Contextual modulation of cochlear hearing desensitization depends on the type of loud sound trauma

Bandwidth determines modulatory effects of centrifugal pathways on cochlear hearing desensitization caused by loud sound

“…It is also notable that threshold shifts to loud 11–13 kHz noise were modulated by background atraumatic white noise (Fig. 4; Rajan, 2006), by as much as it modulated threshold shifts to loud 8–13 kHz noise. It has been argued (Rajan, 2005) that background white noise and OC pathways modulate threshold shifts by acting on the same site to reduce the desensitization of the electromotile ‘active’ process in outer hair cells that causes threshold shifts (Patuzzi et al ., 1984, 1989; Cooper & Rhode, 1992; Patuzzi, 1992; Ruggero et al ., 1993, 1996; Zhang & Zwislocki, 1995; Fridberger et al ., 2002a, b).…”

“…In the next experimental series the loud 11–13‐kHz noise was presented in a background of simultaneous atraumatic white noise at 80 dB SPL. It has been detailed previously that although the white noise itself does not cause threshold shifts, it does modulate threshold shifts caused by loud sounds, as shown (Rajan, 2006) by comparing threshold shifts in OC– ears presented only a test loud sound or that sound together with background white noise. As OC pathways are cut in both conditions any difference in threshold shifts reflects the effect of white noise on cochlear susceptibility to loud sound.…”

“…Figure 4 summarizes the effects in our previous study (Rajan, 2006) examining how such white noise alters threshold shifts to a loud 11–13‐kHz noise, a loud tone (loud 13‐kHz tone, from Rajan, 2000) and a loud 8–13‐kHz noise (from Rajan, 2005). Figure 4A plots, for each of the three traumata, the difference between frequency‐specific mean threshold shifts in OC– ears tested either with the particular traumatic sound by itself or in the presence of the atrautmatic background WN.…”

“…It was noted above that the absence of modulation of threshold shifts to loud 11–13 kHz noise by OC pathways is not an artefact or because this noise did not cause some critical amount of threshold shifts. Also, threshold shifts to the loud 11–13 kHz noise were modulated by atraumatic white noise (Rajan, 2006), by as much as it modulates threshold shifts to loud 8–13 kHz noise, and it has been argued (Rajan, 2005) that both atraumatic white noise and OC pathways modulate threshold shifts by acting on the same site to reduce desensitization of the electromotile ‘active’ process in outer hair cells. If so, the absence of OC‐mediated modulation, but not white noise modulation, of threshold shifts to loud 11–13 kHz noise cannot be related to any cochlear factors.…”