Scientists begin to map hearing loss in the brain.


Prolonged exposure to loud noise alters how the brain processes speech, potentially increasing the difficulty in distinguishing speech sounds, according to neuroscientists at The University of Texas at Dallas.  The researchers demonstrated for the first time how noise-induced hearing loss affects the brain’s recognition of speech sounds.

Noise-induced hearing loss (NIHL) reaches all corners of the population, affecting an estimated 15 percent of Americans between the ages of 20 and 69, according to the National Institute of Deafness and Other Communication Disorders (NIDCD).  Exposure to intensely loud sounds leads to permanent damage of the hair cells, which act as sound receivers in the ear. Once damaged, the hair cells do not grow back, leading to NIHL.

As the populace have made machines and electronic devices more powerful, the potential to cause permanent damage has grown tremendously.  Even the smaller MP3 players can reach volume levels that are highly damaging to the ear in a matter of minutes.

Before the study, scientists had not clearly understood the direct effects of NIHL on how the brain responds to speech.  To simulate two types of noise trauma that clinical populations face, UT Dallas scientists exposed rats to moderate or intense levels of noise for an hour. One group heard a high-frequency noise at 115 decibels inducing moderate hearing loss, and a second group heard a low-frequency noise at 124 decibels causing severe hearing loss.

For comparison, the American Speech-Language-Hearing Association lists the maximum output of an MP3 player or the sound of a chain saw at about 110 decibels and the siren on an emergency vehicle at 120 decibels. Regular exposure to sounds greater than 100 decibels for more than a minute at a time may lead to permanent hearing loss, according to the NIDCD.

Researchers observed how the two types of hearing loss affected speech sound processing in the rats by recording the neuronal response in the auditory cortex a month after the noise exposure. The auditory cortex, one of the main areas that processes sounds in the brain, is organized on a scale, like a piano. Neurons at one end of the cortex respond to low-frequency sounds, while other neurons at the opposite end react to higher frequencies.

In the group with severe hearing loss, less than one-third of the tested auditory cortex sites that normally respond to sound reacted to stimulation. In the sites that did respond, there were unusual patterns of activity. The neurons reacted slower, the sounds had to be louder and the neurons responded to frequency ranges narrower than normal. Additionally, the rats could not tell the speech sounds apart in a behavioural task they could successfully complete before the hearing loss.

In the group with moderate hearing loss, the area of the cortex responding to sounds didn’t change, but the neurons’ reaction did. A larger area of the auditory cortex responded to low-frequency sounds. Neurons reacting to high frequencies needed more intense sound stimulation and responded slower than those in normal hearing animals. Despite these changes, the rats were still able to discriminate the speech sounds in a behavioural task.

Although the ear is critical to hearing, it is just the first step of many processing stages needed to hold a conversation.  Researchers are beginning to understand how hearing damage alters the brain and makes it hard to process speech, especially in noisy environments.

Source:  The University of Texas at Dallas

 

Tone Frequency Response Organization Defines Rodent Core Auditory Cortex. Optical imaging is another method used to map cortical responses to sound frequency or position of activation on the cochlea. One of the problems with using fMRI to map cochleotopy in auditory cortex is the magnets make a lot of noise. Consequently, it is tricky to determine responses to specific sounds. With optical imaging you can obtain high spatial resolution maps of cortical responses to sound without this large background noise. The map on the left is the optically imaged response to ascending and descending tone scales overlaid with an image of the surface blood vessels for the same region of temporal cortex in the rat. Each color in the colourbar is the a different pure-tone frequency or note in the musical scale used as a sensory stimulus to activate the cortex. Even this small mammal has multiple cochleotopic maps which are complete representations of the cochlear sensory response to tones. On the right is the same functional image overlaid with a vector map. The vector map illustrates the direction and magnitude of change in frequency. The points labeled a, c and e all respond to low frequency tones. The response ascends from low (point a, dark blue) to high (b, green) characteristic frequencies in one representation of the cochlea. Then at point "b" there is a mirror reversal and the response descends from high (b, green) to low (c, dark blue) characteristic frequencies. The anatomy of human auditory cortex and its thalamic inputs remains a bit unclear and therefore it is hard to determine what regions are similar or homologous between human and other mammals. However, it is clear that all mammals have core regions with cochleotopic organization and mirror reversals in that cochleotopy between neighboring core or belt regions. (Figure Source: modified from Higgins et al, 2010)

Tone Frequency Response Organization Defines Rodent Core Auditory Cortex. Optical imaging is another method used to map cortical responses to sound frequency or position of activation on the cochlea. One of the problems with using fMRI to map cochleotopy in auditory cortex is the magnets make a lot of noise. Consequently, it is tricky to determine responses to specific sounds. With optical imaging you can obtain high spatial resolution maps of cortical responses to sound without this large background noise. The map on the left is the optically imaged response to ascending and descending tone scales overlaid with an image of the surface blood vessels for the same region of temporal cortex in the rat. Each color in the colourbar is the a different pure-tone frequency or note in the musical scale used as a sensory stimulus to activate the cortex. Even this small mammal has multiple cochleotopic maps which are complete representations of the cochlear sensory response to tones. On the right is the same functional image overlaid with a vector map. The vector map illustrates the direction and magnitude of change in frequency. The points labeled a, c and e all respond to low frequency tones. The response ascends from low (point a, dark blue) to high (b, green) characteristic frequencies in one representation of the cochlea. Then at point “b” there is a mirror reversal and the response descends from high (b, green) to low (c, dark blue) characteristic frequencies. The anatomy of human auditory cortex and its thalamic inputs remains a bit unclear and therefore it is hard to determine what regions are similar or homologous between human and other mammals. However, it is clear that all mammals have core regions with cochleotopic organization and mirror reversals in that cochleotopy between neighboring core or belt regions. (Figure Source: modified from Higgins et al, 2010)

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.