ASA 127th Meeting M.I.T. 1994 June 6-10

2aPP30. A suppressive ``off-effect'' in the f[sub 2]-f[sub 1] DPOAE response to continuous, moderate-level primary stimulation.

Sharon G. Kujawa

Maureen Fallon

Richard P. Bobbin

Kresge Hearing Res. Lab., LSU Med. Ctr., 2020 Gravier St., Ste. A, New Orleans, LA 70112

Following a short period of amplitude enhancement, the f[sub 2]-f[sub 1] DPOAE can be suppressed during continuous ipsilateral stimulation with primary tones. Evidence for and against efferent control of such response alterations has been presented. Here, an exacerbation of f[sub 2]-f[sub 1] suppression, apparent following a short rest from continuous stimulation, is described and the hypothesis that it is efferent-mediated is tested. The f[sub 2]-f[sub 1] DPOAE to intermittent or continuous primaries ( f[sub 1]=6.25 kHz; f[sub 2]=7.5 kHz; L[sub 1]=L[sub 2]=60 dB SPL) was recorded in urethane-anesthetized guinea pigs with sectioned middle ear muscles before, during, and after perfusion of the cochlear perilymph compartment with antagonists of olivocochlear efferent activity (curare, 1 (mu)M; bicuculline, 10 (mu)M) or of action potentials in general (tetrodotoxin, TTX; 1 (mu)M). Results confirmed an initial f[sub 2]-f[sub 1] amplitude enhancement and gradual suppression during continued stimulation. Following a 1-min rest, however, the DPOAE was further suppressed from its pre-rest level. Thereafter, its amplitude increased rapidly, reaching a maximum within 1--2 min. These amplitude alterations were intensity dependent. Moreover, they were reduced, but not blocked by efferent antagonists and were largely unaffected by TTX, suggesting that additional (e.g., local) alterations in cochlear status impact the cochlear mechanical response to continuous sound stimulation. [Work supported by NIH DC00007; DAMD 17-93-V-3013.]