Different interaural level difference processing with complex sounds and pure tones

Abstract

Interaural level differences (ILDs) are one of the primary cues to sound source position on the horizontal plane. Most studies on human performance with ILDs use simple sounds such as pure tones. However, naturally occurring acoustic stimuli usually have complex waveforms. Here human learning of ILD discrimination was examined with complex waveforms. Sixteen listeners were trained 1 hr/day for 9 days on ILD discrimination with a 4-kHz tone sinusoidally amplitude modulated at 0.3 kHz conveyed through headphones. Before and after training, they were tested, together with sixteen untrained controls, on the trained condition and five related untrained conditions (three with amplitude modulated tones, two with pure tones). The trained listeners improved significantly more than controls on all conditions with amplitude-modulated tones, but not on those with pure tones. The lack of generalization of ILD learning from modulated to pure tones suggests that practice modified ILD processing in a region that encodes complex sounds with a variety of carrier frequencies and modulation rates, but not pure tones. Thus, these findings suggest that waveform complexity is an important factor in ILD processing, and must be considered when evaluating human ILD performance. [Work supported by NIH.]