## 4 Discussion
### The Localisation Dominance Effect  
The results of the present experiment suggest that the only variable that had a significant effect on the localisation threshold was ICTD, with delays of both 1 and 10 ms requiring significantly less level reduction than did 0 ms. However, there was no condition whereby ICTD alone was sufficient for the localisation threshold; ICLD was always necessary. This result indicates that the precedence effect, in which an ICTD greater than 1.1 ms between coherent loudspeakers located on the horizontal plane will cause the resultant sound source to be localised at the exact position of the earlier loudspeaker [36], is not a feature of median plane localisation. This agrees with the conclusions reported in [8; 9; 10; 11]. What is suggested, however, is somewhat of a localisation dominance effect, whereby the presence of an ICTD biases localisation towards the earlier loudspeaker. This can be considered as being similar to summing localisation [31] and has been shown to operate in numerous median plane localisation studies [37; 38; 39]. If it is the case that the earlier loudspeaker becomes dominant in determining perceived source location in the median plane, then this might explain why significantly less ICLD was necessary to meet the  
Figure 11: Difference in spectral energy between the main layer only and phantom image conditions for both presentation methods with 0-dB ICLD between the main and height layers and 9.5-dB ICLD (localisation threshold): (**a**) vertical stereophonic condition; (**b**) vertical quadraphonic condition.  
localisation threshold for the 1 and 10 ms ICTDs compared to for the 0 ms condition. It should be noted, however, that higher localisation thresholds as a result of a localisation dominance effect have not been reported in previous localisation threshold experiments, with both [8] and [9] reporting that there was no significant difference between the localisation thresholds in the range of 0-10 ms.  
The data provided in Experiment Two enables further analysis as to whether or not the significant effect of ICTD was related to the operation of a localisation dominance effect. Figure 13 shows the experimental data for the main and height layer-only conditions alongside those for the 0-dB ICLD conditions (both 0- and 1-ms ICTD). The median perceived elevation for each has been plotted with notch edges. From the results, it is clear that there is no evidence to support the existence of a localisation dominance effect, with the median perceived elevation for all stimuli increasing in the presence of an ICTD. This result is somewhat similar to those of a previous study conducted by the authors [11], in which the perceived elevation of broadband pink noise presented from vertically-arranged stereophonic loudspeakers in anechoic conditions increased as the ICTD increased from 0 to 1 ms. It should also be noted in the present study that the differences between the 0- and 1-ms conditions were not significant. Based on these results, the hypothesis that the localisation thresholds are higher in the presence of an ICTD due to the operation of a localisation dominance effect can be rejected. As a consequence of this, further study would be required in order to adequately explain this result.  
Figure 12: Illustration to show how presentation method would not affect localisation thresholds despite the presence of the phantom image elevation effect.  
Figure 13: Medians and associated notch edges for the verification test results arranged for the analysis of the localisation dominance effect.