The following 4 part series of blogs, overviewing the phenomenon of masking, is written for the musician, and not the audiologist. The first three parts (upwards spread of masking, downwards spread of masking, and temporal masking) relate to the function and structure of the cochlea and associated neural structures, whereas the last part (phase) refers to the acoustics of any room. Strictly speaking, phase issues are not related to “masking” in the typical sense but can be viewed as masking in a more general sense, since it can be responsible for a deletion of important information. In part 1, the characteristics of upwards spread of masking were discussed. In part 2, the phenomenon of downwards spread of masking was discussed. In this part, the characteristics of temporal masking are discussed.
Temporal masking sounds like something out of a Star Trek episode where some time travelling alien has altered the time line and things have changed. The one thing about “time” is that it is linear. Time moves forward at a well-define pace and, short of a black hole or other gravitational threat, cannot be altered.
Despite the great-sounding name of “temporal masking” however, the reality is not so interesting, but it does occur.
Temporal masking is a favorite topic of science fiction…. well maybe temporal infarctions but anything with the word ‘temporal’ in it sounds neat.
The time taken to travel to the local store from your home is about the same- it can however change if you speed (which I am constantly told by my local constabulary that I should not), or if there is some other obstruction that will delay you.
Despite a given distance, the time of arrival can be quite different and this is what happens in our auditory system.
In the case of “upwards spread of masking” in part 1, or its closely related but distorted cousin, “downwards spread of masking” in part 2, these two phenomena were related to the function and the structure of the cochlea.
In the case of “temporal masking” this phenomenon is related to the structure and the function of nerves- in this case the VIII- auditory nerve that carries electrical impulses from the cochlea to the auditory cortex.
There is a well-researched body of evidence, and in fact a mathematical equation, that relates the diameter of the nerve, and whether it has myelin covering (a sheath, much like wire insulation) that defines these parameters with the neural conduction rate or speed of propagation of the nerve impulses. Generally the thicker, and the better insulated a nerve is, the quicker rate of propagation.
Courtesy of www.theodessyonline.com
Since all humans are human, this rate should be rather similar from person to person. There are some pathologies such as Multiple Sclerosis (MS) where there is a gradual de-myelination process that will slow down the speed of the neural impulse and it is therefore no surprise that some of the symptoms of MS are auditory in nature.
Another factor that impinges on neural conduction rate is the sound level of the speech and music- higher sound levels result in quicker neural conduction rates than quieter sounds- vowels and forte musical notes reach the auditory cortex before quieter consonants and quieter pianissimo level musical notes, given the same starting point.
This results in one sound arriving in the brain before another if it is significantly louder than a co-incident sound. This temporal alteration occurs in the normally functioning ear and like the other forms of masking can serve to cover up elements of temporally adjacent sounds.
This is a neural form of masking and unlike the spread of masking mentioned in parts 1 and 2 of this blog series, does not derive from the cochlear structure and function.
Like other forms of masking, too much temporal masking may be a deleterious as too little.
In part 4 of this blog series, we will discuss a fourth type of alteration that to a certain extent can be desirable at one level, but disastrous at another.
