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Biomedical subjects

Y M Cheung

Publications and source records attributed to Y M Cheung.

At least 19 recordsLinked to original sources
PubMed2000

Rival Penalized Competitive Learning based approach for discrete-valued source separation.

This paper presents an approach based on Rival Penalized Competitive Learning (RPCL) rules for discrete-valued source separation. In this approach, we first build a connection between the source number and the cluster number of observations. Then, we use the RPCL rule to automatically find out the correct number of clusters such that the source number is determined. Moreover, we tune the de-mixing matrix based on the cluster centers instead of the observation themselves, whereby the noise interference is considerably reduced. The experiments have shown that this new approach not only quickly and automatically determines the number of sources, but also is insensitive to the noise in performing blind source separation.

Algorithms↗
PubMed2000

Levels-of-processing effects on Chinese character completion: the importance of lexical processing and test cue.

The levels of processing (LoP) effect was studied in Chinese character completion tasks. The same stem cue, either graphemically or semantically related to the target character, was used to generate either a perceptual or a conceptual test. Participants received either direct or indirect instructions, so that the same stem cue also produced either an explicit or an implicit test of memory. This allowed us to examine simultaneously the roles of perceptual versus conceptual test and retrieval intentionality in the LoP effect. No LoP effects were found when the memory test was implicit. LoP effects were also not found for stems and characters related graphemically (perceptually), but were found for stems and characters related semantically (conceptually) when the memory test was explicit. The results are discussed in terms of the importance of lexical processing for character completion tests and the importance of reinstatement of the study process during retrieval for explicit memory tests.

Adult↗
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↗
PubMed1991

Rhythmic release pattern of pineal melatonin in rodents.

The plasma melatonin in the confluens sinuum of anesthetized rabbits and rats exhibited an episodic release pattern, with pulses superimposed on a basal level. The rhythmicity of pineal melatonin release revealed by our experimental paradigm was dependent on the blood sampling interval. In rabbits, it was found that the plasma melatonin level in the confluens sinuum was 7-15 times higher than that obtained from the plasma collected at the same time from the peripheral artery where the melatonin level also showed a pulsatile pattern. Diurnal variation in pineal melatonin level in the confluens sinuum was observed only in acutely blinded rabbits and rats. Electrical stimulation of the unilateral cervical sympathetic trunk of rabbits resulted in elevation of the level of plasma melatonin in the confluens sinuum, while the frequency of pulse peaks of plasma melatonin revealed at 4-min intervals was not affected. Similar results were obtained in rabbits with systemic administration of beta-adrenergic agents. These findings suggest that the release of pineal melatonin into the confluens sinuum is under the regulation of the cervical sympathetic system. The sympathetic influence on pineal melatonin secretion seems to vary with age. Furthermore, age-related fluctuation of systemic blood melatonin appeared to be the results of changes in the rate of pineal melatonin secretion, body growth and ageing.

Aging↗
PubMed1991

Response of medullary reticular neurons of cat to off-vertical axis rotations.

To investigate whether neurons in the medullary reticular formation of decerebrate cats can encode spatial orientation during 360 degrees off-vertical axis rotations (OVAR), responses of these units were studied during constant velocity OVAR in clockwise (CW) and counter-clockwise (CCW) directions. Tilt-sensitive units, some of which were activated by antidromic stimulation of the thoracic cord, were found to respond to 360 degrees OVAR with position-and direction-dependent modulation in discharge rates. For the individual responsive unit there was a difference in the spatial location of the CW and CCW discharge maxima, while a comparable response gain was obtained for rotations in either direction. The orientation of the best response of each unit was not significantly modified by changes in the amplitude of head tilt and/or velocity of rotation. Such characterized directional axes of the medullary reticular units were found to have a fairly equal distribution over the 360 degrees. The role that neurons in the medullary reticular formation plays in encoding otolith inputs with respect to head movements is discussed.

Animals↗
PubMed1990

Simultaneous determination of some active ingredients in cough-cold syrups by gas-liquid chromatography.

A simple, efficient and accurate gas-liquid chromatography (GLC) method for the simultaneous determination of eight active ingredients in cough-cold syrups has been developed. The active ingredients under study were bromhexine, chlorpheniramine, codeine, dextromethorphan, diphenhydramine, ephedrine/pseudoephedrine, guaiphenesin and papaverine. Before injection, the active ingredients were first separated, from the excipients present in the cough-cold syrups, with chloroform, from alkaline medium. They were then separated by GLC on a glass column (5 ft x 2 mm i.d.) packed with 3% of OV-25 supported on Supelcoport (80-100 mesh). The column temperature was maintained at 170 degrees C for 1 min, then programmed to 265 degrees C at a rate of 10 degrees C min-1, and maintained at this temperature for 10 and 1 min, respectively, for samples with and without papaverine. The flow-rate of the nitrogen carrier gas was 30 ml min-1. A flame-ionisation detector was used for detection, and clomipramine hydrochloride was used as the internal standard. The recoveries of the drugs ranged from 96.0 to 99.7%, and the relative standard deviations for ten replicate determinations ranged from 0.49 to 4.7%. Results are reported for nine commercially available cough-cold syrups.

Chromatography, Gas↗
PubMed1990

Patterns of pineal melatonin secretion in rabbits: diurnal variation of basal and pulsatile release.

The level and release pattern of plasma melatonin in the confluens sinuum of 28 sighted and 18 blinded (i.e. acute bilateral orbital enucleation) rabbits anesthetized with pentobarbital were studied. The animals had been adapted to a 12:12 h light-dark regime. Blood samples were collected from the cannulated confluens sinuum and/or the femoral artery at either 2- or 4-min intervals in both the light and dark phases. Plasma melatonin was determined by radioimmunoassay. In all rabbits studied, plasma melatonin in the confluens sinuum exhibited an episodic release pattern, with pulses superimposed on a basal level. At 4-min sampling intervals, the pulsatile release of melatonin in sighted rabbits were 3.8 peaks/h in the light phase; shorter sampling interval (2-min) revealed more frequent pulsatile release of melatonin (5.1 peaks/h). In sighted animals in the light phase, the level of melatonin in the plasma of confluens sinuum was 7-15 times higher than that obtained from the plasma collected at the same time from the peripheral artery where the level of melatonin also exhibited pulsatile pattern. In blinded animals, melatonin levels in terms of mean concentration, mean maximum level and mean minimum (or baseline) level obtained in the dark phase were 12-14 times higher than those obtained in the light phase. These results suggest that the level of melatonin exhibits a diurnal rhythm in the confluens sinuum of rabbits.

Animals↗
PubMed1989

Elevation of pineal melatonin secretion by electrical stimulation of the cervical sympathetic trunk in rabbits.

The influence of electrical stimulation of the unilateral sympathetic trunk on the release patterns of plasma melatonin in the confluens sinuum of anesthetized rabbits was studied. Plasma melatonin exhibited an episodic release pattern, with pulses superimposed on a basal level. The mean concentration, mean maximum and minimum levels of plasma melatonin in the confluens sinuum were found to be significantly elevated when the sympathetic trunk was electrically stimulated. The frequency of pulse peaks of plasma melatonin sampled at 4-min intervals was not affected by electrical stimulation. Our results suggest that the diurnal rhythmic secretory pattern of pineal melatonin into the confluens sinuum of rabbits is regulated by the cervical sympathetic system.

Animals↗
PubMed1989

Light diffraction intensity from muscle fibres in different osmotic solutions: measurement of equilibration time.

A new method for evaluating the osmotic equilibration time of striated muscle fibres is described. Single fibres from the semitendinosus muscles of frog were illuminated normally with a He-Ne laser. The first-order diffraction intensity was measured continuously after the tonicity of the fibre's bathing solution was changed abruptly. Hypertonicity decreased while hypotonicity increased the intensity. The time course of the intensity decrease or increase followed closely a simple exponential with a single time constant. It is proposed that the time constant associated with the diffraction intensity transient is a true representation of the equilibration process. A simple diffraction model provides a relationship between diffraction intensity and the diameter of the muscle fibre. This relationship is used to explain the diffraction intensity of fibres in different osmotic solutions.

Animals↗
PubMed1989

The origin and terminal arbors of individual uncrossed retinogeniculate fibers in rabbits.

The distribution and morphology of individual uncrossed retinogeniculate fibers in both normal and monocularly enucleated adult Dutch-belted rabbits were studied using horseradish peroxidase and wheat germ agglutinin conjugated to horseradish peroxidase as neuronal markers. The results showed that the uncrossed retinogeniculate fibers were distributed almost entirely in the ipsilateral segment of the dorsal nucleus of the lateral geniculate body, and the extent of terminal distribution of the fibers observed in rabbits with one eye enucleated during the young adult stage was essentially the same as that in the normal rabbit. Most of the uncrossed retinogeniculate fibers appeared to arise as collateral branches of optic tract fibers which were apparently destined for the pretectum or the superior colliculus. The uncrossed retinogeniculate fibers labeled by the wheat germ agglutinin conjugated to horseradish peroxidase passed through the dorsal nucleus of the lateral geniculate body without any branching until they reached the ipsilateral segment. There they could be divided into several morphological types although the possibility that they may represent different classes of a continuum cannot be ruled out.

Animals↗
PubMed1989

Simultaneous determination of ascorbic acid, caffeine and paracetamol in drug formulations by differential-pulse voltammetry using a glassy carbon electrode.

A simple, rapid and accurate method for the simultaneous determination of ascorbic acid, caffeine and paracetamol in drug formulations has been developed. Peak currents were measured with a glassy carbon electrode at +0.350, +0.618 and +1.425 V versus a saturated calomel electrode for ascorbic acid, paracetamol and caffeine, respectively. Perchloric acid (0.1 M) - methanol (1 + 1) was used both as a solvent and supporting electrolyte. The optimum modulation amplitude, pulse repeat time and scan rate of the polarographic analyser were found to be 50 mV, 0.5 s and 5 mV s-1, respectively and the linear calibration ranges for ascorbic acid, caffeine and paracetamol were 0-35, 0-50, and 0-55 micrograms ml-1, respectively. The relative standard deviations for 9.30 micrograms ml-1 of ascorbic acid, 8.50 micrograms ml-1 of caffeine and 7.30 micrograms ml-1 of paracetamol were 1.3, 2.5 and 0.7%, respectively. Results are reported for several commercially available drugs.

Acetaminophen↗
PubMed1987

Response characteristics of neurons in the cat vestibular nuclei during slow and constant velocity off-vertical axes rotations in the clockwise and counterclockwise rotations.

The responses to slow constant velocity rotations in the clockwise (CW) and counterclockwise (CCW) directions about an axis tilted 10 degrees from the earth's vertical were studied in static tilt-sensitive neurons in the vestibular nuclei of decerebrate cats. Each unit responded to any 360 degrees unidirectional rotation with a position-dependent discharge maximum. The location of the maximum, obtained by rotation in one direction, differed from that obtained by an oppositely directed rotation (phase difference). In about 80% of the units such phase difference (up to 160 degrees in second-order neurons) in response to oppositely directed rotations was unaffected by different amplitudes of head displacement (5-25 degrees). Units were thus classified into two groups depending on the location of the CW discharge maximum relative to the CCW counterpart. The direction of rotation had no influence on the response gains of these units.

Animals↗
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↗
PubMed1985

Effect of tilt on the response of neuronal activity within the cat vestibular nuclei during slow and constant velocity rotation.

The responses of neurons sensitive to static tilt in the vestibular nuclei were examined in decerebrate cats during slow and constant velocity rotations about an axis tilted at various angles from the vertical. During any 360 degrees rotation, each unit showed a modulation of their firing rate, with a position-dependent maximum and minimum. Changes in amplitude of head displacement from 5 degrees to 25 degrees decreased the response gain of the units without affecting the locations of the discharge maxima and minima.

Animals↗
PubMed1983

Determination of myofibrillar diameter by light diffractometry.

Distinctive oscillations in the diffraction line intensity were observed when a laser beam was directed at selected spots on single skeletal muscle fibres of frog and normally to the fibres. These intensity oscillations were interpreted as the diffraction from cylindrical myofibrils because they followed the first-order Bessel function. This interpretation allowed a direct determination of the myofibrillar diameter from the first intensity minimum of the zerothorder diffraction line. The hypothesis that the intensity oscillations were related to the myofibrillar diameter was substantiated by further experiments. At fixed sarcomere length the measured myofibrillar diameter increased when the fibre was immersed in hypotonic solution and decreased in hypertonic solution. In another experiment the diameter decreased and the sarcomere volume remained constant when the fibre was stretched passively. Furthermore, there was excellent agreement between the myofibrillar diameters measured by light diffractometry and electron microscopy.

Animals↗
PubMed1981

Estimation of changes in single muscle fibre diameter in different solutions by diffraction studies.

Studies of the effects of varying tonicity on the contraction of muscle fibres requires a knowledge of the corresponding changes in fibre diameters, if the observed mechanical changes are to be attributed to various factors, including that of the interfilamental spacing. Hitherto, measurements of such changes in fibre diameters involved tedious measurements of the diameters of the fibres at numerous points along its length. We now describe a technique whereby the change in fibre diameter can be deduced from measurements of the corresponding intensity ratio of laser diffraction lines when the fibre is bathed in solutions of differing tonicities.

Animals↗