13,257 research outputs found
Abnormal negative feedback processing in first episode schizophrenia: evidence from an oculomotor rule switching task
Background. Previous studies have shown that patients with schizophrenia are impaired on executive tasks,
where positive and negative feedbacks are used to update task rules or switch attention. However, research to date
using saccadic tasks has not revealed clear deficits in task switching in these patients. The present study used an
oculomotor ‘ rule switching ’ task to investigate the use of negative feedback when switching between task rules in
people with schizophrenia.
Method. A total of 50 patients with first episode schizophrenia and 25 healthy controls performed a task in which the association between a centrally presented visual cue and the direction of a saccade could change from trial to trial. Rule changes were heralded by an unexpected negative feedback, indicating that the cue-response mapping
had reversed.
Results. Schizophrenia patients were found to make increased errors following a rule switch, but these were almost entirely the result of executing saccades away from the location at which the negative feedback had been presented on the preceding trial. This impairment in negative feedback processing was independent of IQ.
Conclusions. The results not only confirm the existence of a basic deficit in stimulus–response rule switching in
schizophrenia, but also suggest that this arises from aberrant processing of response outcomes, resulting in a failure to appropriately update rules. The findings are discussed in the context of neurological and pharmacological
abnormalities in the conditions that may disrupt prediction error signalling in schizophrenia
Assembly bias and the dynamical structure of dark matter halos
Based on the Millennium Simulation we examine assembly bias for the halo
properties: shape, triaxiality, concentration, spin, shape of the velocity
ellipsoid and velocity anisotropy. For consistency we determine all these
properties using the same set of particles, namely all gravitationally
self-bound particles belonging to the most massive sub-structure of a given
friends-of-friends halo. We confirm that near-spherical and high-spin halos
show enhanced clustering. The opposite is true for strongly aspherical and
low-spin halos. Further, below the typical collapse mass, M*, more concentrated
halos show stronger clustering whereas less concentrated halos are less
clustered which is reversed for masses above M*. Going beyond earlier work we
show that: (1) oblate halos are more strongly clustered than prolate ones; (2)
the dependence of clustering on the shape of the velocity ellipsoid coincides
with that of the real-space shape, although the signal is stronger; (3) halos
with weak velocity anisotropy are more clustered, whereas radially anisotropic
halos are more weakly clustered; (4) for all highly clustered subsets we find
systematically less radially biased velocity anisotropy profiles. These
findings indicate that the velocity structure of halos is tightly correlated
with environment.Comment: 5 pages, 2 figures, accepted for publication in Ap
Experiments to investigate particulate materials in reduced gravity fields
Study investigates agglomeration and macroscopic behavior in reduced gravity fields of particles of known properties by measuring and correlating thermal and acoustical properties of particulate materials. Experiment evaluations provide a basis for a particle behavior theory and measure bulk properties of particulate materials in reduced gravity
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
Density mapping with weak lensing and phase information
The available probes of the large scale structure in the Universe have
distinct properties: galaxies are a high resolution but biased tracer of mass,
while weak lensing avoids such biases but, due to low signal-to-noise ratio,
has poor resolution. We investigate reconstructing the projected density field
using the complementarity of weak lensing and galaxy positions. We propose a
maximum-probability reconstruction of the 2D lensing convergence with a
likelihood term for shear data and a prior on the Fourier phases constructed
from the galaxy positions. By considering only the phases of the galaxy field,
we evade the unknown value of the bias and allow it to be calibrated by lensing
on a mode-by-mode basis. By applying this method to a realistic simulated
galaxy shear catalogue, we find that a weak prior on phases provides a good
quality reconstruction down to scales beyond l=1000, far into the noise domain
of the lensing signal alone.Comment: 11 pages, 9 figures, published in MNRA
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Semantic fluency in deaf children who use spoken and signed language, in comparison to hearing peers
Background
Deafness has an adverse impact on children’s ability to acquire spoken languages. Signed languages offer a more accessible input for deaf children, but because the vast majority are born to hearing parents who do not sign, their early exposure to sign language is limited. Deaf children as a whole are therefore at high risk of language delays.
Aims
We compared deaf and hearing children’s performance on a semantic fluency task. Optimal performance on this task requires a systematic search of the mental lexicon, the retrieval of words within a subcategory, and, when that subcategory is exhausted, switching to a new subcategory. We compared retrieval patterns between groups, and also compared the responses of deaf children who used British Sign Language (BSL) to those who used spoken English. We investigated how semantic fluency performance related to children’s expressive vocabulary and executive function skills, and also re-tested semantic fluency in the majority of the children nearly two years later, in order to investigate how much progress they had made in that time.
Methods and procedures
Participants were deaf children aged 6-11 years (N=106, comprising 69 users of spoken English, 29 users of BSL and 8 users of Sign Supported English) compared to hearing children (N=120) of the same age who used spoken English. Semantic fluency was tested for the category “animals”. We coded for errors, clusters (e.g., “pets”, “farm animals”) and switches. Participants also completed the Expressive One-Word Picture Vocabulary Test and a battery of six non-verbal executive function tasks. In addition, we collected follow-up semantic fluency data for 70 deaf and 74 hearing children, nearly 2 years after they were first tested.
Outcomes and results
Deaf children, whether using spoken or signed language, produced fewer items in the semantic fluency task than hearing children, but they showed similar patterns of responses for items most commonly produced, clustering of items into subcategories and switching between subcategories. Both vocabulary and executive function scores predicted the number of correct items produced. Follow-up data from deaf participants showed continuing delays relative to hearing children two years later.
Conclusions and implications
We conclude that semantic fluency can be used experimentally to investigate lexical organisation in deaf children, and that it potentially has clinical utility across the heterogeneous deaf population. We present normative data to aid clinicians who wish to use this task with deaf children
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