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At least 973 records · Page 54Linked to original sources
PubMed1991

The local border mechanism in grating induction.

Both White's effect and the grating induction effect are examples of brightness contrast phenomena. Models to account for these effects have either explicitly rejected local border mechanisms (such as retinal ganglion cells) in favour of cortical mechanisms, or explicitly rejected elongated cortical filters in favour of local mechanisms. We have argued that any viable model must include both classes of mechanism. In this paper we present some novel versions of induction effects, and describe the explanatory power of a model couched solely in terms of the operation of local spatial filters. The model employs filters at different spatial scales whose outputs are then averaged. Using this approach it is possible to give a good account not only for the novel demonstrations we present, but also for the pattern of results reported by others concerning various manipulations of the spatial parameters of induction displays.

Contrast Sensitivity↗
PubMed1993

Masked detection of gratings: the standard model revisited.

According to the standard model of masked detection of gratings, the observers's task is to decide whether the response of some critical mechanism is reliably increased by the addition of the test stimulus to the masker. To test this conception, experiments were performed in which the contrast levels in the masker alone and test-plus-masker intervals of a forced-choice trial were randomly perturbed on each presentation. The perturbations in the two intervals of a trial were either independent, positively correlated, or negatively correlated. Such perturbations had the expected effect on the detectability of 10 c/deg test on a 10 c/deg masker. However, they had no effect at all on the detectability of 2 or 8 c/deg test stimuli on the same masker. These results contradict the standard model of masking, but are compatible with the assumption that the test stimulus is detected by some changes it produces in the local spatial features of the phenomenal appearance of the masker.

Adult↗
PubMed1993

Processing of linear elements in stereopsis: effects of positional and orientational distinctiveness.

The stereoscopic processing of small linear elements is probed through the comparative analysis of stereograms containing needles or crosses, differing in the local spatial arrangement and orientation of the elements, and the presence or absence of slant. Depending upon the details of the textural design, depth analysis may proceed faster with crosses than with needles, or the reverse. It proceeds faster with vertical than with horizontal needles, except in the case of unslanted regularly-spaced needles. On the whole the data suggest that the elements to be matched in a stereogram are first processed along a common pathway, in which positional regularity has a detrimental effect. In the presence of small linear elements, orientation-tuned neurons would be recruited and their participation would lead either to an inhibition effect when the elements are all similarly oriented, or to a facilitation effect when there is sufficient orientational diversity among the elements. Here, slant plays an indirect role, by widening the orientation spectrum in otherwise regularly oriented textures. Positional irregularity is useful to suppress false matches, while orientational diversity helps to stabilize the perceived surfaces.

Depth Perception↗
PubMed1995

The critical role of relative luminance relations in White's effect and grating induction.

It has been proposed that both White's effect and the grating induction effect are examples of brightness induction phenomena modeled in terms of local spatial filters. We have shown that for these illusions to occur it is necessary that the luminance of the gray target elements falls between that of the inducing stripes of the square-wave pattern. This critical role of luminance relationships is not predicted by existing models of these illusions.

Color Perception↗
PubMed1996

The binocular computation of visual direction.

How is a single visual direction assigned to a binocular feature for which the left and right eyes are signaling different directions? According to geometrical principles, binocular visual direction is the average of the visual directions measured from the left and right eyes. Contrary to this prediction, we have found that the relative visual direction between two Gabor targets presented at different stereoscopic depths could be manipulated by varying the contrast ratio between the left and right images. This finding is consistent with a new model in which the relative alignment of depth features is determined from a maximum-likelihood combination of the direction signals from the left and right eyes. In a second experiment we provide support for this model, showing that the magnitude of the contrast-dependent bias in visual direction is predicted by the uncertainty for spatial localization in the left and right images. Lastly we show that visual direction and stereopsis have different dependencies on interocular contrast differences, suggesting that the computation of stereo depth and visual direction are mediated via different mechanisms.

Contrast Sensitivity↗
PubMed1976

Neuropeptide MSH/ACTH 4-10 enhances attention in the mentally retarded.

Twenty adult mentally retarded men were randomly assigned to receive MSH/ACTH 4-10 or a vehicle control solution in a double blind procedure. After an intravenous injection the subjects were presented with an orienting sequence and a series of behavioral tests. Treatment with the peptide resulted in a significant decelerative heart rate response during the test stimulus of the orienting sequence. Improved performance of the intradimensional and extradimensional shift of a visual discrimination procedure was significant for subjects receiving MSH/ACTH 4-10. The pattern of response on the subproblem analysis of the extradimensional shift reflected greater dimensional attention in the subjects treated with the peptide. Attentive subjects given MSH/ACTH 4-10 evidenced significant improvement on a rhythm matching test, a test of spatial localization and a visual retention test. The data were interpreted as indicating that the peptide resulted in improved attention of stimulus processing. It was speculated that MSH/ACTH 4-10 may be unquely coded for perceptor/attentional functioning and may be useful as a treatment for disorders of attention.

Adrenocorticotropic Hormone↗
PubMed1992

Analysis of a spatially regulated phosphotyrosine phosphatase identifies tyrosine phosphorylation as a key regulatory pathway in Dictyostelium.

We have cloned a Dictyostelium phosphotyrosine phosphatase (PTP1) with a catalytic domain showing approximately 38%-50% amino acid identity to those of other PTPs. PTP1 contains an approximately 99 amino acid insert and bacterially produced PTP1 possesses PTP activity. PTP1 is expressed at a very low level in vegetative cells, induced by 4 hr, and maximally expressed at the tight aggregate stage. PTP1-lacZ studies indicate that PTP1 is spatially localized to prestalk and anterior-like cell types. PTP1 gene disruptants show accelerated development, whereas strains overexpressing PTP1 to a high level fail to aggregate. Strains overexpressing moderate levels exhibit severe morphological defects following aggregation, including multiply tipped aggregates and morphologically aberrant fruiting bodies. Western blot analysis using anti-phosphotyrosine antibodies shows specific changes in the mutant strains when compared with wild-type cells. The results indicate that reversible protein-tyrosine phosphorylation and PTP1 play important regulatory roles during Dictyostelium development.

Amino Acid Sequence↗
PubMed1993

Spatial segregation of odorant receptor expression in the mammalian olfactory epithelium.

The signal elicited by the interaction of odorous ligands with receptors on olfactory sensory neurons must be decoded by the brain to determine which of the numerous receptors have been activated. We have examined the patterns of odorant receptor expression in the rat olfactory epithelium to determine whether the mammalian olfactory system employs spatial segregation of sensory input to encode the identity of an odorant stimulus. In situ hybridization experiments with probes for 11 different odorant receptors demonstrate that sensory neurons expressing distinct receptors are topologically segregated into a small number of broad, yet circumscribed, zones within the olfactory epithelium. Within a given zone, however, olfactory neurons expressing a specific receptor appear to be randomly distributed, rather than spatially localized. The complex mammalian olfactory system may therefore compartmentalize the epithelium into anatomically and functionally discrete units, such that each zone expresses only a subset of the entire receptor repertoire.

Animals↗
PubMed1993

Interaction between dorsal and ventral cells in the imaginal disc directs wing development in Drosophila.

The adult appendages of Drosophila develop from imaginal discs. An early step in disc patterning involves the formation of developmental boundaries that subdivide the discs into compartments. Anterior and posterior compartments are established in the embryo. Later in development a new boundary originates to subdivide the wing disc into dorsal and ventral compartments, which correspond to the dorsal and ventral surfaces of the adult wing. We report here that spatially localized expression of the homeobox gene apterous (ap) specifies the identity of dorsal cells in the wing. The boundary of cell lineage restriction between dorsal and ventral compartments coincides with the limit of the domain of ap expression. Using genetic mosaics, we show that juxtaposition of dorsal and ventral cells induces formation of the wing margin. We present evidence that the dorsal-ventral boundary promotes growth and serves as a pattern-organizing center in the wing disc.

Animals↗
PubMed1983

Central auditory processing. III. The "cocktail party" effect and anterior temporal lobectomy.

The capacity to selectively attend to only one of multiple, spatially separated. simultaneous sound sources--the "cocktail party" effect--was evaluated in normal subjects and in those with anterior temporal lobectomy using common environmental sounds. A significant deficit in this capacity was observed for those stimuli located on the side of space contralateral to the lobectomy, a finding consistent with the hypothesis that within each anterior temporal lobe is a mechanism that is normally capable of enhancing the perceptual salience of one acoustic stimulus on the opposite side of space, when other sound sources are present on that side. Damage to this mechanism also appears to be associated with a deficit of spatial localization for sounds contralateral to the lesion.

Attention↗
PubMed1989

In vivo transillumination of the hand using near infrared laser pulses and differential spectroscopy.

In vivo dual wavelength differential spectrography was performed on the hand of an adult male, using a collimated transillumination device. A pulsed laser with sufficiently high peak power and sufficiently low energy was employed so that transillumination could be realized without thermal damage. Spectrochemical analysis based on the absorbance of oxygen transporting molecules (OTM), i.e. haemoglobin in blood vessels and myoglobin in muscles, was performed along a 70 mm scanning line within the near-red infrared range. The two wavelengths used (675 and 800 nm) were chosen on the basis of the absorption spectrum of haemoglobin. The profiles computed with differential spectrography data related to the oxygen saturation of OTM are closely correlated with X-ray densitometry of the most vascularized tissues in the hand along the scanning line. In addition, at 675 nm, the profiles are rapidly modified as a function of the oxygen supply to the hand. Considering the accuracy obtained for the spatial localization of the OTM redox state, it is expected that these results could be applied to imaging.

Adult↗
PubMed1984

Long-term changes of orienting behavior in cats with artificial divergent strabismus.

Orienting behavior was measured in two monocular spatial localization tasks in normal cats and in cats with artificially induced monocular divergent strabismus. Immediately, and for several weeks after the operation, cats made large "past-pointing errors" with the deviated eye in a direction opposite to the misalignment. These errors were replaced later by "over compensation errors" in the same direction as the strabismus deviation, which persisted for many months after the operation. Over-compensation did not occur when the non-deviated eye was sutured in early life. Electrophysiological measures like grating VEP and A17 single unit responses demonstrated the dominance of the non-deviated eye in the strabismic cats. It is suggested that long-term binocular exposure may lead to a reorganization of visual direction for the deviated eye in divergent strabismus.

Amblyopia↗
PubMed1994

Covariability of visually evoked potentials and simple motor reaction times.

Visually evoked potentials (VEPs) were recorded during simple motor reaction time (RT) experiments using spatially localized, contrast modulated visual stimuli. VEPs were averaged in groups, called "subensembles," depending on the speed of the subject's motor response. To a specific visual stimulus, the VEP response time of a subensemble covaries with motor response latency according to the same linear relationship found to describe conventionally averaged VEPs and RTs over a wide range of stimuli.

Electroencephalography↗
PubMed1989

The egg came first, of course! Anterior-posterior pattern formation in Drosophila embryogenesis and oogenesis.

The anterior-posterior body pattern of the Drosophila embryo is initiated through the action of maternal gene products. In particular, three groups of maternally acting genes (the anterior, posterior and terminal groups) have been shown to direct the synthesis and spatial restriction of the three major organizing activities in the egg. The initial spatial localizations of the maternal organizing activities are established during oogenesis. After fertilization these activities regulate zygotic gene activity along the anterior-posterior axis of the egg.

Animals↗
PubMed1995

Clustering and co-localization of immunogold double labelled neural cell adhesion molecule isoforms in chick forebrain.

A quantitative immunoelectron microscopic study was carried out in the intermediate portion of hyperstriatum ventrale of chick forebrain (a region exhibiting learning-related morphological plasticity) in order to examine the fine distribution of the neural cell adhesion molecule (N-CAM). Antibodies against alpha 2,8 polysialic acid (PSA), characteristic for embryonic N-CAM, and those against the protein backbones of all N-CAM isoforms were sequentially labelled (post-embedding) with 5- and 15-nm gold particles, and binding to their epitopes was analyzed using the statistics of point processes. The results showed that: (1) PSA tends to form clusters (up to 200 nm in size), in contrast to the protein backbones of all N-CAM isoforms, and (2) the two tissue bound antibodies are co-localized spatially in clusters of sizes up to 100 nm. The data suggest that there is a spatial sub-cellular domain enriched in both PSA and protein isoforms of N-CAM.

Animals↗
PubMed1988

Structure and neural expression of a zebrafish homeobox sequence.

A genomic library of zebrafish was constructed and screened with homeobox-containing probes. One of the positive clones contains a transcribed region which shares extensive sequence homology with the murine Hox-1.4 and Hox-2.6 genes and the human HHO.c13 gene. Characterization of this zebrafish homologue (ZF-13) with respect to expression demonstrated that it is transcribed during embryogenesis where a major RNA species of 2.5 kb and a minor transcript of 4.6 kb are detected. The highest concentration of both transcripts was found in embryos at the stage of somite formation. By in situ hybridization the spatial localization of expression was analysed in hatching embryos. Hybridization signals were mainly detected throughout the neural tube and in the brain. A small amount of RNA derived from ZF-13 was localized in differentiated muscle cells. Our results suggest that homeobox genes of distantly related vertebrate species are very similar with respect to structure and function.

Animals↗
PubMedDate not supplied

NMR in medicine.

Nuclear magnetic resonance (NMR) offers a powerful new probe of the body's internal anatomy and function. The technique utilizes a combination of static and radio frequency (RF) magnetic fields to excite a weak resonant magnetic field emission from various selected naturally abundant nuclei in the body such as hydrogen (1H), phosphorus (31P) and carbon (13C). The emissions can be spatially encoded by application of magnetic field gradients, to enable the generation of high resolution anatomical images which reflect the nuclear density distribution, and/or spatial variations in the molecular level and chemical environments of the nuclei as measured by the NMR relaxation times and chemical shifts. 1H NMR relaxation time images are proving useful for the detection of a wide spectrum of disorders, whilst spatially localized 31P and 13C NMR chemical shift spectra measure directly the metabolic status of living tissue. A qualitative explanation of NMR, NMR imaging, relaxation times, and chemical shifts is presented, and the historical development, future of the technology, and its range of applications explored.

Animals↗
PubMed1982

Basic concepts for nuclear magnetic resonance imaging.

The basic concepts necessary to understand the physical basis of NMR imaging are presented in this didactic article. It is intended as a starting point for the radiologist or medical physicist who is addressing the topic of NMR for the first time. The basic of the NMR phenomena is described with introduction of the concepts of magnetic moment, magnetic fields, magnetic resonance, net magnetic moment of a sample, NMR excitation and NMR emission. The equipment necessary to observe these NMR properties of matter is summarized as well as the procedures for basic pulsed NMR experiments. The physical concepts for spatial localization of NMR emissions are introduced with physical analogies to stringed musical instruments. Several alternative imaging modalities are compared with greatest emphasis on the inversion recovery technique which yields images weighted by tissue T values. The six subsystems of an NMR imaging device (primary magnet, computer, radio equipment, magnetic gradient, data storage and display subsystems) are described in an overview fashion. The paper is followed by a series of study questions to test the reader's comprehension of basic NMR imaging concepts.

Humans↗