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PubMed1983

Spatial and temporal properties of luminosity horizontal cells in the turtle retina.

Luminosity horizontal cells in the turtle retina respond approximately linearly to visual stimuli with contrast levels spanning a large part of the physiological range. We characterized the response properties of these cells under conditions of low photopic background illumination by measuring their spatial and temporal frequency transfer functions. Our experimental results indicate in two ways that, under these conditions, feedback from luminosity horizontal cells to cones does not play a major role in the mechanisms underlying the spatial and temporal tuning of horizontal cell responses. First, the shape of the spatial transfer function depended only weakly on the temporal frequency with which it was measured. Second, the shape of the temporal transfer function depended only weakly on the spatial frequency with which it was measured.

Animals↗

PubMed2004

Fundamental study on diffuse reflective optical tomography.

We report a simulation study on diffuse reflective optical computed tomography, in which continuous-wave sources and detectors are placed on the plane surface of a semi-infinite body. We adopted a simple Tikhonov regularization in the inverse problem and demonstrated the feasibility of three-dimensional reconstruction of the absorption coefficient change. The spatial resolution of the reconstructed image was shown to be degrading markedly with the depth. The regularization parameter should be chosen appropriately considering the trade-off between the reconstructed image noise and the spatial resolution. We analysed the dependence of the spatial resolution of the reconstructed image on the regularization parameter and the depth, and also the behaviour of the reconstructed image noise on the regularization parameter and the depth.

Absorption↗

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↗

PubMed2006

Density dependence in marine fish populations revealed at small and large spatial scales.

Experimental manipulation of population density has frequently been used to demonstrate demographic density dependence. However, such studies are usually small scale and typically provide evidence of spatial (within-generation) density dependence. It is often unclear whether small-scale, experimental tests of spatial density dependence will accurately describe temporal (between-generation) density dependence required for population regulation. Understanding the mechanisms generating density dependence may provide a link between spatial experiments and temporal regulation of populations. In this study, I manipulated the density of recently settled kelp rockfish (Sebastes atrovirens) in both the presence and absence of predators to test for density-dependent mortality and whether predation was the mechanism responsible. I also examined mortality of rockfish cohorts within kelp beds throughout central California to evaluate temporal (between-generation) density dependence in mortality. Experiments suggested that short-term behavioral responses of predators and/or a shortage of prey refuges caused spatial density dependence. Temporal density dependence in mortality was also detected at larger spatial scales for several species of rockfish. It is likely that short-term responses of predators generated both spatial and temporal density dependence in mortality. Spatial experiments that describe the causal mechanisms generating density dependence may therefore be valuable in describing temporal density dependence and population regulation.

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↗

PubMedDate not supplied

Artificial scotoma-induced perceptual distortions are orientation dependent and short lived.

Conditioning human observers with an "artificial scotoma"-a small retinal area deprived of patterned stimulation within a larger area of dynamically textured noise-results in contractions and expansions of perceived space that are thought to reflect receptive-field changes among cells in the primary visual cortex (Kapadia et al., 1994). Here we show that one-dimensional counter-phase flickering grating patterns are also potent stimuli for producing artificial scotomata capable of altering three-element bisection ability analogous to those results reported earlier. Moreover, we found that the magnitude of the induced spatial distortions depends critically on the relative orientations of peri-scotomatous and test-stimulus spatial contrast. In addition, the perceptual distortions are found to be relatively short lived, decaying within 660 ms. The results support the hypothesis that artificial scotoma-induced perceptual distortions are generated by dynamic alteration of connection efficacy within a network linking cortical areas of similar orientation specificity, consistent with established anatomical and physiological results.

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↗

PubMed1993

Spatio-temporal frequency separability in area 18 neurons of the cat.

Spatial frequency tuning curves were obtained at a series of temporal frequencies and/or velocities from neurons of Areas 17 and 18 of the cat, in order to assess the interaction between these spatial and temporal parameters. In the great majority of instances, for neurons in both Areas 17 and 18, we found the optimal spatial frequency invariant with the temporal frequency or velocity at which a neuron was tested. Thus in this respect, responses of neurons in Area 18, like those of Area 17, are separable in their dependence on spatial and temporal frequency.

Animals↗

PubMed2001

Visual imagery without visual experience: evidence from congenitally totally blind people.

We explored the ability of congenitally totally blind people (who were contrasted with age-, sex- and education matched blindfolded sighted subjects) to perform tasks which are mediated by visual mental imagery in sighted people. In the first (pictorial) task, subjects had to mentally compare the shape of the outline of three named objects and to indicate the odd-one-out. In the second (spatial) task the participants were asked to memorise the position of a target cube in two- and three-dimensional matrices, based on a sequence of spatially based imagery operations. In addition, during half of the trials of both imagery tasks subjects were required to perform a concurrent finger tapping task, to investigate whether the blind subjects would be more dependent on spatial processing. Although blind participants made significantly more errors than sighted participants, they were well able to perform the spatial imagery task as well as the pictorial imagery task. Interference from the concurrent tapping task affected both groups to the same extent. Our results shed new light on the question whether early visual experience is necessary for performance on visual imagery tasks, and strongly suggest that vision and haptics may share common representations.

Adult↗

PubMed2003

Spatial and temporal tuning of motion in depth.

We used the Pulfrich effect to investigate perception of motion in depth. Independent manipulation of spatial and temporal frequency content in stereoscopic motion stimuli revealed the tuning characteristics of motion-in-depth perception. Sensitivity to interocular phase difference between sinusoidally oscillating sine-wave gratings was measured in four observers who judged direction of motion in depth. Discrimination thresholds in terms of interocular phase difference were determined to investigate spatial and temporal tuning characteristics of a system that is based on interocular phase difference, interocular delay, binocular disparity and velocity difference. Temporal frequency tuning of interocular phase difference thresholds was band pass and relatively dependent on spatial frequency variation. These results together with evidence from two control experiments support the idea that sensitivity to direction of motion in depth is limited by a stereo-motion system that monitors binocular horizontal disparity and motion rather than interocular phase difference, interocular delay, or interocular velocity difference.

Computer Simulation↗

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↗

PubMed2004

Experimental characterization of simultaneous spatial and spectral filtering by an optical resonant filter.

Simultaneous spatial and spectral filtering by an optical resonant filter has been characterized experimentally to furnish additional insight into the operation and applications of optical resonant filters. Our experimental study can be useful for applications that depend on spatial filtering, spectral filtering, spatial-spectral filtering, and polarization selectivity. One significant application is the integration of an optical resonant filter with semiconductor lasers to control the spatial-spectral radiation for optimum performance.

Journal Article↗

PubMed2002

Long-term potentiation and spatial learning are associated with increased phosphorylation of TrkB and extracellular signal-regulated kinase (ERK) in the dentate gyrus: evidence for a role for brain-derived neurotrophic factor.

In this study, the authors investigate changes in the presynaptic terminal of the dentate gyrus that accompany 2 types of hippocampal-dependent plasticity: spatial learning and long-term potentiation (LTP). Parallel changes occurred in the dentate gyrus of rats that had undergone training in the Morris water maze and had sustained LTP. In both cases, KCl-induced brain-derived neurotrophic factor release was increased, and this was accompanied by increased phosphorylation of TrkB and the mitogen-activated protein kinase, ERK. Glutamate release was also enhanced, and the data suggest that this may be a consequence of increased activation of TrkB and ERK. Because the data indicate that similar cellular modifications are shared by these 2 forms of plasticity, they provide circumstantial evidence that LTP satisfies some of the requirements of a memory-inducing cellular substrate.

Animals↗

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↗

PubMed2005

Modulational instability in systems with integrating nonlinearity.

A new type of modulational instability for coherent as well as partially coherent light in systems with integrating nonlinearity caused by an irreversible process is investigated both experimentally and theoretically. In such systems plane waves never reach the stationary limit and exhibit a nontrivial time dependence resulting in new features of the modulational instability. For example, the modulational frequency of the nonexponentially increasing perturbation with maximum gain decreases while the wave is propagating. The threshold for vanishing modulational instability due to a finite degree of spatial coherence depends only on system parameters and not on the light intensity.

Journal Article↗