Relating the absence of binaural pitch percept to retro-cochlear impairment

Binaural pitch stimuli, created by introducing an interaural phase difference over a narrow band of otherwise diotic white noise, produce an immediate tonal sensation with a pitch close to the centre of the phase-shifted band. In Santurette and Dau [Hear. Res. 223(1-2):29-47, 2007], it was shown that the salience of binaural pitch was affected by hearing impairment. Specifically, for subjects with a sensorineural impairment, binaural pitch perception was weaker than the normal-hearing average but the pitch sensation was immediately present. In contrast, no binaural pitch sensation at all was found for the (only) two subjects with damage at central stages. The aim of the present study is to clarify whether such a sharp distinction between levels of impairment can be made using binaural pitch stimuli. A pitch detection test was performed by three groups of subjects with: 1) normal hearing; 2) a cochlear impairment with no sign of retro-cochlear impairment; and 3) a diagnosed retro-cochlear impairment. Subjects were asked to judge the pitch direction of series of five notes of equal duration (300, 600 or 900 ms), ranging from 523 to 784 Hz, presented either in an ascending, descending, or constant sequence. The results from two stimulus configurations, namely Huggins’ pitch stimuli and pure tones presented in diotic white noise, were compared. In addition to the pitch detection experiment, measures of frequency selectivity, fine structure and envelope processing, binaural interaction, and cognitive abilities, were obtained in order to investigate the correlation between these outcomes and results from the binaural pitch test. As no spectral cues are provided by binaural pitch stimuli, their perception is expected to heavily depend on the acuity of fine structure coding and the accuracy of the binaural system in combining the input from both ears. Overall, the absence of any binaural pitch percept is expected to be found only among subjects from group 3, while deficits at cochlear level are expected not to be sufficient to eliminate the perception of binaural pitch. If so, a binaural pitch test would be an interesting indicator of retro-cochlear deficit and useful for characterising the auditory profile of individual hearing-impaired listeners.