85 research outputs found
Development of spatial coarse-to-fine processing in the visual pathway
The sequential analysis of information in a coarse-to-fine manner is a
fundamental mode of processing in the visual pathway. Spatial frequency (SF)
tuning, arguably the most fundamental feature of spatial vision, provides
particular intuition within the coarse-to-fine framework: low spatial
frequencies convey global information about an image (e.g., general
orientation), while high spatial frequencies carry more detailed information
(e.g., edges). In this paper, we study the development of cortical spatial
frequency tuning. As feedforward input from the lateral geniculate nucleus
(LGN) has been shown to have significant influence on cortical coarse-to-fine
processing, we present a firing-rate based thalamocortical model which includes
both feedforward and feedback components. We analyze the relationship between
various model parameters (including cortical feedback strength) and responses.
We confirm the importance of the antagonistic relationship between the center
and surround responses in thalamic relay cell receptive fields (RFs), and
further characterize how specific structural LGN RF parameters affect cortical
coarse-to-fine processing. Our results also indicate that the effect of
cortical feedback on spatial frequency tuning is age-dependent: in particular,
cortical feedback more strongly affects coarse-to-fine processing in kittens
than in adults. We use our results to propose an experimentally testable
hypothesis for the function of the extensive feedback in the corticothalamic
circuit.Comment: 20 pages, 7 figures; substantial restructuring from previous versio
The Limiting Speed of the Bacterial Flagellar Motor
Recent experiments on the bacterial flagellar motor have shown that the
structure of this nanomachine, which drives locomotion in a wide range of
bacterial species, is more dynamic than previously believed. Specifically, the
number of active torque-generating units (stators) was shown to vary across
applied loads. This finding invalidates the experimental evidence reporting
that limiting (zero-torque) speed is independent of the number of active
stators. Here, we propose that, contrary to previous assumptions, the maximum
speed of the motor is not universal, but rather increases as additional
torque-generators are recruited. This result arises from our assumption that
stators disengage from the motor for a significant portion of their
mechanochemical cycles at low loads. We show that this assumption is consistent
with current experimental evidence and consolidate our predictions with
arguments that a processive motor must have a high duty ratio at high loads.Comment: 8 pages, 3 figures (main text); 7 pages, 3 figures (supplementary
Universal features in panarthropod inter-limb coordination during forward walking
Terrestrial animals must often negotiate heterogeneous, varying environments.
Accordingly, their locomotive strategies must adapt to a wide range of terrain,
as well as to a range of speeds in order to accomplish different behavioral
goals. Studies in \textit{Drosophila} have found that inter-leg coordination
patterns (ICPs) vary smoothly with walking speed, rather than switching between
distinct gaits as in vertebrates (e.g., horses transitioning between trotting
and galloping). Such a continuum of stepping patterns implies that separate
neural controllers are not necessary for each observed ICP. Furthermore, the
spectrum of \textit{Drosophila} stepping patterns includes all canonical
coordination patterns observed during forward walking in insects. This raises
the exciting possibility that the controller in \textit{Drosophila} is common
to all insects, and perhaps more generally to panarthropod walkers. Here, we
survey and collate data on leg kinematics and inter-leg coordination
relationships during forward walking in a range of arthropod species, as well
as include data from a recent behavioral investigation into the tardigrade
\textit{Hypsibius exemplaris}. Using this comparative dataset, we point to
several functional and morphological features that are shared amongst
panarthropods. The goal of the framework presented in this review is to
emphasize the importance of comparative functional and morphological analyses
in understanding the origins and diversification of walking in Panarthropoda
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Investigations in avocado breeding
Thesis (M.S.)--Massachusetts Agricultural College, 1922.Mode of access: Internet
Mechanics of torque generation in the bacterial flagellar motor
The bacterial flagellar motor (BFM) is responsible for driving bacterial
locomotion and chemotaxis, fundamental processes in pathogenesis and biofilm
formation. In the BFM, torque is generated at the interface between
transmembrane proteins (stators) and a rotor. It is well-established that the
passage of ions down a transmembrane gradient through the stator complex
provides the energy needed for torque generation. However, the physics involved
in this energy conversion remain poorly understood. Here we propose a
mechanically specific model for torque generation in the BFM. In particular, we
identify two fundamental forces involved in torque generation: electrostatic
and steric. We propose that electrostatic forces serve to position the stator,
while steric forces comprise the actual 'power stroke'. Specifically, we
predict that ion-induced conformational changes about a proline 'hinge' residue
in an -helix of the stator are directly responsible for generating the
power stroke. Our model predictions fit well with recent experiments on a
single-stator motor. Furthermore, we propose several experiments to elucidate
the torque-speed relationship in motors where the number of stators may not be
constant. The proposed model provides a mechanical explanation for several
fundamental features of the flagellar motor, including: torque-speed and
speed-ion motive force relationships, backstepping, variation in step sizes,
and the puzzle of swarming experiments
Co-registration of paired histological sections and MRI scans of the rabbit larynx
Co-registering images of different modalities, termed intermodal image registration, is an important tool in improving our understanding of how certain features detectable in one modality might manifest in the other. However, structural changes ā usually the result of tissue processing or noise in image acquisition ā can make matching difficult. In this thesis, I outline a pre-processing protocol for co-registration of paired histological sections and MRI scans as well as discuss different co-registration strategies using the rabbit larynx as a model system
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Kinematics and coordination of moth flies walking on smooth and rough surfaces
The moth fly, Clogmia albipunctata, is a common synanthropic insect with a worldwide range that lives in nearly any area with moist, decaying organic matter. These habitats comprise both smooth, slippery substrates (e.g., bathroom drains) and heterogeneous, bumpy ground (e.g., soil in plant pots). By using terrain of varying levels of roughness, we focus specifically on how substrate roughness at the approximate size scale of the organism affects kinematics and coordination in adult moth flies. Finally, we compare and contrast our characterizations of locomotion in C. albipunctata with previous work of insect walking in naturalistic environments
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