Comparative study of fixed time versus intensity trade and fixed intensity versus time trade tests in sound lateralization

Objective In the perception of sound lateralization a sound source exists in the side that the sound reaches an ear earlier in time or louder in intensity than the other ear. It is an imaginary phenomenon where the direction of sound is lateralized by an interaural time difference (ITD) and an interaural intensity difference (IID) of sound by both ears. It is speculated from pathophysiological findings that ITD and IID are processed by different pathways, but it has been not yet proved which of them is predominant in sound lateralization. The time difference can be traded by the intensity difference (fixed time versus intensity trade), and vice versa (fixed intensity versus time trade). In order to investigate predominance in this trade, we measured possible differences in effects by ITD and IID using two opposite trade phenomena of time versus intensity trade and intensity versus time trade. Methods The fixed time versus intensity trade test for selected narrow-band noise was investigated in 30 subjects with normal hearing. Using headphones, the subjects were instructed to push the button when the sound bias generated by fixed ITD (4 dB, 6 dB, 8 dB, 10 dB, 12 dB, 14 dB, 16 dB, 18 dB, 20 dB) was traded by gradually increased IID. The plot figure with fixed ITD and required IID to trade was made. It was compared with similar fixed intensity versus time trade test in 12 subjects with normal hearing. The fixed ITD (200–600 μs) was traded by gradually increased IID and the plot figure was made. Results In fixed time versus intensity trade test, each fixed ITD could be traded with an average of 6 dB of IID. On the other hand, in fixed intensity versus time trade test, ITD required to trade fixed IID increased, in proportion to fixed IID increased. Conclusion It is concluded that predominance exists. In fixed time versus intensity trade test, the uniform amount of IID is required to trade the different ITD. However, in the novel fixed intensity versus time trade test, the phenomenon was in completely different manner that ITD in proportion to the given IID is required to trade.