scholarly journals Non-relativistic D-brane from T-duality along null direction

2019 ◽  
Vol 2019 (10) ◽  
Author(s):  
J. Klusoň
Keyword(s):  
Null Direction

scholarly journals Substructure of Direction-Selective Receptive Fields in Macaque V1

2003 ◽  
Vol 89 (5) ◽  
pp. 2743-2759 ◽  
Author(s):  
Margaret S. Livingstone ◽  
Bevil R. Conway
Keyword(s):  
Receptive Fields ◽  
Direction Selectivity ◽  
Positive Interactions ◽  
Null Direction ◽  
Simple Cells ◽  
Complex Cells ◽  
Preferred Direction ◽  
Orientation Axis ◽  
Interaction Regions ◽  
Simple And Complex Cells

We used two-dimensional (2-D) sparse noise to map simultaneous and sequential two-spot interactions in simple and complex direction-selective cells in macaque V1. Sequential-interaction maps for both simple and complex cells showed preferred-direction facilitation and null-direction suppression for same-contrast stimulus sequences and the reverse for inverting-contrast sequences, although the magnitudes of the interactions were weaker for the simple cells. Contrast-sign selectivity in complex cells indicates that direction-selective interactions in these cells must occur in antecedent simple cells or in simple-cell-like dendritic compartments. Our maps suggest that direction selectivity, and on andoff segregation perpendicular to the orientation axis, can occur prior to receptive-field elongation along the orientation axis. 2-D interaction maps for some complex cells showed elongated alternating facilitatory and suppressive interactions as predicted if their inputs were orientation-selective simple cells. The negative interactions, however, were less elongated than the positive interactions, and there was an inflection at the origin in the positive interactions, so the interactions were chevron-shaped rather than band-like. Other complex cells showed only two round interaction regions, one negative and one positive. Several explanations for the map shapes are considered, including the possibility that directional interactions are generated directly from unoriented inputs.

scholarly journals Can a false vacuum bubble remove the singularity inside a black hole?

2020 ◽  
Vol 80 (8) ◽  
Author(s):  
Suddhasattwa Brahma ◽  
Dong-han Yeom
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Thin Shell ◽  
Apparent Horizon ◽  
False Vacuum ◽  
De Sitter ◽  
Singularity Formation ◽  
Null Direction ◽  
Singularity Resolution ◽  
Hole Model

Abstract We investigate a regular black hole model with a de Sitter-like core at its center. This type of a black hole model with a false vacuum core was introduced with the hope of singularity-resolution and is a common feature shared by many regular black holes. In this paper, we examine this claim of a singularity-free black hole by employing the thin-shell formalism, and exploring its dynamics, within the Vaidya approximation. We find that during gravitational collapse, the shell necessarily moves along a space-like direction. More interestingly, during the evaporation phase, the shell and the outer apparent horizon approach each other but, unless the evaporation takes place very rapidly, the approaching tendency is too slow to avoid singularity-formation. This shows that albeit a false vacuum core may remove the singularity along the ingoing null direction, there still exists a singularity along the outgoing null direction, unless the evaporation is very strong.

A spacetime calculus based on a single null direction

1996 ◽  
Vol 13 (6) ◽  
pp. 1579-1587 ◽  
Author(s):  
M P Machado Ramos ◽  
J A G Vickers
Keyword(s):  
Null Direction

Potassium-evoked directionally selective responses from rabbit retinal ganglion cells

1996 ◽  
Vol 13 (4) ◽  
pp. 705-719 ◽  
Author(s):  
Ralph J. Jensen
Keyword(s):  
Retinal Ganglion Cells ◽  
Apparent Motion ◽  
Ganglion Cells ◽  
Short Pulse ◽  
Retinal Ganglion ◽  
Null Direction ◽  
Sequence Dependent ◽  
Motion Responses ◽  
A Cell ◽  
Gaba Antagonist

AbstractPrevious studies have shown that directionally selective (DS) retinal ganglion cells cannot only discriminate the direction of a moving object but they can also discriminate the sequence of two flashes of light at neighboring locations in the visual field: that is, the cells elicit a DS response to both real and apparent motion. This study examines whether a DS response can be elicited in DS ganglion cells by simply stimulating two neighboring areas of the retina with high external K+. Extracellular recordings were made from ON-OFF DS ganglion cells in superfused rabbit retinas, and the responses of these cells to focal applications of 100 mM KCl to the vitreal surface of the retina were measured. All cells produced a burst of spikes (typically lasting 50–200 ms) when a short pulse (10–50 ms duration) of KCl was ejected from the tip of a micropipette that was placed within the cell's receptive field. When KCl was ejected successively from the tips of two micropipettes that were aligned along the preferred-null axis of a cell, sequence-dependent responses were observed. The response to the second micropipette was suppressed when mimicking motion in the cell's null direction, whereas an enhancement during apparent motion in the opposite direction frequently occurred. Sequence discrimination in these cells was eliminated by the GABA antagonist picrotoxin and by the Ca2+-channel blocker ω-conotoxin MVIIC, two drugs that are known to abolish directional selectivity in these ganglion cells. The spatiotemporal properties of the K+-evoked sequence-dependent responses are described and compared with previous findings on apparent motion responses of ON-OFF DS ganglion cells.

scholarly journals An Optimization framework for Nonspiking Neuronal Networks and Aided Discovery of a Model for the Elementary Motion Detector

2019 ◽  
Author(s):  
Arunava Banerjee
Keyword(s):  
Optic Lobe ◽  
Optimization Procedure ◽  
Theoretical Models ◽  
Direction Selectivity ◽  
Recurrent Networks ◽  
Motion Detector ◽  
Null Direction ◽  
Optimization Framework ◽  
Preferred Direction ◽  
Low Pass

AbstractWe present a general optimization procedure that given a parameterized network of nonspiking conductance based compartmentally modeled neurons, tunes the parameters to elicit a desired network behavior. Armed with this tool, we address the elementary motion detector problem. Central to established theoretical models, the Hassenstein-Reichardt and Barlow-Levick detectors, are delay lines whose outputs from spatially separated locations are prescribed to be nonlinearly integrated with the direct outputs to engender direction selectivity. The neural implementation of the delays—which are substantial as stipulated by interomatidial angles—has remained elusive although there is consensus regarding the neurons that constitute the network. Assisted by the optimization procedure, we identify parameter settings consistent with the connectivity architecture and physiology of the Drosophila optic lobe, that demonstrates that the requisite delay and the concomitant direction selectivity can emerge from the nonlinear dynamics of small recurrent networks of neurons with simple tonically active synapses. Additionally, although the temporally extended responses of the neurons permit simple synaptic integration of their signals to be sufficient to induce direction selectivity, both preferred direction enhancement and null direction suppression is necessary to abridge the overall response. Finally, the characteristics of the response to drifting sinusoidal gratings are readily explained by the charging-up of the recurrent networks and their low-pass nature.

scholarly journals IDENTIFICATION OF DIRECTIONALLY SELECTIVE MOTION-DETECTING NEURONES IN THE LOCUST LOBULA AND THEIR SYNAPTIC CONNECTIONS WITH AN IDENTIFIED DESCENDING NEURONE

1990 ◽  
Vol 149 (1) ◽  
pp. 21-43 ◽  
Author(s):  
F. CLAIRE RIND
Keyword(s):  
Preceding Paper ◽  
Apparent Movement ◽  
Single Step ◽  
Resting Membrane Potential ◽  
Null Direction ◽  
Excitatory Connection ◽  
Anatomy And Physiology ◽  
Preferred Direction ◽  
Presynaptic Spike ◽  
Postsynaptic Neurone

The anatomy and physiology of two directionally selective motion-detecting neurones in the locust are described. Both neurones had dendrites in the lobula, and projected to the ipsilateral protocerebrum. Their cell bodies were located on the posterio-dorsal junction of the optic lobe with the protocerebrum. The neurones were sensitive to horizontal motion of a visual stimulus. One neurone, LDSMD(F), had a preferred direction forwards over the ipsilateral eye, and a null direction backwards. The other neurone, LDSMD(B), had a preferred direction backwards over the ipsilateral eye 1. Motion in the preferred direction caused EPSPs and spikes in the LDSMD neurones. Motion in the null direction resulted in IPSPs 2. Both excitatory and inhibitory inputs were derived from the ipsilateral eye 3. The DSMD neurones responded to velocities of movement up to and beyond 270°s−1 4. The response of both LDSMD neurones showed no evidence of adaptation during maintained apparent or real movement 5. There was a delay of 60–80 ms between a single step of apparent movement, either the preferred or the null direction, and the start of the response 6. There was a monosynaptic, excitatory connection between the LDSMD(B) neurone and the protocerebral, descending DSMD neurone (PDDSMD) identified in the preceding paper (Rind, 1990). At resting membrane potential, a single presynaptic spike did not give rise to a spike in the postsynaptic neurone

scholarly journals Unidirectional Rotation Neurones in the Optomotor System of the Crab, Carcinus

1971 ◽  
Vol 54 (2) ◽  
pp. 507-513
Author(s):  
C. A. G. WIERSMA ◽  
L. FIORE
Keyword(s):  
Visual Stimulation ◽  
Large Diameter ◽  
Visual Input ◽  
Null Direction ◽  
Dorsoventral Axis ◽  
Eye Muscles ◽  
Dorsal Axis

1. Among the optomotor fibres to the eye muscles in Carcinus a class was found which responds to unidirectional fast rotations around various body axes. All had large signals and are therefore of large diameter. 2. In one set of these fibres which fires especially for rotations around the dorsoventral axis, it could be shown that discharges take place especially during accelerations and that, when a rotation in the null direction is suddenly stopped, a short discharge occurs. The fibres for other axes behave in a similar manner. 3. For rotations around the ventro-dorsal axis, but not for other directions, mediumsized fibres are present which, in contrast to the fast fibres, respond to visual stimulation, as well as to body rotations in darkness, thus combining the input properties of the unidirectional fast rotatory and the unidirectional purely optokinetic small fibres. Their sensitivity to visual input is for low rotation velocities, to body rotations is for high rotation velocities.

scholarly journals Receptoral Mechanisms for Fast Cholinergic Transmission in Direction-Selective Retinal Circuitry

2020 ◽  
Vol 14 ◽  
Author(s):  
Joseph Pottackal ◽  
Joshua H. Singer ◽  
Jonathan B. Demb
Keyword(s):  
Synaptic Transmission ◽  
Amacrine Cells ◽  
Direction Selectivity ◽  
Gabaergic Inhibition ◽  
Temporal Difference ◽  
Cholinergic Transmission ◽  
Null Direction ◽  
Gabaergic Transmission ◽  
Temporal Properties ◽  
Synaptic Mechanisms

Direction selectivity represents an elementary sensory computation that can be related to underlying synaptic mechanisms. In mammalian retina, direction-selective ganglion cells (DSGCs) respond strongly to visual motion in a “preferred” direction and weakly to motion in the opposite, “null” direction. The DS mechanism depends on starburst amacrine cells (SACs), which provide null direction-tuned GABAergic inhibition and untuned cholinergic excitation to DSGCs. GABAergic inhibition depends on conventional synaptic transmission, whereas cholinergic excitation apparently depends on paracrine (i.e., non-synaptic) transmission. Despite its paracrine mode of transmission, cholinergic excitation is more transient than GABAergic inhibition, yielding a temporal difference that contributes essentially to the DS computation. To isolate synaptic mechanisms that generate the distinct temporal properties of cholinergic and GABAergic transmission from SACs to DSGCs, we optogenetically stimulated SACs while recording postsynaptic currents (PSCs) from DSGCs in mouse retina. Direct recordings from channelrhodopsin-2-expressing (ChR2+) SACs during quasi-white noise (WN) (0-30 Hz) photostimulation demonstrated precise, graded optogenetic control of SAC membrane current and potential. Linear systems analysis of ChR2-evoked PSCs recorded in DSGCs revealed cholinergic transmission to be faster than GABAergic transmission. A deconvolution-based analysis showed that distinct postsynaptic receptor kinetics fully account for the temporal difference between cholinergic and GABAergic transmission. Furthermore, GABAA receptor blockade prolonged cholinergic transmission, identifying a new functional role for GABAergic inhibition of SACs. Thus, fast cholinergic transmission from SACs to DSGCs arises from at least two distinct mechanisms, yielding temporal properties consistent with conventional synapses despite its paracrine nature.

scholarly journals Subthreshold Membrane Conductances Enhance Directional Selectivity in Vertebrate Sensory Neurons

2010 ◽  
Vol 104 (1) ◽  
pp. 449-462 ◽  
Author(s):  
Maurice J. Chacron ◽  
Eric S. Fortune
Keyword(s):  
Directional Selectivity ◽  
Null Direction ◽  
Directional Bias ◽  
Membrane Conductances ◽  
Preferred Direction ◽  
Receptor Array ◽  
Central Nervous Systems ◽  
Direction Of Movement ◽  
Nonlinear Integration

Directional selectivity, in which neurons respond preferentially to one “preferred” direction of movement over the opposite “null” direction, is a critical computation that is found in the central nervous systems of many animals. Such responses are generated using two mechanisms: spatiotemporal convergence via pathways that differ in the timing of information from different locations on the receptor array and the nonlinear integration of this information. Previous studies have showed that various mechanisms may act as nonlinear integrators by suppressing the response in the null direction. Here we show, through a combination of mathematical modeling and in vivo intracellular recordings, that subthreshold membrane conductances can act as a nonlinear integrator by increasing the response in the preferred direction of motion only, thereby enhancing the directional bias. Such subthreshold conductances are ubiquitous in the CNS and therefore may be used in a wide array of computations that involve the enhancement of an existing bias arising from differential spatiotemporal filtering.

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