272,602 research outputs found
Laminar Cortical Dynamics of Visual Form and Motion Interactions During Coherent Object Motion Perception
How do visual form and motion processes cooperate to compute object motion when each process separately is insufficient? A 3D FORMOTION model specifies how 3D boundary representations, which separate figures from backgrounds within cortical area V2, capture motion signals at the appropriate depths in MT; how motion signals in MT disambiguate boundaries in V2 via MT-to-Vl-to-V2 feedback; how sparse feature tracking signals are amplified; and how a spatially anisotropic motion grouping process propagates across perceptual space via MT-MST feedback to integrate feature-tracking and ambiguous motion signals to determine a global object motion percept. Simulated data include: the degree of motion coherence of rotating shapes observed through apertures, the coherent vs. element motion percepts separated in depth during the chopsticks illusion, and the rigid vs. non-rigid appearance of rotating ellipses.Air Force Office of Scientific Research (F49620-01-1-0397); National Geospatial-Intelligence Agency (NMA201-01-1-2016); National Science Foundation (BCS-02-35398, SBE-0354378); Office of Naval Research (N00014-95-1-0409, N00014-01-1-0624
The hydrodynamic force on a rigid particle undergoing arbitrary time-dependent motion at small Reynolds number
The hydrodynamic force acting on a rigid spherical particle translating with arbitrary time-dependent motion in a time-dependent flowing fluid is calculated to O(Re) for small but finite values of the Reynolds number, Re, based on the particle's slip velocity relative to the uniform flow. The corresponding expression for an arbitrarily shaped rigid particle is evaluated for the case when the timescale of variation of the particle's slip velocity is much greater than the diffusive scale, a^2/v, where a is the characteristic particle dimension and v is the kinematic viscosity of the fluid. It is found that the expression for the hydrodynamic force is not simply an additive combination of the results from unsteady Stokes flow and steady Oseen flow and that the temporal decay to steady state for small but finite Re is always faster than the t^-œ behaviour of unsteady Stokes flow. For example, when the particle accelerates from rest the temporal approach to steady state scales as t^-2
Motion clouds: model-based stimulus synthesis of natural-like random textures for the study of motion perception
Choosing an appropriate set of stimuli is essential to characterize the
response of a sensory system to a particular functional dimension, such as the
eye movement following the motion of a visual scene. Here, we describe a
framework to generate random texture movies with controlled information
content, i.e., Motion Clouds. These stimuli are defined using a generative
model that is based on controlled experimental parametrization. We show that
Motion Clouds correspond to dense mixing of localized moving gratings with
random positions. Their global envelope is similar to natural-like stimulation
with an approximate full-field translation corresponding to a retinal slip. We
describe the construction of these stimuli mathematically and propose an
open-source Python-based implementation. Examples of the use of this framework
are shown. We also propose extensions to other modalities such as color vision,
touch, and audition
Buoyancy-driven motion of a deformable drop toward a planar wall at low Reynolds number
The slow viscous motion of a deformable drop moving normal to a planar wall is studied numerically. In particular, a boundary integral technique employing the Green's function appropriate to a no-slip planar wall is used. Beginning with spherical drop shapes far from the wall, highly deformed and âdimpledâ drop configurations are obtained as the planar wall is approached. The initial stages of dimpling and their evolution provide information and insight into the basic assumptions of film-drainage theory
The role of terminators and occlusion cues in motion integration and segmentation: a neural network model
The perceptual interaction of terminators and occlusion cues with the functional processes of motion integration and segmentation is examined using a computational model. Inte-gration is necessary to overcome noise and the inherent ambiguity in locally measured motion direction (the aperture problem). Segmentation is required to detect the presence of motion discontinuities and to prevent spurious integration of motion signals between objects with different trajectories. Terminators are used for motion disambiguation, while occlusion cues are used to suppress motion noise at points where objects intersect. The model illustrates how competitive and cooperative interactions among cells carrying out these functions can account for a number of perceptual effects, including the chopsticks illusion and the occluded diamond illusion. Possible links to the neurophysiology of the middle temporal visual area (MT) are suggested
3D view of transient horizontal magnetic fields in the photosphere
We infer the 3D magnetic structure of a transient horizontal magnetic field
(THMF) during its evolution through the photosphere using SIRGAUS inversion
code. The SIRGAUS code is a modified version of SIR (Stokes Inversion based on
Response function), and allows for retrieval of information on the magnetic and
thermodynamic parameters of the flux tube embedded in the atmosphere from the
observed Stokes profiles. Spectro-polarimetric observations of the quiet Sun at
the disk center were performed with the Solar Optical Telescope (SOT) on board
Hinode with Fe I 630.2 nm lines. Using repetitive scans with a cadence of 130
s, we first detect the horizontal field that appears inside a granule, near its
edge. On the second scan, vertical fields with positive and negative polarities
appear at both ends of the horizontal field. Then, the horizontal field
disappears leaving the bipolar vertical magnetic fields. The results from the
inversion of the Stokes spectra clearly point to the existence of a flux tube
with magnetic field strength of G rising through the line forming
layer of the Fe I 630.2 nm lines. The flux tube is located at around
at =0 s and around
at =130 s. At =260 s the horizontal part is already above
the line forming region of the analyzed lines. The observed Doppler velocity is
maximally 3 km s, consistent with the upward motion of the structure as
retrieved from the SIRGAUS code. The vertical size of the tube is smaller than
the thickness of the line forming layer. The THMF has a clear
-shaped-loop structure with the apex located near the edge of a
granular cell. The magnetic flux carried by this THMF is estimated to be
Mx.Comment: 35 pages, 9 figures, Accepted for publication in Ap
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