The range of control of the afferent system of the flexor reflex]

The control processes caused by the action of electric inhibition of the nerve fibres and depolarized and hyperpolarized influences in the neurones structures of the spinal cord are discussed. Three typical bands of the amplitude control of motor apparatus in the multineuronic reflex ring are distinguished. The transmission of the action potentials in the battle control bands is carried out by the double way modulation. The first band is characterized by the most effective control with amplitude of the signal control in the limit of three thresholds. In the third band the transmission of information is possible in the inversion regime with unsteady polarity of the reflex movement. The second band unfits for the transmission of control commands.     

Decreasing motion artifacts in calcium-dependent fluorescence transients from the perfused mouse heart using frequency filtering

A strategy has been developed for the removal of motion artifact and noise in calcium-dependent fluorescence transients from the perfused mouse heart using frequency filtering. An analytical model indicates that the spectral removal of motion artifacts is independent of the phase shift of the motion waveform in the frequency domain, and thus to the time shift (or delay) of motion in the time domain. This is based on the "shift theorem" of Fourier analysis, which avoids erroneous correction of motion artifact when using the motion signal obtained using reflectance from the heart. Several major steps are adopted to implement this model for elimination of motion as well as detection noise from the fluorescence transient signals from the calcium-sensitive probe Rhod-2. These include (1) extracting the fluorescence calcium transient signal from the raw data by using power spectrum density (PSD) in the frequency domain by subtracting the motion recorded using the reflectance of excitation light, (2) digitally filtering out the random noise using multiple bandpass filters centralized at harmonic frequencies of the transients, and (3) extracting high frequency noise with a Gaussian Kernel filter method. The processed signal of transients acquired with excessive motion artifact is comparable to transients acquired with minimal motion obtained by immobilizing the heart against the detection window, demonstrating the usefulness of this technique.     

Reflex depression in rhythmically active monosynaptic reflex pathways

A study has been made of the depression that occurs when a monosynaptic reflex pathway is subjected to repetitive stimulation. Reflex depression has a dual origin. High frequency or early depression is postsynaptic in origin and results from subnormality in the motoneurons. Low frequency, late, or enduring depression is presynaptic in origin. The conditioning volley-test volley technique and the frequency-mean monosynaptic reflex amplitude relation yield similar information concerning reflex depression. Each method has its advantages and for some purposes one or the other of the methods necessarily must be employed. The results of a variety of experiments are consistent with the proposition that reflex depression in the monosynaptic reflex pathway originates by action in the group IA afferent fibers of muscle origin that are responsible for monosynaptic reflex transmission. Depression is present at a frequency of 0.1 per second (6 per minute) and absent at a frequency of 0.05 per second (3 per minute). Thus it is impractical for most purposes to employ repetition rates that satisfy the requirement for designation as "single shock" stimulations. The temporal course of enduring depression has been determined. It is identical with that for a number of other phenomena observable in monosynaptic reflex pathways, which suggests a common origin. The mechanism of low frequency or enduring depression is discussed in the light of this suggestion.     

Noise-limited frequency signal transmission in gene circuits

To maintain normal physiology, cells must properly process diverse signals arising from changes in temperature, pH, nutrient concentrations, and other factors. Many physiological processes are controlled by temporal aspects of oscillating signals; that is, these signals can encode information in the frequency domain. By modeling simple gene circuits, we analyze the impact of cellular noise on the fidelity and speed of frequency-signal transmission. We find that transmission of frequency signals is "all-or-none", limited by a critical frequency (f(c)). Signals with frequencies f(c) are severely corrupted or completely lost in transmission. We argue that f(c) is an intrinsic property of a gene circuit and it varies with circuit parameters and additional feedback or feedforward regulation. Our results may have implications for understanding signal processing in natural biological networks and for engineering synthetic gene circuits.     

Frequency encoded biochemical regulation is more accurate than amplitude dependent control

It is generally recognised in engineering that encoding information in a frequency provides resistance to degradation by noise and an enhanced precision of control. This paper demonstrates how the same arguments can be applied to biochemical control networks. It shows that the conversion of an analogue demand signal to an oscillation is stable against corruption by noise in the input and even against corruption by certain internal chaotic motions. The paper also argues that intracellular transmission of frequency encoded information is robust against noise. These arguments are proposed as a partial explanation of why so many biological regulatory systems are periodic.     

Coding of time-dependent stimuli in homogeneous and heterogeneous neural populations

We compare the information transmission of a time-dependent signal by two types of uncoupled neuron populations that differ in their sources of variability: i) a homogeneous population whose units receive independent noise and ii) a deterministic heterogeneous population, where each unit exhibits a different baseline firing rate (’disorder’). Our criterion for making both sources of variability quantitatively comparable is that the interspike-interval distributions are identical for both systems. Numerical simulations using leaky integrate-and-fire neurons unveil that a non-zero amount of both noise or disorder maximizes the encoding efficiency of the homogeneous and heterogeneous system, respectively, as a particular case of suprathreshold stochastic resonance. Our findings thus illustrate that heterogeneity can render similarly profitable effects for neuronal populations as dynamic noise. The optimal noise/disorder depends on the system size and the properties of the stimulus such as its intensity or cutoff frequency. We find that weak stimuli are better encoded by a noiseless heterogeneous population, whereas for strong stimuli a homogeneous population outperforms an equivalent heterogeneous system up to a moderate noise level. Furthermore, we derive analytical expressions of the coherence function for the cases of very strong noise and of vanishing intrinsic noise or heterogeneity, which predict the existence of an optimal noise intensity. Our results show that, depending on the type of signal, noise as well as heterogeneity can enhance the encoding performance of neuronal populations.     

Information transmission by isolated frog muscle spindle

Transmission of sensory information was calculated for the isolated frog muscle spindle receptor, using Shannon's information measure. Sinusoidal movements, random noise stretches, and sinusoids with superimposed auxiliary noise were applied as stimuli. In addition, the static prestretch level of the intrafusal muscle bundle was adjusted between resting length (L0) and L0 + 600 micron, so that the analysis of the information transmission properties covered the entire dynamic range of the sensory receptor organ. Sinusoidal stretches below 2 Hz evoked smoothly modulated cycle histograms, which were approximately linearly related to the stimulating sinewave. The transinformation rates under these conditions were generally low (5-17 bit X s-1), regardless of the amplitude of the applied movement. Increasing prestretch enhanced the modulation depth of the cycle histograms considerably, but increased the transinformation rates by less than 10 bit X s-1. By contrast, sinusoids above 2 Hz evoked clearly nonlinear cycle histograms, because each action potential was firmly phase-locked to a small segment of the stretch cycle. Under these conditions the transinformation rates grew larger with increasing stimulus frequency and approached 130 bit X s-1 at 60 Hz. Small amplitude sinusoidal stretches, however, evoked considerable transinformation rates in the high frequency region only then, when the spindle receptor was extended to higher prestretch levels. Random stretches evoked transinformation rates between 5 and 30 bit X s-1 depending on both the prestretch level and the intensity of the noise stimulus. The linear response components carried only about 25% of the transinformation rates transmitted by both the linear and nonlinear response components. Auxiliary noise stimuli greatly improved the information transmission of sinusoidal stretches. For example, a pure sinusoid evoked 5 bit X s-1. Adding a noise signal with equal energy to the sinusoidal movement elicited 20 bit X s-1. This facilitation effect of auxiliary noise was restricted to low frequency sinusoidal stimuli. The present results are discussed with respect to the information transmission properties of various sensory systems evaluated by either the same or different information processing procedure as that used in the present study. The functional significance of high transinformation rates sent by the muscle spindle to the central nervous system is discussed with respect to motor control.     

A communication channel model for information transmission in the blowfly photoreceptor

Biological photoreceptors transduce and communicate information about visual stimuli to other neurons through a series of signal transformations among physical states such as concentration of a chemical species, current, or the number of open ion channels. We present a communication channel model to quantify the transmission and degradation of visual information in the blowfly photoreceptor cell. The model is a cascade of linear transfer functions and noise sources that are derived from fundamental principles whenever possible, and whose parameters are estimated from physiological data. We employ the model to calculate the information capacity of blowfly phototransduction; our results compare favorably with estimates of the capacity derived from experimental measurements by de Ruyter van Steveninck and Laughlin (Nature 379 (1996) 642-645) and Juusola (J. Gen. Physiol. 104 (1994) 593-621). The model predicts that photon shot noise and ion channel noise are the dominant noise sources that limits information transmission in the blowfly photoreceptor.     

Early post-tetanic potentiation and low frequency depression of some group I reflex actions

Group I reflex functions, namely monosynaptic reflex transmission, facilitation of synergists, and direct and disynaptic inhibition, show early post-tetanic potentiation following conditioning with a brief, high frequency, tetanus. Of these reflex functions, monosynaptic transmission always shows low frequency depression. Direct inhibitory pathways, and therefore inhibitory junctions, are insensitive to low frequency depression. The fact that direct inhibition can be potentiated shows it to be sufficiently labile that a decrease in efficacy at inhibitory junctions during repetitive activity should be revealed. Disynaptic inhibition often shows low frequency depression. As it is likely that inhibitory junctions in the direct and disynaptic pathways are similar, the low frequency depression of disynaptic inhibition is probably due to the properties of the excitatory relay between afferent fibers and interneurons in that pathway. Facilitation between synergists is often more depressed when the conditioning and testing volleys are nearly simultaneous than when they are separated by 1 to 1.5 msec. This result indicates that an early and rapid phase of action, responsible for homonymous and heteronymous transmission, is more sensitive to low frequency depression than is residual facilitation. In general, reflex transmission is more sensitive than are other aspects of action by group I fibers to events concurrent with and following repetitive activation.     

Fisher information as a metric of locally optimal processing and stochastic resonance

The origins of Fisher information are in its use as a performance measure for parametric estimation. We augment this and show that the Fisher information can characterize the performance in several other significant signal processing operations. For processing of a weak signal in additive white noise, we demonstrate that the Fisher information determines (i) the maximum output signal-to-noise ratio for a periodic signal; (ii) the optimum asymptotic efficacy for signal detection; (iii) the best cross-correlation coefficient for signal transmission; and (iv) the minimum mean square error of an unbiased estimator. This unifying picture, via inequalities on the Fisher information, is used to establish conditions where improvement by noise through stochastic resonance is feasible or not.     

Considerations on mechanisms of focussed signal transmission in the multi-channel muscle stretch reflex system

This paper presents theoretical considerations on the possibility of topographically ordered signal transmission in the control system of the muscle stretch reflex. It is investigated how correlations between Ia fibres from primary muscle spindle endings in conjunction with an appropriate connectivity of Ia fibres and motoneurones enable the stretch reflex system to trace local routes through the spinal cord. The complex data processing capabilities of the motoneuronal soma-dendritic membrane system are fully taken into account, and it is argued that correlations between inputs to this system may play an important role for signal transmission through the spinal cord.     

Signal/noise ratio at the visual recognition threshold of incomplete figures

We measured recognition thresholds of incomplete figure perception (the Gollin test). This test we regarded as a visual masking problem. Digital image processing permits us to measure the spatial properties and spatial frequency spectrum of the absent part of the image as the mask. Using a noise paradigm, we have measured the signal/noise ratio for Incomplete Figure. Recognition was worse with better spectral "similarity" between the figure and the "invisible" mask. At threshold, the spectrum of the fragmented image was equally similar to that of the "invisible" mask and complete image. We think the recognition thresholds for Gollin stimuli reflect the signal/noise ratio.     

Filtering the surface EMG signal: Movement artifact and baseline noise contamination

The surface electromyographic (sEMG) signal that originates in the muscle is inevitably contaminated by various noise signals or artifacts that originate at the skin-electrode interface, in the electronics that amplifies the signals, and in external sources. Modern technology is substantially immune to some of these noises, but not to the baseline noise and the movement artifact noise. These noise sources have frequency spectra that contaminate the low-frequency part of the sEMG frequency spectrum. There are many factors which must be taken into consideration when determining the appropriate filter specifications to remove these artifacts; they include the muscle tested and type of contraction, the sensor configuration, and specific noise source. The band-pass determination is always a compromise between (a) reducing noise and artifact contamination, and (b) preserving the desired information from the sEMG signal. This study was designed to investigate the effects of mechanical perturbations and noise that are typically encountered during sEMG recordings in clinical and related applications. The analysis established the relationship between the attenuation rates of the movement artifact and the sEMG signal as a function of the filter band pass. When this relationship is combined with other considerations related to the informational content of the signal, the signal distortion of filters, and the kinds of artifacts evaluated in this study, a Butterworth filter with a corner frequency of 20 Hz and a slope of 12 dB/oct is recommended for general use. The results of this study are relevant to biomechanical and clinical applications where the measurements of body dynamics and kinematics may include artifact sources.     

Ambient noise and the design of begging signals

The apparent extravagance of begging displays is usually attributed to selection for features, such as loud calls, that make the signal costly and hence reliable. An alternative explanation, however, is that these design features are needed for effective signal transmission and reception. Here, we test the latter hypothesis by examining how the begging calls of tree swallow (Tachycineta bicolor) nestlings and the response to these calls by parents are affected by ambient noise. In a field study, we found that call length, amplitude and frequency range all increased with increasing noise levels at nests. In the laboratory, however, only call amplitude increased in response to the playback of noise to nestlings. In field playbacks to parents, similar levels of noise abolished parental preferences for higher call rates, but the preference was restored when call amplitude was increased to the level that nestlings had used in the laboratory study. Our results show that nestling birds, like other acoustic signallers, consistently increase call amplitude in response to ambient noise and this response appears to enhance discrimination by receivers. Thus, selection for signal efficacy may explain some of the seemingly extravagant features of begging displays.     

Frequency selectivity of hearing in the green treefrog,Hyla cinerea

Frequency selectivity of hearing was measured in the green treefrog, Hyla cinerea. A psychophysical technique based on reflex modification was used to obtain masked threshold estimates for pure tones (300-5,400 Hz) presented against two levels of broadband masking noise. A pure tone (S-1) presented 200 ms prior to a reflex-eliciting stimulus (S-2) inhibited the motor reflex response to S-2. The magnitude of this reflex modification effect varied systematically with the sound pressure level (SPL) of S-1, and threshold was defined as the SPL of S-1 at which the reflex modification effect disappeared. Masked thresholds were used to calculate critical ratios, an index of the auditory system's frequency selectivity. The frequency selectivity of the treefrog's hearing is greatest and critical ratios are lowest (22-24 dB) at about 900 and 3,000 Hz, the two spectral regions dominant in the male treefrog's species-specific advertisement call. These results suggest that the treefrog's auditory system may be specialized to reject noise at biologically-relevant frequencies. As in other vertebrates, critical ratios remain constant when background noise level is varied; however, the shape of the treefrog's critical ratio function across frequencies differs from the typical vertebrate function that increases with increasing frequency at a slope of about 3 dB/octave. Instead, the treefrog's critical ratio function resembles its pure tone audiogram. Although the shape of the treefrog's critical ratio function is atypical, the critical ratio values themselves are comparable to those of many other vertebrates in the same frequency range. Critical ratio values here measured behaviorally do not match critical ratio values previously measured physiologically in single eighth nerve fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sometimes noise is beneficial: stream noise informs vocal communication in the little torrent frog <Emphasis Type="Italic">Amolops torrentis</Emphasis>

Many kinds of environmental noise can interfere with acoustic communication and efficient decision making in terrestrial species. Here we identified an exception to this generalization in a streamside species, the little torrent frog (Amolops torrentis) which communicates in a stream noise environment. To determine whether stream noise can act as a cue regarding the microhabitat characteristics of senders, we performed phonotaxis experiments using stimulus pairs constructed with synthetic male calls (high or low dominant frequency) and stream noise with varied signal-to-noise ratios. We found that females prefer calls with high amplitude stream noise added compared to those with low amplitude stream noise added for both high and low dominant frequency stimulus pairs; however, stream noise itself was not attractive in the absence of calls. These results show that stream noise can function as a cue that may be used by females for enhancing the attractiveness of calls. Stream noise associates closely with rocks, topographies and vegetation and may thus provide useful microhabitat information for signal receivers, thereby acting on sexual selection. These data therefore contribute to our understanding of how the perception of mate attractiveness in heterogeneous ecological environments can evolve.     

Your attention please: increasing ambient noise levels elicits a change in communication behaviour in humpback whales (Megaptera novaeangliae)

High background noise is an important obstacle in successful signal detection and perception of an intended acoustic signal. To overcome this problem, many animals modify their acoustic signal by increasing the repetition rate, duration, amplitude or frequency range of the signal. An alternative method to ensure successful signal reception, yet to be tested in animals, involves the use of two different types of signal, where one signal type may enhance the other in periods of high background noise. Humpback whale communication signals comprise two different types: vocal signals, and surface-generated signals such as ‘breaching’ or ‘pectoral slapping’. We found that humpback whales gradually switched from primarily vocal to primarily surface-generated communication in increasing wind speeds and background noise levels, though kept both signal types in their repertoire. Vocal signals have the advantage of having higher information content but may have the disadvantage of loosing this information in a noisy environment. Surface-generated sounds have energy distributed over a greater frequency range and may be less likely to become confused in periods of high wind-generated noise but have less information content when compared with vocal sounds. Therefore, surface-generated sounds may improve detection or enhance the perception of vocal signals in a noisy environment.     

A maximum entropy criterion of filtering and coding for stationary autoregressive signals: Its physical interpretations and suggestions for its application to neural information transmission

The operations of encoding and decoding in communication agree with filtering operations of convolution and deconvolution for Gaussian signal processing. In an analogy with power transmission in thermodynamics, an autoregressive model of information transmission is proposed for representing a continuous communication system which requires a pair of an internal noise source and a signal source to encode or decode a message. In this model transinformation (informational entropy) equals the increase in stationary nonequilibrium organization formed through the amplification of white noise by a positive feedback system. The channel capacity is finite due to the existence of inherent noise in the system. The maximum entropy criterion in information dynamics corresponds to the 2^nd law of thermodynamics. If the process is stationary, the communication system is invertible, and has the maximum efficiency of transformation. The total variation in informational entropy is zero in the cycle of the invertible system, while in the noninvertible system the entropy of decoding is less than that of encoding. A noisy autoregressive coding which maximizes transinformation is optimum, but is also ideal.     

生物电阻抗特征参数测量数据采集系统的研究

为了实现生物电阻抗参数成像,我们建立了一个快速、精确的生物电阻抗特性参数测量的数据采集系统,采取多频率组合激励并扫基频的方法,在多个激励频率不同测量阻抗信息,同时采用了一种新的高精度解调方法;正交序列数字解调,并对阻抗模型参数的估算方法进行了改进。结果表明,系统可以在1．6kHz-380kHz的频带范围内以小于100Hz的跨度编程选择工作频率,最多时可同时在四种频率下测量阻抗信息,系统的噪声水平在－80dB左右。在每周期采样64点的情况下,解调算法可将信噪比提高5．6倍以上。     

Communication in a noisy environment: short-term acoustic adjustments and the underlying acoustic niche of a Neotropical stream-breeding frog

Acoustically active animals may show long- and short-term adaptations in acoustic traits for coping with ambient noise. Given the key role of calls in anurans’ life history, long- and short-term adaptations are expected in species inhabiting noisy habitats. However, to disentangle such adaptations is a difficult task, incipiently addressed for Neotropical frogs. We investigated if males of a stream-breeding frog (Crossodactylus schmidti) adjust call traits according to the background noise, and if the signal-to-noise ratio (SNR) varies between call harmonics and along call notes. We measured sound pressure levels of calls and noise in the field and used a fine-scale acoustic analysis to describe the signal and noise structure and test for noise-related call adjustments. The multi-note harmonic call of C. schmidti greatly varied in the spectral structure, including a trend for increasing note amplitude along the call, a wide frequency bandwidth of the 2nd harmonic, a minor call frequency modulation due to a trend for increasing note frequency within the same harmonic, and a major call frequency modulation due to the variable location of the dominant harmonic along the call. Calls had significantly higher frequencies than the noise at the range of the 1st and the 2nd call harmonics, and significantly louder sound pressure than the noise at the range of all harmonics. Males emitted the majority of call notes showing positive SNR, and though males also emitted some notes with negative SNR, when a given harmonic was negative the other harmonics in the same note did not tend to be SNR-negative. Our results indicate that male C. schmidti show short-term acoustic adjustments that make the advertisement call effective for coping with the interference of the stream-generated noise. We suggest that the call spectral plasticity serves for coping with temporary changes in the background noise, whilst we also discuss the possibility that the redundant, harmonic-structured call may have evolved to diminish masking interference on the acoustic signal by the background noise. This is the first study to uncouple noise-related acoustic adjustments and putative long-term acoustic adaptations for a Hylodidae, providing insights on behavioral plasticity and signal evolution of stream-breeding frogs.