Do perceptual consequences of spontaneous otoacoustic emissions reflect a central plasticity effect?

Frequency difference limens (FDLs) have been found to improve in the vicinity of spontaneous otoacoustic emissions (SOAEs). This effect has been observed ipsilaterally and contralaterally to the emission ear, suggesting that prolonged ongoing stimulation of nerve cells tuned to the SOAE frequency lead to a central oversensitivity to that frequency (Norena et al., 2002, Hear.

Res.). However, it is known that a tone close to an SOAE frequency can “entrain” the emission to oscillate at the tone frequency (Long and Tubis, 1988, Hear. Res.), thus FDLs near SOAEs might also be affected by this peripheral process. An alternative hypothesis explaining FDL performance in terms of peripheral entrainment and beating between external tones and SOAEs is proposed here.

SOAE entrainment patterns were obtained in seven subjects with an ipsilateral SOAE and no neighboring contralateral SOAE. Ipsi- and contralateral FDLs were measured at frequencies covering the individual entrainment and beating regions. Ipsilateral FDLs were lowest in the entrainment region and worsened significantly when beating occurred.

However, no changes in contralateral FDLs were found. Contralateral FDLs also remained unaffected by continuous ipsilateral presentation of a pure tone aimed at emulating an SOAE. These findings suggest a peripheral rather than central plasticity origin for perceptual consequences of SOAEs.