Crosshearing Tutorial
Crossover is the term used to describe the phenomenon in which sound energy presented to one ear may ultimately stimulate the opposite ear. For instance, if I present a 50 dB air conduction signal to a patient's right ear using standard supraaural earphones, I can expect that the opposite ear will be stimulated with a signal perhaps as great as 10 dB. (More on how to calculate this later.) This phenomenon occurs because sound energy delivered through the earphone actually strikes the skull as it travels into the ear canal. As the energy strikes the skull some of the energy actually causes the skull to vibrate. This vibration is essentially equal at all areas of the skull. Since the cochlea is embedded within the temporal bone, the vibration of the skull will cause vibration of the fluids in both cochleae. Thus, even though the signal is being delivered to only one ear, the non-test ears can potentially be stimulated.
An important point to remember is that crossover does not necessarily mean crosshearing. Crosshearing occurs when the signal that has crossed over the skull is of sufficient intensity to be audible in the opposite ear. From the example above, assume that the 50 dB signal presented to the right ear crossed over and stimulated the left ear at 10 dB and that the threshold for the left ear is O dB. We can deduce from this situation that the 10 dB crossed signal would be audible as it is greater (i.e., more intense) than the individual's threshold in the left ear. In this situation both crossover and crosshearing have taken place.
The phenomena of crossover and crosshearing are of considerable concern in audiometry. When an individual responds to the presentation of a tone it would be nice if we could state with considerable conviction that we know which ear was actually stimulated by the signal!
Crossover is fairly well understood phenomenon. In other words we can predict when crossover is a potential threat to the validity of our testing (i.e., when the opposite ear may be the one actually responding to the signal). Generally, crossover is a potential problem any time the signal being presented through the transducer is of sufficient intensity to cause the skull to vibrate. If we consider the procedures used in bone conduction testing, a vibrator is placed on the skull (either on the mastoid portion of the temporal bone or on the forehead) and energy is introduced to the vibrator. The energy flowing from the vibrator causes the skull to vibrate. This vibrational energy will stimulate the cochlea through one of three mechanisms. If the energy is of sufficient intensity to create a traveling wave and adequately stimulate cochlear hair cells, hearing will occur. The main benefit of bone conduction testing is that by delivering energy to the cochlea through vibration of the skull, we can bypass the air conduction pathways and obtain a clearer picture of the function of the cochlea without contamination by outer and or middle ear processes.
While bone conduction testing is of unquestioned utility, the techniques employed to obtain bone conduction responses (directly vibrating the skull with the transducer) must automatically introduce concern regarding crossover. Remember that crossover will occur any time the signal delivered from the transducer is of sufficient intensity to vibrate the skull. By definition, bone conduction testing involves direct vibration of the skull. When we place a bone conduction vibrator on an individual's skull we cannot be sure which ear is actually being stimulated as the signal can be considered to be stimulating both cochleae. Again, although we will assume that crossover will occur any time we are testing by bone conduction, we cannot assume that crosshearing is taking place. The occurrence of crosshearing depends on the relative sensitivity of the two ears; we will deal with the determination of crosshearing later in this tutorial.
The situation is a little different when dealing with air conduction testing. The earphones traditionally used in air conduction testing sit over the ears. The diaphragm in the middle of the earphone emits an air conducted signal that is 'directed' towards the opening of the ear canal and ultimately to the tympanic membrane. However, as the sound wave travels from the diaphragm to the TM, some of the energy comes in contact with the concha and the cartilaginous and bony portions of the ear canal. This situation provides the means for transferring the energy of the sound wave to the bones of the skull in the form of vibrational energy. However, in contrast to the bone conducted signal, the purpose of the air conducted signal is not to vibrate the skull; its purpose is to vibrate the tympanic membrane and initiate a cochlear response via the air conduction pathway. The impedance of the air in the ear canal is much lower than the impedance of the cartilage and bone lining the ear canal. We know that energy transfer across an impedance boundary such as exists between the air and bone is very inefficient. Thus, an air conducted signal is not a efficient at vibrating the skull as is a bone conducted signal.
In order to cause vibration of the skull, an air conducted signal must be of considerably greater amplitude than a bone conducted signal. Numerous studies have described how intense an air conducted signal must be in order to cause crossover. These values are shown in the table below. Note that this table displays minimum, maximum and average values (in dB) and that the effect is frequency dependent; as we move higher in frequency more energy is generally needed to achieve crossover.
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Other studies have reported comparable results for this phenomenon. When one examines all of the studies that have explored this topic, one finds that the lowest value reported that can cause crossover is around 40 dB. Thus, for clinical purposes we will assume that any air conducted signal of 40 dB or greater can cause crossover.
In discussing this phenomenon, the term traditionally associated with the foregoing discussion is interaural attenuation. Interaural attenuation literally translates to loss of energy as a signal crosses from one ear to the other. In other words, if I present a 50 dB air conducted signal to the right ear and we assume an interaural attenuation of 40 dB we can conclude that a 10 dB signal will ultimately cross to the left ear. The traditional definition is that the 50 dB signal 'lost' 40 dB as it crossed from one side to the other. (This definition is not completely accurate. The signal actually 'lost' very little energy as it crossed the skull. The signal did not have sufficient energy to vibrate the skull until it reached 40 dB. Thus, the first 40 dB of the signal was ineffective (from a crossover standpoint) and only 10 dB of the signal was effective at vibrating the skull. Thus the effective signal transmitted across the skull has an amplitude of 10 dB. Note that no energy was actually 'lost' as the term interaural attenuation suggests. Although the term interaural attenuation is technically inaccurate, the concept has merit in making the phenomenon somewhat more understandable. Therefore, we will use the term interaural attenuation (IA) in discussing the occurrence of crossover.)
From the foregoing discussions we will conclude that the IA for bone conducted signals is O dB and the IA for air conducted signals is 40 dB. Now that we know when crossover is a concern, we need to determine when crossover may be leading to crosshearing. For crossover to occur we must present a signal to one ear (the test ear) and have that signal vibrate the skull with sufficient amplitude that it creates a mechanical response in the cochlea on the opposite side (the non-test ear). Crosshearing will only occur when crossover has occurred; hopefully this should be obvious. We can further define when crosshearing will occur by stating that crosshearing will only occur when the signal that has crossed over has sufficient amplitude that it can be 'heard' by the cochlea in the non-test ear. Thus, to predict if crosshearing will occur in response to a specified signal we need to know how much energy will cross to the non-test ear and if that amount of energy can be heard by the non-test cochlea. The above discussion provided a means of answering the first part of this question. We can predict the strength of the crossed signal by taking the test signal and subtracting the IA value from it. For instance, assume that we are testing the right ear via air conduction and we present a signal at 60 dB HL. We have already decided that we will assume that IA for all air conducted signals is at least 40 dB. Thus, the 60 dB signal will 'lose' 40 dB as it travels from the test ear to the non-test ear. We can assume that the signal crossed to the non-test ear will have an effective amplitude of 20 dB.
Now that we can predict the amplitude of the crossed signal, we have to predict if the individual will be able to hear it. Remember that, by definition, an audiometric threshold is the lowest sound pressure at which an individual can detect the presence of a sound (i.e., hear) at least 50% of the time. Additionally, crosshearing will stimulate the ear via bone conduction so our concern is with the bone conduction threshold of the non-test ear. Therefore, if the amplitude of the crossed signal is greater than or equal to the bone conduction thresholds of the non-test ear then crosshearing is a threat. The formula we use to determine if crosshearing is a threat is:
where ACte air conduction threshold in the test ear, IA is the interaural attenuation value we will be using, and BCnte is the bone conduction threshold in the non-test ear. In terms of the above discussion, the left part of the equation determines the amount of sound that may be crossing over to the non test ear. We then compare that value to the BC threshold in the non-test ear to determine if the crossed signal may be audible.If the above equation evaluates to true then crosshearing is a threat and we will need to begin masking the non-test ear to assure that it will not be able to respond to the crossed signal.
One final point in relation to the above equation involves the bone conduction threshold value that we use in the equation. If we have not yet completed the bone conduction testing but we have air conduction thresholds for both ears we can still use the above equation to determine if crosshearing is a threat. Remember that, at least in theory, the bone conduction thresholds for an ear will not be worse than the air conduction thresholds. Therefore, if I have an air conduction threshold of 40 dB for a given ear, I can be sure that the bone conduction threshold for that ear will be 40 dB or less. If I observe a 40 dB difference between the air conduction threshold in my test ear and the air conduction thresholds in the non-test ear, I can be sure that the difference between the air conduction in the test ear and the bone conduction in the non-test ear will be at least 40. Thus, I can use the air conduction threshold in the non-test ear as an acceptable estimate of the bone conduction threshold in the ear in order to determine if crosshearing is a threat. However, realize that after completing bone conduction testing in the ears, a 40 dB difference between the air conduction in the test ear and bone conduction in the non-test ear may be apparent and I will need to go back and mask at that point.
The threat from crosshearing is a little simpler to deal with when testing bone conduction. Remember that the IA value for bone conduction is considered 0 dB. Thus, crossover is a threat any time the bone conduction threshold in the test ear is greater than or equal to the bone conduction threshold in the non-test ear. (Using the formula above we can substitute the BCte value for the ACte value since we are testing bone conduction. Since IA for bone conduction is 0 dB the formula looks like:
Recognize that since the IA value for bone conduction is 0 this situation should always occur when testing bone conduction. In other words, the better ear will be the one to respond regardless of where I place the bone conduction oscillator. We can conclde then that crosshearing is always a threat with bone conduction testing)
However, we will only be concerned about crosshearing whenever we observe an air-bone gap in the test ear. The reasoning behind this is that with bone conduction testing we are attempting to determine if a suspected hearing loss is conductive (air-bone gaps are present) or sensorineural (no air-bone gaps). If my test ear demonstrates an air-bone gap I would suspect a conductive loss. However, if the bone conduction threshold in the test ear is actually a crossed response I may be making an errant diagnosis. I will need to mask the non-test ear to determine if the initial bone conduction threshold was accurate. If it wasn't and the air-bone gap closes once the non-test ear is masked I will change my diagnosis to a sensorineural loss. If the test ear demonstrates no air-bone gaps in the unmasked condition, masking the non-test ear would make no difference to my diagnosis as masking the non-test ear would never make my bone conduction threshold in the test ear better. This may be a little confusing to you at first but if you spend some time chewing it over it may become quite a bit clearer. (For those of you going into Audiology, I promise that it will become a lot clearer.) The bottom line here is that when testing bone conduction, crosshearing is a threat anytime the bone conduction in the test ear is greater than or equal to the bone conduction in the non-test ear but we are only concerned about crosshearing when I observe an air-bone gap in the test ear.