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PubMed1984

The contrast dependence of spatial frequency channel interactions.

Interactions between spatial frequency channels were tested in two ways: we measured the discriminability by the visual system between two compound spatial-frequency gratings, of components with spatial frequencies in the ratio 1:3, when the difference between the two gratings was an increase (or decrease) in contrast of both components of the compound grating (contrast discrimination), or when the difference between the two gratings was an increase in contrast of one component and a decrease in contrast of the other component (pattern discrimination). We found that the contrast: pattern discriminability ratio differs significantly from unity in most conditions. Furthermore, this ratio is generally greater when the components of the grating are in peaks-add relative phase than when in peaks-subtract phase. On the other hand, the ratio was close to unity for grating components of spatial frequencies 1 and 9 cycles/deg. These results suggest that the human visual system contains spatial frequency channels with bandwidths of between 1.6 and 3.2 octaves and that these relatively broad channels have peaks-add spatial profiles. The channels appear linear at intermediate contrasts and spatial frequencies, but super-linear at high spatial frequencies and contrasts. Contrast and spatial frequency may be interchangeable for the determination of the linearity of the visual system.

Discrimination, Psychological↗
PubMed1990

Infant spatiotemporal vision: dependence of spatial contrast sensitivity on temporal frequency.

Effects of temporal frequency on infant spatial contrast sensitivity were studied with forced-choice preferential looking, using localized 1.0 and 0.35 c/deg grating patches. Between 4 and 8 months, contrast sensitivity at 1.0 c/deg increased significantly at 8-17 Hz but showed little change at 2-4 Hz, so temporal tuning became increasingly bandpass with age. At 4 months, temporal tuning was lowpass at 1.0 c/deg and bandpass at 0.35 c/deg. Control data on adults indicate that the differences between the infant and adult data cannot be accounted for solely by reduced photoreceptor density and quantal catch, and suggest developmental changes in tuning and/or relative sensitivities of spatiotemporal mechanisms.

Adult↗
PubMed1991

Orientation discrimination depends on spatial frequency.

Thresholds were measured for discriminating the orientation of sinusoidal gratings of varying spatial frequency, and found to decrease monotonically with increasing spatial frequency. For discrimination of high-contrast (10 times threshold) near-vertical gratings, thresholds ranged from about 1 deg at 0.04 c/deg to 0.5 deg at 0.2 c/deg, after which there was little improvement. At lower contrasts and for discriminations around a mean of 45 deg, thresholds varied more so, and continued to improve until 1 c/deg. The variation of orientation discrimination thresholds with spatial frequency follows a similar trend to the variation in orientation bandwidth of visual units over the same range of spatial frequencies. Thus the present results are consistent with recent "opponent-process" models of orientation discrimination, that predict that thresholds to be limited (at least in part) by the maximum slope of orientation selectivity of visual detectors. That thresholds for high contrast vertical gratings did not improve for frequencies higher than 0.2 c/deg implies that orientation bandwidth and noisiness of oriented detectors may not be the sole factor limiting orientation discrimination, and suggests the existence of more central noise sources.

Contrast Sensitivity↗
PubMed1995

An illusion of relative motion dependent upon spatial frequency and orientation.

Observers scanned a stationary pattern comprising a tilted sine-wave grating completely surrounding another grating of similar spatial frequency but tilted in the opposite direction (Fig. 2). They reported an illusory "sliding" motion of the inset grating with respect to the surround grating and the effect was clearly strongest for angles between the gratings of less than 60 degrees and for spatial frequencies between 6-11 cpd. In a second experiment, a similar pattern was moved (2.0 deg/sec) either up or down for a presentation time of 167 msec. Simultaneously, the inset grating was drifted at different speeds in each of its two directions. Using the method of constant stimuli, it was shown that the relative motion illusion could be cancelled by physically drifting the grating in the opposite direction to the illusory movement. The illusion arises because there is a failure to integrate two motion signals into the single motion vector which characterises rigid motion.

Contrast Sensitivity↗
PubMed2004

Dorso-ventral asymmetric functions of teashirt in Drosophila eye development depend on spatial cues provided by early DV patterning genes.

The teashirt (tsh) gene has dorso-ventral (DV) asymmetric functions in Drosophila eye development: promoting eye development in dorsal and suppressing eye development in ventral by Wingless mediated Homothorax (HTH) induction [Development 129 (2002) 4271]. We looked for DV spatial cues required by tsh for its asymmetric functions. The dorsal Iroquois-Complex (Iro-C) genes and Delta (Dl) are required and sufficient for the tsh dorsal functions. The ventral Serrate (Ser), but not fringe (fng) or Lobe (L), is required and sufficient for the tsh ventral function. We propose that DV asymmetric function of tsh represents a novel tier of DV pattern regulation, which takes place after the spatial expression patterns of early DV patterning genes are established in the eye.

Animals↗
PubMed2003

Sensitivity to contrast modulation: the spatial frequency dependence of second-order vision.

We consider the overall shape of the second-order modulation sensitivity function (MSF). Because second-order modulations of local contrast or orientation require a carrier signal, it is necessary to evaluate modulation sensitivity against a variety of carriers before reaching a general conclusion about second-order sensitivity. Here we present second-order sensitivity functions for new carrier types (low pass (1/f) noise, and high pass noise) and demonstrate that, when first-order artefacts have been accounted for, the shape of the resulting MSFs are similar to one another and to those for white and broad band noise. They are all low pass with a likely upper frequency limit in the range 10-20 c/deg, suggesting that detection of second-order stimuli is relatively insensitive to the structure of the carrier signal. This result contrasts strongly with that found for (first-order) luminance modulations of the same noise types. Here the noise acts as mask and each noise type masks most those frequencies that are dominant in its spectrum. Thus the shape of second-order MSFs are largely independent of the spectrum of their noise carrier, but first-order CSFs depend on the spectrum of an additive noise mask. This provides further evidence for the separation of first- and second-order vision and characterises second-order vision as a low pass mechanism.

Artifacts↗
PubMed1996

Hippocampal LTP Depends on Spatial and Temporal Correlation of Inputs.

We studied the LTP inducing factors using temporally and spatially modulated stimuli given to the hippocampal neural network. It was found that when the spatial factors were maintained to be constant the positive correlation in the successive inter-stimulus intervals contributes to produce larger LTP. On the other hand, if the temporal factors are kept constant, the spatial coincidence contributes to produce larger LTP. We propose a learning rule by which these experimental results can be consistently interpreted. Copyright 1996 Elsevier Science Ltd.

Journal Article↗
PubMedDate not supplied

A Monte Carlo investigation of artifacts caused by liver uptake in single-photon emission computed tomography perfusion imaging with technetium 99m-labeled agents.

BACKGROUND: Significant hepatobiliary accumulation of technetium 99m-labeled cardiac perfusion agents has been shown to cause alterations in the apparent localization of the agents in the cardiac walls. A Monte Carlo study was conducted to investigate the hypothesis that the cardiac count changes are due to the inconsistencies in the projection data input to reconstruction, and that correction of the causes of these inconsistencies before reconstruction, or including knowledge of the physics underlying them in the reconstruction algorithm, would virtually eliminate these artifacts. METHODS AND RESULTS: The SIMIND Monte Carlo package was used to simulate 64 x 64 pixel projection images at 128 angles of the three-dimensional mathematical cardiac-torso (MCAT) phantom. Simulations were made of (1) a point source in the liver, (2) cardiac activity only, and (3) hepatic activity only. The planar projections and reconstructed point spread functions (PSFs) of the point source in the liver were investigated to study the nature of the inconsistencies introduced into the projections by imaging, and how these affect the distribution of counts in the reconstructed slices. Bull's eye polar maps of the counts at the center of the left ventricular wall of filtered back-projection (FBP) and maximum-likelihood expectation-maximization (MLEM) reconstructions of projections with solely cardiac activity, and with cardiac activity plus hepatic activity scaled to have twice the cardiac concentration, were compared to determine the magnitude and location of apparent changes in cardiac activity when hepatic activity is present. Separate simulations were made to allow the investigation of stationary spatial resolution, distance-dependent spatial resolution, attenuation, and scatter. The point source projections showed significant inconsistencies as a function of projection angle with the largest effect being caused by attenuation. When consistent projections were simulated, no significant impact of hepatic activity on cardiac counts was noted with FBP, or 100 iterations of MLEM. With inconsistent projections, reconstruction of 180 degrees resulted in greater apparent cardiac count losses than did 360 degrees reconstruction for both FBP and MLEM. The incorporation of attenuation correction in MLEM reconstruction reduced the changes in cardiac counts to that seen in simulations in which attenuation was not included, but resulted in increased apparent localization of activity in the posterior wall of the left ventricle when scatter was present in the simulated images. CONCLUSIONS: The apparent alterations in cardiac counts when significant hepatic localization is present is due to the inconsistency of the projections inherent in imaging. Prior correction of these, or accounting for them in the reconstruction algorithm, will virtually eliminate them as causes of artifactual changes in localization. Attenuation correction and scatter correction are both required to overcome the major sources of apparent count changes in the heart associated with hepatic uptake.

Artifacts↗
PubMed2006

Persistence of spatial populations depends on returning home.

There is a need for better description and heuristic understanding of the sustainability of populations connected over space by a dispersing stage, both for management purposes and to increase our basic knowledge of the dynamics of these populations. We show that persistence of such a population of connected subpopulations depends on whether the sum of the reproductive gains through all possible closed, between-patch reproductive paths through multiple generations, relative to the shortfall in self-persistence in each path, exceeds unity plus extra terms, which only appear if there are four or more patches. These extra terms have the heuristic explanation that they avoid double counting of reproductive paths that arise with four or more patches because there can be nonoverlapping subnetworks. Thus only those patterns of reproduction and connectivity which eventually lead to descendants returning to the patch from which they originate contribute to persistence. This result provides the basis for evaluating connectivity and habitat heterogeneity to understand reserve design, the effects of human fragmentation, the collapse of marine fisheries, and other conservation issues.

Animals↗
PubMed2002

Cancer induction by radiotherapy: dose dependence and spatial relationship to irradiated volume.

The present study was initiated to identify radiotherapy-related parameters influencing the development of second malignancies. Between 1969 and 1989, about 31000 patients were treated with radiotherapy in Dresden. Of these, 203 were re-admitted for radiotherapy of a newly developed malignancy. Based on completeness of documentation, definition of site of origin and exclusion of recurrent or metastatic tumours, 85 patients were selected for analysis of spatial relation between new tumour and primary treatment fields, and of dose at the site of origin. The majority of second tumours were seen within the margin region of the treatment volume, defined as the volume from 2.5 cm inside to 5 cm outside the margin of the planning target volume (PTV). Fewer than 10% of the second tumours occurred inside the PTV and 11% developed outside the margin region. The majority of second tumours were observed in the volume receiving <6 Gy. Similar results were obtained in a group of 31 patients initially irradiated for benign diseases. This should be considered in radiotherapy, particularly with multiple-field techniques or intensity-modulated radiotherapy, where the volume receiving <6 Gy is substantially increased.

Humans↗
PubMed1985

Dependence of spatial heterogeneity of myocardial blood flow on mean blood flow rate in the rabbit heart.

The relationship between spatial heterogeneity of myocardial blood flow and the rate of perfusion was examined in anaesthetised-chest rabbits. Blood flow determinations employed the radioactive microsphere technique. Five tissue samples were obtained from each of four regions of the left ventricle: left septum, right septum, subendocardium and subepicardium. Standard deviation (SD) and the coefficient of variation (CV = SD/mean flow x 100), an index of spatial heterogeneity, were computed. A wide range of mean coronary flows was obtained in seven groups by inspiration of room air, 8% O2, 8% O2 and 10% CO2, 100% O2, 100% O2 and hyperventilation, and by administration of adenosine or chromomar HCl. Significant differences in CV were found between treatment groups at the upper and lower ends of the flow range. A significant positive linear correlation between overall SD and mean coronary flow was found (r = 0.760). A significant inverse linear relationship between CV and mean flow was found over the entire range of flows studied (r = -0.482). An improved correlation between CV of calculated coronary vascular resistance and mean vascular resistance (r = 0.742) amongst these treatments suggests that spatial heterogeneity is better described by the variability in calibre of large arterioles. This decreased heterogeneity accompanying high flows or reduced vascular resistance may be beneficial in terms of O2 supply and demand under conditions of myocardial stress.

Adenosine↗
PubMed2005

The combination of vision and touch depends on spatial proximity.

The nervous system often combines visual and haptic information about object properties such that the combined estimate is more precise than with vision or haptics alone. We examined how the system determines when to combine the signals. Presumably, signals should not be combined when they come from different objects. The likelihood that signals come from different objects is highly correlated with the spatial separation between the signals, so we asked how the spatial separation between visual and haptic signals affects their combination. To do this, we first created conditions for each observer in which the effect of combination--the increase in discrimination precision with two modalities relative to performance with one modality--should be maximal. Then under these conditions, we presented visual and haptic stimuli separated by different spatial distances and compared human performance with predictions of a model that combined signals optimally. We found that discrimination precision was essentially optimal when the signals came from the same location, and that discrimination precision was poorer when the signals came from different locations. Thus, the mechanism of visual-haptic combination is specialized for signals that coincide in space.

Adult↗
PubMed1999

Temporal detection in human vision: dependence on spatial frequency.

In the study of perception of temporal changes in luminance, it is customary to model perceptual performance as based on one or more linear filters. The task is then to estimate the temporal impulse responses or the representation of the impulse response in the frequency domain. Previously, temporal masking data have been used to estimate the properties and numbers of these temporal mechanisms (filters) in central vision for 1-cycle-per-degree (cpd) targets [Vision Res. 38, 1023 (1998)]. The same methods have been used to explore how properties of the estimated filters change with stimulus contrast energy [J. Opt. Soc. Am. A 14, 2557 (1997)]. We present estimated properties for temporal mechanisms that detect low spatial-frequency patterns. The results indicate that two filters provide the best model for performance when mask contrast is significant. There are also differences between properties for mechanisms that detect signal spatial frequencies of 1 cpd and 1/3 cpd. The sensitivity of the low-pass mechanism relative to the bandpass mechanism is reduced at 1/3 cpd, consistent with previous findings.

Artifacts↗