MSD from NCR was much lower for NPN than both NPO and 4 kHz conditions. An independent measures t-test used on each condition determined the relationship between obtained MSD from NCR and the MSD from NCR expected by chance (4), evaluated against an alpha level of .01. Difference from chance was significant in both NPN and NPO conditions; t(11) = -29.54, p<.01, two-tailed, and t(11) = -4.70, p<.01, two-tailed, respectively. However, there was no significant difference between the 4kHz condition and chance; t(11) = -.27, p>.01, two-tailed. Details of t-test calculations are presented in Appendix E.
Unexpected significance, of NPO in the MSD from NCR data, prompted additional analysis to examine whether there was a significant difference between NPN and NPO, in the MSD from NCR data, a paired samples t-test was performed. This difference was significant; t(11) = -12.58, p<.01, two-tailed. Details of additional t-test calculations are presented in Appendix F. Discussion The aim of this study was to reproduce aspects of previous experiments, in which the convolutions of the pinnae were demonstrated to affect the accuracy of sound localisation in the vertical plane, in a simpler design.
As hypothesized, it was found that participants had better performance judging vertical sound localisation of spectrally complex sounds than expected by chance. Also, as predicted, participants performed no better than expected by chance when judging the vertical sound localisation of pure tones. However, the third and final hypothesis, that the irregularities of the pinna would facilitate significantly increased vertical sound localisation performance compared to those results expected by chance, was rejected.
It was found, contrary to prediction that both the NPN and NPO conditions performed significantly better than expect by chance in the MSD from NCR data (which took error responses into account). This result initially suggests that the convolutions of the pinnae had no effect on vertical sound localisation. Further analysis was carried out to examine this unexpected occurrence, investigating whether there was a significant difference between vertical sound localisation performance in NPN and NPO conditions. This analysis yielded that there was significantly superior performance in the NPN compared to the NPO condition; establishing that the convolutions of the pinnae did have a significant effect on vertical sound localisation performance, even though the NPO condition performed better than expected by chance.
There are several possible explanations for why the NPO data performed better than expected by chance. In a study undertaken by Hebrank & Wright (1974) it was found that participants who had an earplug placed in their ear canal (before being administered vertical sound localisation trials) did not retain impaired localisation ability for an extended period of time, instead participants became accustom to the device and after several trials were yielding results similar to if they were not wearing the plug at all. These results suggest a practice effect. Even though the participants in the current study were required to have both pinnae totally occluded, the presence of a practice effect as a result of the 50 consecutive trials with the pinnae occluded cannot be overlooked as a possibility.
Another explanation, although few studies relating to vertical localisation have been published on it, is that the 400ms duration of the sound bursts may have been simply too long, overriding the pinnae in its vertical sound localisation role and giving way to other spatial cues such as reflections from interior aspects of the room, such as tables, chairs, walls, floor, ceiling and onlookers (Hartmann, 1983). Because the rooms in which the tests were administered in (university class rooms) were not under anechoic conditions this is a plausible explanation.
The current study possessed several methodological shortcomings which, if managed appropriately in future studies, may alter and add additional insight to the results. These shortcomings included non-anechoic conditions as discussed previously, possibility of practice effects, and the fixed testing positions of the speakers. The possibility of a practice effect as described earlier could have been eliminated if the conditions were administered in a random mixed order (e.g. trial one 4kHz, trial 2 NPN, trial 3 NPO, trial 4 NPN etc.); although this randomization would depend on the type of pinnae occlusion (it would be troublesome to be placing/removing silicon ear impression compound from the pinnae for each trial). Additionally the fixed position of the speakers may have served as a localisation clue, influencing results; as demonstrated in Gardiner and Gardiner (1973) the speakers could be interchanged at random intervals to counteract this effect.
The results of this study confirm previous research that present the pinnae as an important element of vertical sound localisation, as well as illustrating the observation that complex sounds are far more localizable in the vertical plane than pure tones. These finding have the potential to be used in real life situations, for example the use of high frequency sounds in emergency devices such as whistles, alarms and sirens may be more effective than use of low frequency sounds, however further investigation into the horizontal sound localisation plane in addition to the vertical sound localisation plane would be needed.
References
Batteau, D. W. (1967). The role of the pinna in human localization. Proceedings of the Royal Society of London, 168, 158-180. Blauert, J. (1983). Spatial hearing: The psychophysics of human sound localization. Cambridge: MIT Press. Butler, R. A. (1969). Monaural and binaural localization of noise bursts vertically in the median saggital plane. Journal of Auditory Research, 3, 230-235.