The overall goal of this procedure is to measure hearing sensitivity in two to four year old children. Children are trained to perform play-based responses when they hear a target signal. Thresholds are then estimated in a two interval, two alternative forced choice paradigm based on observations of the child's behaviors.
It is difficult to obtain reliable estimates of behavioral hearing thresholds from toddlers and preschoolers. Consequently, there are significant gaps in our understanding of human auditory development between infancy and early school age. The method described here efficiently estimates hearing thresholds in toddlers and preschoolers using a play-based, two interval, forced choice behavioral testing procedure.
The main advantage for using a forced choice paradigm is that it guards against some forms of response bias, thus permitting direct comparisons across different ages and/or stimuli using an efficient adaptive procedure. This method can be used to answer key questions about how the auditory system changes during the toddler and preschooler years, a time period where we know little about auditory behavior. Testing is performed in a sound isolating testing room.
The room is configured with a table and chairs. The parent sits here and the child sits here. Testing toddlers and preschoolers requires two experimenters, an assistant who sits inside of the booth with the child and an observer who is in the adjacent control room.
The testing space is equipped with a two way communication system between the observer and the assistant. The assistant inside of the booth trains the child to perform a play-based motor response whenever the target sound is heard. Re-instruction is provided whenever necessary throughout testing.
The assistant socially engages the child and may elect to change the game the child is playing based on the child's level of interest. The assistant provides social reinforcement following trials in which the child performs the targeted motor response. The observer outside of the booth watches the child and assistant inside of the booth through a window.
The observer initiates test trials and indicates whether the child heard the sound in the first or second listening interval, based solely on the child's responses. In this example, custom software controls a realtime processor, which routs the sound to a transducer. The signal is a 1, 000 hertz warble tone with a duration of 500 milliseconds that is presented through an appropriately calibrated insert earphone.
The stimulus sounds like this. The software controls the timing of each trial, which contains two observation intervals that are separated by an inter-stimulus interval. In this example, each observation interval is 1065 milliseconds in duration.
The intervals are separated with a 300 millisecond inter-stimulus interval. Thus, the total duration of each trial is 2, 430 milliseconds. From the observer's perspective, a testing trial looks like this.
One, two. Both the observer and the assistant hear an audio prompt indicating the two intervals. The observer can also see a visual display indicating the two intervals.
Neither the assistant nor the observer is given an indication of which interval contains the signal. The observer uses these time lock interval indicators in conjunction with observations of the listener's behavior to determine which interval contained the target sound. At the end of each trial, the observer is prompted to select which interval contained the signal.
Feedback is provided to the observer after each trial. The testing protocol consists of three stages. The purpose of stage one, the operant conditioning stage, is to teach the listener to perform a play-based motor response as soon as he or she hears the signal.
In this stage, the signal is always presented at a clearly audible level and always in interval two. After three trials in stage one, the observer is prompted to either repeat stage one or initiate stage two. The purpose of stage two, the verification stage, is to ensure that the observer can reliably judge the interval containing the signal based solely on the child's behavior.
The signal occurs in either interval one or interval two, with equal a priori probability. Stage two continues until the observer correctly judges the observation interval containing the signal on four out of five consecutive trials. In stage three, experimental data are collected.
In stage three, the signal occurs in either interval one or interval two, with equal a priori probability. The present example uses a two down, one up adaptive tracking procedure to estimate the signal level associated with 71%correct detection. Probe trials are also presented in stage three.
A probe trial is a signal presented at the same clearly audible level used during training. Probe trials are randomly presented on two out of every 12 trials. Responses on probe trials reflect consistency of responses to supra-threshold signals across the testing session.
For toddlers and preschoolers, the assistant provides social reinforcement to help keep the child motivated and focused on the task. If additional reinforcement is desired at any time during testing, the software also controls two mechanical toys with lights that are located in dark Plexiglas boxes in the testing room. In the previous section, we described the key features of the custom software used to control the experiment.
In this section, we will discuss how children are tested during a session and provide further details about the role of the assistant and of the observer. Prior to the child's arrival to the laboratory, the environment is prepared, including selecting four to six games for the testing session and a listing check. As soon as the child and parent arrive, informed consent for participation in the study is obtained.
The child and parent enter the booth. Children typically prefer for a parent to be sitting next to them inside of the booth, and in some cases, the child will sit in the parent's lap. Using child friendly language, an overview of the task is given to the child and the transducer is placed.
Prior to presenting the first trial, the assistant gives the child a toy. The child is trained to place the object against his or her torso or cheek. This is called the ready position.
Throughout testing, the observer seated outside of the booth only initiates a trial if the child is in the ready position. The observer initiates the conditioning stage, stage one. The goal of this stage is to condition the child to make a clear, time-locked motor response when he or she hears the signal.
To facilitate learning the task, the assistant and child listen together initially. The assistant guides the child's responses using a hand over hand technique. Recall that the signal is always presented in interval two during this stage of testing, so the assistant can use the auditory interval markers to appropriately time this response.
Assistance gradually reduces as the child demonstrates mastery of the task. Stage one is continued until the observer determines that one, the child is able to perform the motor task without assistance, and two, the child is able to make a fast, time-locked response to the signal that the observer can judge. A minimum of three trials is required, but stage one continues until the observer is confident that the child's responses are under stimulus control.
Stage two of testing is initiated by the observer. In this stage, neither the observer nor the assistant knows which interval contains the signal. The observer must accurately judge four out of five trials to successfully complete this stage.
The child is considered trained with the successful completion of stage one and stage two. If needed, the testing session can be suspended at this point. In stage three, experimental data are collected.
As in stage two, the observer continues to closely watch the listener and judge which interval contained the signal. In addition to performing the motor-based play activity, children often exhibit subtle behaviors to sound, including eye or head movements, changes in facial expression and quieting of activity. The observer can use these subtle behaviors to judge which interval contained the signal when the signal level is near a child's threshold.
The assistant switches the game every 10 to 15 trials to maximize the child's interest and reduce the risk of habituation. The assistant provides frequent social reinforcement to the child throughout testing to engage the child and to shape and maintain the child's behavior. At the conclusion of testing, a tympanogram is completed to verify normal middle ear function.
The participant is compensated and a followup visit is scheduled if needed. This is an example of an adapted track from a child who is four years and 11 months of age. Black circles are probe trials.
Blue and yellow circles are signal trials, with yellow circles indicating reversals. Three features of the track are examined. First, the detection threshold is calculated.
Threshold is the average of the signal level at the last six reversals of the adapted track. Second, the standard deviation of the last six reversal levels is also computed. In order for a threshold to be considered valid, the standard deviation must be below a criterion established based on pilot data.
Third, the hit rate for probe trials is calculated by determining the proportion of probe signal trials that were correctly judged by the observer during stage three of testing. In order for a threshold to be considered valid, listeners must detect 80%of probe trials. 33 children were recruited.
Exclusion criteria included a known history of hearing loss, more than three episodes of otitis media or an episode of otitis media in the month prior to testing. This group included eight two year olds, 18 three year olds and seven four year olds. Shown in blue in the figure, a threshold was measured for 63%of two year olds, 94%of three year olds and 100%of four year olds.
Thresholds were not obtained for four children. One child did not successfully complete training and three others would not tolerate the insert earphone. This figure shows individual children's thresholds as a function of age for the warble tone stimulus in quiet.
All listeners shown by the open circles had normal middle ear function based on tympanometry on the day of testing. Listeners shown by filled symbols had abnormal or unknown middle ear status, and their results should be interpreted cautiously. The thresholds obtained from children with normal middle ear status are broadly consistent with previous data on this age group and for this stimulus.
The play-based two interval procedure described here provides reliable behavioral data on the hearing abilities of toddlers and preschoolers. There are several advantages associated with this method. The first advantage is that it can be used to test children who are developmentally as young as two years of age.
The second advantage of this method is that it guards against some forms of response bias. The observer must base his or her judgments solely based on the child's behavior, so prior expectations regarding the child's sensitivity or behavior do not affect the results. The third advantage of this method is that it may be easier for the observer to judge which interval contained the signal than to make a yes/no decision regarding the presence or absence of the signal as is typically required in other paradigms.
This procedure can be modified to test children in complex listening conditions, such as detecting speech and noise. It may also be possible to modify this procedure to test children who are unable to perform conventional motor responses. Thus, this method provides a valuable tool so that researchers can study how the auditory system changes during the toddler and preschooler years.
