Interaural Time Processing When Stimulus Bandwidth Differs at the Two Ears



Fig. 28.1
The relationship of the noise spectra at each ear (L and R) used in Brown and Yost (2011; left panel) and the current study (right panel). In Brown and Yost (2011), the upper cutoff of the band-pass filter was increased at the right ear relative to the left ear. In the current study, the center frequency (CF) of the band of noise in the right ear was shifted upward in frequency relative to the CF of the noise band in the left ear



In this chapter, we used band-pass filtered noises at each ear as shown in the right panel of Fig. 28.1. The bandwidth of the noise was kept constant at both ears, and the center frequency of the noise band at the right ear was shifted upward relative to that at the left ear, reducing the area of interaural spectral overlap (ISO) and increasing the spectral regions of low-frequency (in the left ear) and high-frequency (in the right ear) monotic information. In these conditions the total power and spectrum level is equivalent at both ears in all conditions. These conditions are in contrast to those used in Brown and Yost (2011). There, the overall level in the right ear increased relative to that in the left ear as the upper cutoff frequency of the band-pass noise in the right ear increased, while the ISO region remained constant.



2 Methods



2.1 Listeners


Five listeners with normal hearing participated in the experiment. They wore Sennheiser HD250 headphones while seated in a double-walled sound proof room. Three of the subjects (L1–L3) were the same as in Brown and Yost (2011) and subject L1 was the coauthor, CB. All procedures were approved by the ASU IRB.


2.2 Stimuli


A 200-ms band of noise shaped with a 10-ms raised cosine rise-decay time and centered at 250 Hz was presented to the left ear with a 1/3rd-, 2/3rd-, or 2-octave bandwidth (same center frequency and bandwidths as some of the conditions of Brown and Yost 2011). The overall level of the noise was randomly varied (same at both ears) from trial to trial between 86 and 90 dB SPL. The same noise with the same bandwidth but with different center frequencies was presented to the right ear, and the center frequency was increased in 1/6 octave steps relative to the 250-Hz center-frequency noise in the left ear. Table 28.1 indicates the actual filter cutoffs of the noise bands. The ITDs were whole waveform ITDs, and a different noise was generated for each interval.


Table 28.1
The 27 filter conditions are shown






































































































































































Condition
BW (Oct)
∆CF (Oct)
Center width (Oct)
Band-pass cutoffs (Hz)

Left ear
Right ear

HP
LP
HP
LP

1
1/3
0
2/6
223
281
223
281

2
1/3
1/6
1/6
223
281
250
315

3
1/3
2/6
0
223
281
281
354

4
1/3
3/6
−1/6
223
281
315
397

5
1/3
4/6
−2/6
223
281
354
446

6
1/3
5/6
−3/6
223
281
396
500

7
2/3
0
4/6
198
315
198
315

8
2/3
1/6
3/6
198
315
223
354

9
2/3
2/6
2/6
198
315
250
397

10
2/3
3/6
1/6
198
315
281
446

11
2/3
4/6
0
198
315
315
500

12
2/3
5/6
−1/6
198
315
353
561

13
2/3
6/6
−3/6
198
315
397
630

14
2/3
8/6
−4/6
198
315
500
794

15
2
0
12/6
125
500
125
500

16
2
1/6

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Apr 7, 2017 | Posted by in OTOLARYNGOLOGY | Comments Off on Interaural Time Processing When Stimulus Bandwidth Differs at the Two Ears

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