Azimuth estimates by human subjects under free-field and headphone conditions

Estimates of sound source directions in the horizontal plane (azimuth) were made by human subjects under acoustic free-field (loudspeaker) and dichotic stimulus conditions (headphones). The functional relationship between the binaural cues, interaural time and intensity differences (ITDs and IIDs) and azimuth angle approximately follow a quarter of a full sine wave between 0 degrees straight ahead and the maximum at 90 degrees for ITDs and, depending on frequency for IIDs, at 60-90 degrees lateral (frontal quadrant). Under free-field conditions, these physically determined, interaural difference cues are correctly translated into azimuth angles by all listeners: estimated angles correspond to presented sound directions. In contrast, when providing dichotic stimuli containing ITDs/IIDs, so that sounds come from virtual azimuth angles, only 8% of the listeners estimated according to the expected transfer function of a quarter of a sine wave. However, about 60% of the listeners ranged azimuth directly proportionally to the binaural differences introduced, systematically overestimating virtual angles by up to 20 degrees. The remaining 32% of the listeners systematically overestimated virtual small and midrange sound directions even more. Virtual 40 degrees (corresponding to 12- to 14-dB and/or 450- to 550-microseconds differences) was often estimated as almost 90 degrees. Without corrective feedback, a reduction of the systematic errors was never observed. Central neural mechanisms and conceptual strategies to accomplish the expected transfer from virtual back to free-field directions are proposed to explain the large systematic overestimates: in contrast to our lifelong 'free-acoustic field' experience, we are unable, under dichotic conditions (hence in the absence of complete pinna cues and possibly room acoustics), to use the either innate or, more likely, acquired quarter of a sine interaural difference function necessary to accomplish the correct transfer.