THE ROLE OF BINAURAL AND FUNDAMENTAL-FREQUENCY DIFFERENCE CUES IN THEIDENTIFICATION OF CONCURRENTLY PRESENTED VOWELS

The relative importance of voice pitch and interaural difference cues in facilitating the recognition of both of two concurrently presented synthetic vowels was measured. The interaural difference cues used wer e an interaural time difference (400 musec ITD), two magnitudes of int eraural level difference (15 dB and infinite ILD), and a combination o f ITD and ILD (400 musec plus 15 dB). The results are analysed separat ely for those cases where both vowels are identical and those where th ey are different. When the two vowels are different, a voice pitch dif ference of one semitone is found to improve the percentage of correct reports of both vowels by 35.8% on average. However, the use of intera ural difference cues results in an improvement of 11.5% on average whe n there is a voice pitch difference of one semitone, but only a non-si gnificant 0.1% when there is no voice pitch difference. When the two v owels are identical, imposition of either a voice pitch difference or binaural difference reduces performance, in a subtractive manner. It i s argued that the smaller size of the interaural difference effect is not due to a ''ceiling effect'' but is characteristic of the relative importance of the two kinds of cues in this type of experiment. The po ssibility that the improvement due to interaural difference cues may i n fact be due to monaural processing is discussed. A control experimen t is reported for the ITD condition, which suggests binaural processin g does occur for this condition. However, it is not certain whether th e improvement in the ILD condition is due to binaural processing or us e of the improvement in signal-to-noise ratio for a single vowel at ea ch ear.