Biomedical subjects

G A O'Beirne

Publications and source records attributed to G A O'Beirne.

4 recordsLinked to original sources

PubMed1999

Boltzmann analysis of CM waveforms using virtual instrument software.

We describe a modification to our technique for the rapid analysis of low-frequency cochlear microphonic (CM) waveforms in the basal turn of the guinea pig cochlea (Patuzzi and Moleirinho, 1998). The transfer curve relating instantaneous sound pressure in the ear canal to instantaneous receptor current through the outer hair cells (OHCs) is determined from the distorted microphonic waveform generated in the extracellular fluid near the hair cells, assuming a first-order Boltzmann activation curve. Previously, the analysis was done in real time using custom-built electronic circuitry. Here, the same task is performed numerically using virtual instrument software (National Instruments LabVIEW 4.1) running on a personal computer. The assumed theoretical function describing the CM waveform is Vcm = Voff + Vsat/[1 + exp[(Eo+Z.Po.sin(2pi f + phi(tot)))/kT]], where the six parameters are (i) a DC offset voltage (Voff); (ii) the frequency of the sinusoidal stimulus (f); (iii) the phase of the sinusoidal stimulus (phi(tot)); (iv) the maximal amplitude of the distorted microphonic signal (Vsat); (v) the sensitivity of the transduction process (Z); and (vi) the operating point on the sigmoidal transfer curve (Eo). The software obtains the least-squares fit to the CM waveforms by continuously deriving the six parameters at a speed of about one determination per second. The independent fitting of the frequency and phase allows the data to be analysed off-line from data previously recorded to tape (i.e. the frequency and phase of the microphonic response need not be known accurately beforehand). We present here an outline of the software we have used, and give an example of the changes which can be monitored using the technique (transient asphyxia). The method's advantages and limitations have been discussed in our previous paper. The virtual instrument described here is available from the authors on request.

Animals↗

PubMed1999

Basic properties of the sound-evoked post-auricular muscle response (PAMR).

One objective electrophysiological test for deafness involves presenting a brief acoustic stimulus to a subject and measuring the electrical activity evoked in the muscle located just behind the ear (the post-auricular muscle or PAM). Although this electrical response has been known for many years, it has been ignored by most clinicians and frequently misreported in the literature. This paper presents the fundamental properties of the PAM electrical response (the PAMR) and examines ways in which its measurement can be improved by altering the standard electrode position and filtering. The response consists of a simple bipolar compound action potential with a first peak latency of between 12.5 and 15 ms, depending on the stimulus intensity and PAM muscle tone. The largest recordings can be made with an active electrode over the PAM and with the reference electrode on the dorsal surface of the pinna. It can be obtained with click and tone-burst stimuli within 20 dB of the subjective detection threshold, can be evoked with tone-bursts between 500 Hz and 16 kHz and grows either linearly with the click level or approximately exponentially with the tone-burst level, reaching a maximum of as large as 250 microV pp in some subjects. It has a frequency spectrum mostly between 25 and 200 Hz. The response is often visible in raw recordings, with as few as 20 averages required for obtaining a stable waveform. There is very little amplitude and latency difference in stimulating the ear on the same side or opposite side to the recording electrodes and the binaurally evoked response is similar to the simple arithmetic sum of the waveforms obtained with monaural stimulation. The response latency and duration are longer in very young infants, but reach adult values by 12 months of age. In a companion paper, we describe a method of enhancing the PAMR using lateral eye movement (Patuzzi and O'Beirne, 1999a).

Acoustic Stimulation↗

PubMed1999

Effects of eye rotation on the sound-evoked post-auricular muscle response (PAMR).

One objective electrophysiological test for deafness involves presenting a brief acoustic stimulus to a subject and measuring the electrical activity evoked in the muscle located just behind the ear (the post-auricular muscle or PAM). We describe a method for enhancing this post-auricular muscle response (PAMR) using lateral eye movement, which increases both the tonic EMG activity in the PAM and the magnitude of the PAMR, and decreases response latency. EMG activity in most subjects tested (more than 30) increased almost instantly on rotation of the eyes, and thereafter grew more slowly with maintained lateral gaze, with the largest increase occurring with eye rotation towards rather than away from the measurement electrodes over the PAM. The EMG activity returned rapidly to near pre-rotation levels when the eyes were returned to the forwards position, with full recovery taking some minutes. While there was a similar increase and return of the PAMR amplitude with eye rotation, the time-course of these changes was somewhat different, largely because the EMG activity and the PAMR amplitude were not proportional. Rather the PAMR amplitude was a saturating function of EMG level, so that the PAMR response did not fall as markedly as the EMG when the eyes were returned to a forwards gaze, and the recovery of the PAMR amplitude to pre-rotation levels appeared to take longer. We discuss the neural mechanisms that may be responsible for this PAMR potentiation with eye movement and discuss its probable role in increasing variability in early studies which did not control for eye movement. We also discuss the utility of eye rotation in potentiating and stabilising the PAMR to allow its use in screening for deafness.

Acoustic Stimulation↗

PubMed1999

A correlation method for detecting the sound-evoked post-auricular muscle response (PAMR).

We have made detailed measurements of the sound-evoked post-auricular muscle response (PAMR) in four adults and two infants, in an attempt to understand the inter-relationships between sound level, potentiation of the PAMR with voluntary PAM contraction or eye rotation, electromyographic (EMG) noise, amplitude of the PAMR, and a correlation measure of the presence of the PAMR. We have found that the amplitude of the PAMR is a simple linear function of the decibel level of a monophasic click (0.1 ms duration), and that the PAMR amplitude is also a saturating power function of the level of tonic EMG. As a result, PAMR=PAM(o).SL. (EMG-EMG(noise))(2)/[(EMG-EMG(noise))(2)+beta(2)], where SL is the decibel level of a click above subjective threshold, PAM(o) is a parameter accounting for the differing PAMR amplitude across individuals or with altered electrode placement, EMG(noise) is the component of EMG not associated with PAMR potentiation, and beta determines the initial rate of growth of PAMR at low levels of PAM activation. We have also found that the correlation measure (C) of the PAMR follows a saturating power function of the signal-to-noise ratio (SNR=PAMR/EMG), with C=SNR(2)/(SNR(2)+delta(2)), where delta determines the onset of saturation in the correlation as a function of SNR. The combination of these two relationships means that correlation is a non-monotonic function of the EMG (PAM activation): it can be large for moderate levels of EMG, but small for high levels of EMG, because the PAMR amplitude saturates but the EMG does not. The correlation is a fast, convenient means of detecting the PAMR, whether using clicks or tone-bursts, and can be used effectively in adults or infants, as long as the reflex is moderately activated. This moderate activation is most effectively produced by eye rotation towards the recording electrodes.

Acoustic Stimulation↗