Use of the pinna reflex as a test of hearing in mutant mice.

Although it is a gross measure, the pinna reflex test is easily administered and is, therefore, incorporated as a general screening tool in mutagenesis programs. Our recent application of this approach indicated that mutant mice lacking one of the small Maf proteins, in this case MafG, failed to exhibit a pinna reflex. In contrast, littermate controls, with the same mixed 129/CD1 background, and including both wild type and heterozygous mutant animals, passed the test. Because previous studies indicate that mafG is expressed in both cochlear and vestibular parts of the mouse inner ear, the source of this 'presumed deafness' was further assessed by making round window recordings to determine compound action potential thresholds. Auditory brainstem responses were also acquired to assess function along portions of the central auditory pathway. In all cases, responses in homozygous mutants (-/-) were comparable to those obtained from littermate controls, either wild type (+/+) or heterozygous mutants (+/-). Gross anatomy of the organ of Corti was also found to be similar in all three groups of mice. Hence, the lack of a pinna reflex must relate to nonauditory causes.

Intermodulation components in inner hair cell and organ of Corti responses.

Two-tone responses are recorded from inner hair cells and from the organ of Corti fluid space in second and third turns of the guinea pig cochlea where best frequencies (BF) are approximately 4000 and 1000 Hz, respectively. This allows both ac and dc response components to be obtained and facilitates comparisons with psychophysical investigations that have traditionally been conducted at low and moderate frequencies. The measurements of ac responses in the organ of Corti fluid space also allow comparisons with mechanical results because the cochlear microphonic is proportional to basilar membrane displacement. By using a constant frequency ratio (f2/f1) of 1.4, local distortion products generated at the recording location are prominent when the two primaries are near the BF of the cell. However, when the primary pairs increase above BF, quadratic and cubic difference tones are recorded even when responses to the primaries are not measurable. The presence of these traveling distortion products is consistent with the idea that both f2-f1 and 2f1-f2 have their own traveling waves. Notches in the existence regions of quadratic and cubic difference tones were also observed and found to be influenced by mutual suppression between the two inputs.