Showing papers by "University of Houston published in 2002"

Adaptive optics scanning laser ophthalmoscopy

Abstract: We present the first scanning laser ophthalmoscope that uses adaptive optics to measure and correct the high order aberrations of the human eye. Adaptive optics increases both lateral and axial resolution, permitting axial sectioning of retinal tissue in vivo. The instrument is used to visualize photoreceptors, nerve fibers and flow of white blood cells in retinal capillaries.

The role of fair treatment and rewards in perceptions of organizational support and leader-member exchange.

Abstract: This study examined a model of the antecedents and consequences of perceived organizational support (POS) and leader-member exchange (LMX). It was predicted that organizational justice (procedural and distributive justice) and organizational practices that provide recognition to the employee (feelings of inclusion and recognition from upper management) would influence POS. For LMX, it was predicted that leader reward (distributive justice and contingent rewards) and punishment behavior would be important antecedents. Results based on a sample of 211 employee-supervisor dyads indicated that organizational justice, inclusion, and recognition were related to POS and contingent rewards were related to LMX. In terms of consequences, POS was related to employee commitment and organizational citizenship behavior, whereas LMX predicted performance ratings.

Coherent structure generation in near-wall turbulence

Abstract: We present a new mechanism for generation of near-wall streamwise vortices – which dominate turbulence phenomena in boundary layers – using linear perturbation analysis and direct numerical simulations of turbulent channel flow. The base flow, consisting of the mean velocity profile and low-speed streaks (free from any initial vortices), is shown to be linearly unstable to sinuous normal modes only for relatively strong streaks, i.e. for wall inclination angles of streak vortex lines exceeding 50°. Analysis of streaks extracted from fully developed near-wall turbulence indicates that about 20% of streak regions in the buffer layer exceed the strength threshold for instability. More importantly, these unstable streaks exhibit only moderate (twofold) normal-mode amplification, the growth being arrested by self-annihilation of streak-flank normal vorticity due to viscous cross-diffusion. We present here an alternative, streak transient growth (STG) mechanism, capable of producing much larger (tenfold) linear ampliflcation of x-dependent disturbances. Note the distinction of STG – responsible for perturbation growth on a streak velocity distribution U(y, z) – from prior transient growth analyses of the (streakless) mean velocity U(y). We reveal that streamwise vortices are generated from the more numerous normal-mode-stable streaks, via a new STG-based scenario: (i) transient growth of perturbations leading to formation of a sheet of streamwise vorticity ωx (by a ‘shearing’ mechanism of vorticity generation), (ii) growth of sinuous streak waviness and hence ∂u/∂x as STG reaches nonlinear amplitude, and (iii) the ωx sheet’s collapse via stretching by ∂u/∂x (rather than rollup) into streamwise vortices. Significantly, the three-dimensional features of the (instantaneous) streamwise vortices of x-alternating sign generated by STG agree well with the (ensemble-averaged) coherent structures educed from fully turbulent flow. The STG-induced formation of internal shear layers, along with quadrant Reynolds stresses and other turbulence measures, also agree well with fully developed turbulence. Results indicate the prominent – possibly dominant – role of this new, transient-growth-based vortex generation scenario, and suggest interesting possibilities for robust control of drag and heat transfer.

Energy dependence of pion and kaon production in central Pb+Pb collisions

Abstract: Measurements of charged pion and kaon production in central Pb+Pb collisions at 40, 80, and 158 A GeV are presented. These are compared with data at lower and higher energies as well as with results from $p+p$ interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with ${s}_{\mathrm{NN}}^{1/4}$ with a change of slope starting in the region 15--40 A GeV. The change from pion suppression with respect to $p+p$ interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40A GeV. A nonmonotonic energy dependence of the ratio of ${K}^{+}$ to ${\ensuremath{\pi}}^{+}$ yields is observed, with a maximum close to 40A GeV and an indication of a nearly constant value at higher energies. The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central Pb+Pb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.

Chiral 2,2'-bipyridines, 1,10-phenanthrolines, and 2,2':6',2' '-terpyridines: syntheses and applications in asymmetric homogeneous catalysis.

Abstract: Although chiral phosphines have enjoyed a longtime popularity in the design of enantioselective catalytic systems, there has been a recent renaissance in the use of nitrogen ligands for this same purpose.1 This renaissance partly arises from several distinct advantages presented by nitrogen-containing ligands. First, they can often be employed in catalytic processes where the use of phosphines may be incompatible with the reaction conditions. Second, many nitrogen ligands are now available in enantiomerically pure form. Third, ligands that bind through nitrogen are known to coordinate with a wide variety of metal ions, and considerable progress has been made in understanding the role which these ligands play in affecting catalytic processes.

Institution-Based Trust in Interorganizational Exchange Relationships: The Role of Online B2B Marketplaces on Trust Formation

Abstract: Given the uncertainty of online transactions, the digital economy encourages the creation of institutional structures that assure online interorganizational exchange relationships. This research examines how institution-based trust develops in online B2B marketplaces to facilitate interorganizational trust (buyers’ trust in sellers). Building upon the notion of institutional trust [Zucker,1986], this study proposes how specific institution-based structures help engender interorganizational trust and indirectly influence transaction success in B2B marketplaces. Five specific institution-based mechanisms are proposed — perceived monitoring, perceived legal bonds, perceived accreditation, perceived feedback, and perceived cooperative norms. To assess the influence of the proposed two dimensions of interorganizational trust — credibility and benevolence — in buyer–seller relationships, three trust outcomes are examined — satisfaction, perceived risk, and continuity. The proposed model is tested on data from 102 organizational buyers in an online B2B marketplace. The results support the proposed model, delineating the relationship between institution based trust and interorganizational trust. The study provides evidence on how specific institutional mechanisms build trust in online B2B marketplaces, stressing the ability of institution-based trust to build a trustworthy trading environment in the digital economy. The paper discusses the theoretical and managerial implications of this study and proposes several suggestions for future research.

Perceived Information Security, Financial Liability and Consumer Trust in Electronic Commerce Transactions

Abstract: Electronic commerce (EC) transactions are subject to multiple information security threats. Proposes that consumer trust in EC transactions is influenced by perceived information security and distinguishes it from the objective assessment of security threats. Proposes mechanisms of encryption, protection, authentication, and verification as antecedents of perceived information security. These mechanisms are derived from technological solutions to security threats that are visible to consumers and hence contribute to actual consumer perceptions. Tests propositions in a study of 179 consumers and shows a significant relationship between consumers’ perceived information security and trust in EC transactions. Explores the role of limited financial liability as a surrogate for perceived security. However, the findings show that there is a minimal effect of financial liability on consumers’ trust in EC. Engenders several new insights regarding the role of perceived security in EC transactions.

Corporate Governance, Takeovers, and Top-Management Compensation: Theory and Evidence

Abstract: We examine, both theoretically and empirically, top-management compensation in the presence of agency conflicts when shareholders have delegated governance responsibilities to a self-interested Board of Directors (BOD). We develop a theoretical framework that explicitly incorporates the BOD as a strategic player, models the negotiation process between the CEO and the BOD in designing CEO compensation, and considers the impact of potential takeovers by large shareholders monitoring the CEO-BOD negotiations. In equilibrium, internal governance by the BOD and external takeover threats by a large shareholder act as substitutes in imposing managerial control, especially in constraining management's profligacy in awarding equity-based compensation to itself. The model emphasizes factors in the design of compensation contracts that are rarely considered in the literature, such as equity ownership of the largest outside shareholder and the firm's bankruptcy risk. It also provides new perspectives on factors that are often considered in the literature, such as firm size, firm performance, equity ownership of the BOD, and BOD structure. Our empirical tests lend considerable support for our theoretical predictions. Equity ownership of the largest external shareholder, that of the BOD, and the default risk, are strongly negatively related to the size of CEO equity compensation. Consistent with the theoretical model, these factors do not significantly influence the growth of fixed (or non-performance-related) compensation. We also find that the equity ownership of the BOD is more important in managerial compensation control than other BOD related variables, such as BOD size or the proportion of outside directors.

Dyslexia-specific brain activation profile becomes normal following successful remedial training

Abstract: Objectives: To examine changes in the spatiotemporal brain activation profiles associated with successful completion of an intensive intervention program in individual dyslexic children. Methods: The authors obtained magnetic source imaging scans during a pseudoword reading task from eight children (7 to 17 years old) before and after 80 hours of intensive remedial instruction. All children were initially diagnosed with dyslexia, marked by severe difficulties in word recognition and phonologic processing. Eight children who never experienced reading problems were also tested on two occasions separated by a 2-month interval. Results: Before intervention, all children with dyslexia showed distinctly aberrant activation profiles featuring little or no activation of the posterior portion of the superior temporal gyrus (STGp), an area normally involved in phonologic processing, and increased activation of the corresponding right hemisphere area. After intervention that produced significant improvement in reading skills, activity in the left STGp increased by several orders of magnitude in every participant. No systematic changes were obtained in the activation profiles of the children without dyslexia as a function of time. Conclusions: These findings suggest that the deficit in functional brain organization underlying dyslexia can be reversed after sufficiently intense intervention lasting as little as 2 months, and are consistent with current proposals that reading difficulties in many children represent a variation of normal development that can be altered by intensive intervention.

Does the Mortality Decline Promote Economic Growth

Abstract: This paper analyzes qualitatively and quantitatively the effects of declining mortality rates on fertility, education and economic growth. The analysis demonstrates that if individuals are prudent in the face of uncertainty about child survival, a decline in an exogenous mortality rate reduces precautionary demand for children and increases parental investment in each child. Once mortality is endogenized, population growth becomes a hump-shaped function of income per capita. At low levels of income population growth rises as income per capita rises leading to a Malthusian steady-state equilibrium, whereas at high levels of income population growth declines leading to a sustained growth steady-state equilibrium.

Elastography: Imaging the elastic properties of soft tissues with ultrasound

Abstract: Elastography is a method that can ultimately generate several new kinds of images, called elastograms. As such, all the properties of elastograms are different from the familiar properties of sonograms. While sonograms convey information related to the local acoustic backscatter energy from tissue components, elastograms relate to its local strains, Young's moduli or Poisson's ratios. In general, these elasticity parameters are not directly correlated with sonographic parameters, i.e. elastography conveys new information about internal tissue structure and behavior under load that is not otherwise obtainable. In this paper we summarize our work in the field of elastography over the past decade. We present some relevant background material from the field of biomechanics. We then discuss the basic principles and limitations that are involved in the production of elastograms of biological tissues. Results from biological tissues in vitro and in vivo are shown to demonstrate this point. We conclude with some observations regarding the potential of elastography for medical diagnosis.

Retinal Origins of the Primate Multifocal ERG: Implications for the Human Response

Abstract: Purpose To better understand the cellular contributions to the human multifocal ERG (mfERG), rhesus monkey and human mfERGs were recorded using the same stimulus conditions. The monkey mfERGs were recorded before and after injections of pharmacologic agents known to selectively block activity of particular cells and circuits in the retina. Methods Photopic mfERGs were recorded with Dawson-Trick-Litzkow (DTL) fiber electrodes from 16 eyes of 10 anesthetized adult rhesus monkeys (Macaca mulatta) and from 4 normal humans. The display consisted of 103 equal-sized hexagons within 17 degrees of the fovea. Monkey mfERGs were obtained before and after inner retinal responses were suppressed with intravitreal injections of tetrodotoxin (TTX), TTX+N-methyl-D-aspartic acid (NMDA), TTX+NMDA with the gamma-aminobutyric acid (GABA(A&C)) antagonist picrotoxin (PTX), or the inhibitory amino acid GABA and after L-2 amino-4-phosphonobutyric acid (APB) to block signal transmission to ON-bipolar cells. Finally, a combination of APB and cis-2,3 piperidine dicarboxylic acid (PDA) was used to isolate the contributions from the cone photoreceptors. Results TTX, which blocks sodium-based action potentials, removes a large contribution from the monkey's mfERG, but it does not remove all inner retinal influences. After administration of TTX, the mfERG is further modified by the addition of NMDA. TTX+NMDA, TTX+NMDA+PTX, or GABA alone have similar effects, suggesting that, at the concentrations used, they are largely removing the inner retinal contributions. After removing the inner retinal influences, the monkey's mfERG is mainly composed of ON- and OFF-bipolar contributions, as revealed after APB and PDA were injected. The leading edge of the first negative potential (N1) is largely shaped by the initial hyperpolarization of the OFF-bipolar cells. The photoreceptors also contribute to the leading edge of N1, but this contribution is small, except in the central 6 degrees. The depolarization of the ON-bipolars and the recovery of the OFF-bipolars contribute to the leading edge of the major positive component (P1), with the recovery of the ON-bipolars being the dominant influence on the trailing edge. The waveform of the human mfERG most closely resembles the rhesus monkey's mfERG after administration of TTX. Conclusions The monkey's mfERG is shaped by large contributions from ON- and OFF-bipolar cells, combined with both spiking and nonspiking inner retinal contributions, and a small contribution from the photoreceptors. In comparison, the human mfERG resembles the monkey's mfERG after reduction of inner retinal contributions. Based on the pharmacologic dissection of the monkey's mfERG, a model of the waveform of the human mfERG is proposed. This model suggests that the waveform can be understood as a combination of overlapping ON- and OFF-bipolar cell contributions combined with smaller contributions from inner retina and photoreceptors.

The depression anxiety stress scale-21: Spanish translation and validation with a Hispanic sample

Abstract: The English-language version of the Depression Anxiety Stress Scale-21 (DASS-21) was professionally translated into Spanish and field-tested among 98 bilingual Hispanic adults. Participants who were diagnosed with an anxiety disorder on the Anxiety Disorders Interview Schedule-IV completed the DASS-21, the Beck Depression Inventory-II, and the Beck Anxiety Inventory. Results indicated strong indices of internal consistency and expected patterns of discriminant, convergent, and structural validity. A confirmatory factor analysis compared a model fit of a first order 1-factor model, a first order 3-factor model, and a second order factor model. The latter 2 models were significantly better than the 1-factor model. Psychometric data were comparable to those of an English version. Clinicians and researchers in need of a brief, Spanish-language, screening measure of general psychopathology may want to consider this newly translated DASS-21.

Solvation and hydration of proteins and nucleic acids: a theoretical view of simulation and experiment.

Abstract: Many theoretical, computational, and experimental techniques recently have been successfully used for description of the solvent distribution around macromolecules. In this Account, we consider recent developments in the areas of protein and nucleic acid solvation and hydration as seen by experiment, theory, and simulations. We find that in most cases not only the general phenomena of solvation but even local hydration patterns are more accurately discussed in the context of water distributions rather than individual molecules of water. While a few localized or highresidency waters are often associated with macromolecules in solution (or crystals from aqueous liquors), these are readily and accurately included in this more general description. The goal of this Account is to review the theoretical models used for the description of the interfacial solvent structure on the border near DNA and protein molecules. In particular, we hope to highlight the progress in this field over the past five years with a focus on comparison of simulation and experimental results.

Value creation and destruction in cross-border acquisitions: an empirical analysis of foreign acquisitions of u.s. firms

Abstract: We conduct an investigation of the sources of gains and losses in cross-border acquisitions in light of different motives for undertaking these transactions: synergy-seeking, managerialism and hubris. We find that the data are consistent with the expectation that multiple sources of value creation exist in synergistic cross-border acquisitions: asset sharing, reverse internalization of valuable intangible assets, and financial diversification. Gains accrue to bidder firm shareholders only for the least fungible of these sources of gains, i.e., reverse internalization. For value-destroying acquisitions that are expected to be driven by managerialism, we find that the data are consistent with only one of the sources of value destruction that we examine, i.e., risk reduction. In these acquisitions, the evidence also suggests that the relative size of the target to the bidder mitigates the negative effects of risk reduction. Our results underscore the importance of considering the implications of alternative behavioral assumptions in empirical strategy content research. Copyright © 2002 John Wiley & Sons, Ltd.

Structural modulation of molybdenyl iodate architectures by alkali metal cations in AMoO3(IO3) (A = K, Rb, Cs): a facile route to new polar materials with large SHG responses.

Abstract: Three new molybdenyl iodates, KMoO3(IO3) (1), RbMoO3(IO3) (2), and CsMoO3(IO3) (3), have been prepared through the hydrothermal reactions of MoO3 with AIO4 (A = K, Rb, or Cs) at 180 °C. These compounds are isolated as nearly colorless, air-stable crystals. Single-crystal X-ray diffraction experiments reveal that 1 possesses a corrugated layered structure constructed from molybdenum oxide chains that are bridged by iodate anions. The puckering of the layers is caused by the alignment of bent molybdenyl (MoO22+) groups along one side of the molybdenum oxide chains. The K+ cations separate these layers from one another and serve to balance charge. In contrast, compounds 2 and 3, which are isostructural, form three-dimensional structures with small cavities filled with Rb+ or Cs+ cations. The differences between the structures of 1 and those of 2 and 3 are due to rotation of the molybdenyl units as translation occurs down the molybdenum oxide chains in order to accommodate the increased size of the Rb+ and Cs...

Are all aberrations equal

Abstract: PURPOSE: To determine for a fixed RMS error (25 μm, over a 6-mm pupil) how each mode of the normalized Zernike polynomial (second through the fourth radial order) affects high and low contrast logMAR visual acuity. METHODS: Three healthy volunteers served as subjects. CTView was used to generate optically aberrated logMAR charts. Accommodation was paralyzed and pupils dilated. The foveal achromatic axis of the eye was aligned to a 3-mm pupil and the eye was optimally refracted. Aberrated acuity charts were read until five letters were missed. Data were normalized for each subject to the acuity obtained by reading unaberrated charts and plotted as letters lost as a function of Zernike mode. RESULTS: Defocus (Z 0 2 ) decreased letter acuity more than astigmatism (Z 2 2 , Z - 2 2 ). Coma (Z 1 3 , Z - 1 3 ) decreased acuity more than trefoil (Z 3 3 , Z - 3 3 ). Spherical aberration (Z 0 4 ) and secondary astigmatism (Z 2 4 , Z - 2 4 ) decreased acuity much more than quadrafoil (Z 4 4 , Z - 4 4 ). CONCLUSIONS: 1. For an equal amount of RMS error not all coefficients of the Zernike polynomial induce equivalent losses in high and low contrast logMAR acuity. 2. Wavefront error concentrated near the center of the pyramid adversely affects visual acuity more than modes near the edge of the pyramid. 3. Large changes in chart appearance are not reflected in equally large decreases in visual performance (ie, subjects could correctly identify highly aberrated letters). 4. Interactions between modes complicate weighting each Zernike mode for visual impact.

A reversible solid-state crystalline transformation in a metal phosphide induced by redox chemistry.

Abstract: We demonstrate low-potential intercalation of lithium in a solid-state metal phosphide. A topotactic first-order transition between different but related crystal structures at room temperature takes place by an electrochemical redox process: MnP4 ↔ Li7MnP4. The P-P bonds in the MnP4 structure are cleaved at the time of Li insertion (reduction) to produce crystalline Li7MnP4 and are reformed after reoxidation to MnP4, thereby acting as an electron storage reservoir. This is an unusual example of facile covalent bond breaking within the crystalline solid state that can be reversed by the input of electrochemical energy.

The Scotopic Threshold Response of the Dark‐Adapted Electroretinogram of the Mouse

Abstract: The most sensitive response in the dark-adapted electroretinogram (ERG), the scotopic threshold response (STR) which originates from the proximal retina, has been identified in several mammals including humans, but previously not in the mouse. The current study established the presence and assessed the nature of the mouse STR. ERGs were recorded from adult wild-type C57/BL6 mice anaesthetized with ketamine (70 mg kg−1) and xylazine (7 mg kg−1). Recordings were between DTL fibres placed under contact lenses on the two eyes. Monocular test stimuli were brief flashes (λmax 462 nm; -6.1 to +1.8 log scotopic Troland seconds(sc td s)) under fully dark-adapted conditions and in the presence of steady adapting backgrounds (-3.2 to -1.7 log sc td). For the weakest test stimuli, ERGs consisted of a slow negative potential maximal ≈200 ms after the flash, with a small positive potential preceding it. The negative wave resembled the STR of other species. As intensity was increased, the negative potential saturated but the positive potential (maximal ≈110 ms) continued to grow as the b-wave. For stimuli that saturated the b-wave, the a-wave emerged. For stimulus strengths up to those at which the a-wave emerged, ERG amplitudes measured at fixed times after the flash (110 and 200 ms) were fitted with a model assuming an initially linear rise of response amplitude with intensity, followed by saturation of five components of declining sensitivity: a negative STR (nSTR), a positive STR (pSTR), a positive scotopic response (pSR), PII (the bipolar cell component) and PIII (the photoreceptor component). The nSTR and pSTR were approximately 3 times more sensitive than the pSR, which was approximately 7 times more sensitive than PII. The sensitive positive components dominated the b-wave up to > 5 % of its saturated amplitude. Pharmacological agents that suppress proximal retinal activity (e.g. GABA) minimized the pSTR, nSTR and pSR, essentially isolating PII which rose linearly with intensity before showing hyperbolic saturation. The nSTR, pSTR and pSR were desensitized by weaker backgrounds than those desensitizing PII. In conclusion, ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the ‘threshold’ negative and positive (b-wave) responses of the mouse dark-adapted ERG. These results support the use of the mouse ERG in studies of proximal retinal function.

Clock mechanisms in zebrafish

Abstract: Recent research on the circadian system of the zebrafish is reviewed. This teleost has become an attractive model system because of its advantages for genetic analyses. Circadian rhythms of zebrafish behavior, visual system function, and pineal melatonin synthesis have been described, and behavioral and pineal rhythms are being used to identify and characterize clock mutants. Zebrafish heart, kidney, and embryonic cell lines contain circadian oscillators and phototransduction mechanisms for entrainment, suggesting that circadian pacemaking functions may be distributed throughout the animal. Studies of circadian system development in zebrafish have found that a molecular circadian oscillation in unfertilized oocytes persists through embryonic development with its phase intact, but that the pacemakers that drive rhythms of melatonin synthesis and behavior require environmental entraining signals late in development for initial synchronization. Zebrafish homologs of several of the core clock genes identified in other animals have been cloned. Transcripts for most of these are rhythmically expressed in multiple tissues. The interactions of clock gene products are for the most part similar to their interactions in mammals, although there are some potentially interesting differences.

Systematic x‐ray photoelectron spectroscopic study of La1−xSrx‐based perovskite‐type oxides

Abstract: X-ray photoelectron spectroscopy is used to analyse systematically a group of La1−xSrx-based mixed-metal perovskite-type oxides of interest in the development of oxygen separation membranes. Angle- and temperature-resolved studies reveal compositional variations that correlate with the segregation of various elements during annealing. The complex O 1s, Sr 3d5/2 and La 3d5/2 spectra could be curve-fitted systematically with two La 3d5/2, three Sr 3d5/2 and five O 1s contributions, respectively, using fixed binding energies and full widths at half-maximum. The trends are discussed and assignments are made on the basis of peak intensity and composition variations noted during the angle- and temperature-resolved studies, literature data and nearest-neighbor electronegativities. In all cases, elements present at the outermost surface revealed higher binding energies than their bulk-bound counterparts. Copyright © 2002 John Wiley & Sons, Ltd.

Does choice lead to racially distinctive schools? Charter schools and household preferences

Abstract: A persistent fear regarding school choice is that it will lead to more racially distinctive schools A growing number of studies compares choosing households to non-choosing households, but few have examined the possibility that choosers sort themselves out based upon school preferences that are correlated with race and ethnicity This report addresses this issue by analyzing the responses of 1,006 charter school households in Texas It first examines the expressed preferences of choosing households, then compares expressed preferences with behavior A comparison of the characteristics of the traditional public schools that choosers leave with the characteristics of the charter schools they choose indicates that race is a good predictor of the choices that choosing households make Whites, African Americans, and Latinos transfer into charter schools where their groups comprise between 11 and 14 percentage points more of the student body than the traditional public schools they are leaving © 2002 by the Association for Public Policy Analysis and Management

Ion channel properties underlying axonal action potential initiation in pyramidal neurons.

Abstract: A high density of Na+ channels in the axon hillock, or initial segment, is believed to determine the threshold for action potential initiation in neurons. Here we report evidence for an alternative mechanism that lowers the threshold in the axon. We investigated properties and distributions of ion channels in outside-out patches from axons and somata of layer 5 pyramidal neurons in rat neocortical slices. Na+ channels in axonal patches (<30 microm from the soma) were activated by 7 mV less depolarization than were somatic Na+ channels. A-type K+ channels, which were prominent in somatic and dendritic patches, were rarely seen in axonal patches. We incorporated these findings into numerical simulations which indicate that biophysical properties of axonal channels, rather than a high density of channels in the initial segment, are most likely to determine the lowest threshold for action potential initiation.

Suppressive and facilitatory spatial interactions in peripheral vision: peripheral crowding is neither size invariant nor simple contrast masking.

Abstract: Peripheral vision is characterized by reduced spatial resolution and inhibitory spatial interactions that extend over long distances. This work had three goals. (1) We considered whether the extensive crowding in peripheral vision is a consequence of a shift in the spatial scale of analysis. To test this, using a large range of target sizes and spatial frequencies, we measured the extent of crowding for targets that were limited in their spatial frequency content. (2) We considered whether crowding in peripheral vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis, we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments, our targets and flanks were composed of Gabor features, thus allowing us to control the feature contrast, spatial frequency, and orientation. (3) We examined the relationship between the suppressive and facilitatory interactions in peripheral contrast detection and crowding. Our results show that unlike the normal fovea (Levi, Klein, & Hariharan, 2002), peripheral crowding is not scale invariant nor is it attributable to simple contrast masking. Rather, our results suggest that inhibitory spatial interactions in peripheral crowding extend over larger distances than in the fovea for targets of the same size. In peripheral vision, the critical distance for crowding is approximately 0.1 times the target eccentricity. Observers can easily detect the features that compose our targets (Gabor patches) under conditions where crowding is strong. Thus, we speculate that peripheral crowding occurs because the target and flanks are combined or pooled at a second stage, following the stage of feature extraction. In peripheral vision, this pooling takes place over a large distance.

Relationship of Rapid Automatized Naming and Phonological Awareness in Early Reading Development Implications for the Double-Deficit Hypothesis

Abstract: It is widely accepted that deficits in phonological awareness skills are related to reading difficulties. Recently, another source of reading difficulty has been identified that involves naming speed, and combined impairments in phonological skills and naming speed will produce more severe reading deficits than single deficits in either of these cognitive skills. The purpose of this study was to investigate the consequences of grouping children based on the presence or absence of deficits in these skills. We demonstrate that the greater severity of reading impairment found in children with a double deficit could be due in part to a statistical artifact caused by grouping children based on their performance on two correlated continuous variables. This artifact also makes it difficult to establish the relative impact of deficits in naming speed on reading ability independent of deficits in phonological awareness.

What geometric visual hallucinations tell us about the visual cortex

Abstract: Many observers see geometric visual hallucinations after taking hallucinogens such as LSD, cannabis, mescaline or psilocybin; on viewing bright flickering lights; on waking up or falling asleep; in "near-death" experiences; and in many other syndromes. Kluver organized the images into four groups called form constants: (I) tunnels and funnels, (II) spirals, (III) lattices, including honeycombs and triangles, and (IV) cobwebs. In most cases, the images are seen in both eyes and move with them. We interpret this to mean that they are generated in the brain. Here, we summarize a theory of their origin in visual cortex (area V1), based on the assumption that the form of the retino-cortical map and the architecture of V1 determine their geometry. (A much longer and more detailed mathematical version has been published in Philosophical Transactions of the Royal Society B, 356 [2001].)We model V1 as the continuum limit of a lattice of interconnected hypercolumns, each comprising a number of interconnected iso-orientation columns. Based on anatomical evidence, we assume that the lateral connectivity between hypercolumns exhibits symmetries, rendering it invariant under the action of the Euclidean group E(2), composed of reflections and translations in the plane, and a (novel) shift-twist action. Using this symmetry, we show that the various patterns of activity that spontaneously emerge when V1's spatially uniform resting state becomes unstable correspond to the form constants when transformed to the visual field using the retino-cortical map. The results are sensitive to the detailed specification of the lateral connectivity and suggest that the cortical mechanisms that generate geometric visual hallucinations are closely related to those used to process edges, contours, surfaces, and textures.