Ability to discriminate an elevation in sound: The effects of an altered pinna in subjects with monaural hearing

We tested the ability of human listeners to discriminate white noise bursts in the absence of binaural discrimination cues and changed the shape of spectral cues. Minimum audible angle (MAA) thresholds were determined for five subjects in a two-alternative, forced-choice, adaptive paradigm as a function of the orientation of the array of sound sources in the vertical plane (elevation). The subjects were students with normal hearing who wore a monaural earplug to reduce sound level and timing cues and were then fitted with a pinna mold to change the shape of spectral cues. Tests measured discrimination of the vertical angle in eight planes parallel to the median plane (0° position). The results showed that the sound source position could be discriminated in the vertical plane when a subject with normal monaural hearing wore the external ear model. However, the discrimination ability worsened remarkably. The ability to discriminate the orientation of sound sources in the lower hemi-field was greater than in the upper hemi-field at the 0° to 180° reference positions, while the reverse was true for the 225° to 315° reference positions. In addition, the pinna cavities of both the near and far ears play a role in determining the perceived vertical angle of a sound source in any horizontal position, including the median plane. As a sound source shifts laterally away from the median plane, the contribution of the near ear increases, while that of the far ear decreases. For horizontal positions at azimuths greater than 180° from the midline, the far ear mold no longer changes the shape of spectral cues for discriminating the vertical angle.