318 research outputs found
Synaptic plasticity in medial vestibular nucleus neurons: comparison with computational requirements of VOR adaptation
Background: Vestibulo-ocular reflex (VOR) gain adaptation, a longstanding experimental model of cerebellar learning, utilizes sites of plasticity in both cerebellar cortex and brainstem. However, the mechanisms by which the activity of cortical Purkinje cells may guide synaptic plasticity in brainstem vestibular neurons are unclear. Theoretical analyses indicate that vestibular plasticity should depend upon the correlation between Purkinje cell and vestibular afferent inputs, so that, in gain-down learning for example, increased cortical activity should induce long-term depression (LTD) at vestibular synapses.
Methodology/Principal Findings: Here we expressed this correlational learning rule in its simplest form, as an anti-Hebbian, heterosynaptic spike-timing dependent plasticity interaction between excitatory (vestibular) and inhibitory (floccular) inputs converging on medial vestibular nucleus (MVN) neurons (input-spike-timing dependent plasticity, iSTDP). To test this rule, we stimulated vestibular afferents to evoke EPSCs in rat MVN neurons in vitro. Control EPSC recordings were followed by an induction protocol where membrane hyperpolarizing pulses, mimicking IPSPs evoked by flocculus inputs, were paired with single vestibular nerve stimuli. A robust LTD developed at vestibular synapses when the afferent EPSPs coincided with membrane hyperpolarisation, while EPSPs occurring before or after the simulated IPSPs induced no lasting change. Furthermore, the iSTDP rule also successfully predicted the effects of a complex protocol using EPSP trains designed to mimic classical conditioning.
Conclusions: These results, in strong support of theoretical predictions, suggest that the cerebellum alters the strength of vestibular synapses on MVN neurons through hetero-synaptic, anti-Hebbian iSTDP. Since the iSTDP rule does not depend on post-synaptic firing, it suggests a possible mechanism for VOR adaptation without compromising gaze-holding and VOR performance in vivo
A Remarkable Oxygen-rich Asymptotic Giant Branch Variable in the Sagittarius Dwarf Irregular Galaxy
We report and discuss JHKs photometry for Sgr dIG, a very metal-deficient
galaxy in the Local Group, obtained over 3.5 years with the Infrared Survey
Facility in South Africa. Three large amplitude asymptotic giant branch
variables are identified. One is an oxygen-rich star that has a pulsation
period of 950 days, that was until recently undergoing hot bottom burning, with
Mbol~-6.7. It is surprising to find a variable of this sort in Sgr dIG, given
their rarity in other dwarf irregulars. Despite its long period the star is
relatively blue and is fainter, at all wavelengths shorter than 4.5microns,
than anticipated from period-luminosity relations that describe hot bottom
burning stars. A comparison with models suggests it had a main sequence mass
Mi~5 times solar and that it is now near the end of its AGB evolution. The
other two periodic variables are carbon stars with periods of 670 and 503 days
(Mbol~-5.7 and -5.3). They are very similar to other such stars found on the
AGB of metal deficient Local Group Galaxies and a comparison with models
suggests Mi~3 times solar. We compare the number of AGB variables in Sgr dIG to
those in NGC6822 and IC1613, and suggest that the differences may be due to the
high specific star formation rate and low metallicity of Sgr dIG.Comment: 12 pages, 10 figures, accepted for MNRA
Period-Luminosity Relation for Type II Cepheids
We have estimated JHKs magnitudes corrected to mean intensity for LMC type II
Cepheids found in the OGLE-III survey. Period-luminosity relations (PLRs) are
derived in JHKs as well as in a reddening-free VI parameter. The BL Her stars
(P<4d) and the W Vir stars (P=4 to 20d) are co-linear in these PLRs. The slopes
of the infrared relations agree with those found previously for type II
Cepheids in globular clusters within the uncertainties. Using the pulsation
parallaxes of V553 Cen and SW Tau, the data lead to an LMC modulus of
18.46+-0.10 mag, uncorrected for any metallicity effects. We have now
established the PLR of type II Cepheids as a distance indicator by confirming
that (almost) the same PLR satisfies the distributions in the PL diagram of
type II Cepheids in (at least) two different systems, i.e. the LMC and Galactic
globular clusters, and by calibrating the zero point of the PLR. RV Tau stars
in the LMC, as a group, are not co-linear with the shorter-period type II
Cepheids in the infrared PLRs in marked contrast to such stars in globular
clusters. We note differences in period distribution and infrared colors for RV
Tau stars in the LMC, globular clusters and Galactic field. We also compare the
PLR of type II Cepheids with that of classical Cepheids.Comment: To appear in the proceedings for the conference "Stellar Pulsation:
Challenges for Theory and Observations" held in Santa Fe, US
Asymptotic Giant Branch Stars in the Leo I Dwarf Spheroidal Galaxy
Twenty six Asymptotic Giant Branch (AGB) variables are identified in the
Local Group galaxy Leo I. These include 7 Mira and 5 semi-regular variables for
which periods, amplitudes and mean magnitudes are determined. The large range
of periods for the Miras, 158<P<523 days, suggests an AGB spanning a
significant age range. The youngest must be around 1.6 Gyr while the oldest
could be 10 Gyr or more. Two of these old Miras are found in the outer regions
of Leo I (over 490 arcsec from the centre) where stars on the extended AGB are
rare. They could provide an interesting test of third dredge-up and mass loss
in old stars with low metallicity and are worth further detailed investigation.
At least two stars, one a Mira, the other an irregular variable, are undergoing
obscuration events due to dust ejection.
An application of the Mira period-luminosity relation to these stars yields a
distance modulus for Leo I of (m-M)=21.80 \pm 0.11 mag (internal), \pm 0.12
(total) (on a scale that puts the LMC at 18.39 mag) in good agreement with
other determinations.Comment: 10 pages, 8 figures, accepted for publication in MNRAS
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