10,592 research outputs found
An oscillatory interference model of grid cell firing
We expand upon our proposal that the oscillatory interference mechanism proposed for the phase precession effect in place cells underlies the grid-like firing pattern of dorsomedial entorhinal grid cells (O'Keefe and Burgess (2005) Hippocampus 15:853-866). The original one-dimensional interference model is generalized to an appropriate two-dimensional mechanism. Specifically, dendritic subunits of layer 11 medial entorhinal stellate cells provide multiple linear interference patterns along different directions, with their product determining the firing of the cell. Connection of appropriate speed- and direction- dependent inputs onto dendritic subunits could result from an unsupervised learning rule which maximizes postsynaptic firing (e.g. competitive learning). These inputs cause the intrinsic oscillation of subunit membrane potential to. increase above theta frequency by an amount proportional to the animal's speed of running in the "preferred" direction. The phase difference between this oscillation and a somatic input at theta-frequency essentially integrates velocity so that the interference of the two oscillations reflects distance traveled in the preferred direction. The overall grid pattern is maintained in environmental location by phase reset of the grid cell by place cells receiving sensory input from the environment, and environmental boundaries in particular. We also outline possible variations on the basic model, including the generation of grid-like firing via the interaction of multiple cells rather than via multiple dendritic subunits. Predictions of the interference model are given for the frequency composition of EEG power spectra and temporal autocorrelograms of grid cell firing as functions of the speed and direction of running and the novelty of the environment. (C) 2007 Wiley-Liss, Inc
Theta-modulated place-by-direction cells in the hippocampal formation in the rat
We report the spatial and temporal properties of a class of cells termed theta-modulated place-by-direction (TPD) cells recorded from the presubicular and parasubicular cortices of the rat. The firing characteristics of TPD cells in open-field enclosures were compared with those of the following two other well characterized cell classes in the hippocampal formation: place and head-direction cells. Unlike place cells, which code only for the animal's location, or head-direction cells, which code only for the animal's directional heading, TPD cells code for both the location and the head direction of the animal. Their firing is also strongly theta modulated, firing primarily at the negative-to-positive phase of the locally recorded theta wave. TPD theta modulation is significantly stronger than that of place cells. In contrast, the firing of head-direction cells is not modulated by theta at all. In repeated exposures to the same environment, the locational and directional signals of TPD cells are stable. When recorded in different environments, TPD locational and directional fields can uncouple, with the locational field shifting unpredictably ("remapping"), whereas the directional preference remains similar across environments
A comparison of the responses of mature and young clonal tea to drought.
To assist commercial producers with optimising the use of irrigation water, the
responses to drought of mature and young tea crops (22 and 5 years after field
planting respectively) were compared using data from two adjacent long-term
irrigation experiments in Southern Tanzania. Providing the maximum potential
soil water deficit was below about 400-500 mm for mature, and 200-250 mm for
young plants (clone 6/8), annual yields of dry tea from rainfed or partially
irrigated crops were similar to those from the corresponding well-watered crops.
At deficits greater than this, annual yields declined rapidly in young tea (up
to 22 kg (ha mm)-1) but relatively slowly in mature tea (up to 6.5 kg (ha mm)-
1). This apparent insensitivity of the mature crop to drought was due
principally to compensation that occurred during the rains for yield lost in the
dry season. Differences in dry matter distribution and shoot to root ratios
contributed to these contrasting responses. Thus, the total above ground dry
mass of well-irrigated, mature plants was about twice that for young plants.
Similarly, the total mass of structural roots (>1 mm diameter), to 3 m depth,
was four times greater in the mature crop than in the young crop and, for fine
roots (<1 mm diameter), eight times greater. The corresponding shoot to root
ratios (dry mass) were about 1:1 and 2:1 respectively. In addition, each unit
area of leaf in the canopy of a mature plant had six times more fine roots (by
weight) available to extract and supply water than did a young plant. Despite
the logistical benefits resulting from more even crop distribution during the
year when crops are fully irrigated, producers currently prefer to save water
and energy costs by allowing a substantial soil water deficit to develop prior
to the start of the rains, up to 250 mm in mature tea, knowing that yield
compensation will occur later
Clinical trials in South Africa : need for capacity building and training
The original publication is available at http://www.samj.org.zaPublishers' Versio
Clinical trials in South Africa : need for capacity building and training
The original publication is available at http://www.samj.org.zaPublishers' Versio
Cosmology and Static Spherically Symmetric solutions in D-dimensional Scalar Tensor Theories: Some Novel Features
We consider scalar tensor theories in D-dimensional spacetime, D \ge 4. They
consist of metric and a non minimally coupled scalar field, with its non
minimal coupling characterised by a function. The probes couple minimally to
the metric only. We obtain vacuum solutions - both cosmological and static
spherically symmetric ones - and study their properties. We find that, as seen
by the probes, there is no singularity in the cosmological solutions for a
class of functions which obey certain constraints. It turns out that for the
same class of functions, there are static spherically symmetric solutions which
exhibit novel properties: {\em e.g.} near the ``horizon'', the gravitational
force as seen by the probe becomes repulsive.Comment: Revtex. 21 pages. Version 2: More references added. Version 3: Issues
raised by the referee are addressed. Results unchanged. Title modified; a new
subsection and more references added. Verison to appear in Physical Review
Braneworld inflation
We discuss various realizations of the four dimensional braneworld inflation
in warped geometries of string theory. In all models the inflaton field is
represented by a Dp probe brane scalar specifying its position in the warped
throat of the compactification manifold. We study existing inflationary throat
local geometries, and construct a new example. The inflationary brane is either
a D3- or a D5-brane of type IIB string theory. In the latter case the
inflationary brane is wrapping a two-cycle of the compactification manifold. We
discuss some phenomenological aspects of the model where slow-roll conditions
are under computational control.Comment: 31 pages + 6 figures, v2: published PRD versio
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