Sustento teórico

Thresholds in all studies were estimated using an adaptive three Alternative Forced Choice (3AFC) task with feedback. The adaptive 3AFC method of obtaining

psychometric thresholds, also referred to as the “oddball” or “odd-man-out” paradigm, has been used extensively with children and adults. Instructions for the task are simple and easily understood by children. The listener is presented with three signals in series. Two signals are identical (or standard signals) and a third is the target. The third is different from the standards. The target can occur in any of the three intervals with equal a priori probability. The target stimulus is varied on the feature of interest in such a way that the just noticeable difference threshold can be identified.

A two-down, one-up adaptive procedure as described by Levitt (1971), tracking the 70.7% correct response level, was employed. With this procedure the signal feature of interest in the target is reduced (the difference between the target and standard is

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decreased) after two consecutive correct responses or increased (the difference between the target and standard is made larger) following one incorrect response. For the signal feature being evaluated, the starting value for the target was very different from the value in the standard signal so that the target signal could be easily detected by all listeners. The target signal feature was varied adaptively with an initial large step size until the first reversal. A reversal is the trial response which meets the criteria for a change in the direction of the adaptive procedure that increases or decreases the distance of the target from the standard signals. Each subsequent reversal resulted in a change of the increase or decrease step size by the target toward or from the standard signal until an established minimum was achieved. The first reversal point was not included in the threshold calculation.

The forced choice paradigm employed in this study was presented in a video game format similar to the one developed by Wightman et al. (1989). A number of different animated and colourful graphics were available to assist in maintaining participant interest in the task. During each trial, the listener was presented with a 3 item visual graphic on the touchscreen monitor. Graphics included flowers, rain clouds, fish, clowns, or balloons in the foreground and the background was a scene appropriate to the item in the foreground or a solid colour with no graphics. Initiation of every trial was clearly indicated by the sequential appearance of the three identical foreground graphic items. In succession, each graphic changed colour or animation to indicate signal presentation (one target and two standard signals). The listener’s task was to touch the graphic they believe corresponded with the target signal presentation. The target stimulus was presented in either the first, second, or third position and the computer employed an a priori probability of 0.33. Following the listener’s selection, feedback was provided for that trial. The graphic items then exited the screen, clearly marking the end of the trial at which point the next trial would commence. For each block of trials a small indicator would track progress by moving from right to left horizontally or from bottom to top vertically, allowing the children to easily identify how far they had advanced in the task. Figure 4 shows an example of the series of computer graphics employed for the listening tasks.

Figure 4 Sample screen

elements of the signal encoding task

backdrop (left); three fish swimming into view

begin (middle); tone presentation associated with the fish opening mouth (right). The lower three slides demonstrate the bouncing, smiling fish that is offered as positive feedback for a correct response (left); fish swimming out of view indicating the end of the trial (middle); end of the block with the crab that travelled from left to right as an indication of the block progression.

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Sample screen-shots from one of the video game graphics show different elements of the signal encoding task. The upper three slides demonstrate

fish swimming into view indicating that a new trial is about to (middle); tone presentation associated with the fish opening mouth (right). The lower three slides demonstrate the bouncing, smiling fish that is offered as positive

rect response (left); fish swimming out of view indicating the end of the trial (middle); end of the block with the crab that travelled from left to right as an indication of the block progression.

w different upper three slides demonstrate the opening

indicating that a new trial is about to (middle); tone presentation associated with the fish opening mouth (right). The lower three slides demonstrate the bouncing, smiling fish that is offered as positive

rect response (left); fish swimming out of view indicating the end of the trial (middle); end of the block with the crab that travelled from left to right as an

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During testing the listener and examiner were seated comfortably at a small table in the sound isolation room. The touchscreen monitor was located on the table facing the listener. The listener was instructed that their task was to watch the monitor, listen carefully and then touch the graphic on the monitor that “sounded different”. An Etymotic Research ER-3 earphone with a foam-tip coupler was placed in the listener’s right ear and an E.A.R. soft regular size earplug was placed in the left ear to reduce the interference of any possible extraneous HVAC noise during the test session. Listeners completed a minimum of three blocks for each different test condition. Listeners were administered additional blocks of trials if there was a technological malfunction or if the block was not completed. The researcher sat with the listeners and monitored the

progress of trial blocks and performance. Trial blocks were discontinued if requested by the listener. Trial blocks were also discontinued if the listener and/or examiner reported a problem with the test signals. For example, the child or examiner may have reported that the signals could no longer be heard or that the signals didn’t sound the way they had previously. By monitoring the number of correct or incorrect target identifications made by the listener, the examiner was able to discontinue and restart the block of trials if the listener was clearly being inattentive to the task or was not following task

instructions. The ability to discontinue and restart trial blocks provided the opportunity to reduce the likelihood that inattention contributed to poor thresholds. Each block was composed of 30 trials and enabled an estimation of threshold. For each study the

listener completed a minimum total of 90 trials organized into 3 blocks. Participants had a scheduled break during the session between measures and were allowed to take

additional rest breaks upon request. Actual test time took approximately 20 minutes for each condition which included three blocks of thirty trials. Following each measure the participants were commended for the successful completion of the task and, in

compliance with the approved protocol, they were given a small toy or school supply as thanks for their participation in the study upon completion of all measures.

Thresholds were calculated upon completion of trial blocks. The threshold for a block of trials was obtained from an average of the midpoints between the reversal points in the block of trials. Trial blocks were considered for threshold calculation if a minimum

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of 4 reversal points were achieved. In this project, all participants achieved a minimum of 4 reversals on all trial blocks. Thresholds for each block of trials were averaged to achieve a single threshold for each condition completed by the listener.