Head Movement

The Effects of Head Movement and Pinnae Distortion on Sound Localisation

Abstract:
Human’s ability to locate sounds on their midline can be effected by the distortion of the pinnae, as well as the restriction of head movement. The study investigated the effects of head moment and pinnae distortion on sound localisation through the presentation of a sound stimulus along three points of the participants midline, in front, above or behind the participants head. The study was carried out under four conditions including having a normal pinnae with no head movement, a distorted pinnae with no head movement, a normal pinnae with head movement and a distorted pinnae with head movement. It was hypothesised that head movement being allowed, in comparison
to head movement being restricted, will in turn lead to a higher rate of correct sound localisations. It was further hypothesised that the distortion of the participants pinnae, will lead to a lower number or correct sound localisations, compared to a normal pinnae. The study supported these hypotheses.

The Effects of Head Movement and Pinnae Distortion on Sound Localisation
Can you hear that clock ticking beside you? How is it that you can locate where the sound is coming from without turning your head? A human’s ability to locate a sound stimulus in their environment relies on both binaural and monaural cues. Monaural cues use information from one ear, the way a sound is distorted by the pinnae, before entering the canal. However binaural cues use information sourced from both ears, through the use of interaural intensity differences (IID) and interaural time differences (ITD) to locate a sound stimulus. The duplex theory provides explanation of the use of IIDs and ITDs in locating sound. ITDs is the difference in arrival times of a sound to each ear, if a sound is presented to the left of someone, the sound is going to reach the persons left ear before the right, the sound therefore being recognized as coming from the left side. Similarily, the IID, helps locate sounds through the idea that a sound presented to the left of someone, will have a higher intensity when it reaches the left ear than the right. Sounds presented on the midline, however, do not allow the use of binaural cues, and rely solely on monaural cues created by the pinnae. In the present study, the effects of pinnae distortion and head movement on the ability to locate sound were investigated. Participants ability to locate sound stimulus presented along varying positions along their midlines were tested under four different conditions. It was therefore hypothesised that head movement being allowed, in comparison to head movement being restricted, will in turn lead to a higher rate of correct sound localisations. It was further hypothesised that the distortion of the participants pinnae, will lead to a lower number or correct sound localisations, compared to a normal pinnae.

Method:
Participants:
The participants of the study included 186 first-year Psychology students from Australian Catholic University, Brisbane, Melbourne and Strathfield campuses.
Participants attended their allocated weekly tutorial where the study was carried out. The sample consisted of 34.9% males and 65.1% females, and had a mean age of 20.24(SD = 5.65).
Materials:
Both the scorer and experimenter were supplied with instructions; the scorer was also supplied with a score sheet to record correct sound localisations made by the participant. The experimenter was instructed to shake a set of keys in a pre determined sequence one metre above the participants head in three separate positions whilst the participant was sitting in a seat below. All participants experienced the same sequence of locations and were marked by the scorer for incorrect and correct localisations of the sound source.
Procedure:
The participant was instructed by the experimenter to be seated, where the experiment would begin.
The experimenter was given a list of sequenced positions in which they would shake a set of keys for two seconds at equal distances of approximately one metre away from the participants head, either behind, above or in front. The experiment was carried out under four conditions, the conditions included having a normal pinnae with no head movement, a distorted pinnae with no head movement, a normal pinnae with head movement and a distorted pinnae with head movement. Under the first condition the participant was required to shut their eyes and keep their head still, whilst the experimenter shook the keys in a sequenced order. After each time the keys were shaken, the participant was then required to state the position they thought the keys were being shaken at, incorrect and correct localisations were then recorded by the scorer in a table. This was then repeated for the remaining positions and results were recorded. Participant was then asked to relax before beginning the second condition. In the second condition, the participant was instructed to distort their pinnae, by pinching in the top and the middle of the outer ear, whilst making sure that the ear canal was not covered, whilst still closing their eyes and maintaining no head movement. The sequence of keys being shaken was then carried out and results were recorded. The third condition required the participant to keep
there eyes closed, with normal pinnae(undistorted), however were able to move their head if they wished, the experimenter then went through the sequence and results were recorded. Condition four included the participant distorting their ear as described in condition two aswell as being able to move head. Results were recorded and the study was complete.
Results:
The mean number of correct sound localisations was calculated for each of the four conditions. These means are presented in Figure 1. As can be seen in the Figure 1, the mean number of correct sound localisations in the two normal pinnae conditions were significantly higher than the number recalled in the two distorted pinnae conditions (F(1,185)=430.05.55, p