1,243 research outputs found
Noise-induced memory in extended excitable systems
We describe a form of memory exhibited by extended excitable systems driven
by stochastic fluctuations. Under such conditions, the system self-organizes
into a state characterized by power-law correlations thus retaining long-term
memory of previous states. The exponents are robust and model-independent. We
discuss novel implications of these results for the functioning of cortical
neurons as well as for networks of neurons.Comment: 4 pages, latex + 5 eps figure
From synaptic interactions to collective dynamics in random neuronal networks models: critical role of eigenvectors and transient behavior
The study of neuronal interactions is currently at the center of several
neuroscience big collaborative projects (including the Human Connectome, the
Blue Brain, the Brainome, etc.) which attempt to obtain a detailed map of the
entire brain matrix. Under certain constraints, mathematical theory can advance
predictions of the expected neural dynamics based solely on the statistical
properties of such synaptic interaction matrix. This work explores the
application of free random variables (FRV) to the study of large synaptic
interaction matrices. Besides recovering in a straightforward way known results
on eigenspectra of neural networks, we extend them to heavy-tailed
distributions of interactions. More importantly, we derive analytically the
behavior of eigenvector overlaps, which determine stability of the spectra. We
observe that upon imposing the neuronal excitation/inhibition balance, although
the eigenvalues remain unchanged, their stability dramatically decreases due to
strong non-orthogonality of associated eigenvectors. It leads us to the
conclusion that the understanding of the temporal evolution of asymmetric
neural networks requires considering the entangled dynamics of both
eigenvectors and eigenvalues, which might bear consequences for learning and
memory processes in these models. Considering the success of FRV analysis in a
wide variety of branches disciplines, we hope that the results presented here
foster additional application of these ideas in the area of brain sciences.Comment: 24 pages + 4 pages of refs, 8 figure
Indução dos genes da nodulação de Rhizobium tropici pela presença de exsudatos de sementes de leguminosas.
A Search for Planets Transiting the M Dwarf Debris Disk Host, AU Microscopii
We present high cadence, high precision multi-band photometry of the young,
M1Ve, debris disk star, AU Microscopii. The data were obtained in three
continuum filters spanning a wavelength range from 4500\AA to 6600\AA, plus
H, over 28 nights in 2005. The lightcurves show intrinsic stellar
variability due to starspots with an amplitude in the blue band of 0.051
magnitudes and a period of 4.847 days. In addition, three large flares were
detected in the data which all occur near the minimum brightness of the star.
We remove the intrinsic stellar variability and combine the lightcurves of all
the filters in order to search for transits by possible planetary companions
orbiting in the plane of the nearly edge-on debris disk. The combined final
lightcurve has a sampling of 0.35 minutes and a standard deviation of 6.8
millimags (mmag). We performed Monte Carlo simulations by adding fake transits
to the observed lightcurve and find with 95% significance that there are no
Jupiter mass planets orbiting in the plane of the debris disk on circular
orbits with periods, P days. In addition, there are no young
Neptune-like planets (with radii 2.5 smaller than the young Jupiter) on
circular orbits with periods, P days.Comment: accepted to MNRA
How we move is universal: scaling in the average shape of human activity
Human motor activity is constrained by the rhythmicity of the 24 hours
circadian cycle, including the usual 12-15 hours sleep-wake cycle. However,
activity fluctuations also appear over a wide range of temporal scales, from
days to a few seconds, resulting from the concatenation of a myriad of
individual smaller motor events. Furthermore, individuals present different
propensity to wakefulness and thus to motor activity throughout the circadian
cycle. Are activity fluctuations across temporal scales intrinsically
different, or is there a universal description encompassing them? Is this
description also universal across individuals, considering the aforementioned
variability? Here we establish the presence of universality in motor activity
fluctuations based on the empirical study of a month of continuous wristwatch
accelerometer recordings. We study the scaling of average fluctuations across
temporal scales and determine a universal law characterized by critical
exponents , and . Results are highly reminiscent of the
universality described for the average shape of avalanches in systems
exhibiting crackling noise. Beyond its theoretical relevance, the present
results can be important for developing objective markers of healthy as well as
pathological human motor behavior.Comment: Communicated to the Granada Seminar, "Physics Meets the Social
Sciences: Emergent cooperative phenomena, from bacterial to human group
behavior". June 14-19, 2015. La Herradura, Spai
Spared nerve injury rats exhibit thermal hyperalgesia on an automated operant dynamic thermal escape Task
Well-established methods are available to measure thermal and mechanical sensitivity in awake behaving rats. However, they require experimenter manipulations and tend to emphasize reflexive behaviors. Here we introduce a new behavioral test, with which we examine thermal sensitivity of rats with neuropathic injury. We contrast thermal hyperalgesia between spared nerve injury and chronic constriction injury rats. This device is a fully automated thermal sensitivity assessment tool designed to emphasize integrated learned responses to thermal painful and non-painful stimuli that are applied dynamically to a surface on which the animal is standing. It documents escape behavior in awake, unrestrained animals to innocuous and noxious heating of the floor where the animal is located. Animals learn to minimize pain by escaping to the opposite non-heated side; escape latency is recorded. On this device, thermal stimulus-response curves showed > 6°C leftward shift in both groups of neuropathic rats. In contrast, when these animals were tested on hotplate the stimulus-response shift was < 2°C. Spared nerve injury rats showed even less evidence for thermal hyperalgesia when thermal sensitivity was tested by measuring paw withdrawal to infrared heating, plantar test. The implications of test dependent magnitude of thermal hyperalgesia are discussed from the viewpoint of the tests used, as well as the animal models studied. It is argued that the dynamic thermal operant task reveals the relevance of the neuropathic injury associated pain-like behavior in relation to the whole organism
- …