9,067 research outputs found
Grid Cells Form a Global Representation of Connected Environments.
The firing patterns of grid cells in medial entorhinal cortex (mEC) and associated brain areas form triangular arrays that tessellate the environment [1, 2] and maintain constant spatial offsets to each other between environments [3, 4]. These cells are thought to provide an efficient metric for navigation in large-scale space [5-8]. However, an accurate and universal metric requires grid cell firing patterns to uniformly cover the space to be navigated, in contrast to recent demonstrations that environmental features such as boundaries can distort [9-11] and fragment [12] grid patterns. To establish whether grid firing is determined by local environmental cues, or provides a coherent global representation, we recorded mEC grid cells in rats foraging in an environment containing two perceptually identical compartments connected via a corridor. During initial exposures to the multicompartment environment, grid firing patterns were dominated by local environmental cues, replicating between the two compartments. However, with prolonged experience, grid cell firing patterns formed a single, continuous representation that spanned both compartments. Thus, we provide the first evidence that in a complex environment, grid cell firing can form the coherent global pattern necessary for them to act as a metric capable of supporting large-scale spatial navigation
Neural Dynamics Indicate Parallel Integration of Environmental and Self-Motion Information by Place and Grid Cells
Place cells and grid cells in the hippocampal formation are thought to integrate sensory and self-motion information into a representation of estimated spatial location, but the precise mechanism is unknown. We simulated a parallel attractor system in which place cells form an attractor network driven by environmental inputs and grid cells form an attractor network performing path integration driven by self-motion, with inter-connections between them allowing both types of input to influence firing in both ensembles. We show that such a system is needed to explain the spatial patterns and temporal dynamics of place cell firing when rats run on a linear track in which the familiar correspondence between environmental and self-motion inputs is changed. In contrast, the alternative architecture of a single recurrent network of place cells (performing path integration and receiving environmental inputs) cannot reproduce the place cell firing dynamics. These results support the hypothesis that grid and place cells provide two different but complementary attractor representations (based on self-motion and environmental sensory inputs, respectively). Our results also indicate the specific neural mechanism and main predictors of hippocampal map realignment and make predictions for future studies
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
DBI Lifshitz Inflation
A new model of DBI inflation is introduced where the mobile brane, the
inflaton field, is moving relativistically inside a Lifshitz throat with an
arbitrary anisotropic scaling exponent . After dimensional reduction to four
dimension the general covariance is broken explicitly both in the matter and
the gravitational sectors. The general action for the metric and matter field
perturbations are obtained and it is shown to be similar to the classifications
made in the effective field theory of inflation literature.Comment: Version 3: minor typos corrected, the JCAP published versio
D-Brane Dynamics and NS5 Rings
We consider the classical motion of a probe D-brane moving in the background
geometry of a ring of NS5 branes, assuming that the latter are non-dynamical.
We analyse the solutions to the Dirac-Born-Infield (DBI) action governing the
approximate dynamics of the system. In the near horizon (throat) approximation
we find several exact solutions for the probe brane motion. These are compared
to numerical solutions obtained in more general cases. One solution of
particular interest is when the probe undergoes oscillatory motion through the
centre of the ring (and perpendicular to it). By taking the ring radius
sufficiently large, this solution should remain stable to any stringy
corrections coming from open-strings stretching between the probe and the
NS5-branes along the ring.Comment: 17 pages, Latex, 8 figures; References adde
Negative Emotional Content Disrupts the Coherence of Episodic Memories
Events are thought to be stored in episodic memory as coherent representations, in which the constituent
elements are bound together so that a cue can trigger reexperience of all elements via pattern completion.
Negative emotional content can strongly influence memory, but opposing theories predict strengthening
or weakening of memory coherence. Across a series of experiments, participants imagined a number of
person-location-object events with half of the events including a negative element (e.g., an injured
person), and memory was tested across all within event associations. We show that the presence of a
negative element reduces memory for associations between event elements, including between neutral
elements encoded after a negative element. The presence of a negative element reduces the coherence
with which a multimodal event is remembered. Our results, supported by a computational model, suggest
that coherent retrieval from neutral events is supported by pattern completion, but that negative content
weakens associative encoding which impairs this process. Our findings have important implications for
understanding the way traumatic events are encoded and support therapeutic strategies aimed at restoring
associations between negative content and its surrounding context
How environment and self-motion combine in neural representations of space
Estimates of location or orientation can be constructed solely from sensory information representing environmental cues. In unfamiliar or sensory-poor environments, these estimates can also be maintained and updated by integrating self-motion information. However, the accumulation of error dictates that updated representations of heading direction and location become progressively less reliable over time, and must be corrected by environmental sensory inputs when available. Anatomical, electrophysiological and behavioural evidence indicates that angular and translational path integration contributes to the firing of head direction cells and grid cells. We discuss how sensory inputs may be combined with self-motion information in the firing patterns of these cells. For head direction cells, direct projections from egocentric sensory representations of distal cues can help to correct cumulative errors. Grid cells may benefit from sensory inputs via boundary vector cells and place cells. However, the allocentric code of boundary vector cells and place cells requires consistent head-direction information in order to translate the sensory signal of egocentric boundary distance into allocentric boundary vector cell firing, suggesting that the different spatial representations found in and around the hippocampal formation are interdependent. We conclude that, rather than representing pure path integration, the firing of head-direction cells and grid cells reflects the interface between self-motion and environmental sensory information. Together with place cells and boundary vector cells they can support a coherent unitary representation of space based on both environmental sensory inputs and path integration signals
Dynamical Fine Tuning in Brane Inflation
We investigate a novel mechanism of dynamical tuning of a flat potential in
the open string landscape within the context of warped brane-antibrane
inflation in type IIB string theory. Because of competing effects between
interactions with the moduli stabilizing D7-branes in the warped throat and
anti-D3-branes at the tip, a stack of branes gives rise to a local minimum of
the potential, holding the branes high up in the throat. As branes successively
tunnel out of the local minimum to the bottom of the throat the potential
barrier becomes lower and is eventually replaced by a flat inflection point,
around which the remaining branes easily inflate. This dynamical flattening of
the inflaton potential reduces the need to fine tune the potential by hand, and
also leads to successful inflation for a larger range of inflaton initial
conditions, due to trapping in the local minimum.Comment: 23 pages, 9 figures. v2: Updated D3-dependence in potential, small
changes to numerical result
Duality, the Semi-Circle Law and Quantum Hall Bilayers
There is considerable experimental evidence for the existence in Quantum Hall
systems of an approximate emergent discrete symmetry, . The evidence consists of the robustness of the tests of a suite a
predictions concerning the transitions between the phases of the system as
magnetic fields and temperatures are varied, which follow from the existence of
the symmetry alone. These include the universality of and quantum numbers of
the fixed points which occur in these transitions; selection rules governing
which phases may be related by transitions; and the semi-circular trajectories
in the Ohmic-Hall conductivity plane which are followed during the transitions.
We explore the implications of this symmetry for Quantum Hall systems involving
{\it two} charge-carrying fluids, and so obtain predictions both for bilayer
systems and for single-layer systems for which the Landau levels have a spin
degeneracy. We obtain similarly striking predictions which include the novel
new phases which are seen in these systems, as well as a prediction for
semicircle trajectories which are traversed by specific combinations of the
bilayer conductivities as magnetic fields are varied at low temperatures.Comment: 12 pages, 8 figures; discussion of magnetic field dependence modified
and figures and references updated in v
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