Off-frequency listening in subjects with chronic tinnitus

The occurrence of subjective tinnitus has been linked to cochlear damage, as most tinnitus patients have impaired hearing, and animal studies have shown that the induction of hearing loss can lead to behavioural signs of tinnitus. In tinnitus patients, the pure-tone audiogram is the main source of information about cochlear damage, but hearing thresholds alone may not adequately reflect its magnitude. Etchelecou et al. (2011) reported that the majority of patients with acute tinnitus post impulse noise exposure showed off-frequency listening (OFL), which is not readily observed in pure-tone audiograms. We investigated the possibility of OFL occurring in subjects with chronic tinnitus by testing twenty subjects who had experienced tinnitus for more than a year. OFL was assessed by measuring psychophysical tuning curves using a forward-masking paradigm. OFL occurred in 13 out of 20 subjects, 12 of whom also did not perceive frequencies above 8 kHz. Such unresponsive frequencies (UFs) were also present in three subjects without OFL. The tinnitus spectrum generally reached its highest values at the edge of or within the frequency regions with OFL or UFs, but there was no significant correlation between edge frequencies and the frequency with the highest tinnitus pitch similarity rating. When OFL and UFs were taken as evidence for cochlear dead regions, 16/20 subjects passed the criterion for cochlear dead regions. The remaining four subjects showed neither OFL nor UFs.