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At least 127 records · Page 7Linked to original sources

PubMed1990

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex.

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.

Animals↗

PubMed1991

Tonic interocular suppression and binocular summation in human vision.

1. Spatial sensitivity of human foveal vision was examined using sinusoidally modulated gratings. Our primary concern was the influence of interocular light adaptation upon monocular visibility. 2. Interocular adapting influences depend upon spatial frequency and adapting luminance. Interocular adaptation has a negligible influence upon the sensitivity to 1 cycle/deg gratings. Any visible interocular adapting field improves the sensitivity to intermediate spatial frequencies (2-5 cycles/deg). 3. Brighter interocular backgrounds (greater than 0.1 cd/m2) improve sensitivity to higher spatial frequencies (10-20 cycles/deg). 4. The interocular adapting influences summarized in (2) and (3) above cannot be duplicated by monocular or binocular adaptation. Similarly, monocular or binocular adaptation have negligible influences upon binocular visibility. 5. The interocular adapting effect summarized in (3) above can be duplicated by pressure blinding the contralateral eye. We conclude that monocular spatial sensitivity is subject to a tonic interocular suppression (TIS) from the dark-adapted eye. 6. The spatial sensitivity resulting from binocular viewing is nearly identical to that observed by combining monocular viewing with interocular light adaptation. We suggest that the improvement in sensitivity resulting from two-eyed viewing may be attributable to the removal of TIS instead of to binocular physiological summation.

Adaptation, Ocular↗

PubMed1985

Spatial and temporal selectivity of the human motion detection system.

Measurements were made of spatial frequency, orientation and temporal frequency selectivity of the visual motion system. The results suggest: (1) There exists in the motion system mechanisms selective for spatial frequency. The preferred spatial frequency varies considerably and extends down to at least 0.06 c/deg. (2) At all spatial frequencies (from 0.1 to 10 c/deg) there exist detectors selective for orientation which vary in (directed) orientation tuning to encompass 360 degrees. (3) The bandwidth of both spatial frequency and orientation selectivity vary inversely with spatial frequency: the lower the spatial frequency, the broader the bandwidth. (4) There exist two classes of temporally tuned detectors, one lowpass (sustained) and one bandpass (transient), of preferred temporal frequency of 7-13 Hz (depending on spatial frequency).

Form Perception↗

PubMed1987

The two-dimensional spectral structure of simple receptive fields in cat striate cortex.

1. A quantitative, general purpose method was developed for measuring the responses of visual neurons to stimuli distributed with high resolution over the two-dimensional (2D) spatial frequency domain. The stimuli consisted of drifting sinusoidal gratings of nonsaturating contrasts whose spatial frequency and orientation were drawn in random order from a 16 X 16 array of coordinates covering each neuron's responsive area. This method was applied to a population of 36 simple cells in area 17 of cat. 2. The response of each simple cell to drifting sinusoidal gratings appeared as a rectified sinusoidal modulation of the spike frequency. The degree of rectification varied from cell to cell, but for each cell, the form of the response was constant irrespective of stimulus spatial frequency, orientation, or contrast. The amplitude of the average response at the stimulus temporal frequency was used as the response metric at all spectral coordinates. Variations in this amplitude over two spectral dimensions forms a surface that we call the 2D spectral response profile. 3. For each cell, the 2D spectral response profile was localized to a limited region of the complete 2D spatial frequency domain. In bidirectionally responsive cells, there were two lobes in the surface disposed with mirror symmetry about the origin. In all cells, each lobe exhibited a single maximum and the response decayed smoothly in every direction away from the maximum. Isoresponse amplitude contours were elliptical and often, but not always, elongated about an axis of symmetry passing through the origin. 4. We tested the hypothesis that orientation and spatial frequency tuning are independent by forming scaled radial and angular sections through 2D spectral response profiles. In virtually every case polar separability did not obtain, that is, orientation selectivity depended on spatial frequency and vice versa. 5. In contrast, more than half the cells had 2D spectral response profiles that were Cartesian separable. The 2D spectral response profiles of most of the remaining cells were neither polar nor Cartesian separable, because the response profiles were elongated about an axis of symmetry that did not pass through the origin. 6. These results are discussed in terms of the constraints they place on models of the contributions simple cells make toward the neural representation of images.

Animals↗

PubMed1991

Perceived speed and direction of complex gratings and plaids.

Measurements of perceived speed were obtained for a variety of drifting simple and complex gratings, and measurements of perceived speed and direction were obtained for plaids. For sine gratings, perceived speed falls off at high spatial frequencies, the effect of spatial frequency being greatest at high speeds. Speed matches obtained from a variety of one-dimensional complex gratings are in some cases consistent with a simple averaging of the discrepant speeds signaled by their spatial Fourier components when seen alone. However, in other cases the results do not fit such an interpretation but suggest the involvement of a mechanism other than Fourier decomposition and recombination. Measurements of the perceived speed of plaids suggest that the observed spatial-frequency-dependent variations in encoded speed of gratings arise largely at a low level, before the aperture problem is solved. Measurements of the perceived direction of plaids whose components are of different spatial frequencies and hence have different perceived speeds show large deviations from the direction predicted by the intersection-of-velocity-constraints model [Nature 300, 523 (1982)] and are suggestive of a revised two-stage model in which the computation of pattern direction is based on the (sometimes disparate) perceived speeds of the components rather than their actual speeds.

Humans↗

PubMed1985

Lower thresholds of motion for gratings as a function of eccentricity and contrast.

We investigated the lower threshold for motion (LTM) of gratings as a function of position in the visual field, spatial frequency and contrast and we compared motion thresholds for sine wave and square wave luminance profiles. For contrasts below 0.05 the lower threshold for motion was raised; the increase in threshold being dependent upon spatial frequency. At contrast levels above 0.05, LTM was found to be a constant velocity at any given spatial location but increased with eccentricity of view. Raised thresholds for motion at eccentric locations could be compensated by increasing the size of eccentric gratings in proportion to M-1, where M is the cortical magnification factor, a procedure which standardises the cortical representation at differing eccentricities. Thus LTM could be expressed as a constant cortical velocity for grating contrasts above 0.05 at all stimulus locations investigated. We interpret our data as support for a ratio model of velocity coding.

Adult↗

PubMed1991

Interocular mismatch in spatial frequency and directionality characteristics of striate cortical neurones.

Spatial-frequency dependence of directional tuning and directional bias was compared, for both eyes, in four previously established discrete classes of binocular feline striate cortical neurones. Two classes (respectively direction-selective or bidirectional at optimal spatial frequency) were directionality invariant at all spatial frequencies. In the remaining two classes, both direction-biased at optimal spatial frequency, directional bias either altered or reversed with change in spatial frequency. In all four classes, the directional tuning of a majority of neurones sharpened at high spatial frequency through either eye, although the bandpass characteristics were sometimes dissimilar for the two eyes. All neurones were of the same type through either eye. Amongst the two classes of direction-biased neurones, the strength of bias was commonly different through the two eyes. Where reversal of bias occurred, that reversal took place at different spatial frequencies for each eye. Thus, the direction and orientation preferences of cortical neurones are fixed at optimal spatial frequency, but their envelope of tuning to a gamut of spatial frequencies is not. These differences are potentially related to binocular coding of visual perspective, including dynamic object rotation in visual space.

Animals↗

PubMed1986

Conjunction of color and form without attention: evidence from an orientation-contingent color aftereffect.

According to feature-integration theory (Treisman & Gelade, 1980), separable features such as color and shape exist in separate maps in preattentive vision and can be integrated only through the use of spatial attention. Many perceptual aftereffects, however, which are also assumed to reflect the features available in preattentive vision, are sensitive to conjunctions of features. One possible resolution of these views holds that adaptation to conjunctions depends on spatial attention. We tested this proposition by presenting observers with gratings varying in color and orientation. The resulting McCollough aftereffects were independent of whether the adaptation stimuli were presented inside or outside of the focus of spatial attention. Therefore, color and shape appear to be conjoined preattentively, when perceptual aftereffects are used as the measure. These same stimuli, however, appeared to be separable in two additional experiments that required observers to search for gratings of a specified color and orientation. These results show that different experimental procedures may be tapping into different stages of preattentive vision.

Adolescent↗

PubMed1990

In situ binding of bouton zinc reversibly disrupts performance on a spatial memory task.

Neurons with zinc in the presynaptic vesicles innervate much of the telencephalon, but the functional significance of the vesicular zinc has never been established. The present work shows that reversible binding of zinc by drug infusion into the hippocampus produces a time-locked and selective disruption of hippocampal-dependent spatial-working memory. A role for vesicular zinc in neurotransmission or neuromodulation is implied.

Animals↗

PubMedDate not supplied

[Theoretical conformational analysis of methylamide of N-acetyl-L-lysine].

The spatial structure of the methylamide of N-acetyl-L-lysine has been analysed taking into account non-bonded and electrostatic interactions, torsional energy, bond angles distortion and hydrogen bonding. Conformational capacities of the backbone and mutual dependence of spatial structures of the backbone and the side chain was described by conformational maps obtained by energy minimisation, the dihedral angles and the bond angles of the side chain being varied for every phi, psi point. Every possible combination for phi, psi, x1-x5-angles was used corresponding to the stable form of the backbone and to torsion potential minima of the initial approximations in the calculation of preferred conformations of the molecule. Comparisons are made between stable forms of the methylamide of N-acetyl-L-lysine and Lys residues in proteins with known structure.

Amides↗

PubMed1990

[Digital radiography].

Advantages of digital radiography: Improved low contrast imaging; Image processing capability (on line or post-processing); Lower radiation dose for certain applications ("Dose variation"); Digital storage and data transfer; Only one exposure for different imaging characteristics; Optimised real-time image, "digital fluoroscopy" (DBR); Shorter examination times (DBR); Advantages for technically complicated exposures (intensive care, superpositions in the chest region, pediatry) (DLR); High dynamic range, which eliminates over or under exposure; Real time image processing and display (DBR). Disadvantages of digital radiography; Lower spatial resolution is limiting fine structure (max. theoretical resolution is 31p/mm for 1000 and app. 5 lp/mm for 2000 pixels image matrix); Spatial resolution depends from image intensifier diameter or screen format; Lower SNR (Signal to Noise Ratio) for reduced dose, increased noise impression for edge enhancement; Information losses for monitor camera (DBR) -hardcopies; Diagnostic capabilities are reduced by noise and low spatial resolution caused by certain applications (mammography).

Humans↗

PubMedDate not supplied

Effect of finite phosphor thickness on detective quantum efficiency.

In this paper we describe theoretically the relationship between the finite thickness of a phosphor screen and its spatial-frequency-dependent detective quantum efficiency DQE(f-). The finite thickness of the screen causes a variation in both the total number of light quanta emitted from the screen in a burst from a given x-ray interaction and in the spatial distribution of the quanta within the light burst [i.e., shape or point spread function (PSF) of the light burst]. The variation in magnitude of the burst gives rise to a spatial-frequency-independent reduction in DQE, characterized by the scintillation efficiency As. The variation in PSF causes a roll off in DQE with increasing spatial frequency which we have characterized by the function Rc(f). Both As and Rc(f) can be determined from the moments of the distribution of the spatial Fourier spectrum of light bursts emitted from the phosphor and thus they are related: As is a scaling factor for Rc(f). Our theory predicts that it is necessary for all light bursts which appear at the output to have the same magnitude to maximize As and the same shape to maximize Rc(f). These requirements can lead to the result that the fluorescent screen with the highest modulation transfer function will not necessarily have the highest DQE(f) even at high spatial frequencies.

Fourier Analysis↗

PubMed1987

Blockade of spatial learning by the M1 muscarinic antagonist pirenzepine.

Two experiments were conducted to determine the effects of the M1 muscarinic receptor antagonist pirenzepine on place navigation in a water maze. In the first experiment rats were required to learn the location of a hidden platform following intracerebroventricular injections of equimolar doses of pirenzepine or scopolamine methylbromide. Both drugs dose-dependently impaired spatial learning according to both escape latency data and transfer test analysis. Pirenzepine was approximately 3 times less potent than scopolamine, a potency ratio which suggests M1 receptor mediation of the impairment. In the second experiment pirenzepine (1 approximately 92.3 micrograms/rat ICV) was injected prior to training on a simultaneous place discrimination task in the water maze. Impairments of choice accuracy were found with a dose of 20 micrograms/rat in the absence of any marked increases in either errors of omission or choice latency. These data suggest that M1 receptor blockade impairs processes which are involved in spatial learning.

Animals↗

PubMed2026

Direct visualization of MCM helicase activation and replisome coupling in situ.

Deciphering the spatial organization of molecular machines that copy the genome remains a fundamental challenge in biology. Essential for eukaryotic DNA replication, Mini-Chromosome Maintenance (MCM2-7) helicases are loaded during G1 as double hexamers (DHs) to license replication origins. Upon activation in S phase, each DH is thought to split into two single hexamers (SHs) that form the active CMG helicases and travel bidirectionally. However, the field has long been divided: biochemical and structural studies define CMG helicases as autonomous, independent motors, while genomic and cellular imaging assays suggest sister replisomes remain physically coupled within replication factories. Here, we use MINFLUX nanoscopy to localize individual MCM complexes down to nanometer precision in situ, directly resolving DHs in human cells and capturing their separation into SHs upon origin firing. We find that the resulting sister replisomes do not diffuse apart: they remain coupled at a characteristic distance of ~40 nm throughout S phase. Depletion experiments identify two distinct contributions to this coupling: local, protein-mediated tethering by the AND1 scaffold, and higher-order spatial confinement dependent on cohesin, which is dispensable for MCM loading in G1 but required to maintain coupling in S phase. By linking the nanometer-scale architecture of the replisome to the genome-wide topology of replication fountains, these findings provide direct spatial evidence that sister forks are coupled during DNA synthesis and define the molecular forces that organize replisomes within their native nuclear context.

DNA replication↗

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↗