Raising the Bar: What is Normal Hearing?
Normal hearing sensitivity has been defined as being able to hear a range of frequencies at a loudness between 0-25 decibels. Logical thinking says that if your hearing is in this range your ability to understand speech would require little effort in higher areas of the brain. New research is questioning this.
In a study by eNeuro, they monitored the brain activity of adults aged 18-41 years while they listened to spoken sentences and answered a series of questions in relation to these sentences.
The sentences used were varied in their complexity (the level of syntactic processing demands - which is the order and arrangement of words in the sentences provided).
The purpose of this study was to investigate how a person with ‘normal hearing’ and speech comprehension processed spoken sentences and to see if these adults who believe they have ‘normal hearing’ have actually developed some form of hearing loss.
For the study, there were 35 active participants ranging from ages 18 to 41.
All group members were native speakers of American English and none of them were aware of any history of any neurologic disorders and all reported normal hearing.
Audiometric testing determined that all participant’s hearing fell within the range that is considered clinically ‘normal’ which is 0-25dB.
Participants underwent 4 different fMRI (functional magnetic resonance imaging) tests and were given tasks involving spoken sentences and unintelligible noise.
There were 24 base sentences created for this procedure and each sentence included a male or a female character who was doing an action. The action involved a recipient and the job of the participant was to figure out which character in the sentence was doing the action.
Participants were asked to identify the gender of the character performing the action. Indicating the gender of the actor allowed the team to be able to monitor the participants’ level of comprehension and alertness during the fMRI scan.
For example, one of the sentences posed was:
“Kings that appreciate queens with three black horses are good.”
They were asked to find the character who was performing an action in the above sentence and answer what it’s gender was. In the case of this sentence, the actor is the king and the answer is male.
The participants entered their answers by quickly and accurately clicking a button that corresponded with their answers.
Despite the accuracy being quite high, there were still some errors made by participants throughout the test. These mistakes corresponded with increased activity in the area of the brain responsible for attention and accuracy.
The effort needed to hear and process speech is much greater with a loss of hearing sensitivity because it pulls resources from other areas of the brain that are not typically used for hearing.
Researchers discovered the activity in the part of the brain responsible for attention changed depending on the individual’s hearing ability.
The participants with poorer hearing (which was still considered normal) were showing a greater amount of activity in this area of the brain when they successfully understood a sentence. This was because they were using it to pay attention and keep focused on what was being said.
The results of this study suggest that even the slightest variation in the ability to hear can have a significant impact on the areas of the brain that are supporting auditory speech and sentence comprehension.
Meaning, even with what we consider normal hearing a loss of hearing sensitivity can cause the individual to put more strain on the area of their brain used for breaking down a sentence and understanding the meaning behind it. This leads to increased effort and pulls resources from other areas of the brain.
This research may lead us to more sensitive ways of measuring hearing.
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