Spatial perception system

The purpose of this study was to construct a simplified model of previous experiments in which pinnae convolutions were demonstrated to affect the accuracy of sound localisation in the vertical plane. Monash University psychology randomly selected from different laboratory classes formed the data pool of 12. Participants were required to verbally report, after listening to either 4kHz or broadband noise with normal or occluded pinnae, the speaker believed to be the source of the sound. Performance results were compared to results expected by chance. The results confirm the pinnae as an important element of vertical sound localisation, as well as illustrating that spectrally complex sounds are far more localizable in the vertical plane than pure tones.

The external human ear (pinna) and its relationship with localisation of sound has been the basis of much study over the last three decades with limited investigation before this time. It is widely agreed that humans posses a remarkably well developed spatial perception system (Wright, Hebrank, & Wilson, 1974; Hartmann, 1983; Chen, Van Veen, & Hecox, 1992); however until recently it was held that the pinna had only a minor effect on the process of hearing sound, merely acting as a sound funnel to the inner ear (Batteau, 1967). Notwithstanding, recent research studies into sound localisation have established that the pinnae play an important role in the localisation of sound (Batteau, 1967; Butler, 1969; Gardner & Gardner, 1973).

The pinnae are considered by many to be especially important when determining the location of a sound source in the vertical plane (Butler, 1969; Gardner & Gardner, 1973). Batteau (1967) was one of the first to demonstrate the effects of the pinnae on localisation accuracy. This was illustrated by recording sounds, received by small microphones inserted into casts of pinnae (either normal pinnae or occluded pinnae, e.g. placing a small tube in each ear canal and filling the pinnae with clay to prevent sound interacting with its convolutions), then playing the recorded sound back to participants via high-fidelity earphones (sound is heard coming from sources outside the head).

In this study participants were found to be able to make reasonable judgments as to the horizontal and vertical components of the sound when the pinnae were normal; accuracy decreased significantly when the pinnae were occluded. Batteau came to the conclusion that the convolutions of the pinnae resulted in the production of time delay paths, transforming the incoming signal, allowing the listener to predict the source of the sound in horizontal and vertical planes, depending upon the transformation. Further studies have revealed similar results to Batteau (1967); focusing mainly on the spectral transformation caused by the pinnae (by comparing participant judgment from normal pinnae and occluded pinnae conditions) these results support the important role spectral transformations serve in sound localisation in the vertical plane (Gardner & Gardner, 1973; Wright et al, 1974; Blauert, 1983).

Further studies have identified other factors which influence the accuracy of vertical sound localisation. Many have found that when pure tones are used, localisation of sound in the vertical plane is inhibited, however, when spectrally complex sounds (e.g. broadband noise) containing high frequencies are used, localisation of sound is reasonably accurate (Thurlow & Runge, 1967; Gardiner & Gardiner, 1973; Wright et al, 1974; Musicant & Butler; 1984). Musicant and Butler (1984) found that high frequency sounds were crucial for accurate localisation performance, as high frequency sounds undergo elevation-dependant spectral transformations as a result of the structures of the pinnae, using these spectral transformations to judge localisation of sound in the vertical plane.

The present study is based on a simplified modeled of previous experiments in which pinnae convolutions were demonstrated to affect the accuracy of sound localisation in the vertical plane. The primary aim is to demonstrate that participants will have the ability to judge the localisation of spectrally complex sounds, but not pure tones, in the vertical plane; and that these judgments will yield significantly superior performance than those expected by chance. Furthermore, in line with previous research, the irregularities of the pinna are predicted to facilitate a significantly increased vertical sound localisation performance compared to those results expected by chance.

Method Participants Members of a third year psychology course at Monash University participated as part of a laboratory class assessment requirement. The data pool consisted of 12 participants each randomly selected from a different laboratory class. Age and sex of participants was not recorded. None of the participants had a known hearing impairment. See Appendices A-C for raw data.

Materials Five speakers were arranged in a vertical arc, numbered ‘1’ to ‘5’ from top to bottom, to provide a 12 degree separation between speakers equidistant, at a distance of 2 metres, to the participants head. A height adjustable chair was used to counterbalance different heights amongst participants, positioning speaker 3 at the participants’ interaural axis (directly ahead at ear level). The sound generator employed produced sound bursts, of either 4-kHz tone or broadband noise, with durations of 400msec, a rise time of approximately 40ms, and of approximately 70dB (A) SPL at the position of the participants head. The sound stimulus was able to be presented through any one of the 5 speakers on a particular trial though the utilization of a selector switch on the sound generator.

Lengths of rubber tubing (~1.5cm) were used to provide an uninterrupted path to the ear canal when “Otoform” silicon ear impression compound was employed to occlude the pinnae. There was optional use of a blindfold. Procedure The participant, seated in the chair, was administered trials under each of the sound stimulus conditions (4kHz, Noise Normal Pinna, then Noise Pinna Occluded) and verbally reported which speaker they judged the sound stimulus to originated from (“1” to “5”), as each of the five speakers presented the stimulus, in turn, in pseudo-random order (see Response Sheet, Appendix D); their responses were noted on the response sheet. It is important to note that the experimenter started at a different trial number in each condition to prevent the participant from learning the sequence.

Under each condition the participant was given 15 practice trials in which the experimenter gave feedback by indicating whether the response was “correct” or “incorrect” (identifying the correct speaker number if incorrect); practice trial responses were not recorded. Fifty test trials followed for each condition where no feedback was given. In the Noise Pinna Occluded condition the tubing was placed into the ear canals of the participant and the convolutions of the pinnae, around the tubing, filled with Otoform. The trials were then administered as above. Results Means and standard deviations as a function of number of correct responses are presented in Table 1 to test whether spectrally complex sounds with pinna normal can be localized in the vertical plane better than pinna occluded and pure tones, relative to chance.

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