158,599 research outputs found
Global and regional brain metabolic scaling and its functional consequences
Background: Information processing in the brain requires large amounts of
metabolic energy, the spatial distribution of which is highly heterogeneous
reflecting complex activity patterns in the mammalian brain.
Results: Here, it is found based on empirical data that, despite this
heterogeneity, the volume-specific cerebral glucose metabolic rate of many
different brain structures scales with brain volume with almost the same
exponent around -0.15. The exception is white matter, the metabolism of which
seems to scale with a standard specific exponent -1/4. The scaling exponents
for the total oxygen and glucose consumptions in the brain in relation to its
volume are identical and equal to , which is significantly larger
than the exponents 3/4 and 2/3 suggested for whole body basal metabolism on
body mass.
Conclusions: These findings show explicitly that in mammals (i)
volume-specific scaling exponents of the cerebral energy expenditure in
different brain parts are approximately constant (except brain stem
structures), and (ii) the total cerebral metabolic exponent against brain
volume is greater than the much-cited Kleiber's 3/4 exponent. The
neurophysiological factors that might account for the regional uniformity of
the exponents and for the excessive scaling of the total brain metabolism are
discussed, along with the relationship between brain metabolic scaling and
computation.Comment: Brain metabolism scales with its mass well above 3/4 exponen
Volume Integral Formulation for the Calculation of Material Independent Modes of Dielectric Scatterers
In the frame of volume integral equation methods, we introduce an alternative
representation of the electromagnetic field scattered by a homogeneous object
of arbitrary shape at a given frequency, in terms of a set of modes independent
of its permittivity. This is accomplished by introducing an auxiliary
eigenvalue problem, based on a volume integral operator. With this modal basis
the expansion coefficients of the scattered field are simple rational functions
of the permittivity of the scatterer. We show, by studying the electromagnetic
scattering from a sphere and a cylinder of dimensions comparable to the
incident wavelength, that only a moderate number of modes is needed to
accurately describe the scattered far field. This method can be used to
investigate resonant scattering phenomena, including plasmonic and photonic
resonances, and to design the permittivity of the object to pursue a prescribed
tailoring of the scattered field. Moreover, the presented modal expansion is
computationally advantageous compared to direct solution of the volume integral
equation when the scattered field has to be computed for many different values
of the dielectric permittivity, given the size and shape of the dielectric
body
Metabolic analyzer
An apparatus is described for the measurement of metabolic rate and breathing dynamics in which inhaled and exhaled breath are sensed by sealed, piston-displacement type spirometers. These spirometers electrically measure the volume of inhaled and exhaled breath. A mass spectrometer analyzes simultaneously for oxygen, carbon dioxide, nitrogen and water vapor. Computation circuits are responsive to the outputs of the spirometers, mass spectrometer, temperature, pressure and timing signals and compute oxygen consumption, carbon dioxide production, minute volume and respiratory exchange ratio. A selective indicator provides for read-out of these data at predetermined cyclic intervals
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