Monaural sound localization: acute versus chronic unilateral impairment

We tested the ability of human listeners to localize broadband noise bursts in the absence of binaural localization cues. The subject population consisted of five patients, who had normal hearing in one ear and congenital deafness in the other, and seven normal controls, who were tested with both ears open and with one ear plugged. Consistent with previous reports, the introduction of an ear plug unilaterally into control subjects resulted in a prominent lateral displacement in their localization judgements by an average of 30.9 degrees toward the side of the open ear. Vertical localization was less strongly impaired. The five monaural patients showed a considerable range of ability to localize sounds. Two of the patients were essentially indistinguishable from the plugged control subjects in that they showed a prominent displacement of responses toward the side of the hearing ear. The other three subjects localized significantly better than the plugged controls, in that they demonstrated little or no lateral displacement toward the hearing side and that they localized targets on the hearing and on the impaired sides about equally well. The performance of these latter patients demonstrates that monaural cues can provide useful localization information in the horizontal as well as in the vertical dimension.