No effect of inter-stimulus interval on acoustic reflex thresholds

The acoustic reflex, a longstanding component of the audiological test battery, has received renewed attention in the context of noise-induced cochlear synaptopathy - the destruction of synapses between inner hair cells and auditory nerve fibers. Non-invasive proxy measures of synaptopathy are widely sought, and acoustic reflex thresholds (ARTs) correlate closely with synaptic survival in rodents. However, measurement in humans at high stimulus frequencies – likely important when testing for noise-induced pathology – can be challenging; reflexes at 4 kHz are frequently absent or occur only at high stimulus levels, even in young people with clinically normal audiograms. This phenomenon may partly reflect differences across stimulus frequency in the temporal characteristics of the response; later onset of the response, earlier onset of adaptation, and higher rate of adaptation have been observed at 4 kHz than at 1 kHz. One temporal aspect of the response that has received little attention is the inter-stimulus interval (ISI); inadequate duration of ISI might lead to incomplete recovery of the response between successive presentations and consequent response fatigue. The present research aimed to test for effects of ISI on ARTs in normally hearing young humans, measured at 1 and 4 kHz. Contrary to our hypotheses, increasing ISIs from 2.5 to 8.5 s did not reduce ART level, nor raise ART reliability. Results confirm that clinically measured ARTs - including those at 4 kHz - can exhibit excellent reliability, and that relatively short (2.5 s) ISIs are adequate for the measurement of sensible and reliable ARTs.