The procedure is split into three main stages: the start of the trial and sound presentation, the presentation of a reward and the end of the trial. A trial shall be discussed in a step by step process.
Beginning of a Trial and Stimuli Presentation
Responses were judged based on the responses the children gave. By doing this, no button presses or other forms of indication were required and as a result the method can be used with a wide age range of children. One of the issues with developing a method which can be used with children of a wide age range is the types of instructions given to them. For this reason the procedure was re- quired to produce head turn responses towards auditory stimulus without giv- ing instructions to the participant. During pilot testing it was found that when testing normal hearing older children (three plus years) some instructions were required in order to make the children at ease when first visiting the AEC. As a result a simple instruction of′f ind the animal′ was given before the children began their first testing session. For all children under three years of age, no instructions were given. This approach proved successful. The procedure also did not use any form of conditioning so that exposure to the task was limited
enough to sit on their own. If sat on their own, in order to reduce any anxiety the child might have had about the AEC, the parent was to take the child into the AEC and then stand at the back, out of sight of the child, and asked to re- main silent throughout testing.
The method uses no experimenter inside the AEC. The child is engaged in the task using cartoon characters projected in front of and to the sides of the child. A trial begins with a character flashing twice (on and off) at midline for a du- ration of two seconds (on/off time of 500ms). After the second disappearance of the character, a short random delay (approximately 0-100ms) was added so that the child could not learn the exact time the stimulus were presented. After this delay, the sound was presented. The sounds were designed to be as easy to localise as possible. This required them to be above threshold, have a wide frequency range and be modulated in accordance with the literature. The stim- ulus was a child speaking animals name (dog, pig, sheep, goat and sun). The animal names related to the rewards which would be presented. The stimuli were around 1500ms long and presented at 60dB SPL with a random level-rove of±6dB SPL. The stimuli were presented randomly from one of five speaker lo- cations (0◦,±30◦and±70◦) along the azimuth, positive speaker locations were to the right hand side of the child (see Figure 3.1).
Response Judgment
After presentation of an auditory stimulus a response by the child to the stim- ulus was looked for. Responses were judged from a live video feed from inside
Figure 3.1:An overhead schematic of the AEC. The five red speakers correspond to the directions used in the experiment.
the AEC. The video feed allowed the judges to see the front and both sides of the children’s head, eyes and torso. Sound presentation was initiated by the experimenters and the direction of the auditory stimuli were known to them. A response was marked as either correct or incorrect and then categorised to allow for the evaluation of the method. If a response was correct then it cate- gorised into one of the following criteria:
• Head turn towards the sound source from which the stimulus was pre- sented
presented
If a response was scored as incorrect it was then categorised into one of the following:
• Head turn not towards the sound source from which the stimulus was presented
• Eye movement not towards the sound source from which the stimulus was presented
• Hand pointing not towards the sound source from which the stimulus was presented
• Null response.
To reduce errors in the response judgments, the number of response criteria was kept low so that correct and incorrect trials were easy to identify and cat- egorise. The number of speaker locations were kept small for similar reasons. Using the frontal camera, it was possible to judge which side the child turned to and using the side cameras it was possible to see which speaker the child was turning to. Experimenter bias was reduced by keeping the types of responses classified low (head, hand and eye) and by using two experimenters.
Judgments were made shortly after sound presentation by the two experimenters independently and recorded on sheets containing the trial number and response options. If there was a conflict in judgment a ‘null response’ was scored, this was to maintain the flow of the testing and not hold up a trial by deciding what
response was observed. A trial was also scored a ‘null response’ when the child did not seem to be trying to localise the sound, for example, when a sound was presented and the child turned to their parent/guardian (if sat holding the child or sat at the back of the AEC), the child started crying, or tried to get off their parents/guardians lap.
Blocks were terminated early if five no trials were observed in a row, this was to reduce the stress on the child if they were not responding and also provided an indication of how effective the method was without trying for excessive amounts of time.
Reward Presentation
A correct response to the sound resulted in an animated (spinning or waving) visual reward being presented. If the child produced an incorrect responses, the visual reward was still presented but remained static, i.e. it was not ani- mated. Where the visual reward was presented in the visual environment in reference to the auditory stimulus is a topic of interest and will be discussed in the following sections. The visual reward was presented (static or animated) for 2000ms. A short time was used so that exposure to the reward would not reduce the number of responses (as shown by Culpepper et al. [82]). Thomp-
corpus). The rewards presented are dependent on the auditory stimulus, i.e. the word dog, resulted in a picture of a dog being presented.
End of a Trial
At this point in the trial, it is assumed the child has turned to the sound pre- sented off midline (zero degrees). With each trial starting at midline, it is im- portant to get the child looking back there before the next trial can begin. To encourage the child to move back to midline the visual reward, after being pre- sented, scrolls back across the screens to the midline (front).
During the testing of children aged between one to five years, it was found that all children tracked the character back to the midline and were ready for the next trial. It is also noted that even if the child responded incorrectly, the scrolling visual reward is still useful for drawing the child’s attention back to the start of a trial. This allowed the experiment to flow trial by trial without the need for a second experimenter inside the AEC capturing the child’s attention at the start of each trial.