Narrow Bars Research Articles

Two new species of the genus Crenicichla Heckel, 1840 from the upper rio Uruguay drainage (Perciformes: Cichlidae)

Two new species of the genus Crenicichla from the upper rio Uruguay basin are described, Crenicichla hadrostigma and C. empheres. Both are included in the C. missioneira species group along with C. missioneira, C. minuano, C. tendybaguassu, C. igara, and C. jurubi, all occurring in the upper and middle rio Uruguay drainage. Crenicichla hadrostigma is distinguished by the conspicuous ocellated posttemporal blotch, absence of a humeral spot and lateral band, and the presence in males of nine to 16 narrow vertical bars extending from the dorsum to a horizontal line that passes through the lower edge of pectoral axilla. Crenicichla empheres is distinguished from the other species of the missioneira-group by the presence of six to eight horizontally elongate dark blotches along the middle portion of the body, slightly below the upper lateral line, absence of a lateral band, presence in males of numerous dark and irregularly arranged spots on nape, back, and sides of body, and isognathous mouth. A key to the species from rio Uruguay basin is presented.

A New Species of Lycodes (perciformes: Zoarcidae) from the Aleutian Islands

A new species of eelpout, Lycodes akuugun, is described from the Aleutian Islands on the basis of 11 specimens collected during bottom-trawl surveys from 1994 through 2004. This species has a distinctive color pattern, characterized by a dark brown dorsum separated from a light gray ventrum by a distinct border running along the lateral surface of the head and most of the body, as well as several distinct narrow bars on the dorsal fin and a light blotch on the posterodorsal portion of the caudal fin. It is also characterized by having an elongate body, a ventrally placed lateral line, rounded pectoral fins, and relatively high meristics, as well as the presence of an interorbital cephalic sensory pore and the absence of scales on the nape. Among sympatric congeners, L. akuugun is most similar to L. diapterus, L. brevipes, and L. concolor. It differs from all three species in color pattern and in the presence of an interorbital cephalic sensory pore. It can be distinguished from other congeners on the basis of coloration, lateral-line morphology, and meristics.

Magnetization reversal of micropattern Fe bar array: Combination of vector and Bragg magneto-optical Kerr effect measurements

We report on a magneto-optical Kerr effect (MOKE) investigation of alternating wide and narrow Fe bars, forming a two-dimensional periodic array. The magnetization reversal was studied by regular longitudinal Vector-MOKE in specular geometry as well as in Bragg-MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. With Vector-MOKE a two-step switching process for the wide and narrow Fe bars is observed, indicative for a narrow switching field distribution in this array. In addition, hysteresis loops were determined as a function of the diffraction order for the hard and easy axis direction. The loops at different orders of diffraction can qualitatively and quantitatively be described by Fourier transformations of micromagnetic simulations.

Gobiodon prolixus, a new species of gobiid fish (Teleostei: Perciformes: Gobiidae) from the Indo-west Pacific

A new species of coral-dwelling gobiid, Gobiodon prolixus, is described on the basis of material from scattered localities in the Indo-west Pacific (Comores, Rodrigues, Chagos, Thailand, Vietnam, Ponape, Tonga, Rapa, and Tuamotu). Gobiodon prolixus is distinguished from congeners by its unique, more elongate body shape in combination with five narrow blue bars on the lateral surfaces of the head. That pigmentation pattern is otherwise found in G. quinquestrigatus and G. rivulatus, species with which the new species has been confused in the past.

Frequency-narrowed diode array bar

We describe a method to frequency narrow multielement high-power diode bars. Using a commercial 60-W, 49-element, 1-cm-long diode array bar at 795 nm running at 45 W, we narrow the linewidth from 1000 to 64 GHz with only a loss of 33% in output power. The resulting laser light is well suited for spin-exchange optical pumping of noble gas nuclei.

Detection of narrow bars when presented in the right and left visual hemifields

Detection of narrow bars when presented in the right and left visual hemifields

Detection of narrow bars when presented in the right and left visual hemifields

Detection of narrow bars when presented in the right and left visual hemifields

Detection of narrow bars when presented in the right and left visual hemifields

Three series of psychophysical experiments were performed to study the effects of additional images on the contrast thresholds for detection of narrow bars of width 1 arc min and length 1 arc degree presented randomly to the left and right sides of the fixation point with an eccentricity of 4 degrees. The additional images were bars of the same size but with lower contrast, presented to the right and left of the test bar at varying distances, i.e., 1-2 degrees. Different series used different contrast ratios for the test and additional bars. The first series revealed significant predominance of one visual hemifield over the other in performing the bar detection task, though the predominant side varied in different subjects. This predominance disappeared or changed to the opposite side in the second or third series. There was a tendency for the additional images to have asymmetrical influences on the detection threshold for the test bar: additional bars had more extensive inhibitory influences when they were on the peripheral side of the test bar. These data provide evidence for the absence of specificity in hemisphere operation in the detection task. The mechanism of description of peripheral images is discussed.

Technique for patterning macroporous silicon and the fabrication of bars of 2D photonic crystals with vertical walls

A new technological process for fabricating structures with vertical walls in a thick layer of macroporous silicon on a substrate is reported. The problem of photolithography has been solved by patterning on the back side of the wafer. As a result, the silicon substrate itself serves as a mask through which certain parts of the porous layer are removed. Narrow and high bars of macroporous silicon, aligned with macropore rows, have been fabricated. The 2D photonic crystals obtained have been used for in-coupling of light in the direction perpendicular to the channel axes and for recording IR spectra.

Masking effect produced by Mach bands on the detection of narrow bars of random polarity.

Difficulties arise in measuring masking by Mach bands because very-low-contrast signals distort the bands. [J. Opt. Soc. Am. A 17, 1147 (2000).] Adding narrow luminance increments (bright bars) in the dark Mach band widens the dark band; adding decrements (dark bars) narrows the dark band, and conversely in the bright bands. Randomizing signal polarity prevents observers from using the distortion of the Mach bands as a cue to the presence of the signal. We measured (two-alternative-forced-choice) Mach bands' masking of randomly selected bright (incremental) or dark (decremental) bars. Detection was worse in both dark and bright Mach bands than on the neighboring plateaus. Separate analysis of trials containing only one polarity signal revealed 9-cycle/deg oscillations in performance as a function of location. Oscillations in the two polarities were approximately 180 degrees out of phase.

Broadband millimeter‐wave planar antenna array with a waveguide and microstrip‐feed network

AbstractIn this paper, we describe a fixed‐beam broadband planar antenna array at millimeter‐wave frequencies. The antenna is fed by waveguide‐feed and microstrip‐feed networks. In order to ensure fixed beams and a broad bandwidth, a subarray concept is introduced. For efficient coupling, a novel waveguide‐to‐slot transition consisting of a slot pair and a narrow conductive bar is proposed. Simulated and measured bandwidth is found to be more than 7.1% from 40.5–43.5 GHz. The main beam directions are broadside over all frequencies. The maximum measured antenna gain is 30.2 dBi at 42.0 GHz. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 283–287, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20279

A theory of the Benham Top based on center–surround interactions in the parvocellular pathway

A model color-opponent neuron was used to investigate the subjective colors evoked by the Benham Top (BT). Color-opponent inputs from cone-selective parvocellular (P) pathway neurons with center–surround receptive fields were subtracted with a short relative delay, yielding a small transient input in response to a white spot. This transient input was amplified by BT-like stimuli, modeled as a thin dark bar followed by full-field illumination. The narrow bar produced maximal activation of the P-pathway surrounds but only partial activation of the P-pathway centers. Due to saturation, subsequent removal of the bar had little effect on the P-pathway surrounds, whereas the transient input from the P-pathway centers was amplified via disinhibition. Responses to BT-like stimuli became weaker as surround sensitivity recovered, producing an effect analogous to the progression of perceived BT colors. Our results suggest that the BT-illusion arises because cone-selective neurons convey information about both color and luminance contrast, allowing the two signals become confounded.

Monocular discs in the occlusion zones of binocular surfaces do not have quantitative depth--a comparison with Panum's limiting case.

Da Vinci stereopsis is defined as apparent depth seen in a monocular object laterally adjacent to a binocular surface in a position consistent with its occlusion by the other eye. It is widely regarded as a new form of quantitative stereopsis because the depth seen is quantitatively related to the lateral separation of the monocular element and the binocular surface (Nakayama and Shimojo 1990 Vision Research 30 1811-1825). This can be predicted on the basis that the more separated the monocular element is from the surface the greater its minimum depth behind the surface would have to be to account for its monocular occlusion. Supporting evidence, however, has used narrow bars as the monocular elements, raising the possibility that quantitative depth as a function of separation could be attributable to Panum's limiting case (double fusion) rather than to a new form of stereopsis. We compared the depth performance of monocular objects fusible with the edge of the surface in the contralateral eye (lines) and non-fusible objects (disks) and found that, although the fusible objects showed highly quantitative depth, the disks did not, appearing behind the surface to the same degree at all separations from it. These findings indicate that, although there is a crude sense of depth for discrete monocular objects placed in a valid position for uniocular occlusion, depth is not quantitative. They also indicate that Panum's limiting case is not, as has sometimes been claimed, itself a case of da Vinci stereopsis since fusibility is a critical factor for seeing quantitative depth in discrete monocular objects relative to a binocular surface.

Timing characteristics of scintillator bars

The proposed Hall D detector at Jefferson Lab will have a time-of-flight detector composed of long and narrow scintillator bars. We have evaluated the time resolution of two bar prototypes in particle beams at the Institute for High Energy Physics in Protvino, Russia. The bars are 2.0 m long and have square cross-sections of size 2.5 and 5.0 cm 2 . In this paper, we present results on how the time resolution of each of these bars depends on the entry position of the beam into the scintillator, on the material used for scintillator wrapping and on the phototube used for the readout.

Feature detection algorithm based on a visual system model

An algorithm for the detection of visually relevant luminance features is presented. The algorithm is motivated and directed by current models of the visual system. The algorithm detects edges (sharp luminance transitions) and narrow bars (luminance cusps) and marks them with the proper polarity. The image is first bandpass filtered with oriented filters at a number of scales an octave apart. The suprathreshold image contrast details at each scale are then identified and are compared across scales to find locations in which the signal polarity (sign) is identical at all scales, representing a minimal level of phase congruence across scales. These locations maintain the polarity of the bandpass-filtered image. The result is a polarity-preserving features map representing the edges with pairs of light and dark lines or curves on corresponding sides of the contour. Similarly, bar features are detected and represented with single curves of the proper polarity. The algorithm is implemented without free (fitted) parameters. All parameters are directly derived from visual models and from measurements on human observers. The algorithm is shown to be robust with respect to variations in filter parameters and requires no use of quadrature filters or Hilbert transforms. The possible utility of such an algorithm within the visual system and in computer vision applications is discussed.

Persistent currents and dissipation in narrow bilayer quantum Hall bars

Bilayer quantum Hall states support a flow of nearly dissipationless staggered current which can only decay through collective channels. We study the dominant finite-temperature dissipation mechanism which in narrow bars is driven by thermal nucleation of pseudospin solitons. We find the finite-temperature resistivity, predict the resulting staggered current-voltage characteristics, and calculate the associated zero-temperature critical staggered current and gate voltage.

The joint contributions of saccades and ocular drift to repeated ocular fixations

We measured the joint contributions of different sized saccades and ocular drift to overall performance in an alternating fixation task. Subjects repeatedly shifted fixation between the centres of a pair of bars of width 2°–2 arc min, either mentally selected or electronically cut from a stationary sine grating display. Eye movement patterns exhibited consistent features across all displays, and pairs of widely separated bars were studied most. Variability (S.D.) and relative accuracy (under/overshooting bias) were estimated from the concentration of eye positions over the two target bars. Overall variability, i.e. for eye movements as a whole, reached a minimum of 5 min for bar widths less than 20 min across subjects, displays and tasks. This was consistent, as were several other aspects of the study, with a constant 20-min diameter goal zone hypothesis. For wide bars, overall variability increased nearly as the square root of bar width, and for narrow bars, was independent of bar separation. A typical between-bar crossing saccade was tightly constrained in departure point but widely scattered in landing position, the associated variability increasing with bar separation. The final high overall precision was achieved largely by within-bar saccades of greater than 7.5 min effective amplitude that were present at a rate of 1 (range 0.3–3) per crossing saccade. This is consistent with views that very small saccades (the smaller microsaccades) make little obvious contribution to oculomotor performance.

Off-axis electron holography of patterned magnetic nanostructures.

Magnetization reversal processes in lithographically patterned magnetic elements that have lateral dimensions of 70-500 nm, thicknesses of 3-30 nm and a wide range of shapes and layer sequences have been followed in situ using off-axis electron holography in the transmission electron microscope. This technique allows domain structures within individual elements and the magnetic interactions between them to be quantified at close to the nanometre scale. The behaviour of 30 nm-thick Co elements was compared with that of 10 nm-thick Ni and Co elements, as well as with Co/Au/Ni trilayers. The hysteresis loops of individual elements were determined directly from the measured holographic phase images. The reproducibility of an element's domain structure in successive cycles was found to be affected by the out-of-plane component of the applied magnetic field and by the exact details of its initial magnetic state. Close proximity to adjacent elements led to strong intercell coupling, and remanent states with the in-plane magnetic field removed included domain structures such as solenoidal (vortex) states that were never observed during hysteresis cycling. Narrow rectangular bars reversed without the formation of end domains, whereas closely separated magnetic layers within individual elements were observed to couple to each other during field reversal.

Responses of rats to noxious mechanical stimulation of their tails during tail reperfusion following transient ischaemia

We investigated whether conscious rats exhibit hyperalgesia to noxious mechanical stimuli during tail reperfusion following transient tail ischaemia. Mechanical stimuli were delivered by three mechanical algometers, differing in contact area, geometry and rate of application. Tail ischaemia was induced by applying a pneumatic tourniquet to the base of the tail until the rats exhibited escape behaviour. Control animals had a sham tourniquet placed on their tails. During tail reperfusion, there was significant hyperalgesia when the noxious mechanical stimulus was applied by a narrow bar transverse to the tail, less when applied by a rubber piston, and there was no hyperalgesia when the stimulus was punctate, similar to a von Frey hair. Repeated application of the mechanical stimuli themselves, at 30-min intervals, did not induce hyperalgesia. Rats subjected repetitively to the combination of the bar algometer and tail ischaemia at intervals of at least 48 h, for a total period of three weeks, exhibited progressively enhanced hyperalgesia to the bar algometer, and also enhanced hyperalgesia to noxious ischaemic and thermal stimuli. Rats therefore do exhibit hyperalgesia to mechanical stimuli following conditioning ischaemia, but the nature of the hyperalgesia depends critically on the geometry of the algometer, and the time course of the hyperalgesia was different to that which manifests to a noxious thermal stimulus.

Parameters affecting specimen flatness of two-dimensional crystals for electron crystallography

The flatness of two-dimensional (2D) crystals on the support film is a critical factor in protein electron crystallography. The influence of the carbon support film and of different grid makes and materials on flatness was investigated, using as a criterion the sharpness of diffraction spots perpendicular to the tilt axis of electron diffraction patterns of purple membrane tilted in the microscope at 45°. In a quantitative test, carbon film that had been evaporated without sparks forming gave a much larger proportion of flat crystals than “sparked” carbon. Titanium grids were superior to copper, probably because they introduce less cryo-crinkling of the carbon film when the sample is cooled to liquid nitrogen temperature, as their thermal expansion coefficient is closer to that of carbon. While the molybdenum grids from Plano were unsuitable for data collection because of their tendency of break the carbon, molybdenum grids from Pacific GridTech gave a much larger yield of flat crystals than the titanium grids. Scanning electron microscope images of the grids as supplied by the manufacturer showed that the Plano grids had very narrow and irregular grid bars, while the Pacific GridTech grids were very smooth with a large surface-to-hole ratio.