Characteristics Of Spontaneous Activity Research Articles

When do bursts matter in the primary motor cortex? Investigating changes in the intermittencies of beta rhythms associated with movement states.

Brain activity exhibits significant temporal structure that is not well captured in the power spectrum. Recently, attention has shifted to characterising the properties of intermittencies in rhythmic neural activity (i.e. bursts), yet the mechanisms that regulate them are unknown. Here, we present evidence from electrocorticography recordings made over the motor cortex to show that the statistics of bursts, such as duration or amplitude, in the beta frequency (14-30Hz) band, significantly aid the classification of motor states such as rest, movement preparation, execution, and imagery. These features reflect nonlinearities not detectable in the power spectrum, with states increasing in nonlinearity from movement execution to preparation to rest. Further, we show using a computational model of the cortical microcircuit, constrained to account for burst features, that modulations of laminar specific inhibitory interneurons are responsible for the temporal organisation of activity. Finally, we show that the temporal characteristics of spontaneous activity can be used to infer the balance of cortical integration between incoming sensory information and endogenous activity. Critically, we contribute to the understanding of how transient brain rhythms may underwrite cortical processing, which in turn, could inform novel approaches for brain state classification, and modulation with novel brain-computer interfaces.

Deterministic characteristics of spontaneous activity detected by multi-fractal analysis in a spiking neural network with long-tailed distributions of synaptic weights.

Cortical neural networks maintain autonomous electrical activity called spontaneous activity that represents the brain's dynamic internal state even in the absence of sensory stimuli. The spatio-temporal complexity of spontaneous activity is strongly related to perceptual, learning, and cognitive brain functions; multi-fractal analysis can be utilized to evaluate the complexity of spontaneous activity. Recent studies have shown that the deterministic dynamic behavior of spontaneous activity especially reflects the topological neural network characteristics and changes of neural network structures. However, it remains unclear whether multi-fractal analysis, recently widely utilized for neural activity, is effective for detecting the complexity of the deterministic dynamic process. To verify this point, we focused on the log-normal distribution of excitatory postsynaptic potentials (EPSPs) to evaluate the multi-fractality of spontaneous activity in a spiking neural network with a log-normal distribution of EPSPs. We found that the spiking activities exhibited multi-fractal characteristics. Moreover, to investigate the presence of a deterministic process in the spiking activity, we conducted a surrogate data analysis against the time-series of spiking activity. The results showed that the spontaneous spiking activity included the deterministic dynamic behavior. Overall, the combination of multi-fractal analysis and surrogate data analysis can detect deterministic complex neural activity. The multi-fractal analysis of neural activity used in this study could be widely utilized for brain modeling and evaluation methods for signals obtained by neuroimaging modalities.

In vivo two-photon imaging of the embryonic cortex reveals spontaneous ketamine-sensitive calcium activity

Prior to sensory experience spontaneous activity appears to play a fundamental role in the correct formation of prominent functional features of different cortical regions. The use of anaesthesia during pregnancy such as ketamine is largely considered to negatively affect neuronal development by interfering with synaptic transmission. Interestingly, the characteristics of spontaneous activity as well as the acute functional effects of maternal anaesthesia remain largely untested in the embryonic cortex in vivo. In the present work, we performed in vivo imaging of spontaneous calcium activity and cell motility in the marginal zone of the cortex of E14-15 embryos connected to the mother. We made use of a preparation where the blood circulation from the mother through the umbilical cord is preserved and fluctuations in intracellular calcium in the embryonic frontal cortex are acquired using two-photon imaging. We found that spontaneous transients were either sporadic or correlated in clusters of neuronal ensembles at this age. These events were not sensitive to maternal isoflurane anaesthesia but were strongly inhibited by acute in situ or maternal application of low concentration of the anaesthetic ketamine (a non-competitive antagonist of NMDA receptors). Moreover, simultaneous imaging of cell motility revealed a correlated strong sensitivity to ketamine. These results show that anaesthetic compounds can differ significantly in their impact on spontaneous early cortical activity as well as motility of cells in the marginal zone. The effects found in this study may be relevant in the etiology of heightened vulnerability to cerebral dysfunction associated with the use of ketamine during pregnancy.

Effects of high-frequency stimulation of the internal pallidal segment on neuronal activity in the thalamus in parkinsonian monkeys.

Deep brain stimulation of the internal globus pallidus (GPi) is a major treatment for advanced Parkinson's disease. The effects of this intervention on electrical activity patterns in targets of GPi output, specifically in the thalamus, are poorly understood. The experiments described here examined these effects using electrophysiological recordings in two Rhesus monkeys rendered moderately parkinsonian through treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), after sampling control data in the same animals. Analysis of spontaneous spiking activity of neurons in the basal ganglia-receiving areas of the ventral thalamus showed that MPTP-induced parkinsonism is associated with a reduction of firing rates of segments of the data that contained neither bursts nor decelerations, and with increased burst firing. Spectral analyses revealed an increase of power in the 3- to 13-Hz band and a reduction in the γ-range in the spiking activity of these neurons. Electrical stimulation of the ventrolateral motor territory of GPi with macroelectrodes, mimicking deep brain stimulation in parkinsonian patients (bipolar electrodes, 0.5 mm intercontact distance, biphasic stimuli, 120 Hz, 100 μs/phase, 200 μA), had antiparkinsonian effects. The stimulation markedly reduced oscillations in thalamic firing in the 13- to 30-Hz range and uncoupled the spiking activity of recorded neurons from simultaneously recorded local field potential (LFP) activity. These results confirm that oscillatory and nonoscillatory characteristics of spontaneous activity in the basal ganglia receiving ventral thalamus are altered in MPTP-induced parkinsonism. Electrical stimulation of GPi did not entrain thalamic activity but changed oscillatory activity in the ventral thalamus and altered the relationship between spikes and simultaneously recorded LFPs.

Quantification of fetal magnetoencephalographic activity in low-risk fetuses using burst duration and interburst interval

ObjectiveTo identify quantitative MEG indices of spontaneous brain activity for fetal neurological maturation in normal pregnancies and examine the effect of fetal state on these indices. MethodsSpontaneous MEG brain activity was examined in 22 low-risk fetal recordings with gestational age (GA) ranging from 30 to 37weeks. As major quantitative characteristics of spontaneous activity, burst duration (BD) and interburst interval (IBI) were studied in correlation with GA and fetal state. ResultsIBI showed a decrease with gestational age (−0.21 s/week, P=0.0031). This trend was only maintained in the quiet-sleep state. With respect to BD, no significant trends were detected with GA and state. ConclusionIBI can be quantified as a fetal brain maturational parameter. The decrease in IBI over gestation was similar to the trend reported in the preterm neonatal EEG studies. Quiet sleep could be the optimal state to study such MEG maturational indices. SignificanceWith further investigation, indices extracted from spontaneous fetal brain activity may serve as an early warning for fetal neurological distress.

Spontaneous activity in a self-organizing recurrent network reflects prior learning

Event Abstract Back to Event Spontaneous activity in a self-organizing recurrent network reflects prior learning Andreea Lazar1*, Gordon Pipa2 and Jochen Triesch1 1 Frankfurt Institute for Advanced Studies, Germany 2 Max Planck Institute for Brain Research, Germany In the neocortex, spontaneous activity in the absence of sensory input exhibits nonrandom spatiotemporal patterns [Tsodyks et al., 1999, Fiser et al., 2004]. Following the repetitive presentation of a given visual stimulus, the spontaneous activity patterns show similarities with the sensory-evoked responses [Han et al., 2008]. It has been hypothesized that spontaneous activity reflects prior information, learned via plasticity based on past experience, which when integrated with sensory-evoked activity enables Bayesian inference [Berkes et al., 2009]. We explore the characteristics of spontaneous activity following unsupervised learning of spatio-temporal stimuli in a self-organizing recurrent network (SORN) shaped by synaptic and neuronal plasticity. The SORN model [Lazar et al., 2009] consists of a population of excitatory cells and a smaller population of inhibitory cells. The connectivity among excitatory units is sparse and subject to a simple spike-timing-dependent plasticity rule. Additionally, synaptic normalization keeps the sum of an excitatory neuron's afferent weights constant, while intrinsic plasticity regulates a neuron’s firing threshold to maintain a low average activity level. The network receives input sequences composed of different letters and learns the structure embedded in these sequences in an unsupervised manner. Following a learning interval, we omit the input and analyse the characteristics of spontaneous activity. We find that the network revisits states similar to those embedded during input stimulation and that it follows similar trajectories through its high-dimensional state space. Furthermore we show that the spontaneous activity reflects the statistical properties of the data: during spontaneous activity the network preferentially visits states that are similar to evoked activity patterns for inputs with a higher prior probability. Our results establish a novel link between STDP-based unsupervised learning in recurrent networks and concepts of statistical inference. Acknowledgments: This work was supported by the Hertie Foundation, grant PLICON (EC MEXT-CT-2006-042484), and GABA Project (EU-04330). We thank Sophie Deneve for literature suggestions. Conference: Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010. Presentation Type: Poster Presentation Topic: Poster session I Citation: Lazar A, Pipa G and Triesch J (2010). Spontaneous activity in a self-organizing recurrent network reflects prior learning. Front. Neurosci. Conference Abstract: Computational and Systems Neuroscience 2010. doi: 10.3389/conf.fnins.2010.03.00336 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 19 Feb 2010; Published Online: 19 Feb 2010. * Correspondence: Andreea Lazar, Frankfurt Institute for Advanced Studies, Franffurt am Main, Germany, andreea.lazar@brain.mpg.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Andreea Lazar Gordon Pipa Jochen Triesch Google Andreea Lazar Gordon Pipa Jochen Triesch Google Scholar Andreea Lazar Gordon Pipa Jochen Triesch PubMed Andreea Lazar Gordon Pipa Jochen Triesch Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Characteristics of Spontaneous Activity in the Bladder Trigone

BackgroundDuring bladder filling, the trigone contracts help keep the ureteral orifices open and the bladder neck shut. The trigone generates spontaneous activity as well as responding to neuromuscular transmitters, but the relationship between these phenomena are unclear. ObjectivesTo characterise the cellular mechanisms that regulate and modify spontaneous activity in trigone smooth muscle. Design, setting, and participantsMuscle strips from the superficial trigone of male guinea-pigs were used for tension experiments and immunofluorescent studies. MeasurementsIn isolated trigonal cells, intracellular Ca2+ was measured by epifluorescence microscopy using the fluorescent Ca2+ indicator Fura-2. Results and limitationsSpontaneous intracellular Ca2+ transients and contractions were observed in trigonal single cells and strips and were significantly higher compared to the bladder dome. Ca-free superfusate and verapamil terminated spontaneity. T-type Ca2+ channel block with NiCl2 depressed slightly Ca2+ transients but not spontaneous contractions. Neither the BKCa channel blocker iberiotoxin nor the SKCa channel blocker apamin had any effect on single cell activity. By contrast, the Cl− channel blocker niflumic acid attenuated significantly both Ca2+ transients and muscle contractions. Agonist stimulation (carbachol, phenylephrine) up-regulated activity. Gap junction labelling (Cx43) was approximately 5 times denser in the trigone than in detrusor smooth muscle. The gap junction blocker 18-ß-glycyrrhetinic acid modulated spontaneous contractions in the trigone but not in the bladder dome. ConclusionsTrigone myocytes employ membrane L-type-Ca2+ channels and Cl− channels to generate spontaneous activity. Intercellular electrical coupling ensures its propagation and, thus, sustains contraction of the whole trigone.

Deviation of spontaneous firing in frog medullar auditory units from the renewal point process

Statistical characteristics of spontaneous activity (distribution of interspike intervals, hazard function, autocorrelation function, autocorrelation function for the process with mixed intervals, and interdependence between adjoining intervals) were analyzed for 123 neurons of the frog medullar dorsal nucleus (homolog of the mammalian cochlear nucleus). In the majority of cells, this activity was distinct from the Poisson process, and firing periodicity was noticed in some cases. In addition, deviations of the spontaneous activity from the renewal process were usually observed. Weak yet reliable positive interspike-interval correlation was typical of most neurons; however, a negative correlation between short adjoining intervals was recorded for some units. These data suggest the effects of memory in the activity of single neurons of the auditory system.

Thermosensitivity Of Large Primary Sensory Neurons

Spontaneous activity originating in the injured nerve or the dorsal root ganglion (DRG) has been implicated in the development and maintenance of neuropathic pain. The inherent characteristics of spontaneous activity and the causal factors that modulate its firing pattern and frequency are not fully understood. We attempted to assess the thermosensitivity of spontaneous activity in dorsal root ganglion (DRG) neurons in normal rats and in rat, with cl-ironic compression of the DRG (CCD) in an in vitro nerve-DRG preparation. Extracellular, dorsal root recording from 66 spontaneously active CCD Abeta fibers indicate that: (1) decreasing bath temperature from 37 to 36-26 °C significantly decreased the firing rate (FR) in 85% (56/66) of fibers tested, of which 19 fibers (34%) responded to temperature change at physiological range (36-37 °C), whereas the remaining fibers responded at lower temperature levels (26-36 °C); (2) cooling of the DRG increased the FR in 12% (8/66) of fibers tested; (3) similarly, the firing rate of 21/26 spontaneously active Abeta fibers from normal rats was decreased following temperature decrease; (4) intracellular recordings from 38 normal neurons revealed that cooling the DRG significantly increased the action potential (AP) threshold, A-P duration, AP amplitude and afterhyperpolarization (AHP) duration, but decreased AHP amplitude, maximal depolarizing and repolarizing rates. There was no significant change in the rheobase currents or the resting membrane potential. The present study indicates that large sensory neurons with myelinated axons are temperature dependent. It also suggests that maintenance of a stable temperature is critical for reliable characterization of spontaneous activity of sensory neurons.

Thermosensitivity of large primary sensory neurons

Spontaneous activity originating in the injured nerve or the dorsal root ganglion (DRG) has been implicated in the development and maintenance of neuropathic pain. The inherent characteristics of spontaneous activity and the causal factors that modulate its firing pattern and frequency are not fully understood. We attempted to assess the thermosensitivity of spontaneous activity in dorsal root ganglion (DRG) neurons in normal rats and in rats with chronic compression of the DRG (CCD) in an in vitro nerve-DRG preparation. Extracellular, dorsal root recording from 66 spontaneously active CCD Aβ fibers indicate that: (1) decreasing bath temperature from 37 to 36–26 °C significantly decreased the firing rate (FR) in 85% (56/66) of fibers tested, of which 19 fibers (34%) responded to temperature change at physiological range (36–37 °C), whereas the remaining fibers responded at lower temperature levels (26–36 °C); (2) cooling of the DRG increased the FR in 12% (8/66) of fibers tested; (3) similarly, the firing rate of 21/26 spontaneously active Aβ fibers from normal rats was decreased following temperature decrease; (4) intracellular recordings from 38 normal neurons revealed that cooling the DRG significantly increased the action potential (AP) threshold, AP duration, AP amplitude and afterhyperpolarization (AHP) duration, but decreased AHP amplitude, maximal depolarizing and repolarizing rates. There was no significant change in the rheobase currents or the resting membrane potential. The present study indicates that large sensory neurons with myelinated axons are temperature dependent. It also suggests that maintenance of a stable temperature is critical for reliable characterization of spontaneous activity of sensory neurons.

An Antagonist of cADP-ribose Inhibits Arrhythmogenic Oscillations of Intracellular Ca2+ In Heart Cells

Oscillations of Ca2+ in heart cells are a major underlying cause of important cardiac arrhythmias, and it is known that Ca2+-induced release of Ca2+ from intracellular stores (the sarcoplasmic reticulum) is fundamental to the generation of such oscillations. There is now evidence that cADP-ribose may be an endogenous regulator of the Ca2+ release channel of the sarcoplasmic reticulum (the ryanodine receptor), raising the possibility that cADP-ribose may influence arrhythmogenic mechanisms in the heart. 8-Amino-cADP-ribose, an antagonist of cADP-ribose, suppressed oscillatory activity associated with overloading of intracellular Ca2+ stores in cardiac myocytes exposed to high doses of the beta-adrenoreceptor agonist isoproterenol or the Na+/K+-ATPase inhibitor ouabain. The oscillations suppressed by 8-amino-cADP-ribose included intracellular Ca2+ waves, spontaneous action potentials, after-depolarizations, and transient inward currents. Another antagonist of cADP-ribose, 8-bromo-cADP-ribose, was also effective in suppressing isoproterenol-induced oscillatory activity. Furthermore, in the presence of ouabain under conditions in which there was no arrhythmogenesis, exogenous cADP-ribose was found to be capable of triggering spontaneous contractile and electrical activity. Because enzymatic machinery for regulating the cytosolic cADP-ribose concentration is present within the cell, we propose that 8-amino-cADP-ribose and 8-bromo-cADP-ribose suppress cytosolic Ca2+ oscillations by antagonism of endogenous cADP-ribose, which sensitizes the Ca2+ release channels of the sarcoplasmic reticulum to Ca2+.

Cationic trypsinogen produced by human pancreatic ductal cancer has the characteristics of spontaneous activation and gelatinolytic activity in the presence of proton.

We examined whether human pancreatic ductal cancer cells express and secrete pancreatic cationic trypsinogen in vitro which can be spontaneously converted into active trypsin at acidic pH (pH 4.5-5. 5), in contrast to anionic trypsinogen. Cationic trypsinogen expression at the mRNA level was observed in differentiated Capan-1 and BxPC-3 cell lines. However, expression was not detected in either poorly-differentiated Panc-1 or undifferentiated MIAPaCa-2 cell line. The gelatinolytic activity of the activated form of trypsinogen in each conditioned medium in the presence of enterokinase (1.0 microg/ml) (a band with a molecular weight of approximately 23 kDa) corresponded well to the level of cationic trypsinogen mRNA. The spontaneous activation of trypsinogen also was observed by gelatin zymography of the acid-loaded conditioned medium (pH 5.5). These findings suggest that trypsinogen produced by human pancreatic ductal cancer has the characteristics of spontaneous activation and gelatinolytic activity in the presence of proton.

Sympathectomy does not affect the early ectopic discharge from myelinated fibres in ferret inferior alveolar nerve neuromas

Ectopic neural activity from damaged axons is thought to contribute to the development of sensory disorders following nerve injury. Here we have studied the role of sympathetic fibres in the generation or potentiation of this abnormal activity by determining the effect of predegenerating these fibres. Twelve adult ferrets were used in the study and six of them underwent sympathectomy by removal of the left superior cervical ganglion. Electrophysiological recordings were made from myelinated axons in fine filaments dissected from the inferior alveolar nerve, 3 days after it had been ligated further distally, and the level of spontaneous activity and mechanical sensitivity was determined. There was no significant difference between the level or characteristics of spontaneous activity, or the level of mechanical sensitivity, in the two groups of animals. We conclude that, in this animal model, the absence of sympathetic nerve fibres does not affect the development or characteristics of ectopic neural activity in the early period following nerve injury.

Functional integration of the rat hippocampal tissue, transplanted into the rabbit septum

Embryonal tissue of the rat hippocampus (E17–18) was grafted into small acute cavity within the dorsal septum of adult rabbit after interruption of septo-hippocampal connections. Histological analysis (Nissl stain), performed 6–8 weeks later, revealed surviving hippocampal grafts in 14 out of 29 grafted animals (48%). Some of the grafts were displaced into the ventricle and had only minor contact with the host septum; some others, residing in the septum, were separated from the host tissue by continuous glial scar. Six grafts (33%) were morphologically well integrated with the host septum. The grafts contained mature pyramidal neurons but their organization into layer was absent. No signs of rejection were observed in the grafts, which were not subjected to microelectrode investigation in chronic conditions. The grafts, which for 5–9 successive days were penetrated by microelectrodes, were heavily infiltrated by lymphocytes and had necrotic areas. Electrophysiological testing revealed the absence of spontaneous neuronal activity, or the presence of hypersynchronous epileptiform bursts in isolated grafts. The 4 integrated grafts, investigated electrophysiologically, contained neurons with normal spontaneous activity of two types: low-frequency discharges with complex spikes and high-frequency single spikes, sometimes with short periods of weak rhythmic modulation in the θ-range. Characteristics of spontaneous activity of the xenografted hippocampus are very close to those described in the rat hippocampus in situ. Rhythmic θ-modulation of neuronal activity appeared or increased in stability after physostigmine injection to the host, after electrical stimulation of the host's midbrain reticular formation and ventral part of diagonal band, and after multimodal sensory stimulation of the host. The frequency of θ-bursts in the grafts was identical to that of neurons of the host septum (4.5–6.5 Hz). Besides the θ-modulation sensory stimuli evoked tonic suppression or activation of discharges, phasic reactions and on-effects. Electrical stimulation of lateral septum and diagonal band evoked orthodromic and antidromic discharges of the grafted cells. Normal spontaneous and evoked activity of the grafted cells deteriorated after 5–11 successive days of microelectrode investigation. It is concluded that xenografts of nervous tissue, developing without immunosuppression, have a low rate of survival and are vulnerable to immune response after mechanical interruption of integrity of the blood-brain barrier. Nevertheless their normal activity and close functional integration with the host brain, apparently depending on development of chimeric synapses, is possible.

Physiological characteristics of the first order horizontal canal neurons in guinea pigs. 1. Resting discharge characteristics

Physiological characteristics of first order horizontal canal neurons, especially in their spontaneous activity, were studied in 11 unpigmented guinea pigs. Ninety-three single neurons were recorded and their inter-spike intervals and regularity of the resting discharge were analyzed. Mean inter-spike intervals was 40.5 msec with a range of 11 to 227 msec. The neurons were classified into three groups according to their regularity of resting dis- charge which was determined by coefficient of variation (CV). Fourty neurons (43.0%) showed regular type of resting discharge and their mean inter-spike intervals was 20.2 msec. Thirty-six (38.7%) neurons exhibited intermediate type and had 32.2 msec mean inter-spike intervals. Remaining seventeen neurons (18.7%) were classified as irregular type and showed mean interspike interval of 95.5 msec. The basic physiological characteristics of spontaneous activity in unpigmented guinea pigs were same as pigmented guinea pigs reported by Curthoys (1982), but the mean inter-spike intervals in unpigmented guinea pigs was clearly greater than that of pigmented guinea pigs.

Types of spontaneous discharges of muscle receptors

Characteristics of discharges of 462 muslce spindle receptors located in the deefferented triceps surae muscle with a divided tendon were analyzed in anesthetized cats. Of the total number, 205 units had spontaneous activity which, depending on its stability, could be divided into three types. A long continuous spike train was generated by 87% of units. Distributions of interspike intervals were unimodal and symmetrical. Spontaneous activity of this type was more regular in secondary than in primary endings. Action potentials in 8% of units were grouped in volleys and interval distributions were bimodal. Spontaneous activity of the remaining 5% of units was characterized by sporadically appearing spikes with long intervals between them. The first two types of spontaneous activity cannot be placed in the category of stochastic processes. Threshold discharges during static stretching of the muscle sufficient to cause the appearance of a long spike train were studied in 231 of 257 initially "silent" units. By the same criteria these discharges were divided into corresponding types. Comparison of the characteristics of spontaneous activity and evoked responses suggests that they share the same mechanism of origin. It is concluded that the spontaneous discharge can be regarded as a static response of the receptors due not to an externally applied stretching load, but to the action of internal factors, depending on interaction between extrafusal and intrafusal muscle fibers.

Effect of stimulation of hypothalamic preoptic region on activity in single fibers of the rabbit optic tract

The statistical characteristics of spontaneous activity and activity evoked by flashes in single fibers of the optic tract were compared before and after stimulation of the preoptic region of the hypothalamus in rabbits immobilized with listhenon. Analysis of the results shows that the effect of high-frequency stimulation of the hypothalamic preoptic region correlates with the character of responses to flashes recorded from fibers of the optic tract. More often stimulation of the hypothalamic preoptic region led to an increase in the spontaneous discharge frequency in the optic tract fiber and to facilitation of the response to flashes. Responses of phasic type to flashes with a short latent period (42 msec), consisting of a long series of spikes, and responses of tonic type with a long latent period (155 msec) were facilitated to a greater degree. Responses of phasic type to flashes with a short latent period (35 msec), consisting of a short series of spikes, usually were not facilitated after stimulation of the hypothalamic preoptic region. The results suggest the presence of influences of complex character on the rabbit retina from the preoptic region of the hypothalamus.

Characteristics of spontaneous activity in tsetse flies.

THE word “spontaneous” is often applied to the activity of tsetse in the field without it being certain that such activity is not environmentally stimulated. During a laboratory study of circadian rhythms in adult tsetse, however, it has now been possible to confirm that flies do move in the near environmental vacuum of constant temperature, humidity, darkness, noise and, probably, smell.

Motoneuron excitability in man: A novel method of evaluation by modulation of tonic muscle activity

The excitability of a population of motoneurons in humans has been investigated classically by the recovery curve of the monosynaptic reflex following a conditioning stimulus. The classical technique is applicable only to a few large muscles with prominent stretch reflexes. The inherent variability of the monosynaptic reflex makes this method both arduous and time-consuming. An alternative means of excitability testing is described. The technique is based upon the observation that during tonic activity, motor unit discharge increased with facilitation of the reflex and decreased with inhibition of the reflex. A system was developed for graphic recording of the modulation of electromyographic activity following a conditioning stimulus. The electromyogram is passed through an absolute-value rectifier circuit and then summed by computer whose sweeps are triggered by the conditioning stimulus. An X-Y plot is then made of the summed potentials (“modulation curve”) representing the graph of activity changes following the conditioning stimulus. The modulation curve resembled but did not reflect precisely changes in central excitability recorded by the classical recovery curve. The modulation curve is rapidly and easily performed. It can be elicited from any muscle, flexor or extensor, and is not restricted to the few large muscles with prominent stretch reflexes as is true in the classical technique. The method is especially adapted to the study of the response characteristics of spontaneous activity in clinical electromyography.