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At least 1,765 records · Page 98Linked to original sources
PubMed2000

Motion energy versus position tracking: spatial, temporal, and chromatic parameters.

The fundamental question in motion perception is whether motion is an interpretation imposed on an object or feature perceived at separate positions at sequential instants, or whether it is the response of direction-sensitive detectors that can extract the motion-energy in the stimulus, i.e. the orientation of spatio-temporal energy. To answer this question we constructed stimuli whose position changed in one direction while the motion energy contained in the same spatial frequency moved in the same or the opposite direction (by superimposing moving sinusoidal gratings on stationary gratings of the same spatial frequency and orientation). In every case tested (0.25-25 Hz temporal frequency; 0.25-1.0 cyc/deg spatial frequency; achromatic and equiluminant contrast), the perceived direction of motion was in the direction of motion energy, indicating the existence of neurons which compute motion direction without explicitly computing spatial position. The measurements also confirmed that motion-energy computations can be modeled as separable in spatial and temporal frequency.

Adult↗
PubMed2001

Energy model for contrast detection: spatial-frequency and orientation selectivity in grating summation.

Models of spatial vision usually assume a "front-end" of spatial-frequency and orientation selective channels. Subthreshold-summation studies have provided some of the strongest support for this notion. We applied a single-channel energy model and a multiple-channels probability-summation model to explore subthreshold-summation phenomena. We measured the contrast thresholds for detection of two superimposed Gabor patches as a function of the spatial-frequency and orientation difference between the components. The stimuli were centred 7.5 deg above the fixation point and were windowed by a Gaussian function with one of two different spatial spreads. We have shown that the spatial-frequency and orientation selectivity in subthreshold summation of Gabor patches is determined by the similarity (cross-correlation) between the stimulus components. A single-channel energy model as well as a multiple-channels probability-summation model could explain the summation data.

Contrast Sensitivity↗
PubMed2001

Spatial neural modulation transfer function of human foveal visual system for equiluminous chromatic gratings.

To determine the spatial modulation transfer function (MTF) of the human foveal visual system for equiluminous chromatic gratings we measured contrast sensitivity as a function of retinal illuminance for spatial frequencies of 0.125-4 c/deg with equiluminous red-green and blue-yellow gratings. Contrast sensitivity for chromatic gratings first increased with luminance, obeying the Rose-DeVries law, but then the increase saturated and contrast sensitivity became independent of light level, obeying Weber's law. Critical retinal illuminance (I(c)) marking the transition point between the laws was found to be independent of spatial frequency at 165 phot. td. According to our detection model of human spatial vision the MTF of the retina and subsequent neural visual pathways (P(c)) is directly proportional to radicalI(c). Hence, P(c) is independent of spatial frequency, reflecting the lack of precortical lateral inhibition for equiluminous chromatic stimuli in spatiochromatically opponent retinal ganglion cells and dLGN neurons.

Adult↗
PubMed2001

Integrating electrophysiology and neuroimaging of spatial selective attention to simple isolated visual stimuli.

Visual-spatial attention involves modulations of activity in human visual cortex as indexed by electrophysiological and functional neuroimaging measures. Prior studies investigating the time course and functional anatomy of spatial attention mechanisms in visual cortex have used higher-order discrimination tasks with complex stimuli (e.g. symbol matching in bilateral stimulus arrays, or letter discrimination), or simple detection tasks but in the presence of complex distracting information (e.g. luminance detection with superimposed symbols as distractors). Here we tested the hypothesis that short-latency modulations of incoming sensory signals in extrastriate visual cortex reflect an early spatially specific attentional mechanism. We sought evidence of attentional modulations of sensory input processing for simple, isolated stimuli requiring only an elementary discrimination (i.e. size discrimination). As in prior studies using complex symbols, we observed attention-related changes in regional cerebral blood flow in extrastriate visual cortex that were associated with changes in event-related potentials at a specific latency range. These findings support the idea that early in cortical processing, spatially-specific attentional selection mechanisms can modulate incoming sensory signals based on their spatial location and perhaps independently of higher-order stimulus form.

Adult↗
PubMed2001

Spatial-frequency and contrast properties of crowding.

Crowding, the difficulty in recognizing a letter flanked by other letters, has been explained as a lateral masking effect. The purpose of this study was to examine the spatial-frequency and contrast dependencies of crowding, and to compare them with the properties of pattern masking. In experiment 1, we measured contrast thresholds for identifying the middle letters in strings of three randomly chosen lower-case letters (trigrams), for a range of letter spacings. Letters were digitally filtered using a set of bandpass filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter. Bandwidth of the filters was 1 octave. Frequencies of the target and flanking letters were the same, or differed by up to 2 octaves. Contrast of the flanking letters was fixed at the maximum value. Testing was conducted at the fovea and 5 degrees eccentricity. We found that crowding exhibits spatial-tuning functions like masking, but with generally broader bandwidths than those for masking. The spatial extent of crowding was found to be about 0.5 deg at the fovea and 2 deg at 5 degrees eccentricity, independent of target letter frequency. In experiment 2, we measured the contrast thresholds for identifying the middle target letters in trigrams for a range of flanking letter contrasts at 5 degrees eccentricity. At low flanker contrast, crowding does not show a facilitatory region, unlike pattern masking. At high flanker contrast, threshold rises with contrast with an exponent of 0.13-0.3, lower than corresponding exponents for pattern masking. In experiment 3, we varied the contrast ratio between the flanking letters and the target letters, and found that the magnitude of crowding increases monotonically with contrast ratio. This finding contradicts a prediction based on a grouping explanation for crowding. Our results are consistent with the postulation that crowding and masking may share the same first stage linear filtering process, and perhaps a similar second-stage process, with the additional property that the second-stage process in crowding pools information over a spatial extent that varies with eccentricity.

Analysis of Variance↗
PubMed2003

Comparison of performance on memory-guided saccade and delayed spatial match-to-sample tasks in monkeys.

To investigate the sources of spatial error in memory-guided saccades (MGS), we have trained monkeys on two different tasks: a MGS task and a delayed spatial match-to-sample (MTS) task. We first tested the effect of introducing a post-saccadic visual feedback on the accuracy of MGS. We found that visual feedback had a pronounced effect on the systematic saccade error, but less of an effect on the variable error. Visual feedback can improve the accuracy of saccadic eye movements over several days, while feedback removal can decrease accuracy in a reversible way. These effects also depend both on target eccentricity and the duration of the memory delay. To test whether saccade error is due to the accuracy of spatial memory storage or arises downstream from that memory, we measured behavioral performance on a spatial MTS task both before and after training with visual feedback. The results showed no significant difference in performance of the MTS task before and after feedback training despite significant changes in MGS accuracy. The results suggest that the accuracy of spatial memory is not the source of the systematic errors that accompany MGS.

Analysis of Variance↗
PubMed2003

Motion detection and the coincidence of structure at high and low spatial frequencies.

We used filtered random dot kinematograms and natural images to examine how motion detection depends the relative locations of structures defined at low and high spatial frequencies. The upper displacement limit of motion (D(max)), the lower displacement limit (D(min)) and motion coherence thresholds were unaffected by the degree of spatial coincidence between high and low spatial frequency structures i.e. whether they were consistent or inconsistent with a single feature. However motion detection was possible between band-pass filtered random dot patterns whose peak frequencies were separated by up to 4 octaves. The first result implicates spatial frequency selective motion detectors that operate independently. The second result implicates a motion system that can integrate the displacements of edges defined by widely separated spatial frequencies. Both are required to account for the two results, and they appear to operate under very similar conditions.

Contrast Sensitivity↗
PubMed2003

The detection of direction-defined and speed-defined spatial contours: one mechanism or two?

It is now accepted that the visual system integrates local orientation information across space to define spatial contours [Vision Research 33 (1993) 173]. More recently, it has been shown that similar integration occurs for the direction of local motion signals, in different parts of the visual field, if they are aligned along the axis of a spatial contour [Vision Research 42 (2002) 653]. Here we ask whether similar spatial-linking rules hold for contours comprised of local elements that share only a common speed (but not direction), in the presence of background elements which collectively have the same mean speed as the contour but considerable random variation in the speeds of the individual elements. Furthermore we investigate the detection of spatial contours that are defined by a common speed that is different (both locally and globally) from that of the background elements. The results show that there is a significant, albeit relatively weak, speed-association field with preferential linking between spatially proximal elements that have similar speeds. Although a salient speed difference between the contour and the background elements enhances detection performance for motion-defined contours, it does so primarily via a different route to that of direction linking. We suggest that for motion-defined contours the Gestalt notions of "common fate" and "good continuity", that describe the parsing of local velocity information into objects, boundaries and contours, are mediated via separate underlying perceptual mechanisms.

Humans↗
PubMed1999

One spatial filter limits speed of detecting low and middle frequency gratings.

Reaction times for detecting sinusoidal gratings depend jointly on grating contrast and spatial frequency. We examine whether the effect of spatial frequency results from low-pass filtering in a single channel or reflects processing of different frequencies by two or more different processing streams. Observers performed a speeded two-alternative spatial forced-choice detection. Errors and reaction times were measured. Contrasts varied from 0.05 to 0.67, and spatial frequencies from 0.72 to 6.51 cpd. No effect of uncertainty about spatial frequency was found, arguing against multiple channels. The data are well fit by a single channel model driven by a low pass filter.

Adult↗
PubMed1999

Spatial integration and effective spectral density of one-dimensional noise masks.

When masking one-dimensional gratings, the strongest masking effect is achieved by using one-dimensional spatial noise, which can be regarded as a special case of two-dimensional noise where the noise check height is equal to the grating height. The extent of spatial integration in the human visual system is limited, however. Hence, our aim was to investigate whether the effective height of noise checks of one-dimensional noise is similarly limited. We measured detection thresholds for vertical sinusoidal gratings with added spatial noise. The width of the noise checks remained constant, but their height increased until equal to the height of the image window which made noise one-dimensional. The contrast energy thresholds increased in direct proportion to increasing noise check height and the spectral density of noise, calculated by taking into account both the height and the width of the noise checks. The increase levelled off, however, after the critical noise check height (nyc). The critical noise check height in grating cycles changed as a function of spatial frequency (f) as nyc = 4.7 [1 + (1.4/f)2]-0.5. According to our results the effective height of noise checks was thus limited in accordance with studies on spatial integration, showing scale invariance above 1.4 c/deg.

Adult↗
PubMed1999

Development of spatial and temporal vision during childhood.

Using the method of limits, we measured the development of spatial and temporal vision beginning at 4 years of age. Participants were adults, and children aged 4, 5, 6, and 7 years (n = 24 per age). Spatial vision was assessed with vertical sine-wave gratings, and temporal vision was assessed with an unpatterned luminance field sinusoidally modulated over time. Under these testing conditions, spatial contrast sensitivity at every frequency increased by at least 0.5 log units between 4 and 7 years of age, at which point it was adult-like. Grating acuity reached adult values at 6 years of age. Temporal vision was more mature: at 4 years of age temporal contrast sensitivity at higher temporal frequencies (20 and 30 Hz) and critical flicker fusion frequency were already adult-like. Sensitivity at lower temporal frequencies (5 and 10 Hz) increased by 0.25 log units after the age of 4 to reach adult levels at age 7. The results suggest that temporal vision matures more rapidly than spatial vision during childhood. Thus, spatial and temporal vision are likely mediated by different underlying neural mechanisms that mature at different rates.

Adolescent↗
PubMed1999

Detection of temporal structure depends on spatial structure.

Observers can more easily detect correlated patterns of temporal contrast modulation within hybrid visual images composed of two components when those components are drawn from the same original picture (Blake, R., & Yang, Y. (1997). Proceedings of the National Academy of Science, 94, 7115-7119). To learn whether spatial phase is a mediating variable, we measured thresholds for detection of contrast modulation over time among component gratings while manipulating spatial phase among those components. In Experiment 1, observers more easily detected correlated contrast modulation when two component gratings were aligned in peaks-subtract phase. Experiment 2 showed that this phase-dependent detectability of synchronized contrast modulation is mediated by the phase-dependent, non-linear interaction among spatial frequency channels. The rigorous evaluation of several a priori reasonable hypotheses indicates that the phase-dependent detectability is not based on local spatial features such as local luminance, contrast or luminance gradient. Taken together, our results indicate that the spatial phase relationship and the temporal correlation of contrast modulation of two component gratings are both important for triggering facilitatory interaction between neural analyzers tuned to those gratings.

Contrast Sensitivity↗
PubMed2000

Sensitivity to contrast modulation depends on carrier spatial frequency and orientation.

We consider how the detection of second-order contrast structure depends on the orientation and spatial frequency of first-order luminance structure. For patterns composed of a bandpass noise carrier multiplied by a contrast envelope function, we show that sensitivity to the envelope varies in proportion to the spatial frequency of the carrier. For oriented carriers at low spatial-frequencies, detection of the contrast envelope is easier when the envelope and carrier are perpendicular, but this dependency diminishes as the spatial frequency of the carrier increases. These differences are not attributable to either the detection of side-bands, or the presence of spurious contrast structure in unmodulated carrier images. A final experiment measured envelope detection in the presence of noise masks. Results indicate that orientationally and spatially-band pass filtering precedes the detection of second-order structure.

Contrast Sensitivity↗
PubMed2000

Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human infants.

This study concerns the spatial-frequency-tuned channels underlying infants' contrast sensitivity functions (CSFs) for red-green chromatic stimuli, and their relationship to the channels underlying infants' CSFs for luminance-modulated stimuli. Behavioral (forced-choice preferential-looking) techniques and stationary stimuli were used. In experiment 1. contrast thresholds were measured in 4- and 6-month-olds, using isoluminant red-green gratings with spatial frequencies ranging from 0.27 to 1.53 c deg. In experiment 2. contrast thresholds were measured in 4-month-olds. using both red-green and luminance-modulated gratings in the same low spatial frequency range. Covariance analyses of individual differences were performed. Experiment 1 revealed one dominant covariance channel for the detection of red-green gratings, with a second channel contributing to detection of the highest spatial frequencies used. Experiment 2 revealed two to three channels serving color and luminance: but surprisingly these channels were not statistically separable for luminance versus chromatic stimuli. Thus, covariance channels for color and luminance that are independent for adults [Peterzell & Teller (2000). Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human adults. Vision Research, 40, 417-430] are apparently interdependent in infants. These data suggest that for infants, detection thresholds for chromatic and luminance-modulated stimuli may be limited by common mechanisms.

Child Development↗
PubMed2001

Pattern recognition techniques for the evaluation of spatial and temporal variations in water quality. A case study: Suquía River Basin (Cordoba-Argentina).

We report a comparative study using three different chemometric techniques to evaluate both spatial and temporal changes in Suquía River water quality, with a special emphasis on the improvement obtained using discriminant analysis for such evaluation. We have monitored 22 parameters at different stations from the upper, middle, and beginning of the lower river basin during at least two years including 232 different samples. We obtained a complex data matrix, which was treated using the pattern recognition techniques of cluster analysis (CA), factor analysis/principal components (FA/PCA). and discriminant analysis (DA). CA renders good results as a first exploratory method to evaluate both spatial and temporal differences, however it fails to show details of these differences. FA/PCA needs 13 parameters to point out 71% of both temporal and spatial changes, consequently data reduction from FA/PCA in this case is not as considerable as expected. However, FA/PCA allows to group the selected parameters according to common features as well as to evaluate the incidence of each group on the overall change in water quality, specially during the analysis of temporal changes. DA technique shows the best results for data reduction and pattern recognition during both temporal and spatial analysis. DA renders an important data reduction using 6 parameters to afford 87% right assignations during temporal analysis. Besides, it uses only 5 parameters to yield 75% right assignations during the spatial analysis of four different basin areas. DA allowed us to greatly reduce the dimensionality of the starting data matrix, pointing out to a few parameters that indicate the biggest changes in water quality as well as variation patterns associated with seasonal variations, urban run-off, and pollution sources, presenting a novel approach for water quality assessments.

Argentina↗
PubMed2003

Key factors affecting spatial variation of methane emissions from freshwater marshes.

To understand the mechanism for spatial variation of CH(4) emissions from marshes grown with different type of plants in a region and plots within a certain marsh grown with one type of plants, we measured CH(4) emissions from a region in which eutrophic freshwater marshes were divided into three types: Carex lasiocarpa, Carex meyeruana and Deyeuxia angustifolia according to plant type as well as CH(4) concentration in porewater, aboveground plant biomass and stem density in situ in Sanjiang Plain of Northeast China in August 2001. Spatial variation of CH(4) emissions from both different marshes in a region and different plots within a certain marsh was high. The flux rates of CH(4) emissions from three marshes ranged from 17.2 to 66.5 mg CH(4) m(-2)h(-1) with 34.76% of variation coefficient, whereas the values in Carex lasiocarpa, Carex meyeriana and Deyeuxia angustifolia marshes varied from 21.6 to 66.5 (39.61%), from 17.2 to 45.0 (29.26%) and from 19.1 to 33.0 mg CH(4) m(-2)h(-1) (17.51%), respectively. Both the flux rates and spatial variation of CH(4) emissions strongly increased as standing water depth increased significantly. Standing water depth greatly governed the spatial variation of CH(4) emissions from different marshes in a region by changing the amount of plant litters inundated in standing water, which provided labile organic C for methanogens and controlled CH(4) concentrations in porewater. Moreover, the aboveground plant biomass determined spatial variation of CH(4) emissions from plots within a certain marsh by controlling the pathways (stem density) of CH(4) emissions from the marsh into the atmosphere.

Biomass↗
PubMed2003

Spatial indoor radon distribution in Mexico City.

We present a spatial analysis of residential radon concentrations in the Mexico City Metropolitan Area, which we intend to use to assign radon exposure in an ongoing case-control study. As part of a probabilistic household survey, carried out between May and June 1999, 501 dwellings were selected for indoor placement of solid state nuclear track detectors (LR 115) in a cup array over a period of approximately 90 days. As part of the sampling design, the city was grid partitioned into nine zones and a sample of dwellings was selected in each zone. All zones were simultaneously surveyed. The stratified sampling design allowed us to obtain radon geometric means, adjusted for household characteristics, week of detector placement and number of days of measurement for these zones. Additionally, adjusted geometric means were estimated for the 100 census tracts surveyed and this information was used to obtain a more detailed spatial distribution of residential radon levels through kriging interpolation and surface contouring. Radon levels depended on the room of placement, the floor level and the ventilation habits but not on building materials. Regarding the city zone, the highest adjusted geometric mean was found in the southwest (136 Bqm(-3)), where 46% of the households had an estimated radon level in excess of 200 Bqm(-3). In the rest of the city, the geometric mean concentration ranged between 41 and 98 Bqm(-3). A more detailed spatial distribution showed that, in general, most of the eastern and middle zones of the city had estimated radon geometric means below 74 Bqm(-3), while the western ones had geometric means above this concentration. Very high geometric means, exceeding 111 Bqm(-3) and even reaching 288 Bqm(-3), are estimated for some areas located in the southern and western zones of Mexico City. The obtained spatial distribution shows that the areas with very high estimated residential radon concentrations are close to inactive volcanic mountains. We believe that the geo-statistical techniques, we have used, offer reasonably good estimates of the average spatial residential radon distribution in Mexico City under average ventilation in homes. The use of this indirect approach for radon exposure measurement in epidemiological studies is an inexpensive alternative to direct radon exposure measurement but may be subject to non-differential misclassification error. The effect of such error on the detection of a real increase in lung cancer risk from indoor radon remains to be determined.

Air Pollution, Indoor↗
PubMed2000

Spatial statistics and geographical information systems in epidemiology and public health.

This chapter surveys the principles behind spatial statistics and geographic information systems (GIS), and their application to epidemiology and public health. Like the other introductory chapters, it is aimed mainly to facilitate understanding in the chapters specific to certain diseases that follow, and to provide a short introduction to the field. A brief overview of spatial statistics and GIS is provided in the introduction. The sections that follow explore the ways in which we can map the distribution of disease, ways in which we can look for spatial patterns in the distribution of disease, and ways in which we can apply spatial statistics and GIS to the problem of identifying the causal factors of observed patterns. In the last section I discuss some of the ways in which these techniques have been applied to assist decision making for disease intervention, and conclude by discussing future developments in the field, and some of the issues surrounding the integration of spatial statistics and GIS.

Animals↗