Aperiodic Activity Research Articles

Ionic mechanisms of wavebreak in fibrillation

out an intermediate band of resting tissue. This causes extinction of the segment of wavefront that hits this functional obstacle and the formation of two singularity points (white dots, 33 ms). The two resulting wavefronts pivot around the points of the wavebreaks (53‐73 ms) and make a complete rotation in approximately 83 ms. The results clearly demonstrate that at the high frequencies of VF, excitation wavefronts frequently encounter and interact with functional and/or anatomic obstacles, giving way to intermittent propagation block patterns, with consequent fragmentation of the wavefront into new daughter wavelets. Therefore, irrespective of the underlying mechanisms, wavebreak and fibrillatory conduction account for the complex patterns of propagation, wavelet formation, and apparent aperiodic activity that typify fibrillation. 11 The question that arises, however, is: What mechanisms underlie wavebreak formation at the ionic/molecular level?

Periodicity, Mixed-Mode Oscillations, and Quasiperiodicityin a Rhythm-Generating Neural Network

We studied patterns of oscillatory neural activity in the network that generates respiratory rhythm in mammals. When isolated in vitro, this network spontaneously generates an inspiratory-related motor rhythm, with stable amplitude from cycle to cycle. We show that progressively elevating neuronal excitability in vitro causes periodic modulation of this inspiratory rhythm, evoking (in order): mixed-mode oscillations, quasiperiodicity, and ultimately disorganized aperiodic activity. Thus, the respiratory network oscillator follows a well defined sequence of behavioral states characterized by dynamical systems theory, which includes discrete stages of periodic and quasiperiodic amplitude modulation and progresses (according to theory) to aperiodic chaos-like behavior. We also observed periodic, mixed-mode periodic, and quasiperiodic breathing patterns in neonatal rodents and human infants in vivo, suggesting that breathing patterns generated by the intact nervous system reflect deterministic neural activity patterns in the underlying rhythm-generating network.

Optimization of olfactory model in software to give 1/ f power spectra reveals numerical instabilities in solutions governed by aperiodic (chaotic) attractors

We present a general connectionist model for an olfactory system. The dynamical behavior of each node (neural ensemble) of the model is governed by a second-order ordinary differential equation (ODE) followed by an asymmetric sigmoidal function, relating the aggregate activity of neurons to system parameters and stimuli from an outside environment. A digital implementation of the general connectionist model simulates the characteristics of a mammalian olfactory system having modifiable synaptic connections and spatio-temporal interactions among neural ensembles. Each of four distributed delay terms is represented by a second-order ODE. A parameter optimization algorithm is an integral component of the model. The parameter optimization discussed in this paper results in aperiodic oscillations having a near 1/ f-type power spectrum with a peak in the gamma range, simulating the electro-encephalographic (EEG) potentials from the neural olfactory system. Random optimization is used for a rough search in a global parameter domain and the parameter self-adaptation rule serves for fine tuning within a local domain after the global search. By design the model is started from an unstable zero point by an external impulse input. It requires 650–850 ms to pass through an initializing transient before settling into a strange attractor. The attractor persists for at least 1500 ms, which is 7.5–20 times longer than the duration of the maximal stationary states observed in the EEGs and it is stable under perturbation by simulated sensory inputs giving response amplitudes less than 2 times the basal aperiodic activity. However, the optimization to 1/ f-type activity reveals an inherent limit on digital simulation of chaotic states, owing to attractor crowding such that the size of basins decreases with increasing size of the model, until it approaches the size of digitizing step in computation, here a 64-bit word (∼10 −16). Outputs that are not optimized to approach 1/ f-type power spectra transit earlier to limit cycle activity, usually well before 2000 ms. The duration of stationarity is increased by randomizing the terminal bit of the 64-bit words representing the state variables. The 1/ f-type solutions are also exquisitely sensitive to parameter truncation; parameter values must be saved in their full binary form for re-starting. The implications in terms of numerical instability, chaos, attractor crowding and the shadowing theorem are discussed.

Interareal synchronization in the visual cortex

The primary visual cortex (VI) is part of a highly interconnected network of cortical areas, hierarchically organized but operating concurrently across hierarchical levels. The high degree of reciprocal interconnection among visual cortical areas provides a framework for their interaction during the performance of visual scene analysis. The functional interdependency of visual cortical areas which develops during scene analysis can be investigated by techniques which measure interareal correlated activity. Evidence from monkeys performing a visual pattern discrimination suggests that synchronization of aperiodic activity from neuronal ensembles in cortical areas at different hierarchical levels is a relevant aspect of visual function. The near-periodic nature of the synchronized response to moving light bars in earlier studies may have been a result of the type of stimulus used. Various models of visual cortex are discussed in which interareal synchronization plays a functional role.

Rhythm generation in organotypic medullary cultures of newborn rats

Organotypic transverse medullary slices (obex level) from six-day-old rats, cultured for two to four weeks in chemically defined medium contained rhythmically discharging neurones which were activated by CO2 and H+. The mechanisms underlying this rhythmicity and the spread of excitation and synaptic transmission within this organotypic tissue were examined by modifying the composition of the external solution. Our findings showed that (1) Exposure to tetrodotoxin (0.2 μM) or to high magnesium (6 mM) and low calcium (0.2 mM) concentrations abolished periodic activity. (2) Neither the blockade of GABAergic potentials with bicuculline methiodide (200 μM) and/or hydroxysaclofen (200 μM) nor the blockade of glycinergic potentials with strychnine hydrochloride (100 μM) abolished rhythmicity. (3) While atropine sulphate (5 μM) was ineffective in modulating periodic discharges nicotine (100 μM) — like CO2 — shortened the intervals between the periodic events; hexamethonium (50–100 μM) reduced both periodic and aperiodic activity. (4) Exposure to the NMDA antagonist 2-aminophosphonovaleric acid (50 μM) suppressed periodic events only transiently. In the presence of 2-aminophosphonovaleric acid rhythmicity recovered. However, the AMPA-antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10–50 μM), abolished periodic activity reversibly within less than 5 min. When 6-cyano-7-nitroquinoxaline-2,3-dione and nicotine were administered simultaneously periodic events persisted for up to 10 min. These findings indicate that synaptic excitatory drive is a prerequisite for the generation of rhythmic discharges of medullary neurones in this preparation. This drive may activate voltage-dependent channels or it may facilitate endogenous cellular mechanisms which initiate oscillations of intracellular calcium concentration. To test the latter possibility (5) calcium antagonists were added to the bath saline. The organic calcium antagonists verapamil and flunarizine (50–100 μM each) and the inorganic calcium antagonists cobalt (2 mM) and magnesium (6 mM) suppressed periodic activity and abolished or weakened the chemosensitivity towards CO2/acidosis. (6) Dantrolene (10 μM), an inhibitor of intracellular calcium release decreased the periodicity, while thapsigargin (2 μM) which blocks endoplasmic Ca2+-ATPase, transiently accelerated the occurrence of periodic events. (7) Oscillations of intracellular free calcium concentrations in Fura-2 AM-loaded cells were weakened or abolished by cobalt (2 mM). The results of (5)–(7) indicate that transmembrane calcium fluxes as well as intracellular Ca2+-release and -clearance mechanisms are a prerequisite for intracellular free calcium oscillations which may be important in the generation of rhythmic discharges in medullary neurones.

Survivorship Bias

<h3>Abstract</h3> Diagnosis and symptom severity in schizophrenia is associated with irregularities across neural oscillatory frequency bands, including theta, alpha, beta, and gamma. However, electroencephalographic signals consist of both periodic and aperiodic background activity, characterized by the (1/f) slope of the power spectrum. We compared traditional band-limited periodic oscillatory activity to aperiodic activity, in schizophrenia participants and healthy controls, during an attention task. Classification analysis revealed that the change in slope of the power spectrum better predicted group status than traditional oscillatory power. It even outperformed the predictions made using participants’ own behavioral performance. Additionally, the differences in slope were highly consistent as they were observed across all electrodes. In sum, the aperiodic slope appears to be a more accurate, consistent, and robust metric to differentiate schizophrenic patients from healthy controls. <h3>Significance statement</h3> Understanding the neurobiological origins of schizophrenia and identifying reliable and consistent biomarkers are of critical importance in improving treatment of that disease. Numerous studies have reported disruptions to neural oscillations in schizophrenia patients. This has, in part, led to schizophrenia being characterized as a disease of disrupted neural coordination, reflected by changes in frequency band power. We report however that changes in aperiodic background noise (i.e., spectral slope) can also predict clinical status. Unlike band-limited power though, the aperiodic slope predicts status better than participants’ own behavioral performance. Furthermore, it is a consistent predictor across all electrodes. Alterations in aperiodic noise are consistent with well-established inhibitory neuron dysfunctions associated with schizophrenia, allowing for a direct link between noninvasive EEG and chronic, widespread, neurobiological deficits.

Rhythm generation in brainstem cultures grown in a serum-free medium

Electrophysiological studies were carried out on long term cultured brainstem tissue taken from neonatal rats with the object of investigating mechanisms underlying respiratory rhythm generation. The preparations were derived from 360 microns thick horizontal medullary slices which were explanted into a chemically defined nutrient medium and which remained organotypically intact for ca. 1 month. In 44 of the 50 explants examined both periodic and aperiodic bioelectric activity was detected, the cycle length of the former ranging from 0.5 to 10 s (mean, 2.7 s) at a pH of 7.4 and bath temperature of 32 degrees C. Periodic activity could take several forms, but commonly consisted of regularly repeated, 100-300 ms long, depolarizing (D-) waves or sequences of inhibitory and/or excitatory postsynaptic potentials. Lowering the pH of the superfusate by lowering the bicarbonate concentration, increasing the pCO2 or adding H+ shortened the interval between periodic events, and increased both the amplitude and duration of the D-waves. The interval was also shortened when the bath temperature was increased (Q10: ca.2.5). The mean resting membrane potential of neurons exhibiting periodic activity was -49 mV (n = 62) and not significantly different from that of aperiodically discharging neurons either in the same preparations or in cultured explants from the neocortex. These observations suggest that brainstem cultures constitute a useful 'model' system for studying pH-dependent rhythm generation in small neuronal networks of the medulla.

Low dimensional chaos in cardiac tissue

Chaos is a term used to characterize aperiodic activity arising in a dynamical system, or in a set of equations describing the system's temporal evolution as a result of a deterministic mechanism that has sensitive dependence on initial conditions. Chaos, in that sense, has been proposed to make an important contribution to normal and abnormal cardiac rhythms. To date, however, descriptions of chaos in heart tissue have been limited primarily to periodically forced cardiac pacemakers. Because many cardiac rhythm disturbances, particularly those initiated or perpetuated by re-entrant excitation, originate from within non-pacemaker cardiac tissues, demonstrations of chaos in non-pacemaker tissue might provide a deterministic explanation for a wide variety of complex dysrhythmias. Here we report experimental evidence for chaotic patterns of activation and action potential characteristics in externally driven, non-spontaneously active Purkinje fibres and ventricular muscle. The results indicate that there is an apparent link between the mechanism of low dimensional chaos and the occurrence of reflected responses which could lead to more spatially disorganized phenomena. A detailed mechanism for the low dimensional chaos observed experimentally is pursued using a difference equation model. Critical features of the model include a non-monotonic relationship between recovery time during rhythmic stimulation and the state of membrane properties, and a steeply sloped recovery of membrane properties over certain ranges of recovery times. Besides explaining our results, the analytical model may pertain to irregular dynamics in other excitable systems, particularly the intact dysrhythmic heart.

The structure of locomotor activity in bilobectomized cockroaches (Blaberus fuscus)

The aperiodic activity patterns of bilobectomized cockroaches can largely be described by simple stochastic regularities: a) there are no correlations between the lengths of successive activity bursts and rest pauses, b) the probabilities for the transition from activity to rest and from rest to activity are time-independent. The activity sequence is strongly influenced by singular temperature steps or temperature cycles.

Interactions between bilaterally paired components of the cockroach circadian system

The hypothesis that bilaterally redundant, and mutually coupled circadian pacemakers are located near the lobula of the optic lobes of the cockroach,Leucophaea maderae, was investigated in a series of lesion experiments. 1. Following removal of one optic lobe electrolytic lesions near the lobula in the dorsal 1/3 of the remaining lobe had no effect on the activity rhythm or its entrainment to a light cycle. Lesions in the ventral 2/3 of the lobula region either had no effect or resulted in aperiodic activity (Figs. 3, 4). 2. In animals in which one optic lobe was neurally isolated from its (ipsilateral) compound eye by section of the optic nerve, lesions near the lobula in the dorsal 1/3 of the contralateral lobe had no effect on entrainment of the rhythm. In contrast, lesions in the ventral 2/3's of the lobe, frequently, resulted in loss of entrainment (Figs. 5, 6, 7). Following loss of entrainment, the average freerunning period (\(\bar \tau = 24.03 \pm 0.26 h\)) was longer than that of intact animals (\(\bar \tau = 23.73 \pm 0.2 h\)). 3. In animals with one cut optic nerve, electrolytic or surgical lesions in the ipsilateral optic lobe, intended to destroy the known monosynaptic connections between the two lobes, had no effect on entrainment or on freerunning period (Figs. 9, 10, 11).

Diel Activity Rhythms in the Nocturnal, Bottom-Dwelling Sharks, Heterodontus francisci and Cephaloscyllium ventriosum

The horn shark, Heterodontus francisci, and the swell shark, Cephaloscyllium ventriosum, exhibited distinct nocturnal activity patterns when observed underwater in the natural environment. Swimming activity began shortly after dusk and continued until dawn, after which very little activity was seen. Laboratory records for the horn shark under fixed and shifting LD 12:12 (12 hr light-12 hr dark) regimes indicated an exogenous rhythm, with an activity onset directly controlled by the onset of darkness. Horn sharks showed a high level of aperiodic activity in constant darkness and a low level of aperiodic activity in constant light. Activity in a swell shark appeared initiated by the onset of darkness during fixed LD 12:12 regimes, but slightly preceded darkness during a shifting (1 hr later/day) LD regime. This swell shark exhibited an endogenous (i.e., circadian) rhythm of shortened period (advancing phase drift) in constant darkness, and lengthened period (delaying phase drift) in constant light.

Subtypes of medullary respiratory neurons

The activity of medullary respiratory neurons was recorded in decerebrate and pentobarbital-anesthetized cats with and without vagotomy. Five subtypes of medullary neurons were identified by the relationship of their burst to the respiratory cycle or by their discharge frequency pattern: ( i) I-throughout, active throughout inspiration; ( ii) I-late, active only during the latter half of inspiration; ( iii) I-early, active at the transition between expiration and inspiration; ( iv) E-early, discharging during expiration with a maximum frequency at the beginning; ( v) E-late, discharging during expiration with a frequency varying from constant to a maximum at the end. Most respiratory neurons were I-throughout and E-late types. In cats with intact vagus nerves, neurons inhibited by lung inflation were designated α and those facilitated were designated β, giving as subtypes I-α-throughout (inhibited) and I-β-throughout (facilitated), E-α-late (inhibited) and E-β-late (facilitated). Limited observations showed vagal inhibition of I-late and I-early and facilitation of E-early types. During hypocarbic apnea, neuronal activity ranged from silence to continuous firing and more than half of both the inspiratory and expiratory neurons retained some aperiodic activity, irrespective of neuronal subtype. Rhythmic discharge activity among these active cells was resumed only as spontaneous breathing commenced.

Concerning Community Symmetry

Paralleling the study of organisms, the study of communities may be separated into several artificial categories. A community may be said to have structure. This taxonomico-morphological a spect was the first phase of modern ecology and culminated in the description of community types. This necessary work has been largely accomplished, just as in general morphology of organisms the main outlines have been delineated. Description of the unusual type of community, or community structural detail, will naturally continue, as will the description of new species of organisms, or minute structural details of organisms. Second, the community may be said to have a genetico-evolutionary aspect and here the questions asked are concerned with the identity of the ancestral community, or with the future development of a community. The study of these developmental processes, succession or community evolution, has followed closely the work on community anatomy, and like the latter, the framework has been largely constructed. Third, a community may be said to have a physiology-the study of many functions within and of the whole, culminating in a study of what is called the superorganism, recently examined by Emerson ('39). It is with community physiology that this paper is concerned. Naturally enough the writer's research program on animal activity for the past twelve years (Park, '40) has influenced his reflections upon the integration of the community. For example when I have observed the animals in a forest I have been overly conscious of the imponderable numbers of organisms in terms of their multiform, specifically peculiar, periodic and aperiodic activities. This function pattern was so complex that it defied analysis in toto. A given part could be analyzed, but the stream of activity and interrrelation swirled on about the segment being analyzed, and ultimate continuity with the whole pattern was deranged or lost. The larger the segment analyzed, the less this loss of integration but the more dilute the analysis. The employment of our large and still growing terminology explained nothing. Similarly the use of mathematics, although an excellent tool in skilled hands, was of no value as a mental telescope. These shortcomings were probably my own fault, yet the reality of the community as a whole continued to plague me, and the complex pattern defied complete realization. After considerable thought I have reached the conclusion that at least one unifying point of view is that there is a maximal utilization of space-time, eventuating in an arhythmic, symmetrical disposition of community activity with reference to the twenty-four hour day-night cycle. This concept will be considered at three levels of integration: the organismal, the infra-social, and the social. OrganismXal level.-In general where the environment is periodic with respect to day-night, habitat niches are occupied by animals whose general activity is periodically synchronized with this environmental rhythm. This periodism is specific and individual; it is composite, composed in part of endogenous rhythms which are partially habitual and partially inherent, and in part of exogenous rhythms. In so far as the exogenous activity is concerned, and indirectly the habitual activity, the present and past experience of an individual results from stimuli perceived through its sensory receptors. Underlying this composite periodic activity, which varies with the individual, with latitude and with season, and its concomitant effects on protoplasmic activity, are other vital functions which