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Biomedical subjects

R Meddis

Publications and source records attributed to R Meddis.

At least 19 recordsLinked to original sources
PubMed1992

Modeling the identification of concurrent vowels with different fundamental frequencies.

Human listeners are better able to identify two simultaneous vowels if the fundamental frequencies of the vowels are different. A computational model is presented which, for the first time, is able to simulate this phenomenon at least qualitatively. The first stage of the model is based upon a bank of bandpass filters and inner hair-cell simulators that simulate approximately the most relevant characteristics of the human auditory periphery. The output of each filter/hair-cell channel is then autocorrelated to extract pitch and timbre information. The pooled autocorrelation function (ACF) based on all channels is used to derive a pitch estimate for one of the component vowels from a signal composed of two vowels. Individual channel ACFs showing a pitch peak at this value are combined and used to identify the first vowel using a template matching procedure. The ACFs in the remaining channels are then combined and used to identify the second vowel. Model recognition performance shows a rapid improvement in correct vowel identification as the difference between the fundamental frequencies of two simultaneous vowels increases from zero to one semitone in a manner closely resembling human performance. As this difference increases up to four semitones, performance improves further only slowly, if at all.

Acoustics↗
PubMed1992

A computer model of a cochlear-nucleus stellate cell: responses to amplitude-modulated and pure-tone stimuli.

A computer model of a ventral-cochlear-nucleus (VCN) stellate cell with chop-S type response properties is presented and evaluated. The model is based on a simplified model of spike generation preceded by a stage that simulates dendritic low-pass filtering. Input to the model is in the form of simulated auditory-nerve spikes produced by a model of the auditory periphery [Meddis and Hewitt, J. Acoust. Soc. Am. 89, 2866-2882 (1991)]. Outputs from the stellate-cell model are shown to qualitatively replicate a wide range of typical in vivo responses. These include: (a) realistic onset and steady-state rate-level functions, (b) "chopper"-type post-stimulus time histogram responses; (c) typical "chop-S"-type neuron responses characterized by a low coefficient of variance (CV less than 0.3) of interspike intervals as a function of time; (d) level-dependent amplitude-modulation transfer functions; (e) intrinsic oscillations in responses to pure-tone stimuli; (f) amplitude-modulation encoding over a wide dynamic range; and (g) frequency-limited phase locking to pure tones. It is shown that these responses can be explained primarily by the membrane properties of the cells. More specifically, how the model encodes signal amplitude modulation was studied and an explanation was suggested for the generation of the bandpass modulation transfer functions. Such functions are observed neurally in response to amplitude-modulated stimuli presented at moderate to high signal levels.

Animals↗
PubMed1991

Lateralization of very-short-duration tone pulses of low and high frequencies.

The position and image-width of the simultaneous images produced by very short tone pulses were measured as a function of interaural time difference (ITD) at both low- (250 and 800 Hz) and high- (2500 and 8000 Hz) frequencies using a direct-estimation technique. Primary images are lateralized towards the ear receiving the leading stimulus. At low frequencies image position is proportional to interaural phase-difference (IPD) below 90 degrees and remains at the lead-ear for larger values. At high frequencies images are reported is proportional to ITD up to 500-1000 microseconds. Secondary images are reported on the opposite side of the head for IPDs greater than 180 degrees at low frequencies, and at ITDs greater than 500 microseconds at high frequencies. Image width is approximately constant for all ITDs and both images at a given frequency, but becomes more compact as frequency increases. The data are discussed in terms of onset cues and stimulus fine-structure cues. The best explanation is in terms of an onset mechanism, but one that is calibrated in terms of IPD at low frequencies. The existence of double images is explained in terms of a breakdown in the mechanism determining fusion.

Adult↗
PubMed1991

A computer model of auditory stream segregation.

A computer model is described which simulates some aspects of auditory stream segregation. The model emphasizes the explanatory power of simple physiological principles operating at a peripheral rather than a central level. The model consists of a multi-channel bandpass-filter bank with a "noisy" output and an attentional mechanism that responds selectively to the channel with the greatest activity. A "leaky integration" principle allows channel excitation to accumulate and dissipate over time. The model produces similar results to two experimental demonstrations of streaming phenomena, which are presented in detail. These results are discussed in terms of the "emergent properties" of a system governed by simple physiological principles. As such the model is contrasted with higher-level Gestalt explanations of the same phenomena while accepting that they may constitute complementary kinds of explanation.

Attention↗
PubMed1991

An evaluation of eight computer models of mammalian inner hair-cell function.

Eight computer models of auditory inner hair cells have been evaluated. From an extensive literature on mammalian species, a subset of well-reported auditory-nerve properties in response to tone-burst stimuli were selected and tested for in the models. This subset included tests for: (a) rate-level functions for onset and steady-state responses; (b) two-component adaptation; (c) recovery of spontaneous activity; (d) physiological forward masking; (e) additivity; and (f) frequency-limited phase locking. As models of hair-cell functioning are increasingly used as the front end of speech-recognition devices, the computational efficiency of each model was also considered. The evaluation shows that no single model completely replicates the subset of tests. Reasons are given for our favoring the Meddis model [R. Meddis, J. Acoust. Soc. Am. 83, 1056-1063 (1988)] both in terms of its good agreement with physiological data and its computational efficiency. It is concluded that this model is well suited to provide the primary input to speech recognition devices and models of central auditory processing.

Animals↗
PubMed1990

A prospective evaluation of in-hospital counselling for first time myocardial infarction men.

Self-ratings of anxiety and depression were studied over six months in 60 male patients, under 66 yr of age, who were admitted to a coronary care unit with a first time acute myocardial infarction. Patients were randomly assigned to either a treatment group, where they received a simple programme of in-hospital counselling in addition to routine care, or to a control group, where they received routine care only. All patients completed the Hospital Anxiety and Depression scale and a battery of visual analogue scales measuring anxiety on a range of topics related to recovery from a myocardial infarction. Patients who received in-hospital counselling reported statistically significantly less anxiety and depression than those who received routine care alone. This effect was sustained for six months after leaving hospital. It is concluded that a simple programme of in-hospital counselling, provided by a coronary care nurse, is efficacious and should be routinely offered to first myocardial infarction patients in hospital.

Adaptation, Psychological↗
PubMed1990

Wives' responses to counselling early after myocardial infarction.

Self-ratings of anxiety and depression were studied over six months in 60 wives of first time myocardial infarction patients. Couples were randomly assigned to either a treatment group, where they received a simple programme of education and psychological support in addition to routine care, or to a control group, where they received routine care only. All wives completed the Hospital Anxiety and Depression scale and a battery of visual analogue scales measuring anxiety on a range of topics related to recovery from a heart attack. Wives in the treatment group reported statistically significantly less anxiety than controls. This effect was sustained for six months after the counselling. It is concluded that a simple programme of in hospital counselling is efficacious and should be routinely offered to the wives of coronary patients.

Adaptation, Psychological↗
PubMed1990

In-hospital counselling for first-time myocardial infarction patients and spouses: effects on satisfaction.

Self-ratings of satisfaction were studied over 6 months in 60 male first-time myocardial infarction patients and their wives. Couples were randomly assigned to either a treatment group, where they received a simple programme of education and psychological support in addition to routine care, or to a control group, where they received routine care only. All patients completed visual analogue scales measuring satisfaction regarding their general health, life in general, care and information received. All wives completed visual analogue scales measuring satisfaction regarding information received and care the patient received. Patients and wives in the treatment group reported statistically significantly more satisfaction than those in the control group. This effect was sustained for 6 months after counselling.

Consumer Behavior↗
PubMed1990

Independence and status--two different work methods.

This paper describes a study to establish whether there were hidden benefits from the changes occurring in nursing work methods. Of particular interest were the feelings of independence and status of qualified nursing staff operating within two discrete work environments, namely a) primary nursing, b) task allocation nursing. A nine item questionnaire with Likert scale was used as the measure. It was hypothesised that those nurses working with primary nursing would 1) feel more independence and, 2) have a higher sense of status than their counterparts in the traditional task-centred environment.

Female↗
PubMed1988

Simulation of auditory-neural transduction: further studies.

A computational model of mechanical to neural transduction at the hair cell-auditory-nerve synapse is presented. It produces a stream of events (spikes) that are precisely located in time in response to an arbitrary stimulus and is intended for use as an input to automatic speech recognition systems as well as a contribution to the theory of the origin of auditory-nerve spike activity. The behavior of the model is compared to data from animal studies in the following tests: (a) rate-intensity functions for adapted and unadapted responding; (b) two-component short-term adaptation; (c) frequency-limited phase locking of events; (d) additivity of responding following stimulus-intensity increases and decreases; (e) recovery of spontaneous activity following stimulus offset; and (f) recovery of ability to respond to a second stimulus following offset of a first stimulus. The behavior of the model compares well with empirical data but discrepancies in tests (d) and (f) point to the need for further development. Additional functions that have been successfully simulated in previous tests include realistic interspike-interval histograms for silence and intense sinusoidal stimuli, realistic poststimulus period histograms at various intensities and nonmonotonic functions relating incremental and decremental responses to background stimulus intensity. The model is computationally convenient and well suited to use in automatic recognition devices that use models of the peripheral auditory system as input devices. It is particularly well suited to devices that require stimulus phase information to be preserved at low frequencies.

Acoustic Stimulation↗
PubMed1986

Simulation of mechanical to neural transduction in the auditory receptor.

A probabilistic model is described for transmitter release from hair cells, auditory neuron EPSP's, and discharge patterns. The model assumes that the release fraction of the transmitter is a function of stimulus intensity. It further assumes that some of this transmitter substance is taken back into the cell while some is irretrievably lost from the cleft. These assumptions differ from other recent models which propose multiple release sites, fixed release fractions, and no transmitter reuptake. The model produces realistic mammalian rate intensity functions, interval and period histograms, incremental responses, and adaptation effects. It mimics successfully the adaptation of successive EPSP amplitudes of the afferent neuron of the goldfish sacculus and offers a reinterpretation of the implications of these studies for hair cell synaptic mechanism.

Animals↗
PubMed1975

On the function of sleep.

It is proposed that sleep serves the function of maintaining immobility in animals at times when immobility is an optimum behavioural survival strategem. Sleep is of great evolutionary age and is always carefully tailored to the life-style of each animal; abundant in some species and absent from others. The survival advantages conferred on an animal by the power of sleep to schedule behaviour effectively may justify its existence and persistence in the evolution of species.

Animals↗