regions. 4. Nicotinic agonists decreased evoked quantal output, but did not change the length over which uniquantal EPPs were generated. We conclude they do not block nerve conduction in the terminals. 5. Data sets were obtained with an extracellular electrode and two intracellular electrodes. The extracellular electrode was invariably near the centre of the region in which congruous MEPPs appeared to be generated. However, the range in the calculated positions of the synchronous MEPPs was as long as 0.8 mm. Therefore, it may be possible that extracellular electrodes have a longer recording range than commonly assumed.

Animals↗
PubMed1995

Differences in influence between pitched-from-vertical lines and slanted-from-frontal horizontal lines on egocentric localization.

The visual field exerts powerful effects on egocentric spatial localization along both horizontal and vertical dimensions. Thus, (1) prism-produced visual pitch and visual slant generate similar mislocalizations of visually perceived eye level (VPEL) and visually perceived straight ahead (VPSA) and (2) in darkness curare-produced extraocular muscle paresis under eccentric gaze generates similar mislocalizations in VPEL and VPSA that are essentially eliminated by introducing a normal visual field. In the present experiments, however, a search for influences of real visual slant on VPSA to correspond to the influences of visual pitch on VPEL failed to find one. Although the elevation corresponding to VPEL changes linearly with the pitch of a visual field consisting of two isolated 66.5 degrees-long pitched-from-vertical lines, the corresponding manipulation of change in the slant of either a horizontal two-line or a horizontal four-line visual field on VPSA did not occur. The average slope of the VPEL-versus-pitch function across 5 subjects was +0.40 over a +/- 30 degrees pitch range, but was indistinguishable from 0.00 for the VPSA-versus-slant function over a +/- 30 degrees slant range. Possible contributions to the difference between susceptibility of VPEL and VPSA to visual influence from extraretinal eye position information, gravity, and several retinal gradients are discussed.

Adult↗
PubMed1993

Roughness coding in the somatosensory system.

Roughness perception is coded in the somatosensory system by neurons in the type I slowly adapting (SAI) system. When the fingers scan a surface, an isomorphic representation of the surface is encoded in the discharge patterns of SAI afferents. Central neurons in area 3b of primary somatosensory (SI) cortex spatially filter the peripheral image to compute local spatial variation. The outputs from these neurons converge onto neurons in area 1 and onto neurons in secondary somatosensory (SII) cortex which we believe is the critical processing pathway underlying roughness perception.

Afferent Pathways↗
PubMed1987

Direction specific masking and the analysis of motion in two dimensions.

We measured the effects of moving two-component cosine grating masks on the detectability of a moving spatially localized test pattern with a 1.0 octave spatial frequency bandwidth. Masking was used to distinguish between two-component patterns with fluid motion (blobs) and those with rigid motion (plaids). The two gratings which made up the two-dimensional masking patterns were always of the same spatial frequency and contrast, but moved in different directions. We find that plaid masks consistently produced threshold elevations that are 2.0-4.0 times greater than are produced by a single component mask at twice the contrast. Furthermore, this effect is nearly independent of the angle between the two mask components. For fluid motion, however, masking is determined by the mask component whose direction of motion is closest to that of the test. The results obtained with moving two-dimensional patterns demonstrate that, for blobs, the motion of the pattern as a whole has no effect on the degree of masking, whereas, for plaids, the signals arising from the two components interact in a nonlinear manner, thus producing a substantial enhancement of masking, which is clearly related to the coherent motion of the entire pattern. These data shed light on the properties of higher order motion units (possibly in MT cortex) that respond to the direction of two-dimensional pattern motion, suggesting that they combine, in a nonlinear manner, the outputs of units which respond independently to the direction of each mask component.

Form Perception↗
PubMed1996

What do developmental mapping rules optimize?

Convergence ratios between pre- and postsynaptic cells in the visual system vary widely between cell classes, areas of the visual field, between individuals and between species. Proper stabilization of the convergence and divergence of single visual neurons is critical for visual integration generally, and for specific functions such as those of rod and cone pathways, or the center and peripheral regions of the visual field. In early development, retinal ganglion cells, target cells and all their processes are produced in excess and stabilize at certain mature values. The intent of the investigations described here is to determine what features of cell connectivity are stabilized over normal variability by these developmental processes and how such stabilization is accomplished, using the developing mammalian retinotectal system as an example. Orderly compression of the retinotopic map into a half tectum was induced by a partial tectal ablation at birth in hamsters, increasing the ratio of retinal ganglion cells to superior colliculus target cells. The convergence problem is solved in this case by undersampling the spatial array with respect to normal, preserving local spatial resolution, but potentially reducing sensitivity or introducing aliasing artifacts. Receptive field sizes of single neurons are indistinguishable from normal, and reduction of branching of presynaptic axon arbors is the mechanism of the remapping. Behaviorally, though the entire visual field is still represented in the remaining colliculus, the solution has a cost in decreased probability and increased latency to orient to visual stimuli, particularly in the peripheral visual field. The generality of this solution for retinal and other central convergence regulation problems is evaluated.

Animals↗
PubMed1994

Segmentation of MRS signals using ASPECT (analysis of SPectra using Eigenvector Decomposition of Targets).

Efforts to minimize the effects of partial volume contamination (PVC) in in vivo magnetic resonance spectroscopy (MRS) have focused upon improving the sensitivity and efficiency of spatially localized MRS measurements. Such improvements may improve spatial resolution and reduce the time required to acquire multiple spectra, however, PVC can affect in vivo spectra at any resolution. In this paper, a model for segmenting in vivo MRS signals compromised by PVC in selected applications is introduced. The segmentation algorithm used is linear and is based on filters originally developed for image processing applications. The model is developed from first principles and evaluated using computer simulations. It is suited for segmenting multivoxel or chemical shift imaging data, and can be used with spectra acquired at any spatial resolution. It is used to estimate the size of the partial volumes contributing to a voxel compromised by PVC and the spatially selective signal components that would be expected to arise from these partial volumes if they could be measured directly. Several spectral perturbants present in in vivo MRS measurements violate the linearity assumptions underlying the model and produce systematic errors that must be accounted for. A number of perturbants are discussed, and the potential in vivo applications of the model are illustrated using solvent-suppressed 1H-CSI spectra from the normal human brain.

Brain↗