I can tell you from experience it’s easy to see the effect of polarity when measuring the auditory brainstem response. However, I would not conclude from this that it is important to preserve it.
A little background: The auditory brainstem response (ABR) is a physiologic (as opposed to behavioral) measure which uses scalp electrodes to record the neurological response from acoustic stimuli. Most universal newborn screenings use a version of this method. It’s easiest to record an ABR using a series of acoustic clicks because the whole cochlea responds at the same time. It’s also easier to measure when the patient is asleep because the electrical noise from muscle movement is much lower.
I don’t want to go into measurement details but I can tell you that the waveform resulting from using compression clicks looks different from the one using rarefaction clicks. Most people, by the way, have clearer wave morphology from rarefaction clicks. Honestly, it’s not exactly clear to me why this is the case. It may be purely physiologic, or it may be purely that the two stimuli are not equal in all respects but polarity. I mean, the standard issue Telephonics TDH-49 headphone ain’t perfect. Most often, the series of clicks alternate between compression and rarefaction to average the two resulting waves together.
It is my understanding that the cochlea and some of the nuclei further along the chain preserve polarity information, but it is lost by the time it reaches the brain. I think it gets exchanged for location information when the two ears are compared to each other.
The point is, I think it’s interesting that an objective physiologic measure shows a difference --every time. But at the same time nobody I know of can actually hear the difference. Maybe some of you guys can. I can’t.
