During daytime, there was almost no REM sleep at sound levels greater than 80 dBC; however, N3 sleep persisted.
In all of these experiments, the low and high frequency sounds were presented at sound levels that were equal before being filtered by the transfer functions of the ears, thereby mimicking the stimulus conditions in the behavioral experiment.
For RRTFs, click trains (monopolar, rectangular pulses of 200 µs duration; 500 ms train duration) were systematically presented 15 times for repetition rates from 1 to 38 Hz (1, 6, 10, 14, 18, 22, 26, 30, 34, and 38 Hz) at sound levels of 82 102 dB SPL (peak equivalent).
Previous authors have reported that the lowest thresholds in humans are seen for broadband noise (Penner, 1977; Plomp, 1964) or for sounds containing high-frequency components >5 kHz (Fitzgibbons, 1983 , 1984, as long as the stimulus was presented above a critical level of approximately 50 dB SPL (a factor not affecting our own data, which were collected at sound levels >70 dB SPL).
To provide the stimuli at sound levels as correct as possible, the hearing thresholds for all five stimuli (four test stimuli and one training stimulus) were binaurally determined before each test session, following the standard modified Hughson Westlake procedure (Harrell 2002).
Furthermore, we show that Ntf3 overexpression can elicit regeneration of the synaptic contacts between cochlear nerve terminals and inner hair cells after noise-induced synaptopathy, a type of neural damage which appears to be widespread even in ears exposed at sound levels well below those which cause hair cell damage and permanent threshold shifts (Kujawa and Liberman, 2009).
Despite interneuron variability, the spectral tuning functions of several exemplary neurons in A1 and especially in CM seem to remain confined within about one or two octaves at sound levels below 60 dB and to exhibit a prominent enlargement right between 60 dB and 70 dB sound level (see, e.g., figures from 3 to 6 in [ 21]; see also Figures 1 and 2 in [ 22]).
The auditory stimuli (1 or 2) were presented using an external speaker with a machine voice, the volume was maintained constant during all experiments at a sound level of approximately 50 dB measured at 1 m from the speakers.
Nedwell et al. (2007) found that marine mammals would be at risk of auditory injury, when exposed to noise at a sound level of approximately 130 dBht for a few seconds.
