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PubMed2000

Analysis of spatial patterns of phase in neocortical gamma EEGs in rabbit.

Arrays of 64 electrodes (8 x 8, 7 x 7 mm) were implanted epidurally on the surface of the visual, auditory or somatosensory cortex of rabbits trained to discriminate conditioned stimuli in the corresponding modality. The 64 electroencephalographic (EEG) traces at all times displayed a high degree of spatial coherence in wave form, averaging >90% of the variance in the largest principal components analysis component. The EEGs were decomposed with the fast Fourier transform (FFT) to give the spatial distributions of amplitude and phase modulation (AM and PM) in segments 128 ms in duration. Spatial (2-dimensional) and temporal (1-dimensional) filters were designed to optimize classification of the spatial AM patterns in the gamma range (20-80 Hz) with respect to discriminative conditioned stimuli. No evidence was found for stimulus-dependent classification of the spatial PM patterns. Instead some spatial PM distributions conformed to the pattern of a cone. The location and sign (maximal lead or lag) of the conic apex varied randomly with each recurrence. The slope of the phase gradient varied in a range corresponding to that of the conduction velocities reported of axons to extend parallel to the cortical surfaces. The durations and times of recurrence of the phase cones corresponded to those of the optimally classified spatial AM patterns. The interpretation is advanced that the phase cones are manifestations of state transitions in the mesoscopic dynamics of sensory cortices by which the intermittent AM patterns are formed. The phase cones show that the gamma EEG spatial coherence is not due to volume conduction from a single deep-lying dipole generator nor to activity at the site of the reference lead on monopolar recording. The random variation of the apical sign shows that gamma AM patterns are self-organized and are not imposed by thalamic pacemakers. The half-power radius of the phase gradient provides a useful measure of the soft boundary condition for the formation and read-out of cooperative cortical domains responsible for binding sensory information into the context of prior experience in the process of perception.

Animals↗

PubMed2002

Contrast-dependent changes in spatial frequency tuning of macaque V1 neurons: effects of a changing receptive field size.

Previous studies on single neurons in primary visual cortex have reported that selectivity for orientation and spatial frequency tuning do not change with stimulus contrast. The prevailing hypothesis is that contrast scales the response magnitude but does not differentially affect particular stimuli. Models where responses are normalized over contrast to maintain constant tuning for parameters such as orientation and spatial frequency have been proposed to explain these results. However, our results indicate that a fundamental property of receptive field organization, spatial summation, is not contrast invariant. We examined the spatial frequency tuning of cells that show contrast-dependent changes in spatial summation and have found that spatial frequency selectivity also depends on stimulus contrast. These results indicate that contrast changes in the spatial frequency tuning curves result from spatial reorganization of the receptive field.

Animals↗

PubMed2005

Implicit and explicit memory for spatial information in Alzheimer's disease.

There is abundant evidence that memory impairment in dementia in patients with Alzheimer's disease (AD) is related to explicit, conscious forms of memory, whereas implicit, unconscious forms of memory function remain relatively intact or are less severely affected. Only a few studies have been performed on spatial memory function in AD, showing that AD patients' explicit spatial memory is impaired, possibly related to hippocampal dysfunction. However, studies on implicit spatial memory in AD are lacking. The current study set out to investigate implicit and explicit spatial memory in AD patients (n=18) using an ecologically valid computer task, in which participants had to remember the locations of various objects in common rooms. The contribution of implicit and explicit memory functions was estimated by means of the process dissociation procedure. The results show that explicit spatial memory is impaired in AD patients compared with a control group (n=21). However, no group difference was found on implicit spatial function. This indicates that spared implicit memory in AD extends to the spatial domain, while the explicit spatial memory function deteriorates. Clinically, this finding might be relevant, in that an intact implicit memory function might be helpful in overcoming problems in explicit processing.

Aged↗

PubMed2001

Local and global visual grouping: tuning for spatial frequency and contrast.

Glass patterns are visual textures composed of a field of dot pairs (dipoles) whose orientations are determined by a simple geometrical transformation, such as a rotation. Detection of structure in these patterns requires the observer to perform local grouping (to find dipoles) and global grouping to combine their orientations into a percept of overall shape. We estimated the spatial frequency tuning of these grouping processes by measuring signal-to-noise detection thresholds for Glass patterns composed of spatially narrow-band elements. Local tuning was probed by varying the spatial frequency difference between the two elements comprising each dipole. Global tuning was estimated using dipoles containing one spatial frequency and then estimating masking as a function of the spatial frequency of randomly positioned noise elements. We report that the tuning of local grouping is band-pass (ie, it is responsive to a narrow range of spatial frequencies), but that tuning of global grouping is broad and low-pass (ie, it integrates across a broader range of lower spatial frequencies). Control experiments examined how the contrast and visibility of elements might contribute to these findings. Local grouping proved to be more resistant to local contrast variation than global grouping. We conclude that local grouping is consistent with the use of simple-oriented filtering mechanisms. Global grouping seems to depend more on the visibility of elements that can be affected by both spatial frequency and contrast.

Contrast Sensitivity↗

PubMed2006

A cortical pooling model of spatial summation for perimetric stimuli.

Contemporary models of perimetric sensitivity assume probability summation of retinal ganglion cell sensitivities, ignoring cortical processing. To assess the role of cortical processing in perimetric spatial summation, we used a common form of multiple-mechanism spatial vision model in which the stimulus is sampled by receptive fields analogous to those of simple cells in primary visual cortex. Psychophysical threshold was computed by probability summation across the receptive fields. When the receptive fields were nonoriented (like ganglion cells), the spatial summation function had a large nonmonotonic transitional region that was inconsistent with perimetric spatial summation data. When the receptive fields were orientation tuned (like cortical cells), the model was able to give good fits to perimetric spatial summation data. The predictions of the model were evaluated with a masking study, in which noise masks either enlarged the critical area or changed the shape of the spatial summation functions. We conclude that cortical pooling by multiple spatial mechanisms can account for perimetric spatial summation, whereas probability summation across ganglion cells cannot.

Artifacts↗

PubMed2004

Spatial properties of photophobia.

PURPOSE: To determine the spatial properties of stimuli that elicit photophobia (PP) in normal subjects: Does PP exhibit spatial summation? Are different parafoveal quadrants (superior, inferior, temporal, and nasal) of the retina differentially sensitive in PP? What is the relationship between PP sensitivity and retinal eccentricity? What is the relationship between the spatial properties of PP and the spatial distribution of macular pigment (MP)? METHODS: A Maxwellian-view optical system with a xenon light source was used to present the stimuli. Four normal subjects viewed stimuli of various sizes, retinal locations, and one of two chromatic contents: xenon-white and a broadband orange. The intensity of the test stimulus was increased between trials until the PP threshold was reached. The squinting response corresponding to PP was assessed by electromyography and used as an objective criterion of PP. Three parameters were examined: stimulus size, parafoveal retinal locus (superior, inferior, temporal, and nasal), and retinal eccentricity (extending into the perifovea). Spatial profiles of MP were measured psychophysically using heterochromatic flicker photometry (HFP). RESULTS: Spatial summation for PP was found essentially to adhere to Piper's law (radiance proportional to square root of stimulus area). The PP response was greater to centrally than peripherally viewed targets. In this regard, MP acted as a spatially integrated filter in the attenuation of PP. CONCLUSIONS: The degree of spatial summation found for PP indicates that an increase of 1.0 log unit in field area results in an approximately 0.57-log-unit decrease in the radiance required to elicit PP. PP appears to serve the function of retinal photoprotection.

Adult↗

PubMed2005

Opportunities for using spatial property assessment data in air pollution exposure assessments.

BACKGROUND: Many epidemiological studies examining the relationships between adverse health outcomes and exposure to air pollutants use ambient air pollution measurements as a proxy for personal exposure levels. When pollution levels vary at neighbourhood levels, using ambient pollution data from sparsely located fixed monitors may inadequately capture the spatial variation in ambient pollution. A major constraint to moving toward exposure assessments and epidemiological studies of air pollution at a neighbourhood level is the lack of readily available data at appropriate spatial resolutions. Spatial property assessment data are widely available in North America and may provide an opportunity for developing neighbourhood level air pollution exposure assessments. RESULTS: This paper provides a detailed description of spatial property assessment data available in the Pacific Northwest of Canada and the United States, and provides examples of potential applications of spatial property assessment data for improving air pollution exposure assessment at the neighbourhood scale, including: (1) creating variables for use in land use regression modelling of neighbourhood levels of ambient air pollution; (2) enhancing wood smoke exposure estimates by mapping fireplace locations; and (3) using data available on individual building characteristics to produce a regional air pollution infiltration model. CONCLUSION: Spatial property assessment data are an extremely detailed data source at a fine spatial resolution, and therefore a source of information that could improve the quality and spatial resolution of current air pollution exposure assessments.

Journal Article↗

PubMed2006

Geostatistical analysis of disease data: visualization and propagation of spatial uncertainty in cancer mortality risk using Poisson kriging and p-field simulation.

BACKGROUND: Smoothing methods have been developed to improve the reliability of risk cancer estimates from sparsely populated geographical entities. Filtering local details of the spatial variation of the risk leads however to the detection of larger clusters of low or high cancer risk while most spatial outliers are filtered out. Static maps of risk estimates and the associated prediction variance also fail to depict the uncertainty attached to the spatial distribution of risk values and does not allow its propagation through local cluster analysis. This paper presents a geostatistical methodology to generate multiple realizations of the spatial distribution of risk values. These maps are then fed into spatial operators, such as in local cluster analysis, allowing one to assess how risk spatial uncertainty translates into uncertainty about the location of spatial clusters and outliers. This novel approach is applied to age-adjusted breast and pancreatic cancer mortality rates recorded for white females in 295 US counties of the Northeast (1970-1994). A public-domain executable with example datasets is provided. RESULTS: Geostatistical simulation generates risk maps that are more variable than the smooth risk map estimated by Poisson kriging and reproduce better the spatial pattern captured by the risk semivariogram model. Local cluster analysis of the set of simulated risk maps leads to a clear visualization of the lower reliability of the classification obtained for pancreatic cancer versus breast cancer: only a few counties in the large cluster of low risk detected in West Virginia and Southern Pennsylvania are significant over 90% of all simulations. On the other hand, the cluster of high breast cancer mortality in Niagara county, detected after application of Poisson kriging, appears on 60% of simulated risk maps. Sensitivity analysis shows that 500 realizations are needed to achieve a stable classification for pancreatic cancer, while convergence is reached for less than 300 realizations for breast cancer. CONCLUSION: The approach presented in this paper enables researchers to generate a set of simulated risk maps that are more realistic than a single map of smoothed mortality rates and allow the propagation of cancer risk uncertainty through local cluster analysis. Coupled with visualization and querying capabilities of geographical information systems, animated display of realizations can highlight areas that depart consistently from the general behavior observed across the region, guiding further investigation and control activities.

Cluster Analysis↗

PubMed2005

Egocentric and allocentric spatial memory in idiopathic cervical dystonia.

OBJECTIVE: To investigate whether impaired spatial memory contributes to cognitive deficits in idiopathic cervical dystonia (ICD). METHODS: The authors tested 16 untreated patients with ICD and 16 healthy controls with four variants of a classic spatial memory task, in which subjects were requested to recall the locations of 16 toy objects either immediately after their presentation or after an unfilled delay of 10 minutes. By varying the spatial relationship between subject and toy object array, we tested encoding and memory of spatial relationships between subject and toy objects (egocentric tasks) and of spatial relationships between toy objects and environment (allocentric tasks). RESULTS: In all four tasks, absolute performance of patients did not differ significantly from controls, suggesting that there are no significant spatial memory deficits in ICD. However, significant correlations between egocentric and allocentric task performance were found in patients but not in controls. CONCLUSIONS: While control subjects flexibly used egocentric or allocentric spatial representations in memory tasks, patients with idiopathic cervical dystonia (ICD) solved both task types with a single allocentric strategy. Our findings point to the existence of compensatory mechanisms between distinct neural systems supporting egocentric and allocentric spatial representations in ICD.

Adaptation, Physiological↗

PubMed1983

Independence of orientation and size in spatial discriminations.

This study of form vision explores the relationships between orientation and spatial frequency in suprathreshold discrimination tasks. Orientation discrimination thresholds for sine-wave gratings were 0.3-0.5 deg, much less than the roughly 10-24-deg orientational bandwidth of channels; spatial-frequency discrimination thresholds were 3-7%, much less than the roughly 1.2-octave spatial-frequency bandwidth of channels. We find that spatial-frequency discrimination between two gratings was as acute when the two gratings were orthogonal as when they were parallel. Orientation discrimination between two gratings was as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies. Thus orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing.

Discrimination, Psychological↗

PubMed1994

Spatial-frequency characteristics of letter identification.

To investigate the spatial-frequency components that govern letter identification we compared contrast thresholds for three types of visual stimulus (1) standard Sloan letters, (2) Sloan letters that were spatially bandpass filtered by cosine log filters, and (3) D6 patterns (sixth spatial derivatives of Gaussians). Stimuli were presented on a gray-scale display screen of a Macintosh computer-based testing system at temporal frequencies primarily of 2 and 16 Hz. Contrast thresholds were measured in two subjects with normal visual acuity with use of forced-choice staircases. Contrast sensitivity functions for standard Sloan letters and D6 patterns were comparable at a temporal frequency of 16 Hz but differed systematically at a temporal frequency of 2 Hz. The measurement of contrast sensitivity for cosine log filtered letters presented at a temporal frequency of 2 Hz indicated that the object spatial frequency of maximum sensitivity shifted to lower frequencies as letter size decreased, whereas the retinal spatial frequency of maximum sensitivity remained relatively constant. When letters were spatially bandpass filtered at a peak object spatial frequency of 2.5 cycles/letter, then contrast sensitivity functions for letter identification were equivalent to those for D6 patterns at both temporal frequencies. These results suggest that spatially filtered letters may provide a more appropriate test of visual function than do standard letter optotypes.

Adult↗

PubMed2006

Spatial and temporal properties of cone signals in alert macaque primary visual cortex.

Neurons in the lateral geniculate nucleus cannot perform the spatial color calculations necessary for color contrast and color constancy. Under neutral-adapting conditions, we mapped the cone inputs (L, M, and S) to 83 cone-opponent cells representing the central visual field of the next stage of visual processing, primary visual cortex (V1), to determine how the color signals are spatially transformed. Cone-opponent cells, constituting approximately 10% of V1 cells, formed two populations, red-green (L vs M; 66 of 83) and blue-yellow (S vs L+M; 17 of 83). Many cone-opponent cells (48 of 83) were double-opponent, with circular receptive-field centers and crescent-shaped surrounds (0.63 degree offset) that had opposite chromatic tuning to the centers and a time-to-peak 11 ms later than the centers. The remaining cone-opponent cells were either spatially opponent in only one cone system (20 of 83) or lacked spatial opponency (15 of 83). Cells lacking spatial opponency had smaller receptive fields (0.5-0.7 degrees) than spatial-opponent cell centers (approximately 1 degree). We found that red-green cells received S-cone input, which aligned with M input, and, unlike blue-yellow cells, red-green cells gave push-pull responses: receptive-field centers of red-ON cells were excited by both L increments (bright red) and M decrements (dark red) and were suppressed by both L decrements (dark green) and M increments (bright green). Excitatory responses to decrements were slightly larger than to increments, which may account for the lower detection and discrimination thresholds of decrements shown psychophysically. By virtue of their specialized receptive fields, the neurons described here spatially transform the cone signals and represent the first stage in the visual system at which spatially opponent color calculations are made.

Action Potentials↗

PubMed2005

Spatial analysis of the distribution of leprosy in the State of Ceará, Northeast Brazil.

The aim of this study was to describe spatial patterns of the distribution of leprosy and to investigate spatial clustering of incidence rates in the state of Ceará, Northeast Brazil. The average incidence rate of leprosy for the period of 1991 to 1999 was calculated for each municipality of Ceará. Maps were used to describe the spatial distribution of the disease, and spatial statistics were applied to explore large- and small-scale variations of incidence rates. Three regions were identified in which the incidence of leprosy was particularly high. A spatial gradient in the incidence rates was identified, with a tendency of high rates to be concentrated on the North-South axis in the middle region of the state. Moran's I statistic indicated that a significant spatial autocorrelation also existed. The spatial distribution of leprosy in Ceará is heterogeneous. The reasons for spatial clustering of disease rates are not known, but might be related to an heterogeneous distribution of other factors such as crowding, social inequality, and environmental characteristics which by themselves determine the transmission of Mycobacterium leprae.

Brazil↗

PubMed2006

Spatial analysis of Aedes albopictus (Diptera: Culicidae) oviposition in suburban neighborhoods of a Piedmont community in North Carolina.

Temporal and spatial distribution of egg-laying by Aedes albopictus (Skuse) (Diptera: Culicidae) was investigated in suburban neighborhoods in Raleigh, NC, by using oviposition traps (ovitraps) at fixed sampling stations during the 2002 and 2003 mosquito seasons. Variations in the phenology of oviposition between the two mosquito seasons resulted from differences in the patterns and amounts of rainfall early in the season. Aerial images of each study neighborhood were digitized, and the proportions of specific types of land cover within buffer zones encompassing ovitraps were estimated. Retrospective analyses showed that in some neighborhoods, oviposition intensity was significantly associated with specific types of land cover. However, in general, it seemed that gravid Ae. albopictus searched throughout the landscape for water-filled containers in which to lay eggs. Peridomestic surveys were carried out concurrently with ovitrap collections to estimate production of Ae. albopictus pupae in discarded water-filled containers and the abundance of females in vegetation that made up the resting habitat. Results of linear regression analyses indicated that the mean standing crop of pupae (total and per container) per residence was not a significant predictor of mean egg densities in ovitraps. However, the mean standing crop of adult females was a significant but weak predictor variable, because the magnitude and sign of regression coefficients varied between neighborhoods. Linear spatial regression analyses revealed that oviposition intensity was not spatially dependent on pupal standing crop or the numbers of pupae-positive containers distributed peridomestically. However, a weak spatial dependence on the standing crop of adult females was found in some neighborhoods. Based on spherical variogram models, kriging was carried out to predict the spatial patterns of oviposition in suburban neighborhoods. Focal areas of high and low oviposition intensity were evident in most neighborhoods; however, the spatial patterns of oviposition changed between mosquito seasons. Kriging predictions were evaluated, using cross-validation, by systematically removing each data point from our data set and predicting the removed point by using the remaining points. The root mean square (standardized) error values of best fitting variogram models approximated 1, and plots of standardized PRESS residuals showed no distinct pattern for most neighborhoods, indicating that predictions of the spatial distribution of oviposition intensity were valid. Spherical variogram models are a satisfactory method for describing the spatial distribution of Ae. albopictus oviposition, and kriging can be a useful technique for predicting oviposition intensity at locations that have not been sampled.

Aedes↗

PubMed1997

The neuropsychology of spatial cognition in the rat.

This article provides a review of the neural mechanisms of spatial cognition in the rat. A survey of the literature shows that the rat has spatial capabilities that can be explained only if one assumes that it possesses a representation of some features of the environment. The scope of such a representation may, however, be more limited than what is implied by the hypothesis of a bird's-eye view of the environment. The best documented spatial ability of the rat is illustrated by its efficiency in performing the water maze navigation task. A review of recent neurobiological data collected while a rat was performing this task suggests that several brain structures make unique contributions to spatial navigation. In particular, the hippocampal formation and the associative (posterior) parietal cortex seem to handle different aspects of navigation and to be differentially involved in the various stages of spatial memory formation. Electrophysiological data support the hypothesis that the hippocampal formation is concerned with rapidly building associative memories of spatial relationships within the environment. In contrast, the associative parietal cortex might be involved in more abstract spatial processing, resulting in a metric representation of spatial information collected during movements.

Animals↗

PubMed2003

Visual-spatial ability in Parkinson's disease.

Parkinson's Disease (PD) has traditionally been viewed as primarily a disturbance of motor functioning, typically involving tremor, rigidity, hypokinesia, gait disturbance, and postural instability. More recently, decline in cognitive function has been recognized as a feature of PD. One prominent cognitive symptom of PD involves deficits on tasks of spatial ability. However, findings of visual-spatial deficits in individuals with PD have been inconsistent. There are several methodological issues in this area of research that potentially confound the interpretation of data and need to be taken into consideration, including subject characteristics (e.g., age, sex and education), duration of illness, the current level of disability, the presence of emotional depression, the current level of medications, and the presence of dementia. Further, the tests that have shown visual-spatial deficits in PD are often complex, showing sensitivity to other cognitive processes as well. Another problem in this area of research is the lack of a clear understanding of the brain mechanisms that underlie visual-spatial deficits in PD. One theory of cognitive dysfunction in PD suggests that these cognitive deficits are in some way related to disruption of frontal-basal ganglia neural circuits important in executive functions. However, frontal-basal ganglionic dysfunction does not appear to account entirely for the visual-spatial cognitive deficits seen in PD. Subtle differences in performance on executive function measures appear to dissociate individuals with frontal lobe damage from individuals with PD. Findings from two recent studies indicate that PD is indeed associated with deficits in visual-spatial ability. These findings also indicate that the relationship between visual-spatial ability and frontal-executive function in PD is likely complex, and that the visual-spatial deficits in PD may be sensitive to the sex of the individual with PD.

Humans↗

PubMed2005

Visual and proprioceptive representations in spatial memory.

It has been shown that spatial information can be acquired from both visual and nonvisual modalities. The present study explored how spatial information from vision and proprioception was represented in memory, investigating orientation dependence of spatial memories acquired through visual and proprioceptive spatial learning. Experiment 1 examined whether visual learning alone and proprioceptive learning alone yielded orientation-dependent spatial memory. Results showed that spatial memories from both types of learning were orientation dependent. Experiment 2 explored how different orientations of the same environment were represented when they were learned visually and proprioceptively. Results showed that both visually and proprioceptively learned orientations were represented in spatial memory, suggesting that participants established two different reference systems based on each type of learning experience and interpreted the environment in terms of these two reference systems. The results provide some initial clues to how different modalities make unique contributions to spatial representations.

Adolescent↗

PubMed2005

Spatial versus object visualizers: a new characterization of visual cognitive style.

The visual system processes object properties (such as shape and color) and spatial properties (such as location and spatial relations) in distinct systems, and neuropsychological evidence reveals that mental imagery respects this distinction. The findings reported in this article demonstrate that verbalizers typically perform at an intermediate level on imagery tasks, whereas visualizers can be divided into two groups. Specifically, scores on spatial and object imagery tasks, along with a visualizer-verbalizer cognitive style questionnaire, identified a group of visualizers who scored poorly on spatial imagery tasks but excelled on object imagery tasks. In contrast, a second group of visualizers scored high on spatial imagery tasks but poorly on object imagery tasks. The results also indicate that object visualizers encode and process images holistically, as a single perceptual unit, whereas spatial visualizers generate and process images analytically, part by part. In addition, we found that scientists and engineers excel in spatial imagery and prefer spatial strategies, whereas visual artists excel in object imagery and prefer object-based strategies.

Adolescent↗