Biomedical subjects

L P Morton

Publications and source records attributed to L P Morton.

4 recordsLinked to original sources

PubMed1995

Acoustic reflex measures at high probe frequency: a phasor diagram approach.

Acoustic reflex measures at high probe frequencies (above 660 Hz) have not been extensively reported in the published literature. Studies of the acoustic reflex at low probe frequencies conflict as to the effect of the reflex on resistance, although all researchers agree that the reflex increases the amount of negative reactance (stiffness). These differences may reflect methodological differences across studies. Lutman and his associates (Lutman and Martin 1979; Lutman et al., 1984) have used phasor diagrams to provide evidence for the effect of the reflex being on reactance, with constant resistance for low probe frequencies (220 and 660 Hz). The present study incorporated the use of this method in order to investigate the effect of the reflex at a 1000 Hz probe frequency in 30 normal ears. Phasor diagrams of probe tip susceptance and conductance values obtained for three stimuli at a 1000 Hz probe frequency were analysed qualitatively. Eighty-two out of a total of 90 phasor diagrams obtained were matched to the constant-resistance stiffness-change model, suggesting that similar effects of the reflex are seen at low and high probe frequencies in most cases. This was noted to be the case for ears that were either stiffness-dominated or at their natural frequency at the 1000 Hz probe. However, the effect of the reflex on mass-dominated systems was less clear and needs further clarification.

Acoustic Stimulation↗

PubMed1994

Direction of susceptance and conductance acoustic reflexes at high probe frequencies.

Previous investigations have shown that for low probe frequencies (220 and 660 Hz) the direction of susceptance and conductance reflexes is related to baseline transmission properties. Previously documented reflex patterns, caused by the stiffening of the ossicular chain, are characterised by decreases in both components at 220 Hz (-B-G) and increases in susceptance and decreases in conductance (+B-G) at 660 Hz. In the present study, the direction of susceptance and conductance acoustic reflexes was investigated at a probe frequency of 1000 Hz, where ears are expected to be less stiffness dominated than at the low frequencies, and for which there is little description of reflex patterns in the published literature. The pattern of ipsilateral susceptance and conductance reflexes from 30 normal ears was recorded across a wide intensity range at this probe frequency for three stimulus frequencies (0.5, 1 and 2 kHz). Reflexes were classified as either increases (+) or decreases (-) for each component, at threshold and at suprathreshold intensity levels. Ears were grouped according to transmission properties at 1000 Hz, and reflex patterns observed within each group of subjects were examined. Patterns observed in these subjects at the 1000 Hz probe frequency included: +B-G; +B+G; -B+G and -B-G. There appeared to be a relationship between baseline transmission and reflex patterns. Further, patterns appear to be consistent with theoretical models of the effect of the acoustic reflex, particularly for ears where stiffness is significant in the baseline condition. The effect of the reflex for mass dominated systems requires further investigation.

Acoustic Impedance Tests↗

PubMed1992

Hearing thresholds in CAPD patients.

Ultra high frequency (UHF, 10-20 kHz) and conventional audiometric thresholds (0,25-8 kHz) were obtained from 42 stable CAPD patients. Twenty-one (50%) and 11 (25%) of the patients had UHF and conventional hearing loss respectively, when compared to age related control data. This was unrelated to length of chronic renal failure and number of treatments with ototoxic drugs. Sixteen of these patients were monitored audiometrically using UHF during a course of vancomycin therapy for peritonitis. There was no significant change in hearing thresholds. In conclusion there is high incidence of hearing loss in CAPD patients which is unrelated to ototoxic drugs.

Adult↗

PubMed1991

High frequency thresholds: variations with age and industrial noise exposure.

The effects of age and noise exposure were investigated. 97 normal subjects representing 6 age groups were tested with both conventional pure tone audiometry and high frequency audiometry (10-20 kHz) using a commercial high frequency audiometer. Thresholds were obtained by means of earphones as opposed to a quasi-free field technique. The results were discussed in terms of the age effect. In addition, conventional and high frequency thresholds were obtained for 64 subjects who had a history of industrial noise exposure representing 4 age groups. Significant differences were found between the noise exposed group and the normal subjects. The implications of the results are discussed.

Adolescent↗