455 research outputs found
Pulsed laser deposition growth of Fe3O4 on IIIâV semiconductors for spin injection
We report on the growth of thin layers of Fe3O4 on GaAs and InAs by pulsed laser deposition. It is found that Fe3O4 grows epitaxially on InAs at a temperature of 350 °C. X-ray photoelecton spectroscopy (XPS) studies of the interface show little if any interface reaction resulting in a clean epitaxial interface. In contrast, Fe3O4 grows in columnar fashion on GaAs, oriented with respect to the growth direction but with random orientation in the plane of the substrate. In this case XPS analysis showed much more evidence of interface reactions, which may contribute to the random-in-plane growth
Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magnetic fields
We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets
can also act as efficient magnetic shields. The magnetic screening properties
under an axial DC magnetic field are characterized at several temperatures
below the liquid nitrogen temperature (77 K). Two main shielding properties are
studied and compared with those of Bi-2223, a material that has been considered
in the past for bulk magnetic shields. The first property is related to the
maximum magnetic flux density that can be screened, Blim; it is defined as the
applied magnetic flux density below which the field attenuation measured at the
centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall
thickness of 5 mm and a large ratio of length over radius, Blim is evaluated to
1 T at T = 10 K. This value largely exceeds the Blim value measured at the same
temperature on similar tubes of Bi-2223. The second shielding property that is
characterized is the dependence of Blim with respect to variations of the sweep
rate of the applied field, dBapp/dt. This dependence is interpreted in terms of
the power law E = Ec(J/Jc)^n and allows us to determine the exponent n of this
E(J) characteristics for Bi-2212. The characterization of the magnetic field
relaxation involves very small values of the electric field. This gives us the
opportunity to experimentally determine the E(J) law in an unexplored region of
small electric fields. Combining these results with transport and AC shielding
measurements, we construct a piecewise E(J) law that spans over 8 orders of
magnitude of the electric field.Comment: 16 pages, 7 figure
ERCC1 expression and RAD51B activity correlate with cell cycle response to platinum drug treatment not DNA repair
Background: The H69CIS200 and H69OX400 cell lines are novel models of low-level platinum-drug resistance. Resistance was not associated with increased cellular glutathione or decreased accumulation of platinum, rather the resistant cell lines have a cell cycle alteration allowing them to rapidly proliferate post drug treatment. Results: A decrease in ERCC1 protein expression and an increase in RAD51B foci activity was observed in association with the platinum induced cell cycle arrest but these changes did not correlate with resistance or altered DNA repair capacity. The H69 cells and resistant cell lines have a p53 mutation and consequently decrease expression of p21 in response to platinum drug treatment, promoting progression of the cell cycle instead of increasing p21 to maintain the arrest.
Conclusion: Decreased ERCC1 protein and increased RAD51B foci may in part be mediating the maintenance of the cell cycle arrest in the sensitive cells. Resistance in the H69CIS200 and H69OX400 cells may therefore involve the regulation of ERCC1 and RAD51B independent of their roles in DNA repair. The novel mechanism of platinum resistance in the H69CIS200 and H69OX400 cells demonstrates the multifactorial nature of platinum resistance which can occur independently of alterations in DNA repair capacity and changes in ERCC1
Pitch Enumeration: Failure to Subitize in Audition
Background: Subitizing involves recognition mechanisms that allow effortless enumeration of up to four visual objects, however despite ample resolution experimental data suggest that only one pitch can be reliably enumerated. This may be due to the grouping of tones according to harmonic relationships by recognition mechanisms prior to fine pitch processing. Poorer frequency resolution of auditory information available to recognition mechanisms may lead to unrelated tones being grouped, resulting in underestimation of pitch number. Methods, Results and Conclusion: We tested whether pitch enumeration is better for chords of full harmonic complex tones, where grouping errors are less likely, than for complexes with fewer and less accurately tuned harmonics. Chords of low familiarity were used to mitigate the possibility that participants would recognize the chord itself and simply recall the number of pitches. We found that accuracy of pitch enumeration was less than the visual system overall, and underestimation of pitch number increased for stimuli containing fewer harmonics. We conclude that harmonically related tones are first grouped at the poorer frequency resolution of the auditory nerve, leading to poor enumeration of more than one pitch
Boolean Dynamics with Random Couplings
This paper reviews a class of generic dissipative dynamical systems called
N-K models. In these models, the dynamics of N elements, defined as Boolean
variables, develop step by step, clocked by a discrete time variable. Each of
the N Boolean elements at a given time is given a value which depends upon K
elements in the previous time step.
We review the work of many authors on the behavior of the models, looking
particularly at the structure and lengths of their cycles, the sizes of their
basins of attraction, and the flow of information through the systems. In the
limit of infinite N, there is a phase transition between a chaotic and an
ordered phase, with a critical phase in between.
We argue that the behavior of this system depends significantly on the
topology of the network connections. If the elements are placed upon a lattice
with dimension d, the system shows correlations related to the standard
percolation or directed percolation phase transition on such a lattice. On the
other hand, a very different behavior is seen in the Kauffman net in which all
spins are equally likely to be coupled to a given spin. In this situation,
coupling loops are mostly suppressed, and the behavior of the system is much
more like that of a mean field theory.
We also describe possible applications of the models to, for example, genetic
networks, cell differentiation, evolution, democracy in social systems and
neural networks.Comment: 69 pages, 16 figures, Submitted to Springer Applied Mathematical
Sciences Serie
Sensitivity of the human auditory cortex to acoustic degradation of speech and non-speech sounds
The perception of speech is usually an effortless and reliable process even in highly adverse listening conditions. In addition to external sound sources, the intelligibility of speech can be reduced by degradation of the structure of speech signal itself, for example by digital compression of sound. This kind of distortion may be even more detrimental to speech intelligibility than external distortion, given that the auditory system will not be able to utilize sound source-specific acoustic features, such as spatial location, to separate the distortion from the speech signal. The perceptual consequences of acoustic distortions on speech intelligibility have been extensively studied. However, the cortical mechanisms of speech perception in adverse listening conditions are not well known at present, particularly in situations where the speech signal itself is distorted. The aim of this thesis was to investigate the cortical mechanisms underlying speech perception in conditions where speech is less intelligible due to external distortion or as a result of digital compression.
In the studies of this thesis, the intelligibility of speech was varied either by digital compression or addition of stochastic noise. Cortical activity related to the speech stimuli was measured using magnetoencephalography (MEG). The results indicated that degradation of speech sounds by digital compression enhanced the evoked responses originating from the auditory cortex, whereas addition of stochastic noise did not modulate the cortical responses. Furthermore, it was shown that if the distortion was presented continuously in the background, the transient activity of auditory cortex was delayed. On the perceptual level, digital compression reduced the comprehensibility of speech more than additive stochastic noise. In addition, it was also demonstrated that prior knowledge of speech content enhanced the intelligibility of distorted speech substantially, and this perceptual change was associated with an increase in cortical activity within several regions adjacent to auditory cortex.
In conclusion, the results of this thesis show that the auditory cortex is very sensitive to the acoustic features of the distortion, while at later processing stages, several cortical areas reflect the intelligibility of speech. These findings suggest that the auditory system rapidly adapts to the variability of the auditory environment, and can efficiently utilize previous knowledge of speech content in deciphering acoustically degraded speech signals.Puheen havaitseminen on useimmiten vaivatonta ja luotettavaa myös erittÀin huonoissa kuunteluolosuhteissa. Puheen ymmÀrrettÀvyys voi kuitenkin heikentyÀ ympÀristön hÀiriölÀhteiden lisÀksi myös silloin, kun puhesignaalin rakennetta muutetaan esimerkiksi pakkaamalla digitaalista ÀÀntÀ. TÀllainen hÀiriö voi heikentÀÀ ymmÀrrettÀvyyttÀ jopa ulkoisia hÀiriöitÀ voimakkaammin, koska kuulojÀrjestelmÀ ei pysty hyödyntÀmÀÀn ÀÀnilÀhteen ominaisuuksia, kuten ÀÀnen tulosuuntaa, hÀiriön erottelemisessa puheesta. Akustisten hÀiriöiden vaikutuksia puheen havaitsemiseen on tutkttu laajalti, mutta havaitsemiseen liittyvÀt aivomekanismit tunnetaan edelleen melko puutteelisesti etenkin tilanteissa, joissa itse puhesignaali on laadultaan heikentynyt. TÀmÀn vÀitöskirjan tavoitteena oli tutkia puheen havaitsemisen aivomekanismeja tilanteissa, joissa puhesignaali on vaikeammin ymmÀrrettÀvissÀ joko ulkoisen ÀÀnilÀhteen tai digitaalisen pakkauksen vuoksi.
VÀitöskirjan neljÀssÀ osatutkimuksessa lyhyiden puheÀÀnien ja jatkuvan puheen ymmÀrrettÀvyyttÀ muokattiin joko digitaalisen pakkauksen kautta tai lisÀÀmÀllÀ puhesignaaliin satunnaiskohinaa. PuheÀrsykkeisiin liittyvÀÀ aivotoimintaa tutkittiin magnetoenkefalografia-mittauksilla. Tutkimuksissa havaittiin, ettÀ kuuloaivokuorella syntyneet herÀtevasteet voimistuivat, kun puheÀÀntÀ pakattiin digitaalisesti. Sen sijaan puheÀÀniin lisÀtty satunnaiskohina ei vaikuttanut herÀtevasteisiin. Edelleen, mikÀli puheÀÀnien taustalla esitettiin jatkuvaa hÀiriötÀ, kuuloaivokuoren aktivoituminen viivÀstyi hÀiriön intensiteetin kasvaessa. Kuuntelukokeissa havaittiin, ettÀ digitaalinen pakkaus heikentÀÀ puheÀÀnien ymmÀrrettÀvyyttÀ voimakkaammin kuin satunnaiskohina. LisÀksi osoitettiin, ettÀ aiempi tieto puheen sisÀllöstÀ paransi merkittÀvÀsti hÀiriöisen puheen ymmÀrrettÀvyyttÀ, mikÀ heijastui aivotoimintaan kuuloaivokuoren viereisillÀ aivoalueilla siten, ettÀ ymmÀrrettÀvÀ puhe aiheutti suuremman aktivaation kuin heikosti ymmÀrrettÀvÀ puhe.
VÀitöskirjan tulokset osoittavat, ettÀ kuuloaivokuori on erittÀin herkkÀ puheÀÀnien akustisille hÀiriöille, ja myöhemmissÀ prosessoinnin vaiheissa useat kuuloaivokuoren viereiset aivoalueet heijastavat puheen ymmÀrrettÀvyyttÀ. Tulosten mukaan voi olettaa, ettÀ kuulojÀrjestelmÀ mukautuu nopeasti ÀÀniympÀristön vaihteluihin muun muassa hyödyntÀmÀllÀ aiempaa tietoa puheen sisÀllöstÀ tulkitessaan hÀiriöistÀ puhesignaalia
Insights on the Neuromagnetic Representation of Temporal Asymmetry in Human Auditory Cortex.
Communication sounds are typically asymmetric in time and human listeners are highly sensitive to this short-term temporal asymmetry. Nevertheless, causal neurophysiological correlates of auditory perceptual asymmetry remain largely elusive to our current analyses
and models. Auditory modelling and animal electrophysiological recordings suggest that perceptual asymmetry results from the presence of multiple time scales of temporal integration, central to the auditory periphery. To test this hypothesis we recorded auditory evoked fields (AEF) elicited by asymmetric sounds in humans. We found a strong correlation between perceived tonal salience of ramped and damped sinusoids and the AEFs, as quantified by the amplitude of the N100m dynamics. The N100m amplitude increased with stimulus
half-life time, showing a maximum difference between the ramped and damped stimulus for a modulation half-life time of 4 ms which is greatly reduced at 0.5 ms and 32 ms. This behaviour of the N100m closely parallels psychophysical data in a manner that: i) longer
half-life times are associated with a stronger tonal percept, and ii) perceptual differences between damped and ramped are maximal at 4 ms half-life time. Interestingly, differences in evoked fields were significantly stronger in the right hemisphere, indicating some degree of hemispheric specialisation. Furthermore, the N100m magnitude was successfully
explained by a pitch perception model using multiple scales of temporal integration of auditory
nerve activity patterns. This striking correlation between AEFs, perception, and model predictions suggests that the physiological mechanisms involved in the processing of pitch evoked by temporal asymmetric sounds are reflected in the N100m
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