Effects of short-term exposure to sound at very high and ultrasonic frequencies at subthreshold levels; a double-blind provocation study.

The study comprises an initial screening session, a hearing threshold measurement session, and four test conditions. == Asymptomatic participants == The following procedure applies to asymptomatic participants, as identified by questionnaire in the screening session == Hearing threshold level measurement session == In the hearing-threshold measurement session, the hearing threshold level (HTL) of the participant at 20 kHz will be measured using standard psychoacoustic methods with very short bursts of ultrasound (<0.5 sec per presentation). The HTL is required in order to ensure that an inaudible sound can be presented to the participant during the exposure conditions. The maximum sound pressure level used in this measurement will be 105 dB (re 20 microPa). The measurement will be performed in a sound-treated booth using earphones. The level of the sound is that measured at the reference microphone of the appropriate ear simulator according to IEC 60318. The HTL will be re-measured after the test condition to ensure that it has not altered (for example due to changes in earphone placement). If the HTL has reduced by 10 dB or more, then the test session will be deemed invalid, and will be repeated on another day. == Test conditions == Four test conditions will be carried out. Condition A, the “Ultrasound ON” condition: Ultrasound will be presented to the participant; the participant will be told that ultrasound may or may not be present. Condition B, the “Ultrasound OFF” condition: Sham ultrasound (silence) will be presented to the participant; the participant will be told that ultrasound may or may not be present. Condition X and Y; additional information will be displayed to the participant. The comparison of conditions A and B will be used to estimate any genuine effect of ultrasound presence. The details of the four conditions are given below. Condition A, the “Ultrasound ON” condition. The participant will be seated in a sound proof booth with the earphones placed by the tester, and not removed for the duration of the test session (unless requested by the participant). High-frequency sound will be presented to the participant at a frequency of 20 kHz and at a sound-pressure level that is either 88 dB (re 20 microPa) or is 15 dB below the participant's hearing threshold level at 20 kHz, whichever is the lower level. The participant will be exposed for a period of 20 minutes, during which time they will be asked to perform a computer-based visual recognition task, displayed to them via a small screen. Galvanic skin responses will be recorded from electrodes attached to the fingers. After the 20-minutes of exposure, the participant will asked to remain seated for a further 5-minute recovery period. The participant will be asked to complete subjective ratings of the severity of their symptoms relating to the health conditions stated previously. Ratings are performed at three time points: pre-exposure (baseline), immediately following exposure, and at the end of the recovery period. Testing will be postponed if the participant scores higher than “0” (=”no symptoms”) on any of the health conditions in the baseline condition. Condition B, the “Ultrasound OFF” condition. This condition is identical to condition A in all respects except that silence is presented from the loudspeaker. Condition X and Y: These conditions are identical to conditions A and/or B in all respects except that additional information will be displayed to the participant. == Sequence and timing of test conditions == Participants will be randomly allocated to one of four arms, which determines the order in which the four conditions will be conducted: Arm 1: A X B Y Arm 2: A Y B X Arm 3: B X A Y Arm 4: B Y A X Allocation will be by random selection without replacement in blocks of four participants to achieve a counterbalanced design. There will be a maximum of two conditions per day, such that A and B will be separated by approximately 24 hours (at least), and X and Y will also be separated by approximately 24 hours. If two conditions are conducted on one day, there will be a minimum of 2 hours between the end of the first condition and the start of the second. A test condition will be postponed if the participant scores higher than “0” (=”no symptoms”) on any of the health conditions in the baseline condition. == Conducting the test == For all conditions, the sound will be presented from a computer in an observation room adjacent to thesound-treated both. The tester has no influence on the sounds being presented, and no way of telling whether ultrasound is being presented or not in conditions A and B. After starting the software run, the tester inputs the participant number only. The software uses a random number generator to determine which arm the participant is allocated to, and thus whether genuine ultrasound or sham exposure (silence) is presented in any session. The key indicating which participant was allocated to which arm is stored in a coded computer file that is not opened until after the experimental phase of the study has finished, and the statistical analysis has been completed. Conditions X and Y are not blind to the tester as they are able to see the same display as the participant. == Symptomatic participants == For symptomatic participants, the above procedure will be adapted as follows. They will be asked about the time course of the symptoms they have previously experienced. If the estimated onset and recovery time of these allows, then the exposure time and time between test conditions will be reduced. Additional repeats of the set of four conditions will also be conducted.