Direction of susceptance and conductance acoustic reflexes at high probe frequencies

Abstract

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.