Hearing test opens window on effects of HIV on brain

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Scientists have found a way to track changes in sound processing in the brains of people living with HIV. Their test provides a simple technique to study how HIV affects the central nervous system.

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By measuring brain signals, the researchers observed the impact of HIV on the central nervous system.

Thanks to advances in antiretroviral therapy over the past 20 years, most people living with HIV can now expect to live long and healthy lives.

However, even if treatment is successful in controlling the virus, people can experience cognitive problems from HIV causing damage to their central nervous system.

Up to 45% of people living with HIV can develop these difficulties, which are collectively referred to as HIV-associated neurocognitive disorder (MAIN).

Although the cause of MAIN remains unknown, scientists have several theories. They speculate, for example, that it may be the result of chronic inflammation, persistent damage from the initial infection, or a toxic effect of antiretroviral drugs.

Alternatively, low levels of the virus can persist in the brain due to the blood brain barrier limit the passage of antiretroviral drugs into the brain.

The signs and symptoms of MAIN can be subtle, making it difficult for doctors to diagnose and watch for early.

One of the problems people living with HIV sometimes report is difficulty understanding speech in the presence of background noise.

Researchers at the Dartmouth Geisel School of Medicine in Hanover, New Hampshire, led a team that studied the hearing of a group of people living with HIV in Dar es Salaam, Tanzania.

They collaborated with scientists from Muhimbili University of Health and Allied Sciences in Dar es Salaam and neuroscientists from Northwestern University in Evanston, IL.

“We initially thought we would find out that HIV affects the ear, but what seems to be affected is the brain’s ability to process sound,” says study co-lead Dr Jay Buckey Jr. ., professor of medicine at Geisel.

To test this idea, they used scalp electrodes to monitor the brain waves of 68 HIV-positive people and 59 uninfected people as they listened to sounds.

Specifically, they recorded a brain response known as Speech Evoked Rate Tracking Response (FFR).

Certain characteristics of brain waves, such as their timing and amplitude, reveal how faithfully the brain encodes an auditory stimulus. The FFR has been shown to be particularly useful for studying how the brain processes information in very complex sounds, such as music and speech.

The precision and accuracy of the FFR may improve with experience, such as in skilled musicians, or worsen as a result of illness or injury, such as a concussion.

“There are many acoustic ingredients in speech, such as pitch, timing, overtones, and phrase,” says study co-lead Nina Kraus, Ph.D., professor of communication sciences and of Neurobiology at Northwestern.

“The FFR allows us to play the sounds of speech in the ear of study participants and determine the quality of the brain’s work by processing these different acoustic ingredients.”

The researchers recorded the FFR while the study participants listened to common speech sounds, such as “ba”, “da” and “ga”.

They examined the extent to which the signal encoded two acoustic characteristics of speech, known as harmonics corresponding to the first forming and the fundamental frequency.

The harmonics corresponding to the first forming identify the units of sound, or “phonemes”, which differentiate one word from another, while the fundamental frequency reflects the frequency at which the vocal cords vibrate, which makes it possible to identify the speaker.

The brains of people living with HIV had similar responses to the brains of people without HIV when processing the fundamental frequency.

They also performed well on standard hearing tests, but the FFR found that the brains of people living with HIV were worse at encoding the first forming.

In other words, they had normal hearing thresholds, but their brains could not distinguish the sounds of different words as precisely.

“When the brain processes sound, it’s not like a volume knob where all of the acoustic ingredients are processed right or wrong,” says Kraus. “With FFR, we are able to see which aspects of auditory processing are affected or diminished and ask:”[I]Is there a specific neural signature that aligns with HIV? “

The researchers published their study in the journal Clinical neurophysiology.

The researchers note that as people get older, even though their hearing remains normal, their brains deteriorate to process the same speech characteristics that the brains of people living with HIV seem to struggle with.

They write that the cognitive problems associated with HIV may therefore be related to early aging.

In the future, they hope other scientists will use FFR not only to study brain dysfunctions associated with HIV, but also other conditions, such as concussions and Alzheimer’s disease.

Standard cognitive function tests involve giving people tasks, such as solving math problems or memorizing a list of words. However, these depend on the language people speak and their culture.

“What is important in our results is that the test does not require any action on the [subject’s] part. It’s passively recorded – subjects can even sleep or watch a movie, ”says Dr. Buckey. “We think FFR holds great promise as a way to assess the brain easily and objectively.”

In their article, the researchers note that the test could prove particularly useful in areas of the world most infected with HIV and with limited health resources.

They write: “While previous studies of neuroimaging and electrophysiology have shown differences between HIV-positive and HIV-positive individuals, these approaches are difficult to transport in resource-limited settings like sub-Saharan Africa, where more than 70% of the world’s HIV-positive population lives and where there is evidence that neurocognitive symptoms are more severe.

“Thus, the FFR is promising as a research tool to deepen [central nervous system] health, especially in resource-limited settings.

The study authors recognize that research projects involving more participants will be needed to confirm their results. They say it would also be interesting to follow changes in FFR from the early stages of infection.

Finally, in light of the evidence suggesting that different strains of the virus may have different neurological effects, they would like to see studies that compare FFR testing in different populations around the world.


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