Ramping Activity Research Articles

Ramping activity is an inefficient estimator of time intervals

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So Many Choices: What Computational Models Reveal about Decision-Making Mechanisms

Is decision making in the brain (a) optimal, (b) stochastic, (c) probabilistic, or (d) all of the above? Two papers in this issue of Neuron by Beck et al. and Furman and Wang address these questions by constructing model neural circuits capable of picking one option given multiple perceptual choices.

Similarity Effect and Optimal Control of Multiple-Choice Decision Making

Decision making with several choice options is central to cognition. To elucidate the neural mechanisms of such decisions, we investigated a recurrent cortical circuit model in which fluctuating spiking neural dynamics underlie trial-by-trial stochastic decisions. The model encodes a continuous analog stimulus feature and is thus applicable to multiple-choice decisions. Importantly, the continuous network captures similarity between alternatives and possible overlaps in their neural representation. Model simulations accounted for behavioral as well as single-unit neurophysiological data from a recent monkey experiment and revealed testable predictions about the patterns of error rate as a function of the similarity between the correct and actual choices. We also found that the similarity and number of options affect speed and accuracy of responses. A mechanism is proposed for flexible control of speed-accuracy tradeoff, based on a simple top-down signal to the decision circuit that may vary nonmonotonically with the number of choice alternatives.

Entrained rhythmic activities of neuronal ensembles as perceptual memory of time interval

The ability to process temporal information is fundamental to sensory perception, cognitive processing and motor behaviour of all living organisms, from amoebae to humans. Neural circuit mechanisms based on neuronal and synaptic properties have been shown to process temporal information over the range of tens of microseconds to hundreds of milliseconds. How neural circuits process temporal information in the range of seconds to minutes is much less understood. Studies of working memory in monkeys and rats have shown that neurons in the prefrontal cortex, the parietal cortex and the thalamus exhibit ramping activities that linearly correlate with the lapse of time until the end of a specific time interval of several seconds that the animal is trained to memorize. Many organisms can also memorize the time interval of rhythmic sensory stimuli in the timescale of seconds and can coordinate motor behaviour accordingly, for example, by keeping the rhythm after exposure to the beat of music. Here we report a form of rhythmic activity among specific neuronal ensembles in the zebrafish optic tectum, which retains the memory of the time interval (in the order of seconds) of repetitive sensory stimuli for a duration of up to approximately 20 s. After repetitive visual conditioning stimulation (CS) of zebrafish larvae, we observed rhythmic post-CS activities among specific tectal neuronal ensembles, with a regular interval that closely matched the CS. Visuomotor behaviour of the zebrafish larvae also showed regular post-CS repetitions at the entrained time interval that correlated with rhythmic neuronal ensemble activities in the tectum. Thus, rhythmic activities among specific neuronal ensembles may act as an adjustable 'metronome' for time intervals in the order of seconds, and serve as a mechanism for the short-term perceptual memory of rhythmic sensory experience.

Remembering the time: a continuous clock

The neural mechanisms for time measurement are currently a subject of much debate. This article argues that our brains can measure time using the same dorsolateral prefrontal cells that are known to be involved in working memory. Evidence for this is: (1) the dorsolateral prefrontal cortex is integral to both cognitive timing and working memory; (2) both behavioural processes are modulated by dopamine and disrupted by manipulation of dopaminergic projections to the dorsolateral prefrontal cortex; (3) the neurons in question ramp their activity in a temporally predictable way during both types of processing; and (4) this ramping activity is modulated by dopamine. The dual involvement of these prefrontal neurons in working memory and cognitive timing supports a view of the prefrontal cortex as a multipurpose processor recruited by a wide variety of tasks.

Beyond bistability: Biophysics and temporal dynamics of working memory

Working memory has often been modeled and conceptualized as a kind of binary (bistable) memory switch, where stimuli turn on plateau-like persistent activity in subsets of cells, in line with many in vivo electrophysiological reports. A potentially related form of bistability, termed up- and down-states, has been studied with regard to its synaptic and ionic basis in vivo and in reduced cortical preparations. Also single cell mechanisms for producing bistability have been proposed and investigated in brain slices and computationally. Recently, however, it has been emphasized that clear plateau-like bistable activity is rather rare during working memory tasks, and that neurons exhibit a multitude of different temporally unfolding activity profiles and temporal structure within their spiking dynamics. Hence, working memory seems to be a highly dynamical neural process with yet unknown mappings from dynamical to computational properties. Empirical findings on ramping activity profiles and temporal structure will be reviewed, as well as neural models that attempt to account for it and its computational significance. Furthermore, recent in vivo, neural culture, and in vitro preparations will be discussed that offer new possibilities for studying the biophysical mechanisms underlying computational processes during working memory. These preparations have revealed additional evidence for temporal structure and spatio-temporally organized attractor states in cortical networks, as well as for specific computational properties that may characterize synaptic processing during high-activity states as during working memory. Together such findings may lay the foundations for highly dynamical theories of working memory based on biophysical principles.

Ramp Activity Expert System for Scheduling and Coordination at an Airport

In this project, we have developed the ramp activity coordination expert system (races) to solve aircraft-parking problems. races includes a knowledge-based scheduling system that assigns all daily arriving and departing flights to the gates and remote spots with domain-specific knowledge and heuristics acquired from human experts. races processes complex scheduling problems such as dynamic interrelations among the characteristics of remote spots-gates and aircraft with various other constraints, for example, customs and ground-handling factors, at an airport. By user-driven modeling for end users and near-optimal knowledge-driven scheduling acquired from human experts, races can produce parking schedules for about 400 daily flights in approximately 20 seconds; human experts normally take 4 to 5 hours to do the same. Scheduling results in the form of Gantt charts produced by races are also accepted by the domain experts. races is also designed to deal with the partial adjustment of the schedule when unexpected events occur. After daily scheduling is completed, the messages for aircraft change, and delay messages are reflected and updated into the schedule according to the knowledge of the domain experts. By analyzing the knowledge model of the domain expert, the reactive scheduling steps are effectively represented as the rules, and the scenarios of the graphic user interfaces are designed. Because the modification of the aircraft dispositions, such as aircraft changes and cancellations of flights, is reflected in the current schedule, the modification should be sent to races from the mainframe for the reactive scheduling. The adjustments of the schedule are made semiautomatically by races because there are many irregularities in dealing with the partial rescheduling.

Seafloor slopes at mid-ocean ridges from submersible observations and implications for interpreting geology from seafloor topography

Observations from 145 submersible dives are used to create a database of mid-ocean ridge scarp topography and lithology. Seafloor lithologies are classified into extrusives, basaltic talus, dykes, gabbros and serpentinites, and the dive locations are broadly classified according to whether they are close to transform valleys and as a function of ridge spreading rate. The database is used to determine whether there is any difference in the maximum slope for each rock type, which might relate to differences in rock jointing, cohesion or friction properties. There is a common perception that lower crustal rocks form steeper slopes than shallower crustal rocks but until now there has been little evidence to support or refute this idea. From our analysis we find a tendency for gabbro and dykes to form steeper slopes than serpentinite. The 90th percentile of each lithology slope distribution, used as a measure of limiting slope, is 43°, 39° and 32° for gabbro, dykes and serpentinite, respectively. Lithologic control on slope is weak, however, compared to overall slope variability in these mid-ocean ridge settings so seafloor relief is likely to be a poor guide to underlying geology. We speculate on the structure of eroding fault scarps, outline the implications for attempts to infer active faults from talus ramp activity and discuss more generally factors affecting the geomorphology of mid-ocean ridge slopes.

TECTONICS AND SEDIMENTATION WITHIN A MESSINIAN FOREDEEP IN THE CENTRAL APENNINES, ITALY

The Marche‐Abruzzi thrust system is part of the Central Apennine fold‐and‐thrust belt of mainland Italy. Our study area covers the SE portion of the Umbria‐Marche‐Abruzzi region and the northern part of the Gran Sasso Mountains. A thick pile of turbidites is exposed in this area; they were deposited within a Messinian foredeep in front of the evolving fold‐and‐thrust belt. We have used regional structural interpretations together with unpublished well‐log and seismic data to construct a cross‐section across the external portion of the thrust system.Our work suggests that: (i) deformation of the sediments in the Messinian foredeep began shortly after their deposition, and continued until the late Pliocene; (ii) the restored (pre‐deformation) width of the basin was approximately twice that of the present‐day basin, and the depocentre migrated eastwards at a rate of about 4 cm/yr; (iii) the geometry and physiography of the Messinian foredeep were controlled by a number of longitudinal and transverse structural features which may be related to foreland ramp activity or to differential flexuring of the Apulian lithosphere, respectively.

Expert system for scheduling in an airline gate allocation

Scheduling is an important technique encompassing a wide application area. Because of the complex interrelations among the resources, knowledge, and various other constraints, scheduling has many difficulties. Artificial Intelligence technology has been applied to solve the scheduling problem. As AI techniques are efficient in representing knowledge and dealing with heuristics, it is an adequate approach to model and to solve scheduling problems. We have implemented the ramp scheduling system, called RACES (Ramp Activity Coordination Expert System), to solve complex and dynamic aircraft parking problems. RACES was developed from the domain knowledge and experience which were acquired from the domain experts. Domain knowledge and experience are important factors in controlling the scheduling procedure. RACES divides the problem into sub-problems and experimental heuristics in the knowledge acquisition process. The system independently processes scheduling for the divided sub-problems and shares variables and domains. During the scheduling, the system selects or confines the search space with domain filtering techniques by exploiting the characteristics of various constraints and knowledge. RACES produces a user-driven near-optimal solution by means of a trade-off scheduling method using heuristics between the size of aircraft and the best-fit time. For 400 daily flights, RACES made parking schedules for aircraft in about 20 s compared with 4–5 h by human experts.

Glycine receptor-mediated fast synaptic inhibition in the brainstem respiratory system

The involvement of glycinergic neurotransmission in the central regulation of respiration was investigated by administration of specific receptor agonists and antagonists into the 4th cerebral ventricle of the rabbit. Central respiratory activity was assessed by cycle-triggered averaging of phrenic nerve activity (PNA). Glycine reduced the peak amplitude of PNA and increased the duration of the respiratory phases. Conversely, strychnine decreased the duration of both phases, being more effective in expiration. Strychnine converted inspiratory ramp activity to a decrescendo type of pattern, with the highest discharge activity present at the onset of the inspiratory phase. Strychnine induced tonic PNA lasting throughout expiration with a decrescendo type of activity. In many cases strychnine induce apneusis with tonic PNA being at a level of about 2/3 of its maximum value during control. We conclude that glycinergic neurotransmission is important for the maintenance of the respiratory rhythm as well as the generation of normal respiratory pattern. Glycine could be involved in phasic as well as tonic inhibitory neurotransmission within the central respiratory system.

Concurrent fast and slow synchronized efferent phrenic activities in time and frequency domain

In urethane-anesthetized or decerebrated vagotomized rabbits efferent multifiber activity of the phrenic nerve was investigated for synchronized activities both in time and frequency domains. When respiratory drive was steadily increased by either an elevation of end-tidal CO 2 concentration or i.v. administration of 4-aminopyridine, medium-frequency oscillations (MFO) first increased, then decreased and finally became absent. The power of high-frequency oscillations (HFO) steadily rose with increasing respiratory drive. In contrast to HFO which revealed a unimodal spectral peak of mostly small bandwidth, the MFO spectrum in most cases consisted of a broad complex. This complex in some cases was composed of two distinct peaks, i.e. MFO were heterogenous. The low- and high-frequency fractions of the MFO complex were related predominantly to the first and last third of inspiration, respectively. Examination of the on-going multifiber activity of the phrenic nerve with an expanded time scale revealed that lower frequency MFO probably result from synchronized ramp-like wave activity during early and mid-inspiration. The duration of the observed ramps well matched the corresponding MFO frequency. We suggest that these ramps might result from propagated synchronized waves of high-threshold phrenic motoneurons. During the last part of inspiration, however, MFO, like HFO, resulted from burst-like synchronized discharge of phenic motoneurons. Thus HFO are superimposed on ramp-like and burst-like activity of the MFO. It is assumed that the decline of MFO at high respiratory drive may be due to the increasing strength of HFO bursts which interrupt ramp activity in the MFO range and thus let MFO appear ‘invisible’ to the recording electrode. Both MFO and HFO were visually detectable in postinspiration.

Design of an expert system for aircraft gate assignment

The task of assigning arriving flights at an airport to the available gates is a key activity in airline station operations. With the development of large connecting hub operations, and the resulting volumes of passengers and baggage transferring between flights, the complexity of the task and the number of factors to be considered have increased significantly. Traditional approaches utilizing classical operations research techniques have difficulty with uncertain information and multiple performance criteria, and do not adapt well to the needs of real-time operations support. As a result, several airlines have been exploring the use of expert systems for operational control of ramp activity. This paper discusses the factors that arise in deciding how to allocate flights to gates, and describes the knowledge base structure, data requirements and inference process of an expert system that would recommend gate allocation decisions to ramp control personnel, taking into account the constraints imposed by the available facilities and personnel to handle the aircraft, and the consequences on downstream operations of particular assignment decisions. The paper describes how these concepts have been implemented in a prototype expert system that has been designed to address a restricted set of gate assignment issues within a framework that could be extended to consider a broader range of factors. The operation of the expert system is illustrated through a case study application to a typical flight schedule at a major hub airport.

Postinspiratory-ramp activity of diaphragn under inspiratory resistive load

Inspiratory ramp activity, postramp acitivity (PRA) of diaphragm and respiratory flow were studied in anesthetized rabbits and conscious humans under inspiratory resisitive load. Under load with chloralose-urethane anesthesia neural inspiration lasted 124±12 msec more than under control conditions and end of mechanical inspiration lagged behind that of neural inspiration by 114±4 msec. With pentobarbital-urethane anesthesia corresponding changes under load were 67±18 msec and 44±6 msec; after SO 2 block of slowly adapting stretch receptors in bronchi corresponding changes under load were nil and 65±7 msec. In human corresponding changes under load were about 192 and 127 msec. Both in rabbits and humans time course of PRA under load was essentially unchanged. Hence, under load a considerable part of PRA (or all of it under pentobarbital-urethane anesthesia without SO 2 block) occured during inspiration and that occuring during expiration was smaller than under control conditions. Consequently, under load braking of expiratory flow was smaller and peak expiratory flow occured earlier than under control conditions.

On the central pattern generator for the basic breathing rhythmicity.

Recent advances in several laboratories concerning the respiration-related medullary neurons, their locations, projections, interconnections, morphological and physiological properties, and patterns of inhibitory postsynaptic potentials, excitatory postsynaptic potentials, and discharge rate, on the one hand, and the "systems behavior," on the other, have provided the basis for new hypothesis concerning the neural mechanisms underlying the central pattern generator (CPG) for breathing and its different parts. The onset of the "ramp"-like increase in inspiratory activity is due to an abrupt release of inhibition and a subsequent progressively increasing synaptic excitation of inspiratory premotor neurons. The integration of the excitatory "drive" inputs underlying the ramp inspiratory activity seems to depend on structures in the ventrorostral medulla, including nucleus paragigantocellularis. The termination of this activity by the off-switch mechanisms is actuated when a critical threshold is attained by the excitatory inputs of 1) a slowly increasing inspiration-related activity and 2) the afferent input from the pulmonary stretch receptors. The nature of the former activity is discussed. During the expiratory phase, an inhibitory activity suppresses inspiration-facilitating inputs with a slowly decaying power that controls the expiratory duration. The postinspiration activity, which brakes the rate of exhalation during the first part of the expiratory phase, depends on mechanisms separate from those responsible for the inspiratory ramp activity. The respiratory CPG seems to be organized with considerable amount of redundancy, or "degeneracity."