A parallel solution-adaptive scheme for ideal magnetohydrodynamics

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77232/1/AIAA-1999-3273-200.pd

Formation of microtubule-based traps controls the sorting and concentration of vesicles to restricted sites of regenerating neurons after axotomy

Transformation of a transected axonal tip into a growth cone (GC) is a critical step in the cascade leading to neuronal regeneration. Critical to the regrowth is the supply and concentration of vesicles at restricted sites along the cut axon. The mechanisms underlying these processes are largely unknown. Using online confocal imaging of transected, cultured Aplysia californica neurons, we report that axotomy leads to reorientation of the microtubule (MT) polarities and formation of two distinct MT-based vesicle traps at the cut axonal end. Approximately 100 μm proximal to the cut end, a selective trap for anterogradely transported vesicles is formed, which is the plus end trap. Distally, a minus end trap is formed that exclusively captures retrogradely transported vesicles. The concentration of anterogradely transported vesicles in the former trap optimizes the formation of a GC after axotomy

A cerebellar learning model that reproduces the behavior of vestibulo-ocular reflex adaptation in wild-type and knock-out mice

The cerebellum is crucial for different types of motor learning. Established theories of cerebellar learning posit that the cerebellum learns by adjusting the weights of Parallel Fiber (PF) to Purkinje cells (PC) synapses, thanks to teaching signals provided by Climbing Fiber inputs. While these theories are consistent with a large body of experimental data, in particular on synaptic plasticity in PF to PC synapses, they cannot easily explain a growing body of experimental work, which seems to indicate a significant role of other sites of plasticity. Recent advances in the development of a large number in transgenic animals, as well as behavioral and electrophysiogical comparative studies between these animals and wild-type animals, have opened an unprecedented window into the mechanisms underlying learning in this structure. In particular, it has been shown that specific knock-outs are impaired selectively on difficult variants of the vestibulo-ocular reflex (VOR) adaptation task, one of the most studied cerebellar-dependent motor learning tasks. These impairments can occur even though the classical plasticity mechanisms are left untouched. These data pose significant new challenges for established models of cerebellar learning

Spiral Galaxies with HST/NICMOS. I. Nuclear Morphologies, Color Maps and Distinct Nuclei

This is the first of two papers where we present the analysis of an HST NICMOS-Cam2 near-infared (NIR) snapshot survey in the F160W (H) filter for a sample of 78 spiral galaxies selected from the UGC and ESOLV catalogs. For 69 of these objects we provide nuclear color information derived by combining the H data either with additional NICMOS F110W (J) images or with V WFPC2/HST data. Here we present the NIR images and the optical-NIR color maps. We focus our attention on the properties of the photometrically-distinct `nuclei' which are found embedded in most of the galaxies, and provide measurements of their half-light radii and magnitudes in the H (and when available, in the J) band. We find that: (i) In the NIR, the nuclei embedded in the bright early- to intermediate-type galaxies span a much larger range in brightness than the nuclei which are typically found embedded in bulgeless late-type disks; (ii) Nuclei are found in both non-barred and barred hosts, in large-scale (\gta 1 kpc) as well as in nuclear (up to a few 100pc) bars; (iii) There is a significant increase in half-light radius with increasing luminosity of the nucleus in the early/intermediate types; (iv) The nuclei of early/intermediate-type spirals cover a large range of optical-NIR colors, from $V-H\approx$ -0.5 to 3. Some nuclei are bluer and others redder than the surrounding galaxy, indicating the presence of activity or reddening by dust in many of these systems; (v) Some early/intermediate nuclei are elongated and/or slightly offset from the isophotal center of the host galaxy. On average, however, these nuclei appear as centered, star-cluster-like structures similar to those which are found in the late-type disks. [abridged]Comment: Astronomical Journal, accepted, January 2002 Issue; higher-res Figs 1, 2 and 9 available upon reques

A SAURON study of stars and gas in Sa bulges

We present results from our ongoing effort to understand the morphological and kinematical properties of early-type galaxies using the integral-field spectrograph SAURON. We discuss the relation between the stellar and gas morphology and kinematics in our sub-sample of 24 representative Sa spiral bulges. We focus on the frequency of kinematically decoupled components and on the presence of star formation in circumnuclear rings.Comment: 6 pages, 3 figures; To appear in the proceedings of the "Island Universes: Structure and Evolution of Disk Galaxies" conference held in Terschelling, Netherlands, July 2005, ed. R. de Jong. A high resolution version is available at http://www.strw.leidenuniv.nl/~jfalcon/JFB_terschelling.pdf.g

HST Optical-NIR Colors of Nearby R1/4R^{1/4} and Exponential Bulges

We have analysed $V$, $H$ and $J$ HST images for a sample of early- to late-type spiral galaxies, and reported elsewhere the statistical frequency of $R^{1/4}$-law and exponential bulges in our sample as a function of Hubble type, and the frequency of occurrence and structural properties of the resolved central nuclei hosted by intermediate- to late-type bulges and disks (see references in the text). Here we use these data to show that: {\it (i)} The $V-H$ color distribution of the $R^{1/4}$ bulges peaks around $\sim 1.3$, with a sigma $\Delta (V-H) \sim 0.1$ magnitudes. Assuming a solar metallicity, these values correspond to stellar ages of $\approx 6 \pm 3$ Gyrs. In contrast, the $V-H$ color distribution of the exponential bulges peaks at $\sim 0.9$ and has a sigma $\Delta (V-H) \sim 0.4$ mags. This likely implies significantly smaller ages and/or lower metallicities for (a significant fraction of the stars in) the exponential bulges compared to the $R^{1/4}$-law spheroids. {\it (ii)} Most of the central nuclei hosted by the exponential bulges have $V-H$ and $J-H$ colors which are compatible with relatively unobscured stellar populations. Assuming no or little dust effects, ages \gta 1 Gyrs are suggested for these nuclei, which in turn imply masses of about a few 10$^6$ to a few 10$^7$ M$_\odot$, i.e., sufficient to dissolve progenitor bars with masses consistent with those inferred for the exponential bulges by their luminosities. {\it (iii)} While different bulge-nucleus pairs cover a large range of $V-H$ colors, each bulge-nucleus pair has quite similar $V-H$ colors, and thus possibly similar stellar populations. (abridged)Comment: ApJ, accepted, Dec 10, 2000 Issu

fMRI Activities in the Emotional Cerebellum: A Preference for Negative Stimuli and Goal-Directed Behavior

Several studies indicate that the cerebellum might play a role in experiencing and/or controlling emphatic emotions, but it remains to be determined whether there is a distinction between positive and negative emotions, and, if so, which specific parts of the cerebellum are involved in these types of emotions. Here, we visualized activations of the cerebellum and extracerebellar regions using high-field fMRI, while we asked participants to observe and imitate images with pictures of human faces expressing different emotional states or with moving geometric shapes as control. The state of the emotions could be positive (happiness and surprise), negative (anger and disgust), or neutral. The positive emotional faces only evoked mild activations of crus 2 in the cerebellum, whereas the negative emotional faces evoked prominent activations in lobules VI and VIIa in its hemispheres and lobules VIII and IX in the vermis. The cerebellar activations associated with negative emotions occurred concomitantly with activations of mirror neuron domains such as the insula and amygdala. These data suggest that the potential role of the cerebellum in control of emotions may be particularly relevant for goal-directed behavior that is required for observing and reacting to another person’s (negative) expressions

Role of periodic loading‐unloading in the magnetotail versus interplanetary magnetic field B z flipping in the ring current buildup

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94813/1/jgra19183.pd

A retinal origin of nystagmus—a perspective

Congenital nystagmus is a condition where the eyes of patients oscillate, mostly horizontally, with a frequency of between 2 and 10 Hz. Historically, nystagmus is believed to be caused by a maladaptation of the oculomotor system and is thus considered a disease of the brain stem. However, we have recently shown that congenital nystagmus associated with congenital stationary night blindness is caused by synchronously oscillating retinal ganglion cells. In this perspective article, we discuss how some details of nystagmus can be accounted for by the retinal mechanism we propose