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At least 109 records · Page 6Linked to original sources

PubMed1992

Phosphorylation-dependent changes in the spatial relationship between Ca-ATPase polypeptide chains in sarcoplasmic reticulum membranes.

In order to investigate possible structural changes associated with the coupling mechanisms of the Ca-ATPase in sarcoplasmic reticulum membranes, we have utilized fluorescence resonance energy transfer between spectroscopic probes covalently bound to different domains of the ATPase. Using time-correlated single photon counting, we have directly measured the energy transfer efficiency between 5-[2-[(iodoacetyl)amino]ethyl]aminonaphthalene-1-sulfonic acid (IAEDANS), that is specifically bound to the B trypic fragment at cysteines 670 and 674 and acceptors covalently bound either near the nucleotide binding site, i.e. fluorescein 5-isothiocyanate at lysine 515, also on the B fragment, or maleimide-directed probes specifically located on the A1, tryptic fragment, i.e. 4-dimethylaminoazobenzene-4'-maleimide (DABmal) or fluorescein-5-maleimide (Fmal), probably at cysteines 344 and 364. All of these donor-acceptor pairs exhibit energy transfer both within and between Ca-ATPase molecules allowing us to investigate spatial relationships between the A1 and B domains and between different ATPase polypeptide chains. Differentiation between the intra- and intermolecular components of energy transfer was accomplished in two ways: 1) by comparing the transfer efficiencies in native membranes before and after detergent solubilization and 2) by reconstituting ATPase chains that have already been labeled with either the donor or acceptor chromophores. Using this approach, we find no significant change in the intramolecular transfer efficiency between any of these donor-acceptor pairs either upon binding of calcium to the high affinity sites or upon stabilization of the phosphoenzyme intermediate, indicating that there are no large structural changes within the B tryptic fragment or, alternatively, between the A1 and B fragments. With respect to intermolecular energy transfer, we observe no effect of calcium binding on the unliganded enzyme with either donor-acceptor pair. However, formation of the phosphoenzyme intermediate results in a measurable increase in the transfer efficiency between IAEDANS and DABmal (or Fmal); this increase is reversible upon phosphoenzyme destabilization by subsequent addition of calcium. There is no corresponding change in the intermolecular component of fluorescence resonance energy transfer between IAEDANS and fluorescein 5-isothiocyanate, indicating that the change in fluorescence resonance energy transfer probably occurs as a result of reorientation of associated ATPase polypeptide chains with respect to one another.

Animals↗

PubMed1978

Spatial structures in a reaction-diffusion system--detailed analysis of the "Brusselator".

Continuous dependence of spatially nonuniform concentration profiles for the 'Brussellator" reaction mechanims on the characteristic length of the system is given both for zero flux and fixed boundary conditions. Branches of solutions arising through primary bifurcation form closed curves. Secondary bifurcations giving rise to spatially asymmetric solutions exist for fixed boundary conditions. Results of a stability analysis of individual solutions are discussed. A method of composing complex spatial profiles for higher lengths from elementary solutions for smaller lenghts is suggested and tested in the case of zero flux boundary conditions. Emergence of subsequently more complex stable patterns in dependence on increasing length of the system suggests many similarities to gradual build up of complex morphogenetic patterns.

Biological Transport↗

PubMed1986

Spatial contrast sensitivity in facial recognition.

Recognition and learning of complex images may depend on spatial processing characteristics of the visual system. Prosopagnosia, an impairment of visual learning and recognition of faces, might result from impaired perception in spatial channels carrying crucial information. We studied spatial contrast sensitivity (SCS) in two subjects with stable facial recognition defects. One had relative SCS reduction for high-frequency gratings but could process high frequencies in room light. The other had normal SCS. Both had intact spatial processing relative to image size. The results suggest that impairments in visual spatial channels are not necessary for the development of prosopagnosia.

Adult↗

PubMed1989

Influence of phenomenal time on perceived space.

It is known from work by Helson and King that the apparent spatial distance between stimuli presented in succession depends on the time interval between those stimuli ("tau effect"). It is also known that the apparent time (i.e., the interval of time perceived between stimuli presented in succession) depends on the spatial distance between the stimuli defining the time interval (called "kappa effect" by Cohen, Hansel, and Sylvester). For the "kappa effect," apparent time between lights and stimuli on the skin of the forearm undergoes modification depending on the apparent spatial distance, as presented in Suto's work beginning in the early 1940s. The present work is complementary to Suto's. Its purpose is to test whether apparent spatial distance depends on the objective or subjective (apparent, phenomenal) time interval. To achieve this, our crucial (third) experiment was preceded by two exploratory experiments: the first to verify the illusion in which a subdivided interval appears longer than an empty interval of the same length (Oppel-Kundt temporal illusion); the second to verify the presence of the "tau effect" with simultaneous tactual and auditory stimuli for time intervals between 1500 and 2500 msec. In the third experiment subjects received successive tactual stimuli defining two spatial distance and two time intervals. They also received auditory stimuli that produced the Oppel-Kundt illusion by making the time intervals phenomenally different. The results fill a small experimental gap by showing it is subjective (phenomenal) and not objective (physical) time that influences perceived spatial distances.

Adolescent↗

PubMed1990

Hemispheric differences are found in the identification, but not the detection, of low versus high spatial frequencies.

The processing of sine-wave gratings presented to the left and right visual fields was examined in four experiments. Subjects were required either to detect the presence of a grating (Experiments 1 and 2) or to identify the spatial frequency of a grating (Experiments 3 and 4). Orthogonally to this, the stimuli were presented either at threshold levels of contrast (Experiments 1 and 3) or at suprathreshold levels (Experiments 2 and 4). Visual field and spatial frequency interacted when the task required identification of spatial frequency, but not when it required only stimulus detection. Regardless of contrast level (threshold, suprathreshold), high-frequency gratings were identified more readily in the right visual field (left hemisphere), whereas low-frequency gratings showed no visual field difference (Experiment 3) or were identified more readily in the left visual field (right hemisphere) (Experiment 4). Thus, hemispheric asymmetries in the processing of spatial frequencies depend on the task. These results support Sergent's (1982) spatial frequency hypothesis, but only when the computational demands of the task exceed those required for the simple detection of the stimuli.

Adult↗

PubMed1991

Management of keratoconus: comparative visual assessments.

We compared the visual performance of keratoconus patients whose vision had been corrected with one of the following: RGP lenses alone, penetrating keratoplasty, or epikeratoplasty. Despite similar Snellen visual acuities with optimal correction, significant differences in visual performance were found in the three groups. Visual performance was assessed by measuring contrast thresholds for electronically-generated sinusoidal gratings and by measuring contrast thresholds in the presence of a glare source. These measurements were carried out on six bilateral keratoconus patients such that three eyes were corrected with RGP lenses alone, five eyes were corrected by penetrating keratoplasty, and four eyes by epikeratoplasty. There were statistically significant differences in the visual responses for the three correction types (ANOVA, F2,45 = 5.755, P = 0.0059), with the epikeratoplasty procedure providing the poorest results. Visual performance was further affected in the presence of a glare source. Statistically significant group differences were again found (ANOVA, F2,45 = 7.593, P = 0.0014), with the penetrating keratoplasty eyes being the most affected. These differences were spatial frequency dependent (p = 0.0293), the effect being most marked at medium spatial frequencies. These findings provide a ranking of the quality of vision provided by each management strategy.

Adult↗

PubMed1987

Spatial and temporal selectivity in the suprasylvian visual cortex of the cat.

We recorded from single units in the medial and lateral banks of the posterolateral suprasylvian visual cortex (PMLS/PLLS) of the cat. The responses to drifting high-contrast gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency, were examined quantitatively for a sample of cells, whose receptive fields covered a wide range of eccentricities. The optimum spatial frequencies (average about 0.2 cycles/deg) were low compared to the values reported for striate cortex but similar to those for area 18. The mean spatial bandwidth (about 2 octaves) was slightly broader than that of cells in other cortical visual areas. The cut-off spatial frequencies ("acuities") covered a wide range, from 0.05 to 2.1 cycles/deg, similar to those of cells in area 18. Responses to drifting sinusoidal gratings were usually dominated by an unmodulated elevation of discharge, although some modulation occurred at the temporal frequency of drift, especially at low spatial frequencies. Modulated responses were relatively stronger in PMLS than in PLLS. For those cells that responded to flashed stimuli, stationary, contrast-modulated gratings presented at different spatial positions typically evoked small responses at the fundamental frequency (dependent on spatial phase) and a larger component at the second harmonic of temporal frequency, with no overall "null-position." The optimum spatial frequency was usually higher than would be predicted by simple summation within the dimensions of the receptive field. Thus, neurons in PMLS and PLLS, like complex cells in areas 17 and 18, behave nonlinearly and their spatial selectivity is determined by "subunits" smaller than their receptive fields. The range of preferred temporal frequencies ranged from less than 2.5 Hz to more than 10 Hz. In their temporal selectivity neurons in PMLS resembled cells in area 17, with little attenuation at low temporal frequencies, whereas there was a tendency for cells in PLLS to prefer higher temporal frequencies, as is common in area 18.

Animals↗

PubMed1987

[Relation between the spatial structure and antigenic activity of trophoblast-specific beta1-glycoprotein].

Temperature- and pH-dependence of spatial structure of a native trophoblast-specific beta-glycoprotein (TSG), its desialated and deglycosylated derivatives, as well as of a fragment obtained by partial acid hydrolysis on the temperature and pH variations has been studied using CD and UV spectroscopy. Within the range 45-50 degrees C a conformational transition of the protein moiety of TSG, leading to partially reversible alterations in tertiary and secondary structures of this glycoprotein after cooling the solution to 20 degrees C has been found out. The results of spectral studies of the spatial structure of the TSG protein component have been compared with the data on antigen activity of native, temperature- and pH-denaturated, desialated, and deglycosilated TSG. It has been concluded that the protein moiety of TSG consists mainly of beta-structures; the greater part of antigen determinants of TSG is topographic and belongs to the protein component of TSG, and only 15% of antigen determinants are not connected with TSG's spatial structure.

Circular Dichroism↗

PubMed1986

Steady-state pattern VEP uncorrelated with suprathreshold contrast perception.

Campbell and Maffei (1970) reported a linear relationship between the logarithm of the grating pattern contrast and the amplitude of the visual evoked response (VEP). This enabled them to predict the pattern visibility by extrapolating to zero amplitude. By using a new digital fast sweep technique for acquiring and analysing steady state evoked potentials we show that, at clearly suprathreshold contrast levels, such a linear relationship only exists for certain spatial frequencies. In general, the VEP saturates with increasing contrast in a way that critically depends on stimulus spatial frequency. Such a dependency on spatial frequency is not obtained for suprathreshold contrast perception which is characterized by a remarkable contrast constancy. Thus the amplitude of the pattern VEP does not convey information about visual perception other than contrast detection thresholds.

Animals↗

PubMed1976

Effect of spatial frequency on the visibility of unstructured patterns.

Threshold measurements have been made on random patterns that have been computer filtered to contain only certain spatial frequencies. The dependence of threshold on the spatial-frequency distribution of the energy in the patterns was measured. The results indicate that the visibility of an unstructured pattern is dependent on the spatial-frequency content of the pattern. Near threshold, narrow-band noise is much more visible than wide-band noise. The narrow-band noise visibility is caused by the presence of structure in the pattern, some of which is rotation sensitive. For wider-band noise, the modulation transfer function is circularly symmetric. The visibility of very-wide-band noise near threshold depends only on the luminance variations concentrated within a one-octave spatial-frequency band.

Humans↗

PubMed1985

Simple- and complex-cell response dependences on stimulation parameters.

We studied the response time course and amplitude dependence on stimulation parameters in cat cortical visual neurons to determine their receptive-field spatial-summation characteristics. Response poststimulus time (PST) histograms of cortical simple cells to contrast-reversal grating stimulation generally have a single peak for each stimulus temporal cycle, though the responses appear rectified. In response to contrast-reversal grating stimulation the general PST histogram time course for complex cells is two peaks, though often these peaks are of different amplitudes. The time course of complex-cell responses, and the ratio of these two response peaks often varies with stimulation parameters. The appearance of a single response peak in simple cells is reflected in the dominance of the odd harmonic Fourier portion, whereas the half-wave rectification leads to a considerable even harmonic portion. Still, this even portion is never significantly greater than the odd portion. When complex cell PST histograms have two nearly equal peaks, Fourier transformation reveals almost only even harmonic components. When the histogram contains two peaks of unequal amplitude Fourier analysis reveals large odd and even components. An even:odd Fourier harmonic portion ratio larger than 1 may be seen as a defining characteristic of complex cells, differentiating them from simple cells. Histograms with two unequal peaks appear "mixed," containing something of the "pure" single-peaked response and something of the pure double-peaked response. The degree to which the response is mixed may be measured by the ratio of the even:odd portion amplitudes. There is a great degree of variability with stimulation parameters (both spatial phase and spatial frequency) of the time course of mixed responses as opposed to the case of responses that have two equal peaks independent of stimulation grating phase and frequency. In both simple and complex cells there is a close coincidence of the spatial frequency ranges over which the even and odd portions are substantial, though many complex cells show a periodic variation of the even:odd portions ratio. This spatial-frequency dependence differs from that of LGN Y-cells where the odd portion dominates at low spatial frequencies and the even portion at high spatial frequencies. The ratio of even-to-odd portion cut-off is close to 3:1 in all Y-cells, a characteristic we did not find in cortical simple or complex cells. We suggest, therefore, that the nonlinearity of these complex cells does not derive from that of Y-cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1975

Visibility of low-spatial-frequency sine-wave targets: Dependence on number of cycles.

The number of cycles in a low-frequency sinusoidal display is a crucial variable in determining the visibility of the display. In particular, the threshold contrast is essentially independent of spatial frequency for these displays. We have extended the above experiments, using more cycles and a variety of targets and observer tasks. The results confirm previous findings; they also show that the type of target or task has little influence. For low-frequency sinusoids that contain up to about 3 cycles, the threshold contrast is determined by the number of cycles. For high-number-of-cycles targets with spatial frequencies above 6-10 cycles per degree, visibility is predominantly dependent on the spatial frequency. The results suggest that the low-frequency decrease in reported MTF's is due to the decrease of the number of cycles used in determining them.

Fourier Analysis↗

PubMed1988

Spatial arrangement of DNA-dependent RNA polymerase of Escherichia coli and DNA in the specific complex. A neutron small angle scattering study.

In this paper we demonstrate that neutron small angle scattering is a suitable method to study the spatial arrangement of large specific protein-DNA complexes. We studied the complex of DNA-dependent RNA polymerase of Escherichia coli and a 130 base-pair DNA fragment containing the strong promoter A1 of bacteriophage T7. Contrast variation of the complex with deuterium allowed us to "visualize" either RNA polymerase, or DNA, or both components in situ. From the corresponding scattering curves information was derived about: (1) Conformational changes of RNA polymerase and DNA by complex formation: comparison of the scattering profiles of the isolated and complexed components showed that by specific complex formation the cross-section of RNA polymerase decreases, while the DNA fragment does not undergo a gross conformational change. (2) The spatial arrangement of RNA polymerase and DNA in the specific complex from the cross-sectional radii of gyration of the complex the normal distance dn between the centre of gravity of the RNA polymerase and the axis of the DNA fragment was derived as 5.0 (+/- 0.3) nm. On the basis of these and footprinting data a low resolution model of the RNA polymerase-promoter complex is proposed. The main feature of this model is the positioning of RNA polymerase to only one side of the DNA.

DNA, Bacterial↗

PubMed1990

Differential use of distance and location information for spatial localization.

Five experiments are reported in which subjects judged the movement or spatial location of a visible object that underwent a combination of real and induced (illusory) motion. When subjects attempted to reproduce the distance that the object moved by moving their unseen hands, they were more affected by the illusion than when they pointed to the object's perceived final location. Furthermore, pointing to the final location was more affected by the illusion when the hand movement began from the same position as that at which the object initially appeared, as compared with responses that began from other positions. The results suggest that people may separately encode two distinct types of spatial information: (1) information about the distance moved by an object and (2) information about the absolute spatial location of the object. Information about distance is more susceptible to the influence of an induced motion illusion, and people appear to rely differentially on the different types of spatial information, depending on features of the pointing response. The results have important implications for the mechanisms that underlie spatially oriented behavior in general.

Adult↗

PubMed1987

Linear mechanisms of directional selectivity in simple cells of cat striate cortex.

The role of linear spatial summation in the directional selectivity of simple cells in cat striate cortex was investigated. The experimental paradigm consisted of comparing the response to drifting grating stimuli with linear predictions based on the response to stationary contrast-reversing gratings. The spatial phase dependence of the response to contrast-reversing gratings was consistent with a high degree of linearity of spatial summation within the receptive fields. Furthermore, the preferred direction predicted from the response to stationary gratings generally agreed with the measurements made with drifting gratings. The amount of directional selectivity predicted was, on average, about half the measured value, indicating that nonlinear mechanisms act in concert with linear mechanisms in determining the overall directional selectivity.

Action Potentials↗

PubMed1987

Imagined spatial transformation of one's body.

This study examined two related phenomena: (a) the judgment of whether a human body part belongs to the left or right half of the body and (b) the imagined spatial transformation of one's body. In three experiments, observers made left-right judgments of a part of a body whose orientation differed from their own by a rotation about one of 13 axes. To do so, they imagined themselves passing to the orientation of the stimulus. Time for (a) left-right judgments and (b) accompanying imagined spatial transformations depended on the extent of the orientation difference (OD) between the observer and stimulus. More important, time for phenomena (a) and (b) depended strongly, and in the same way, on the direction of OD. Further results showed that the rate of imagined spatial transformations can vary strongly for different axes and directions of rotation about an axis. These and other results (e.g., Parsons, 1987a) suggest that temporal and kinematic properties of imagined spatial transformations are more object-specific than could be previously assumed.

Body Image↗

PubMed1979

Temporal and intensitive factors in comparative judgments of tactile space.

Temporal imbalances between the successive stimulation of pairs of loci on each forearm altered judgments of the equality of the spatial extents between sets of loci. The spatial judgments became increasingly inaccurate as the temporal imbalance was increased, i.e., the Tau effect. Changes in stimulus intensity also influenced the dependency of spatial judgments on temporal factors. The data suggest that concomitant variations of spatial, temporal, and intensitive components of a vibrotactile stimulus "package" can produce systematic perceptual effects of potential significance in the design of vibrotactile sensory substitution systems.

Electric Stimulation↗

PubMedDate not supplied

[Submicroscopic structure of the membrane apparatus of methanotrophic bakteria].

The fine structure of the membrane apparatus in obligate methanotrophic bacteria, Methylomonas methanica, Methylomonas rubrum 15 and Methylobacter bovis 53b was studied. Complex intracytoplasmic membrane systems of the I type were found in all cultures, but they differed by their structure and spatial organization depending on microorganisms. The cytoplasmic membrane was shown to be involved in the formation of complex membrane structures. A scheme of the spatial organization of the intracytoplasmic membrane systems on the basis of studying serial sections of the methanotrophs is suggested. The formation of the membranex packs is supposed to be a continuous process in Methylomonas methanica and Methylomonas rubrum but a discrete process in Methylobacter bovis.

Cell Membrane↗