Mark Fletcher

Is Off-Frequency Overshoot Caused by Adaptation of Suppression?

This study is concerned with the mechanism of off-frequency overshoot. Overshoot refers to the phenomenon whereby a brief signal presented at the onset of a masker is easier to detect when the masker is preceded by a “precursor” sound (which is often the same as the masker). Overshoot is most prominent when the masker and precursor have a different frequency than the signal (henceforth referred to as “off-frequency overshoot”). It has been suggested that off-frequency overshoot is based on a similar mechanism as “enhancement,” which refers to the perceptual pop-out of a signal after presentation of a precursor that contains a spectral notch at the signal frequency; both have been proposed to be caused by a reduction in the suppressive masking of the signal as a result of the adaptive effect of the precursor (“adaptation of suppression”). In this study, we measured overshoot, suppression, and adaptation of suppression for a 4-kHz sinusoidal signal and a 4.75-kHz sinusoidal masker and precursor, using the same set of participants. We show that, while the precursor yielded strong overshoot and the masker produced strong suppression, the precursor did not appear to cause any reduction (adaptation) of suppression. Predictions based on an established model of the cochlear input–output function indicate that our failure to obtain any adaptation of suppression is unlikely to represent a false negative outcome. Our results indicate that off-frequency overshoot and enhancement are likely caused by different mechanisms. We argue that overshoot may be due to higher-order perceptual factors such as transient masking or attentional diversion, whereas enhancement may be based on mechanisms similar to those that generate the Zwicker tone.

Adverse effects of very high-frequency sound and ultrasound on humans

For many years, workers have reported adverse symptoms resulting from exposure to very high-frequency sound (VHFS) and ultrasound (US), including annoyance, dizziness and difficulty concentrating. Recent work showing the presence of a new generation of VHFS/US sources in public places has reopened the debate about whether adverse effects can be caused by exposure to VHFS/US. Our field measurements of VHF/US sources in public places have identified devices producing up to 100 dB SPL at 20 kHz. Nearly all of the sources measured, including those in places occupied by tens of millions of people each year, are likely to be clearly audible to many young people. We have conducted two studies. The first looked at adverse symptoms resulting from exposure to audible VHFS/US, and the second was a double-blind study of adverse symptoms resulting from exposure to inaudible VHFS/US. In each study, both symptomatic participants, who reported previously experiencing symptoms, and asymptomatics participants, who did not, were tested. We found evidence that symptoms were produced by exposure to audible VHFS/US but not by inaudible sound. It is possible that the substantial effects reported for inaudible VHFS/US exposure were not reproduced because of ethical restrictions on stimulus level and duration.

Does reduction in cochlear gain explain the overshoot effect

Under certain conditions, detection of a masked tone is improved by a preceding sound (a precursor); this is the overshoot effect. Despite over half a century of research, its underlying mechanisms remain unknown. A popular hypothesis links overshoot to reduction in cochlear gain by the medial olivocochlear reflex. This is thought to reduce excitatory masking when the masker is at the signal frequency (within‐channel effect) and suppressive masking when the masker is remote from the signal in frequency (across‐channel effect). This hypothesis was examined in the first of the two experiments presented in this study. The results found no within‐channel overshoot, indicating that the effect must be due to factors other than gain reduction at the signal frequency. While there was substantial across‐frequency overshoot, the pattern of results was inconsistent with reduction in suppressive masking. Interpretation of results from overshoot experiments is often complicated by the possibility that the precursor itself might have a masking effect on the signal. The second experiment presented in this study was designed to overcome this problem.