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At least 1,747 records · Page 97Linked to original sources
PubMed2006

Effect of connectivity and bistability on the visual potentials evoked by illusory figures.

The present study aimed at testing functional hypotheses regarding two brain potentials elicited by illusory figures. Accordingly, the N1 potential indexes mechanisms connecting the separate parts of the illusory form, whereas a subsequent negative potential indexes compensatory processes triggered by perceptual difficulty. Here, perceptual difficulty was induced by bistability; that is, by equating the probability of perceiving the illusory form to that of perceiving the independent separate parts. We compared the brain potentials evoked by a strongly connected illusory square, with almost no bistability, with those evoked by a weakly connected illusory square presenting strong bistability. Consistent with our hypotheses, the latter figure evoked the smallest N1 and a larger negative component peaking at 360 ms (N360). These results strengthen the link between N1 and connection and between negativity to perceptual difficulty and perceptual difficulty.

Adult↗
PubMed2005

Self-channeling of ultrashort laser pulses in materials with anomalous dispersion.

The nonlinear dynamics of femtosecond optical pulses propagating in solid media with anomalous group-velocity dispersion (GVD) is investigated. A map fixing the boundaries of collapse or noncollapse regimes for high-power beams versus the relative strength of GVD is first established. Next, from a nonlinear Schrödinger model accounting for higher-order dispersion, self-steepening, and plasma generation, the possibility of producing extended collapse events that promote a long self-guiding is confirmed, in agreement with recent experiments [K.D. Moll and A.L. Gaeta, Opt. Lett. 29, 995 (2004)]. Three-dimensional collapsing pulses are shown to propagate by emitting quasiperiodically bursts of temporally compressed light bullets, with durations close to the single cycle limit.

Journal Article↗
PubMed2005

Mapping dark matter with cosmic magnification.

We develop a new tool to generate statistically precise dark matter maps from the cosmic magnification of galaxies with distance estimates. We show how to overcome the intrinsic clustering problem using the slope of the luminosity function, because magnification changes strongly over the luminosity function, while intrinsic clustering only changes weakly. This may allow precision cosmology beyond most current systematic limitations. The Square Kilometre Array is able to reconstruct the projected matter density map at smoothing scale approximately 10' with S/N > or = 1, at the rate of 200-4000 deg2 per year, depending on the abundance and evolution of 21 cm emitting galaxies. This power of mapping dark matter is comparable to, or even better than, that of cosmic shear from deep optical surveys or 21 cm surveys.

Journal Article↗
PubMed2005

Cortical representation of space around the blind spot.

The neural mechanism that mediates perceptual filling-in of the blind spot is still under discussion. One hypothesis proposes that the cortical representation of the blind spot is activated only under conditions that elicit perceptual filling-in and requires congruent stimulation on both sides of the blind spot. Alternatively, the passive remapping hypothesis proposes that inputs from regions surrounding the blind spot infiltrate the representation of the blind spot in cortex. This theory predicts that independent stimuli presented to the left and right of the blind spot should lead to neighboring/overlapping activations in visual cortex when the blind-spot eye is stimulated but separated activations when the fellow eye is stimulated. Using functional MRI, we directly tested the remapping hypothesis by presenting flickering checkerboard wedges to the left or right of the spatial location of the blind spot, either to the blind-spot eye or to the fellow eye. Irrespective of which eye was stimulated, we found separate activations corresponding to the left and right wedges. We identified the centroid of the activations on a cortical flat map and measured the distance between activations. Distance measures of the cortical gap across the blind spot were accurate and reliable (mean distance: 6-8 mm across subjects, SD approximately 1 mm within subjects). Contrary to the predictions of the remapping hypothesis, cortical distances between activations to the two wedges were equally large for the blind-spot eye and fellow eye in areas V1 and V2/V3. Remapping therefore appears unlikely to account for perceptual filling-in at an early cortical level.

Adult↗
PubMed1982

Neural correlates of optokinetic nystagmus in the mesencephalon of Rana pipiens: a functional analysis.

The effects of lesions of the anuran mesencephalic retinal terminal fields on horizontal optokinetic nystagmus (OKN) were examined. Lesion sites which produced effects upon OKN responses were as follows: BOR, nBOR, peri-nBOR, the large-celled pretectal nucleus, and the dorsal tegmental gray and deep tectal layers. Transection of BOR generally resulted in an increase in saccadic frequently at the lower stimulus velocities. Lesions of nBOR produced a decrease in the frequency of both head and eye saccades in the middle to high range of stimulus velocities. The only lesions which totally abolished horizontal OKN were those located medical to nBOR, in the peri-nBOR region. Lesions of the large-celled pretectal nucleus and dorsal tegmental gray substantially reduced both head and eye saccades at all stimulus velocities. Small lesions in the deep tectal layers also depressed OKN frequency. These studies indicate that horizontal OKN may be mediated by a number of structures within the anuran mesencephalon.

Animals↗
PubMed2004

Imaging spectrograph for interstellar shocks: a narrowband imaging payload for the far ultraviolet.

We present an imaging spectrometer developed for narrowband imaging at 1035 A with high (approximately 1-arc sec) spatial resolution over a modest field of view (approximately 5 arc min). The instrument is based on a conventional Gregorian telescope with aberration-corrected holographic rulings on the secondary optic. These aberration-correcting rulings enable stigmatic imaging in diffracted light with a minimum number of optical elements, thereby maintaining a high system efficiency. The capabilities of this instrument allow us to map the distribution of UV-emitting material in the hot (approximately 300,000 K) plasma from shocks in supernova remnants. Although this design is optimized for imaging near 1035 A, the basic concept can be applied to provide narrowband imaging or long-slit imaging spectroscopy at any wavelength. In addition, a larger field of view is possible with a corresponding loss in spatial resolution.

Journal Article↗
PubMed2006

Boundaryless finite-difference method for three-dimensional beam propagation.

A two-dimensional optical field paraxial propagation scheme, in Cartesian and cylindrical coordinate systems, is proposed. This is achieved by extending the method originally proposed by Ladouceur [Opt. Lett. 21, 4 (1996)] for boundaryless beam propagation to two-dimensional optical wave fields. With this formulation the arbitrary choice of physical window size is avoided by mapping the infinite transverse dimensions into a finite-size domain with an appropriate change of variables, thus avoiding the energy loss through the artificial physical boundary that is usually required for the absorbing or the transparent boundary approach.

Journal Article↗
PubMed2004

Propagation of a femtosecond pulse in a microresonator visualized in time.

A noninvasive pulse-tracking technique has been exploited to observe the time-resolved motion of an ultrashort light pulse within an integrated optical microresonator. We follow a pulse as it completes several round trips in the resonator, directly mapping the resonator modes in space and time. Our time-dependent and phase-sensitive measurement provides direct access to the angular group and phase velocity of the modes in the resonator. From the measurement the coupling constants between the access waveguides and the resonator are retrieved while at the same time the loss mechanisms throughout the structure are directly visualized.

Journal Article↗
PubMed1999

Relative number of cells projecting from contralateral and ipsilateral nucleus isthmi to loci in the optic tectum is dependent on visuotopic location: horseradish peroxidase study in the leopard frog.

The leopard frog optic tectum is the principal target of the contralateral retina. The retinal terminals form a topographic map of the visual field. The tectum also receives bilateral topographic input from a midbrain structure called nucleus isthmi. In this study we determined the relative strength of n. isthmi projections to different loci in the tectum. Horseradish peroxidase (HRP) was applied at single superficial tectal locations in a series of leopard frogs. The application sites were distributed across the tectum. Retrogradely filled cells were counted in ipsilateral and contralateral nucleus isthmi. Although all regions of the tectum receive input from both n. isthmi, the relative number of labeled cells in the two n. isthmi is dependent on visuotopic location. Input to the rostromedial tectum representing the visual field ipsilateral to the labeled tectum comes primarily from the contralateral n. isthmi. Input to the caudolateral tectum representing the visual field contralateral to the labeled tectum originates mostly from the ipsilateral n. isthmi. Tectal application sites representing the visual midline had approximately equal numbers of labeled cells in the two n. isthmi. The results are similar at postapplication survival times ranging from 2 to 14 days. Using application of HRP to rostral tectum and application of nuclear yellow to caudal tectum, we show that the anisotropy in isthmi labeling is not due to take up of these labels by isthmotectal fibers passing through the application sites that terminate elsewhere.

Animals↗
PubMed2003

Mapping liquid distribution in absorbent incontinence products.

This paper reviews methods available for mapping the distribution of fluid in incontinence pad materials to assist with evaluating existing products and developing new ones, and to provide data for building and validating predictive models. Specifically, the following technologies are considered and their strengths and limitations described: discrete sensors based on conductance, temperature or optical measurements, optical imaging, gravimetric methods, X-ray imaging and magnetic resonance imaging. It is suggested that the ideal method would enable fluid distribution to be mapped in three dimensions with good spatial and time resolution in single materials and composite structures of simple and complex geometries under static and dynamic mechanical loading. It would also allow liquid to be mapped in products when worn by users. It is concluded that, although each existing method meets some of these requirements, and each requirement is met, at least reasonably well, by at least one method, improved techniques are needed. The particular need for methods that can provide some measurement of liquid saturation within absorbent products, both in the laboratory and in real use, is highlighted. In many cases, simple methods used appropriately are sufficient to elicit the important aspects of liquid transport and storage within absorbent products.

Absorption↗
PubMed2000

OPA1, encoding a dynamin-related GTPase, is mutated in autosomal dominant optic atrophy linked to chromosome 3q28.

Autosomal dominant optic atrophy (ADOA) is the most prevalent hereditary optic neuropathy resulting in progressive loss of visual acuity, centrocoecal scotoma and bilateral temporal atrophy of the optic nerve with an onset within the first two decades of life. The predominant locus for this disorder (OPA1; MIM 165500) has been mapped to a 1.4-cM interval on chromosome 3q28-q29 flanked by markers D3S3669 and D3S3562 (ref. 3). We established a PAC contig covering the entire OPA1 candidate region of approximately 1 Mb and a sequence skimming approach allowed us to identify a gene encoding a polypeptide of 960 amino acids with homology to dynamin-related GTPases. The gene comprises 28 coding exons and spans more than 40 kb of genomic sequence. Upon sequence analysis, we identified mutations in seven independent families with ADOA. The mutations include missense and nonsense alterations, deletions and insertions, which all segregate with the disease in these families. Because most mutations probably represent null alleles, dominant inheritance of the disease may result from haploinsufficiency of OPA1. OPA1 is widely expressed and is most abundant in the retina. The presence of consensus signal peptide sequences suggests that the product of the gene OPA1 is targeted to mitochondria and may exert its function in mitochondrial biogenesis and stabilization of mitochondrial membrane integrity.

Amino Acid Sequence↗