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Results for “spatial localization”

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At least 145 records · Page 8Linked to original sources
PubMed2001

An analysis of perceptual errors in reading mammograms using quasi-local spatial frequency spectra.

In this pilot study the authors examined areas on a mammogram that attracted the visual attention of experienced mammographers and mammography fellows, as well as areas that were reported to contain a malignant lesion, and, based on their spatial frequency spectrum, they characterized these areas by the type of decision outcome that they yielded: true-positives (TP), false-positives (FP), true-negatives (TN), and false-negatives (FN). Five 2-view (craniocaudal and medial-lateral oblique) mammogram cases were examined by 8 experienced observers, and the eye position of the observers was tracked. The observers were asked to report the location and nature of any malignant lesions present in the case. The authors analyzed each area in which either the observer made a decision or in which the observer had prolonged (>1,000 ms) visual dwell using wavelet packets, and characterized these areas in terms of the energy contents of each spatial frequency band. It was shown that each decision outcome is characterized by a specific profile in the spatial frequency domain, and that these profiles are significantly different from one another. As a consequence of these differences, the profiles can be used to determine which type of decision a given observer will make when examining the area. Computer-assisted perception correctly predicted up to 64% of the TPs made by the observers, 77% of the FPs, and 70% of the TNs.

Breast Neoplasms↗
PubMed1993

On the time course and accuracy of spatial localization: basic data and a two-process model.

This article addresses the question how fast and accurate the location of a single stimulus can be perceived. In Experiment 1, we measured localization performance in a task which required subjects to perceive and report the location of a single target stimulus ('*' sign) presented in one square of an imaginary 25 x 19 grid. Two factors were varied: stimulus duration and stimulus eccentricity. Stimulus duration was manipulated by employing a backward masking stimulus. Ten intervals (stimulus onset asynchronies) separated target and masking stimulus: 25, 50, 75, 100, 125, 150, 200, 250, 300, and 350 ms. Stimulus eccentricity was manipulated by presenting the target stimulus at five different distances from the fixation point. The observer localized the target stimulus by moving the cursor from the middle of the grid (the initial fixation point) to the perceived target location by pressing the 'arrow' keys on the keyboard. Localization performance showed to be typically related to stimulus duration. That is, two components could be distinguished: The first component represented an initial steep rise in localization performance during the first 50 ms of stimulus duration; the second component represented a gradual rise in localization performance after 50 ms, reaching maximal performance at about 300 ms. We interpreted these two localization performance functions as reflecting the operation of two systems, namely the attentional system for the initial strong increase and the eye movement system for the subsequent gradual increase. In Experiment 2, we measured saccadic eye response latencies to clarify the role of eye movements in localization performance. It was found that in 98.4% of all trials saccades were executed, and, moreover, that saccadic eye response latency decreased with increasing stimulus duration. In Experiment 3, we compared localization performance in the absence and presence of eye movements and demonstrated that localization performance for stimulus durations up to 50 ms was independent of eye movements. Overall, the present findings were interpreted as evidence in support of a two-process model of localization performance in which a shift of attention is followed by a rapid eye movement toward the target location. In line with a continuous flow conception of visual information processing, our model assumes that location information takes time to develop in the visual system; hence, an observer's localization response may be based on qualitatively different processes operating on qualitatively different kinds of information. In case of short duration stimuli, information conveyed by transient cells is used by the attentional system to shift attention toward the target location; this results in course location information being available.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1985

Colliding targets: evidence for spatial localization within the motion system.

The ability to judge the relative location of moving targets is seriously degraded if the targets move in different directions. The vernier acuity for a target in which the two components move in the same direction is not impaired until target velocity exceeds about 4 deg/sec. If the components are moving along trajectories which differ in direction by more than 15 deg, vernier thresholds rise significantly at target speeds greater than 1 deg/sec. The conditions which affect stationary vernier acuity, i.e. separation of target components, duration, and orientation, do not account for the loss in acuity. Our results suggest that localization for moving targets depends on directionally-selective motion detectors.

Female↗
PubMed1985

Accuracy of spatial localizations near the time of saccadic eye movements.

Two-dimensional eye movements were recorded while subjects used a hammer to strike targets that were flashed-on briefly before, during or up to 750 msec after a horizontal saccade. Mean position of hammer blows was 20 min arc (SD = 67 min arc) from the target when the only cue to target location was eye position. Position of responses varied slightly with time of target exposure relative to the saccade. These results show that observers can closely monitor small changes in eye position during and near the time of saccadic eye movements.

Eye Movements↗
PubMed1996

Hyperacuity for spatial localization of contrast-modulated patterns.

The acuity for localizing the position of a grating and other first order patterns which are defined directly by the luminance distribution, is much higher than the resolution for such gratings. This well-described phenomenon usually is referred to as hyperacuity, and is regarded as a cortical function which is not limited by the optics and the sampling properties of the eye. Second order patterns which can be defined by the distribution of local contrast gained some interest because they require more complex processing mechanisms than first order patterns. We investigated how well gratings and bars which are exclusively defined by the variation of the local contrast of static random dot patterns can be localized in space. In this case localization acuity does not reach the precision which is known for first order patterns. However, the localization of contrast-modulated patterns can be almost one order of magnitude better than second order grating resolution, and therefore reaches into the hyperacuity range. In combination with findings for motion-defined or stereo-defined patterns it is concluded that the brain mechanisms responsible for the localization of features in the visual scene have not only access to first order information which is available immediately from the retinal image, but in addition, to second order information which has to be extracted from the retinal intensity distribution by some sort of nonlinear processing.

Adult↗
PubMed1996

Primitives used in the spatial localization of nonabutting stimuli: peaks or centroids.

In order to determine whether simple luminance profiles are located by their peaks or centroids we performed a three element alignment task where the central element's degree of luminance asymmetry was randomly chosen from a flat distribution (skew noise). The central element with its randomly chosen skew was either positioned using the peak or centroid of its distribution. Accuracy is invariant with the magnitude of the skew noise for the centroid but not the peak condition. We conclude that the human visual system assigns position tags using centroids not peaks of luminance distributions for gabors. However this is not the case for Gaussian blobs, where a measure closer to the midpoint is used for our stimulus arrangement.

Humans↗
PubMed1993

Temporal and spatial localization of steroidogenic enzymes in premenopausal human ovaries: in situ hybridization and immunohistochemical study.

In situ hybridization and immunohistochemical localization of cytochrome P450 cholesterol side-chain cleavage (P450scc), 3 beta-hydroxysteroid dehydrogenase (3 beta HSD), cytochrome P450 17 alpha-hydroxylase (P450c17) and cytochrome P450 aromatase (P450arom) was performed in 50 morphologically normal human premenopausal ovaries, and correlated these findings with their endometrial phase. In general, mRNA expression of these enzymes examined by in situ hybridization were in good agreement with immunolocalization examined by immunohistochemistry. Expression of P450scc, 3 beta HSD and P450c17 was observed in large-sized preantral follicles, consisting of more than five layers of granulosa cells, preovulatory follicles, corpora lutea, and some degenerating corpora lutea and atretic follicles in all endometrial phases. Several follicles and/or corpora lutea positive for these enzymes were observed in the same ovary. Expression of P450arom was generally observed in only one follicle (antral or preovulatory follicle) or corpus luteum per case in mid proliferative to premenstrual phase, and was not observed in menstrual to early proliferative phase. These findings indicated that (1) expression of steroidogenic enzymes was associated with the continual human ovarian process including follicular development and atresia, and (2) especially, P450arom expression may occur only in a selected antral follicle and may have an important role in dominant follicular development.

3-Hydroxysteroid Dehydrogenases↗
PubMed1993

Spatially localized neuronal cell lineages in the developing mammalian forebrain.

The role of cell lineage in the organization of the cerebral cortex and striatum of the developing rat forebrain was analysed using retroviral-mediated gene transfer to mark the progeny of individual progenitors. Injections around the onset of neurogenesis (embryonic day 14) produced neuronal- and glial-specific clones in the striatum and cortex. The majority of the neuronal clones were restricted to either the deep or superficial layers of the cortex and to either the striatal patch or matrix compartments of the striatum. Moreover, modeling the distributions of the neuronal clones in various ways revealed that grouping the clones into deep vs superficial cortical compartments and patch vs matrix striatal compartments best accounted for the clone distributions. These results suggest that at the onset of neurogenesis there is a heterogeneity of neuronal progenitors within the proliferative ventricular zone.

Animals↗
PubMed1990

Distinct spatial localization of specific mRNAs in cultured sympathetic neurons.

We examined the subcellular distribution of specific mRNAs in cultured sympathetic neurons. Under appropriate conditions, sympathetic neurons extend both axons and dendrites that are distinguishable by light microscopic and immunocytochemical criteria. In situ hybridization revealed a differential localization of mRNA within dendrites. mRNA encoding MAP2 was abundant in cell bodies and distributed nonhomogeneously throughout the dendritic compartment, but was not detected in axons. In contrast, mRNAs encoding GAP-43 and alpha-tubulin were restricted to the cell body and largely excluded from dendrites as well as axons. Detergent extraction revealed that most dendrite-associated mRNA encoding MAP2 was associated with the Triton X-100 insoluble fraction of the cell. The subset of mRNAs present in the dendritic compartment may encode proteins involved in the morphogenesis and remodeling of dendrites.

Animals↗
PubMed1994

Spatial localization of pre-mRNA transcription and processing within the nucleus.

The organization of transcription, processing, and transport of pre-mRNA within the nucleus is a major unsolved problem in cell biology. Several recent studies have helped to define the localization of specific DNAs, RNAs, and proteins within the nucleus and have led to various models for higher level organization of pre-mRNA metabolism.

Animals↗
PubMed1995

Lateral neck imaging for spatial localization of parathyroid tissue.

Two patients with an ectopic parathyroid adenoma are described. In both cases the lesions were clearly demonstrated in an anterior view on T1-201/Tc-99m scintigraphy, but were not identified during the first surgical exploration. Failure to identify the ectopic adenoma at surgery in the second patient led to oblique and lateral views being obtained. These views permitted correct depth localization. We, therefore, believe that lateral and oblique views can be of assistance in improving preoperative localization of an ectopic parathyroid adenoma.

Adenoma↗