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At least 145 records · Page 8Linked to original sources
PubMed1985

Visual-vestibular interaction studied with stroboscopically illuminated visual patterns.

Horizontal DC-electrooculograms were recorded in subjects rotating on a horizontal turntable sinusoidally at 0.1 Hz and 35 to 40 degrees amplitude. The subjects either fixated a stroboscopically illuminated vertically striped pattern (1.15 to 3.45 degrees period) rotating with the turntable or initiated Sigma-OKN before the rotation began and tried to maintain Sigma-OKN during rotation. In a third paradigm, interaction of vestibulo-ocular reflex (VOR) and Phi-OKN was studied. VOR-suppression by fixation was complete within the limits of EOG-recording precision (+/- 1 degree X s-1) for flash frequencies fs greater than 10 flashes X s-1. VOR-suppression decreased monotonically with fs between 10 and 1 flashes X s-1. A similar dependency on fs was found for VOR-suppression during Sigma- or Phi-OKN. Above 10 flashes X s-1 VOR-suppression remained incomplete; below 5 flashes X s-1 VOR-suppression was stronger with the Sigma-OKN paradigm than during fixation and depended on spatial frequency of the pattern. During sinewave rotation of the subject the perceived speed Vp of Sigma-movement correlated to the movement of gaze in space and not to the movement of the eye in head. In a control experiment with normal optokinetic stimulation, OKN-suppression by fixating a small flashing target was found to depend on fs in a similar way as VOR-suppression in the experiments described above.

Adult↗
PubMed1991

Analogies in the two-dimensional spatial arrangement of adsorbed proteins and adhering bacteria: bovine serum albumin and Streptococcus sanguis 12.

Neither proteins nor bacteria adsorb or adhere homogeneously to a substratum surface. The final two-dimensional spatial arrangement depends on a complicated interplay between protein-protein (bacterium-bacterium) and protein (bacterium)-substratum interactions and the prevailing hydrodynamic conditions. In this paper, results are presented of two separate experiments in which bovine serum albumin (BSA) was adsorbed to, or Streptococcus sanguis 12 was deposited on substrata with different wettabilities in a search for analogies in the two-dimensional spatial arrangement of the absorbed proteins and adhering bacteria. The spatial arrangement of adsorbed BSA, visualized by transmission electron microscopy on replicas of the surface, was island-like on substrata with a low wettability and well distributed on substrata with a high wettability. The spatial arrangement of adhering S. sanguis 12 was observed directly by light microscopy during the experiment and showed a relatively large collection of near-neighbour sites on the low wettability substrata compared with the high wettability substrata. Thus, it seems that the occurrence of island-like structures in protein adsorption is concurrent with a large collection of near-neighbour sites in bacterial adhesion. As a possible explanation for the above analogy, it is suggested that proteins or bacteria are insufficiently immobilized on low wettability substrata owing to weak interaction forces, and they can move over the substratum surface to yield the two-dimensional spatial arrangements observed.

Adsorption↗
PubMedDate not supplied

Spatial resolution of neuromagnetic records: theoretical calculations in a spherical model.

Spatial resolution of magnetoencephalography (MEG) was studied by computer simulations using a spherical conductor model for the head. The accuracy obtainable in the absolute location of a dipole was found by calculating the confidence limits for source location in 3 dimensions. The accuracy in determining the relative locations of two sources was estimated by calculating the smallest shift in source location that could be detected with statistical significance. The results were used to illustrate the dependence of spatial resolution on several factors including noise, source depth, source strength, flux transformer configuration and the choice of the measurement locations. Under optimal conditions, separations of a couple of millimeters in superficial non-simultaneous sources can be detected, whereas for deeper sources the resolution is worse.

Electroencephalography↗
PubMed1988

[The role of tryptophan residues in the antigenic activity of carcinoembryonic antigen].

Antigenic determinants of carcino-embryonic antigen (CEA) were spatially located using N-bromosuccinimide modification of tryptophan residues both in native (acetate buffer solution) and unfolded (guanidinium chloride solution) molecule of the antigen. Modification of exposed tryptophan residues failed to alter CEA antigenic activity and conformation of its protein portion as shown by CD spectroscopy. On the contrary, modification of buried tryptophan residues induced conformational changes of CEA protein portion connected with a considerable loss of its antigenic activity. It was shown that CEA antigenic activity depends on spatial structure of its protein moiety.

Bromosuccinimide↗
PubMed1987

Simple reaction time to sinusoidal grating and perceptual integration time: contributions of perceptual and response processes.

For sinusoidal gratings, two experiments were carried out to examine relation between simple reaction time and perceptual and response processes. First, the simple reaction time (RT) was measured as a function of grating contrast with spatial frequency as a parameter. Second, contrast threshold was determined as a function of exposure duration (ED). On the basis of the RT-contrast function and the ED-contrast threshold function empirically obtained, the difference between the RT and the ED was evaluated as a function of grating contrast. The RT-ED difference, in which the perceptual integration time was eliminated, was considered to reflect the transmission time, delay in the response process, and delay in the motor response. As the grating contrast increased, the RT-ED difference decreased rapidly at first, and then gradually, and finally leveled off. The RT-ED difference vs contrast function depended on spatial frequency. Based on the analysis of the characteristics of the RT-ED functions, the effects of grating contrast and spatial frequency on the simple RT could be ascribed to the changing contributions of the perceptual and the response processes.

Evoked Potentials, Visual↗
PubMed1979

Proprioceptive influences on directional hearing: dependence on the movement type.

Proprioceptive effects (PE) on directional hearing, i.e., sound image shifts relatively to the sound source, produced by changes in head position, were studied in their dependence on head movement type in two groups of listeners. The first group included subjects in whom horizontal head deviation to the right or to the left, performed on instruction before the presentation of clicks, caused sound image displacement of 10-15 degrees. When the same sounds were made signals of movement direction, i.e., of pointing with the head at the sound source, PE became fully inhibited in all subjects. The second group included listeners in whom PE was either absent or only very small; substitution of passive head deviations for active turns resulted (in 63 percent of cases) in PEs which presumably had been previously inhibited. Thus identical muscle activities play different roles in auditory spatial perception depending on the type of movement and on the significance of the acoustic signal for the motor act.

Auditory Perception↗
PubMed1986

Changes in conduction velocity during acute ischemia in ventricular myocardium of the isolated porcine heart.

Conduction velocities along longitudinal (vL) and transverse (vT) fiber axes were determined in isolated porcine hearts from subepicardial activation patterns that were produced by local stimulation and measured with a multiterminal electrode. In some of the experiments extracellular [K+] ([K+]o) and transmembrane potentials were recorded. During normal perfusion vL and vT were (cm/sec) 50.08 +/- 2.13, (SE) and 21.08 +/- 0.97. After 3 to 5 min of global ischemia, vL and vT decreased to approximately 30 and 13 cm/sec. Before the occurrence of total inexcitability propagation became time dependent 2: 1 block developed and centrifugal spread from the stimulus site was partially blocked at short intervals and was normal at long intervals. This suggested that slowed conduction was dependent on spatial nonuniformities of recovery from excitability. Slowing of conduction during ischemia was not explained by accumulation of [K+]o alone, because vL and vT at a given [K+]o were lower during ischemia than during perfusion with elevated K+. In hearts perfused at 20 mM [K+]o "slow responses" were produced by addition of epinephrine (2.5 X 10(-5)M). Resting membrane potentials of slow responses were significantly lower than of depressed action potentials during ischemia. The values vL and vT of slow responses (10 and 5 cm/sec) were much lower than the lowest values during ischemia (20 and 10 cm/sec). This indicates that slow conduction in ischemia is associated with depressed action potentials initiated by a partially inactivated rapid Na+ inward current. The time dependence of nonuniform propagation and the relatively high conduction velocities explain two major characteristics of reentrant tachycardias in acute ischemia: the large diameters of reentrant circuits and the beat-to-beat changes in localization of conduction block.

Action Potentials↗
PubMed1992

Electro-physiological investigation of edge-selective mechanisms of human vision.

This study investigates the spatial and temporal characteristics of human visual mechanisms that respond selectively to the polarity of edges. The technique was to record steady-state visual evoked-potentials (VEPs) while visually stimulating with a sawtooth waveform (a series of edges of the same polarity) periodically reversing in contrast (and hence edge-polarity) at a suitable frequency. To ensure that phase-locked VEPs resulted from polarity reversal (rather than local luminance modulation) the stimuli were randomly jittered to a new position between each contrast reversal. The jittered stimulus elicited strong and reliable second-harmonic modulation, usually about one-fifth the amplitude of standard VEPs under similar conditions. The amplitude and extrapolated thresholds of polarity-specific VEPs (relative to standard VEPs) did not vary with eccentricity (up to 10 degrees) or with stimulus orientation. The dependency on spatial frequency was similar to that of standard VEPs, but the polarity-specific VEPs tended to peak at lower temporal frequencies. Perhaps the clearest difference in the two types of VEPs was in the estimated response latency, about 140 msec for the polarity VEPs, compared with 90 msec for standard VEPs.

Contrast Sensitivity↗
PubMed1989

Measurement of resolution and recovery in recent generation positron tomographs.

In positron tomographic images, the ability to differentiate closely lying structures, the spillover of activity from a region into adjacent regions and the reduction in apparent isotope concentration in small structures are all dependent on spatial resolution. Resolution in the reconstructed image is affected by (i) detector size, (ii) the spatial sampling used (e.g. stationary, wobble), (iii) the amount of smoothing in the reconstruction process (or subsequent to reconstruction) and (iv) the image pixel size. Under ideal conditions, modern commercial tomographs can produce a reconstructed spatial resolution of 5 mm or less. However, this is rarely realizable in a clinical study due to the inadequacy of counting statistics and the amplification of statistical noise. In practice, a smoother filter has to be used. This paper presents a summary of practical measurements of spatial resolution, and the related count recovery, performed on recent generation positron tomographs. It is intended to contribute to the definition of methods of measuring these parameters which is part of an on going concerted action in positron tomography supported by the European Commission.

Image Processing, Computer-Assisted↗
PubMed1977

Influence of hydrogen bonding on the rotamer distribution of the histidine side chain in peptides: 1H NMR and CD studies.

Both 1H NMR and circular dichroism pH titration studies on histidine, His-Gly, Gly-His and Gly-His-Gly indicate that the side-chain spatial orientation depends strongly on the vicinal charges. The arrangement of the imidazole side-chain (rotamer population) is shown by the histidine beta and beta' and the glycine methylene proton chemical shifts as well as the vicinal 1H-1H coupling constants 3JCalpha-H-beta-H, beta'-H. For His-Gly and Gly-His-Gly a good correlation can be found between the ionization of the glycine COOH group and the increase of rotamer III (g-g) which is also visualized by circular dichroism through an enhancement of the ellipticity at 212 nm. In these two peptides a hydrogen bond between the imidazolium and the carboxylate group is supposed to stabilize rotamer III at pH 4-5.

Circular Dichroism↗
PubMed1987

Spinal inputs to the ventral dendrite of the teleost Mauthner cell.

Ascending excitatory inputs from the periphery to the ventral dendrite of the goldfish Mauthner (M)-cell are characterized in this report. Direct stimulation of the spinal cord, at strengths suprathreshold for antidromic activation of the M-axon, evoked a graded excitatory postsynaptic potential (EPSP) in the distal ventral dendrite of the cell. This localization was demonstrated by multiple intracellular recordings from the soma and dendritic loci. The EPSP had a relatively long latency (mean = 3.6 ms) and contained multiple components. Furthermore, the EPSP amplitudes were extremely sensitive to frequency, being reduced by more than 50% at frequencies of 1-2 Hz and maximal with interstimulus intervals of 30-60 s. The spinal input is, therefore, likely to be mediated by a polysynaptic pathway. Direct stimulation of the skin surface evoked similar EPSPs, in terms of latency, wave form, graded nature, frequency dependence and spatial distribution on the M-cell ventral dendrite. Thus, the spinal cord and skin inputs probably relay somatosensory information from the trunk to the M-cell ventral dendrite. This notion was further confirmed by an interaction study of the EPSPs evoked from the two sites. We also report that the ventral dendrite does not support active spike electrogenesis, as indicated by the spatial profile of the M-cell antidromic impulse amplitude.

Afferent Pathways↗
PubMed1987

Functional characteristics of in vitro generated macrophages: a transient refractory state precedes reinducibility of a spatially restricted, possibly contact-dependent, cytostatic mechanism.

Adherent layers of macrophages (M phi-c) generated in vitro from splenic precursors previously have been shown to inhibit proliferation of normal and neoplastic cells by mechanisms that do not involve the secretion of significant quantities of prostaglandins, peroxides, or proteases. In the current report, it is demonstrated that the effective range of the cytostatic effect of adherent M phi-c is so low that cytostasis can be circumvented by preventing the target cells from settling to the adherent layer by incubating the cultures of targets and M phi-c on a rocking platform. Despite the paucity of production of inhibitory mediators, the M phi-c do produce sufficient IL-1 to restore IL-2 production capacity to macrophage-depleted lymphocyte populations. The cytostatic activity of the M phi-c is an inducible event and remains for several days in the continuous presence of mitogen-activated lymphocytes or LPS + lymphokines. Once the activators are removed the cytostatic activity rapidly declines, reaching background levels in 24-48 hr. Maximal cytostatic activity can be reinduced but only 48 hr or longer after the primary activators were removed. A transient period in which the M phi-c are refractory to reactivation can be demonstrated to exist for 1-2 days after removal of the primary activators.

Animals↗
PubMed1985

Crystalline regions in collagen fibrils.

A new image processing technique, content-dependent anisotropic spatial frequency filtering, has been developed to visualize the location and orientation of crystalline regions in collagen fibril cross-sections. The results show that most crystalline regions are oriented with their approximately 4 nm periodicity directed radially from the fibril centre. This periodicity corresponds to the separation between rows of molecular ends in the quasi-hexagonal molecular packing scheme. The extent of crystallinity increases with radius and frequently the lattice is either continuously distorted or interrupted by sharp discontinuities.

Animals↗
PubMed1985

The influence of spatial frequency on the reaction times and evoked potentials recorded to grating pattern stimuli.

The simple reaction times recorded to sine-wave and square-wave grating stimulus patterns of both constant physical contrast and of constant suprathreshold contrast were appreciably delayed by an increase in spatial frequency from 0.5 to 10 c/deg. There was no comparable increase, however, in the peak latency of the initial visual evoked potential component, C1, recorded to the same stimulus patterns. In view of the evidence that C1 has a striate cortical origin, these results suggest that the large spatial-frequency dependent variations in RT do not reflect delays of stimulus-induced neuronal responses in the primary visual pathway from retina to striate cortex.

Evoked Potentials, Visual↗
PubMed1987

Perceptual latency and spatial frequency.

Subjects estimated the perceived onset of sinusoidal gratings by adjusting a visual probe so that grating and probe were phenomenally simultaneous. The first experiment found that perceptual latency increased by an average of 21 msec over a range of 0.5-9.0 c/deg, or approximately 5 msec per octave of spatial frequency. A second experiment found, that when a fundamental frequency and its third harmonic at one third contrast are compared, a lag of between 21 and 25 msec for the higher frequency lower amplitude grating is obtained. These spatial frequency dependent delays are substantially lower than those reported using other methods.

Adult↗
PubMed1992

Failure of rivalry at low contrast: evidence of a suprathreshold binocular summation process.

Presentation of different images to the two eyes normally results in a time-varying alternation between the two images (binocular rivalry). However, we find that when orthogonal gratings are viewed dichoptically at low contrast, a stable summation between the two images is perceived in the form of a dichoptic plaid. The range of perception of the dichoptic plaid depends on spatial frequency, contrast and luminance of the gratings. This phenomenon differs from the "false fusion", a fleeting summation of different images perceived only under very brief presentation of the stimuli. The observations suggest that there exists a neural process that performs a summation of dissimilar images, and that is distinct from the competitive process of suppression and binocular rivalry.

Contrast Sensitivity↗