Search PubMed⌕ Search

Biomedical subjects

P W Poon

Publications and source records attributed to P W Poon.

At least 37 records · Page 2Linked to original sources
PubMed1994

An enhanced approach to adaptive processing of the brain stem auditory evoked potential.

The present paper describes an adaptive signal enhancer (ASE) method for improving signal-to-noise ratio (SNR) and also for tracking the variation of brain-stem auditory evoked potential (BAEP). The enhancer has two inputs: a primary input which is the original data to be processed, consisting of signal badly corrupted by noise, and a reference input. A method called dynamic averaging (DA) is introduced to obtain the reference input. The improved reference signal allows closer tracking of evoked potential in each subsequent trial. For BAEP measurement in human, the SNR is typically very low. It requires about 200 ensembles to generate the reference input acceptable for ASE, and tracking of the variation of BAEP can be obtained satisfactorily. Our results show that while ensemble averaging is still needed, the total number of ensembles is greatly reduced. The adaptive method thus reduces the time of measurement for BAEP compared with the conventional method of ensemble averaging.

Electricity↗
PubMed1993

Spectral characteristics of embolic heart sounds detected by precordial Doppler ultrasound during venous air embolism in dogs.

We have studied the frequency spectrum of embolic heart sounds as detected by precordial Doppler ultrasound after injecting air 0.01-0.1 ml into the external jugular vein of anaesthetized dogs (n = 7). At volumes of 0.05 ml and greater, there was a significant increase in both the maximum frequency of the spectrum and the power of those high frequency components to greater than control values. Both changes correlated significantly with the volume of air injected (P < 0.05). With repeated injections of air at 5-min intervals, there was a small increase in the high frequency components in the control signal between injections (P < 0.05). We conclude that such spectral changes could be used to quantify the volume of air in transit through the heart, and to reflect the short-term history of venous air embolism.

Animals↗
PubMed1993

Sensitivity of auditory nerve fibers to spectral notches.

1. Listeners use direction-dependent spectral cues introduced by the torso, head, and pinnae to localize the source of a sound in space. Among the prominent direction-dependent spectral features in the free field-to-eardrum transfer function are narrow regions of low acoustic energy referred to as spectral notches. In this paper, we studied the sensitivity of single auditory nerve fibers in the barbiturate-anesthetized cat to broadband noise that had been filtered by a function whose shape approximated natural notches in the free field-to-eardrum transfer function. 2. Two experimental paradigms were employed. The first was the repeated presentation of a burst of broadband noise filtered by the simulated-notch function. Center frequency of the notch was held constant at or around the fiber characteristic frequency (CF). We refer to this as a "stationary" notch stimulus. The second paradigm was the repeated presentation of a broadband noise that was constructed from noise segments, each filtered by the simulated notch, whose CF was incremented and then decremented in a systematic way. We refer to this as a "moving" notch stimulus. Results from these two paradigms were compared with respect to notch detection. 3. Data were obtained from 161 single auditory nerve fibers having CFs ranging from 0.4 to 40 kHz. Most fibers studied had CFs > 5 kHz, and they detected the presence of the spectral notch in an intensity- and frequency-dependent manner. Each fiber responded vigorously to the presence of broadband noise. When the CF of the notch encroached on the response area of the fiber, there was a demonstrable reduction in discharge rate. The greatest reduction in discharge rate occurred when the notch was centered at the fiber's CF and when the level of the notch signal was some 15-55 dB above the fiber's noise threshold. There was close association in the frequency-intensity plane between the position of the most effective notch and the fiber's threshold tuning curve. 4. For high-spontaneous rate fibers, a moving-notch stimulus, but not a stationary one, reduced the discharge below the spontaneous rate at and in the immediate vicinity of the most effective notch frequency. This increases sensitivity to a spectral notch and suggests a mechanism by which localization ability is enhanced when there is relative motion between a sound source and the head.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1993

Virtual-space receptive fields of single auditory nerve fibers.

1. Sounds reaching the tympanic membranes are first modified by the acoustic properties of the torso, head, and external ear. For certain frequencies in the incident sound there results a complex, direction-dependent spatial distribution of sound pressure at the eardrum such that, within a sound field, localized areas of pressure maxima are flanked by areas of pressure minima. Listeners may use these spatial maxima and minima in localizing the source of a sound in space. The results presented describe how information about this spatial pressure pattern is transmitted from the cochlea to the central auditory system via single fibers of the auditory nerve. 2. Discharges of single fibers of the auditory nerve were studied in Nembutal-anesthetized cats [characteristic frequencies (CFs) ranged from 0.4 to 40 kHz]. Click stimuli were derived from sound-pressure waveforms that were generated by a loudspeaker placed at 1,800 locations around the cat's head and recorded at the tympanic membrane with miniature microphones. Recorded signals were converted to acoustic stimuli and delivered to the ear via a calibrated and sealed earphone. The full complement of signals is referred to as "virtual acoustic space," and the spatial distribution of discharges to this array of signals is referred to as a "virtual-space receptive field" (VSRF). 3. Fibers detect both pressure maxima and pressure minima in virtual acoustic space. Thus VSRFs take on complex shapes. 4. VSRFs of fibers of the same or similar CF having low spontaneous rates had the same overall pattern as those from high-spontaneous rate (HSR) fibers. For HSR fibers, the VSRF is obscured by the high background spike activity. 5. Comparison of the VSRF and isolevel contour maps of the stimulus derived at various frequencies revealed that auditory nerve fibers most accurately extract spectral information contained in the stimulus at a frequency close to or slightly higher than CF.

Animals↗
PubMed1992

Differences in FM response correlate with morphology of neurons in the rat inferior colliculus.

The response characteristics to linear frequency sweeps were studied in two groups of FM (frequency modulation) sensitive neurons in the rat inferior colliculus. 'FM specialized' cells responded to frequency sweeps but not to pure tones. 'Mixed' cells responded to both frequency sweeps and pure tones. FM specialized cells preferred faster and broader sweeps of higher intensity than did mixed cells and were more directionally selective. In addition, FM specialized cells were more sharply tuned to FM velocity and FM range and had longer response latencies. Physiologically identified FM cells stained intracellularly with horseradish peroxidase revealed differences in morphology correlating with the differences in their responses to tones. FM specialized cells had larger dendritic fields, more dendritic branching and more dendritic spines than did mixed cells. The findings are taken as evidence that the two groups of inferior colliculus neurons are both functionally and morphologically distinct.

Acoustic Stimulation↗
PubMed1992

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.

Algorithms↗
PubMed1992

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.

Algorithms↗
PubMed1991

Basic determinants for FM responses in the inferior colliculus of rats.

The response of 835 click-sensitive neurons in the inferior colliculus (IC) to ramp frequency modulated (FM) tones was studied in the anaesthetized rat. More than 70% of the cells were sensitive to the FM sound, and over 25% were "FM specialized". Systematic variations of the stimulus parameters showed that sweep velocity, sweep range, and intensity of the FM signal were the 3 basic determinants for the unit response. For an "FM specialized" cell, the response pattern to each of the parameters was either monotonic or bell-shaped. The population statistics of response patterns to the FM parameters, including the tuning factors, were generated. A stimulus domain was proposed to represent the "receptive space" of the FM cells.

Acoustic Stimulation↗
PubMed1991

Visual evoked potential measurement by maximum length sequence technique.

The maximum length sequence (MLS) technique is applied to improve data collection of a clinically important long latency visual potential. Given a limited time for measuring the response and with low number of stimulus presentation, the MLS technique exceeded the conventional method of ensemble averaging in attaining a higher signal-to-noise ratio (SNR). The theoretical consideration of such improvement is developed along with the system implementation. Typical examples of the results obtained in a human subject are presented.

Algorithms↗
PubMed1990

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.

Acoustic Stimulation↗
PubMed1990

Frequency and space representation in the inferior colliculus of the FM bat, Eptesicus fuscus.

The tonotopic organization and spatial sensitivity of 217 inferior collicular (IC) neurons of Eptesicus fuscus were studied under free field stimulation conditions. Acoustic stimuli were delivered from a loudspeaker placed 21 cm ahead of the bat to determine the best frequency (BF) and minimum threshold (MT) of isolated IC neurons. A BF stimulus was then delivered as the loudspeaker was moved horizontally across the frontal auditory space of the bat to locate the best azimuthal angle (BAZ) at which the neuron had its lowest MT. The stimulus was then raised 3 dB above the lowest MT to determine the horizontal extent of the auditory space within which a sound could elicit responses from the neurons. This was done by moving the loudspeaker laterally at every 5 degrees or 10 degrees until the neuron failed to respond. These measurements also allowed us to redetermine the BAZ at which the neuron fired maximal number of impulses. Electrodes were placed evenly across the whole IC surface and IC neurons were sampled as many as possible within each electrode penetration. Tonotopic organization and spatial sensitivity were examined among all 217 IC neurons as a whole as well as among IC neurons sequentially sampled within individual electrode penetrations. The whole population of 217 IC neurons is organized tonotopically along the dorsoventral axis of the IC. Thus, low frequency neurons are mostly located dorsally and high frequency neurons ventrally with median frequency neurons intervening in between. The BAZ of these 217 IC neurons tend to shift from lateral to medial portions of the contralateral frontal auditory space with increasing BF.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1990

Changes in power spectrum of electromyograms of masseter and anterior temporal muscles during functional appliance therapy in children.

This study applied frequency analysis to compare changes in the power spectrum density functions of surface electromyograms obtained from the paired masseter and anterior temporal muscles during therapy with three types of functional appliances, namely the Bionator, modified Fränkel type I, and Fränkel type III. Eighteen children were divided into three groups receiving either Bionator, Fränkel type I, or Fränkel type III therapy; a fourth group consisting of six children who received no therapy served as control. Before and after 3, 6, and 12 months of therapy, each child performed maximum voluntary isometric clenches in the position of maximum intercuspation. The mean frequency of the power spectrum was the variable studied. Children treated with the Bionator and Fränkel type I appliances showed greater changes in mean frequency than those treated with the Fränkel type III appliance. Although the muscles of untreated children also showed shifts of mean frequency to lower frequency values as a function of time, there was a greater downward shift of mean frequency in those treated with functional appliances. The downward shifts might have been associated with changes in muscle fiber lengths and/or recruitment patterns as a result of both treatment and normal growth.

Activator Appliances↗
PubMed1989

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.

Adult↗
PubMed1988

Maximum length sequence applied to the measurement of brainstem auditory evoked responses.

An evoked response may be assumed to consist of synchronous electrical activities of groups of neurons in response to a stimulus. Unfortunately, this response is usually weak and often masked by the background EEG activity generated spontaneously by the brain. In order to extract the embedded evoked response we have employed the theory of maximum length sequence, assuming only a temporal linearity of the system from which measurements are taken. The technique has been applied successfully with the aid of a microcomputer. The system also supports the same measurement by a conventional averaging technique. The results of the two are compared, indicating the advantages of the maximum length technique: improved signal-to-noise ratio, repeatability and speed.

Animals↗
PubMed1987

The temporal course of masking and the auditory filter shape.

Recent experiments have shown that frequency selectivity measured in tone-on-tone simultaneous masking improves with increasing delay of a brief signal relative to the onset of a longer duration gated masker. To determine whether a similar improvement occurs for a notched-noise masker, threshold was measured for a 20-ms signal presented at the beginning, the temporal center, or the end of the 400-ms masker (simultaneous masking), or immediately following the masker (forward masking). The notch width was varied systematically and the notch was placed both symmetrically and asymmetrically about the 1-kHz signal frequency. Growth-of-masking functions were determined for each temporal condition, for a noise masker without a spectral notch. These functions were used to express the thresholds from the notched-noise experiment in terms of the level of a flat-spectrum noise which would produce the same threshold. In simultaneous masking the auditory filter shapes derived from the transformed data did not change significantly with signal delay, suggesting that the selectivity of the auditory filter does not develop over time. In forward masking the auditory filter shapes were sharper than those for simultaneous masking, particularly on the high-frequency side, which was attributed to suppression.

Adult↗
PubMed1986

Correlation between auditory reaction time and intelligence.

Discriminative reaction times to auditory stimuli were measured for 150 students of the local Chinese population. Pearson's correlation with scores on the Standard Progressive Matrices and RTs was negative, becoming -0.30 when slower trials were eliminated. The possible basis for improvement was discussed in relation to cognitive processes and the general theory of assessing intelligence.

Adolescent↗
PubMed1986

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.

Action Potentials↗