Are two ears necessary for localization of sound sources on the median plane?

Several investigators have shown that monaural localization of sound sources on the median plane (MP) is inferior to binaural MP localization, causing speculation that two ears are necessary for MP localization, and further, that two ears may allow binaural processing of asymmetrical pinna filtering making localization of unfamiliar sounds possible. The purpose of the two experiments reported in this paper is (1) to test the hypothesis that binaural subjects can localize unfamiliar sounds more accurately than monaural subjects, and (2) to evaluate monaural localization accuracy after training. The results show that binaural and monaural subjects have similar difficulty in localizing unfamiliar sounds and show that monaural subjects can easily be trained to localize as well as they normally localize with two ears. The results indicate MP localization is fundamentally a monaural process.