A novel system for simultaneous monitoring of locomotor and sound activities in animals.

This paper describes a PC-based system for simultaneous monitoring of locomotor and sound activities on small rodents. The displacement and location signals of the animal were first determined across consecutive video-frames, followed by marked data reduction to cater for long-term studies. At the same time, sounds generated by the animal were detected and the sound level was recorded as root-mean-square values at 1 s intervals. Preliminary data showed that such a multi-parametric monitor system could provide comprehensive information on the animal's activity.

Postnatal exposure to tones alters the tuning characteristics of inferior collicular neurons in the rat.

Young rats were exposed to either a 4 or 20 kHz steady tone during the first 3 weeks after birth, and then single unit responses to pure tones were studied at their inferior colliculi (IC). The best frequencies (BF's) of the cells showed strong clustering around the frequency of exposure.

Altered sensitivities of auditory neurons in the rat midbrain following early postnatal exposure to patterned sounds.

Pure tone sensitivity of inferior colliculus (IC) neurons in adult rats was studied electrophysiologically following exposure to a frequency-modulated (FM) tone during the first 5 postnatal weeks. The distribution of best frequencies (BF) and minimum thresholds (MT) of 274 single units, when compared to the control, showed an abnormal clustering centered around the region of the audiogram occupied by the FM tone to which the rats had been exposed. This effect was interpreted as the result of activity-dependent changes of IC during development.

Spectro-temporal receptive fields of midbrain auditory neurons in the rat obtained with frequency modulated stimulation.

Single unit responses at the auditory midbrain of the anesthetized rat were characterized in terms of spectro-temporal receptive field (STRF) using random frequency modulated (FM) tones and peri-spike averaging. STRFs were obtained from 121 FM-sensitive units covering a wide range of characteristic frequency (CF). Roughly half of the neurons showed clearly preferred stimulus time profiles that formed either a single, double or multiple bands. Neurons with a single-band STRF appeared to be sorted into positive or negative directional sensitivity for FM modulation on the basis of their CF either below or above 10 kHz. This directional selectivity is discussed in relation to the most sensitive part of the rat's audiogram.

EMG power spectrum patterns of anterior temporal and masseter muscles in children and adults.

The power spectrum of electromyograms (EMG) has been demonstrated to vary with muscles having different muscle fiber type compositions. This study investigated the variations in EMG power spectrum patterns of the masticatory muscles with age and gender by comparison of the mean power frequency (MPF) of the anterior temporal and masseter muscles in children and adults. Surface EMG signals were sampled bilaterally from the muscles when the subjects were performing maximum voluntary isometric clenches at maximal intercuspal position. The results indicated that MPF values were age-dependent (p less than 0.001), and sexual dimorphism was evident (p less than 0.001), with lower MPF values in male and adult muscles. While male adults had the lowest and female children had the highest MPF values, female adults had MPF values closer to values obtained from male children. These differences or similarities could be attributed to the degree of differentiation of the muscles during growth and development of the craniofacial morphology.

Effect of cage size on ultradian locomotor rhythms of laboratory mice.

The effect of cage size on spontaneous locomotor rhythms of laboratory mice was studied under simulated light-dark (12:12) cycles. On-line image analysis of bodily displacement yielded a locomotor signal over a period of 3 days. Continuous wavelet transform was applied to the signal, and ensemble averaging of eight mice revealed in the time-frequency plot bouts of increased motor activities. Notably, there were two bouts in the dark corresponding to ultradians of periods below 5 h: a first bout at the dark onset (at 0.6-1.0 cycle/h), and a second bout during the second half of the dark period (at 0.4-0.7 cycle/h). These increases of activity were more intense and distinct when the animals were kept inside the larger cage. Furthermore, the first bout disappeared when the animals were kept in the small cage for 3 days.

Detection of interaural time differences for clicks and tone pips: effects of interaural signal disparity.

The ability to detect small interaural time differences (delta t) was determined in 4 subjects using clicks or long tone pips of various interaural signal disparities which are expressed as the extent of interaural spectral overlaps. The interaural signal disparity was varied by changing (a) the interaural pulse duration difference (delta d) for clicks, or (b) the interaural carrier frequency difference (delta f) for tone pips. In either case, the sensitivity to delta t was maximal under diotic presentations and declined with delta d or delta f. The overall sensitivity to delta t was remarkably higher for clicks than for long tone pips. The results indicate that both (a) the extent of binaural spectral overlaps and (b) the structure of acoustic stimuli are important in detecting small interaural time differences of binaural sounds.

Simulation of free-field sound sources and its application to studies of cortical mechanisms of sound localization in the cat.

We synthesized a set of signals (clicks) for earphone delivery whose waveforms and amplitude spectra, measured at the eardrum, mimic those of sounds arriving from a free-field source. The complete stimulus set represents 1816 sound-source directions, which together surround the head to form a 'virtual acoustic space' for the cat. Virtual-space stimuli were delivered via calibrated earphones sealed into the external meatus in cats under barbiturate anesthesia. Neurons recorded in AI cortex exhibited sensitivity to the direction of sound in virtual acoustic space. The aggregation of effective sound directions formed a virtual space receptive field (VSRF). At 20 dB above minimal threshold, VSRFs fell into one of several categories based on spatial dimension and location. Most VSRFs were confined to either the contralateral (59%) or ipsilateral (10%) sound hemifield. Seven percent spanned the frontal quadrants and 16% were omnidirectional. Eight percent fit into no clear category and were termed 'complex'. The size, shape, and location of VSRFs remained stable over many hours of recording. The results are in essential agreement with free-field studies. VSRFs were found to be shaped by excitatory and inhibitory interactions of activity arriving from the two ears. Some cortical neurons were found to preserve the spectral information in the free-field sound which was generated by the acoustical properties of the head and pinna, filtered by the cochlea and transmitted by auditory nerve fibers.

Fast detection of venous air embolism in Doppler heart sound using the wavelet transform.

The introduction of air bubbles into the systemic circulation can result in significant morbidity. Real-time monitoring of continuous heart sound in patients detected by precordial Doppler ultrasound is, thus, vital for early detection of venous air embolism (VAE) during surgery. In this study, the multiscale feature of wavelet transforms (WT's) is exploited to examine the embolic Doppler heart sound (DHS) during intravenous air injections in dogs. As both humans and dogs share similar physiological conditions, our methods and results for dogs are expected to be applicable to humans. The WT of DHS at scale 2j (j = 1, 2) selectively magnified the power of embolic, but not the normal, heart sound. Statistically, the enhanced embolic power was found to be sensitive (P < 0.01 at 0.01 ml of injected air) and correlated significantly (P < 0.0005, r = 0.83) with the volume of injected air from 0.01 to 0.10 ml. A fast detection algorithm of O(N) complexity with unit complexity constant for VAE was developed (processing speed = 8 ms per heartbeat), which confirmed the feasibility of real-time processing for both humans and dogs.

Adaptive filter for event-related bioelectric signals using an impulse correlated reference input: comparison with signal averaging techniques.

Many bioelectric signals result from the electrical response of physiological systems to an impulse that can be internal (ECG signals) or external (evoked potentials). In this paper an adaptive impulse correlated filter (AICF) for event-related signals that are time-locked to a stimulus is presented. This filter estimates the deterministic component of the signal and removes the noise uncorrelated with the stimulus, even if this noise is colored, as in the case of evoked potentials. The filter needs two inputs: the signal (primary input) and an impulse correlated with the deterministic component (reference input). We use the LMS algorithm to adjust the weights in the adaptive process. First, we show that the AICF is equivalent to exponentially weighted averaging (EWA) when using the LMS algorithm. A quantitative analysis of the signal-to-noise ratio improvement, convergence, and misadjustment error is presented. A comparison of the AICF with ensemble averaging (EA) and moving window averaging (MWA) techniques is also presented. The adaptive filter is applied to real high-resolution ECG signals and time-varying somatosensory evoked potentials.

Multiscale characterization of chronobiological signals based on the discrete wavelet transform.

To compensate for the deficiency of conventional frequency-domain or time-domain analysis, this paper presents a multiscale approach to characterize the chronobiological time series (CTS) based on a discrete wavelet transform (DWT). We have shown that the local modulus maxima and zero-crossings of the wavelet coefficients at different scales give a complete characterization of rhythmic activities. We further constructed a tree scheme to represent those interacting activities across scales. Using the bandpass filter property of the DWT in the frequency domain, we also characterized the band-related activities by calculating energy in respective rhythmic bands. Moreover, since there is a fast and easily implemented algorithm for the DWT, this new approach may simplify the signal processing and provide a more efficient and complete study of the temporal-frequency dynamics of the CTS. Preliminary results are presented using the proposed method on the locomotion of mice under altered lighting conditions, verifying its competency for CTS analysis.

Similarities of FM and AM receptive space of single units at the auditory midbrain.

Complex sounds, including human speech, contain time-varying signals like frequency modulation (FM) and amplitude modulation (AM) components. In spite of various attempts to characterize their neuronal coding in the mammalian auditory systems, a unified view of their responses has not been reached. We compared FM and AM coding in terms of receptive space with reference to the input-output relationship of the underlying neural circuits. Using extracellular recording, single unit responses to a novel stimulus (i.e. random AM or FM tone) were obtained at the auditory midbrain of the anesthetized rat. Responses could be classified into three general types, corresponding to selective sensitivity to one of the following aspects of the modulation: (a) steady state, (b) dynamic state, or (c) steady-and-dynamic states. Such response typing was basically similar between FM and AM stimuli. Furthermore, the receptive space of each unit could be characterized in a three-dimensional Cartesian co-ordinate system formed by three modulation parameters: velocity, range and intensity. This representation applies to both FM and AM responses. We concluded that the FM and AM codings are very similar at the auditory midbrain and may likely involve similar neural mechanisms.

Modeling of the response of midbrain auditory neurons in the rat to their vocalization sounds based on FM sensitivities.

Single units were recorded from the inferior colliculus (IC) of anaesthetized rats in response to: (a) an FM tone, the frequency of which was randomly varied, and (b) a digitized rat vocalization sound. We hypothesized that these neurons may have 'orientation-specific' spectrotemporal receptive field (STRF) that can be used to estimate their responses to complex communication signals. Based on the FM response, we first estimated the cell's STRF which was then convolved with the spectrogram of the rat's vocalization call. A simple convolution gave only crude prediction of the cell's response to the vocalization sound. When inhibitory areas were added around certain parts of the STRF, a better match was found. We conclude that for some FM-sensitive neurons of the IC, STRF with inhibitory areas may account for their responses to vocalization sounds.

Detection of brainstem auditory evoked potential by adaptive filtering.

A method of detecting brainstem auditory evoked potential (BAEP) using adaptive signal enhancement (ASE) is proposed and tested in humans and cats. The ASE in this system estimates the signal component of the primary input, which is correlated with the reference input to the adaptive filter. The reference input is carefully designed to make an optimal and rapid estimation of the signal corrupted with noise, such as ongoing EEG. With a good choice of reference input, it is possible to track the variability of BAEP efficiently and rapidly. Moreover, the number of repetitions required could be markedly reduced and the result of the system is superior to that of ensemble averaging (EA). To detect BAEP in cats, only 30 ensemble averages are needed to obtain a reasonable reference input to the adaptive filter, and, for humans, 350-750 ensemble averages are sufficient for a satisfactory result. Using the LMS adaptive algorithm, individual BAEP can be obtained in real-time.

Measurement of human BAERs by the maximum length sequence technique.

The traditional brainstem auditory evoked response (BAER) measurement technique (ensemble averaging) is time-consuming and is not acceptable for some time-critical clinical applications. In the paper the application of a pseudorandom binary sequence, the maximum length sequence, to human BAER measurements is examined. This technique permits a faster click rate to stimulate the test subject, and obtains a higher signal-to-noise ratio (SNR) response through deconvolution. When compared with conventional averaging, the method can result in an improved SNR or in faster measurement of BAER. The theory of the technique and the experimental setup are presented, and theoretical analysis on the SNR improvement by this technique against averaging is also given. Actual measurements of BAER on both humans and cats indicate that this technique is effective, especially when the measurement time is not too long, or the number of trials is not too large.

Evoked potential estimation using modified time-sequenced adaptive filter.

A method called modified time-sequenced adaptive filtering (MTSAF) is applied to estimate evoked potential (EP) signals and track the temporal variations of EPs. The MTSAF consists of a set of adaptive filters (AFs), with each processing a time segment of EP data. After convergence, each AF reaches the best estimation of EP signals over its own time segment in terms of minimum mean squared error (MMSE). Numerical results of simulated and human EP data show that the MTSAF reaches better estimation of EPs than a conventional adaptive signal enhancer (ASE). With the MTSAF, the temporal variations of EPS across trials can be estimated to reveal more subtle variations of EPs, which may be of clinical value.

A tracing evoked potential estimator.

The paper presents an adaptive Gaussian radial basis function neural network (RBFNN) for rapid estimation of evoked potential (EP). Usually, a recorded EP is severely contaminated by background ongoing activities of the brain. Many approaches have been reported to enhance the signal-to-noise ratio (SNR) of the recorded signal. However, non-linear methods are seldom explored due to their complexity and the fact that the non-linear characteristics of the signal are generally hard to determine. An RBFNN possesses built-in non-linear activation functions that enable the neural network to learn any function mapping. An RBFNN was carefully designed to model the EP signal. It has the advantage of being linear-in-parameter, thus a conventional adaptive method can efficiently estimate its parameters. The proposed algorithm is simple so that its convergence behaviour and performance in signal-to-noise ratio (SNR) improvement can be mathematically derived. A series of experiments carried out on simulated and human test responses confirmed the superior performance of the method. In a simulation experiment, an RBFNN having 15 hidden nodes was trained to approximate human visual EP (VEP). For detecting human brain stem auditory EP (BAEP), the approach (40 hidden nodes and convergence rate = 0.005) speeded up the estimation remarkably by using only 80 ensembles to achieve a result comparable to that obtained by averaging 1000 ensembles.

Computerised infrared imaging system for studying thermal activation on the skull following somatic stimulation in small animals.

A computerised infrared imaging system has been developed to measure infrared radiation as a means of functionally mapping the cerebral cortex. In two species of small mammal, rat and gerbil, the authors localised the thermal changes at the skull overlying the somatic sensory cortex following somatic stimulation of the mystacial vibrissae. Though typically small in magnitude, a thermal response could be detected through the skull. To enhance detection sensitivity, a number of measures were taken to improve various aspects of data acquisition, stimulus delivery and control of experimental conditions. Regarding data analysis, a coordinate system based on skull landmarks was adopted to localise thermally-active regions for comparison across animals of the same species. To extract the region of weak temperature changes, a coarse-to-fine detection strategy was developed, which searched automatically for clusters of temporally- and spatially-correlated pixels above a data-driven threshold. Thus, the dynamic aspect of the thermal changes at any region of interest on the skull could be studied efficiently. The detection algorithm was tested against simulated responses in addition to empirical data obtained from animals. All of the above software was integrated in a user-friendly package.

Effects of superfusion of morphine and enkephalins on the activity of single units in the spinal trigeminal nucleus and cuneate nucleus of cat.

The effects of superfusion of morphine, met-enkephalin and D-ala2-met5-enkephalinamide on the spontaneous neural discharge rates of units in the spinal trigeminal nucleus and cuneate nucleus of decerebrate cats were studied. The drugs were superfused onto the dorsum of the exposed surface of the caudal medulla overlying these nuclei. Some of these neurons were identified by their response to innocuous mechanical stimuli delivered to the skin. In the caudal spinal trigeminal nucleus, morphine caused a dose-dependent suppression of the spontaneous discharge rate in the majority of the neurons studied. Endogenous opiate peptide, met-enkephalin or its synthetic analogue, D-ala2-met5-enkephalinamide caused an initial reduction, followed by a rebound of the discharge rate to the control value. These depressant effects of morphine and enkephalins were antagonized by concomitant superfusion of the opiate antagonist naloxone. In the main cuneate nucleus, however, similar doses of morphine, met-enkephalin and D-ala2-met5-enkephalinamide have little if any significant effect on the spontaneous activity of the neurons studied. These results provide electrophysiological evidence for the presence of opiate receptors in the caudal spinal trigeminal nucleus and the relative lack of such receptors in the main cuneate nucleus.

Transient frequency and intensity sensitivities of central auditory neurons determined with sweep tone.

To determine the transient frequency and intensity sensitivities of central auditory neurons, we implemented an exponential sweep tone stimulus (2 sec in period, mean sweep rate 3.3 octave/sec), intensity of which varied systematically across trials. Response of single units to the stimulus was studied at the inferior colliculus (IC) of urethane-anesthetized rats. Most IC units responded to the sweep tone by one or more transient increases in discharge rate. The area of increased discharge, or response area (RA), was delineated on the frequency-intensity plane. The tip of RA gives the best frequency (BF) and minimum threshold (MT) of the cell. We also compared the BF and MT concurrently obtained with another method, viz., the conventional 'audio-visual' method of subjective judgment. Results showed that for the same population of cells (n=130), correlation between the two methods is better for BF (r=0.91) than for MT (r=0.78). Such discrepancy was discussed in relation to the response characteristics of these central auditory neurons.